Publications by authors named "Jan Seuntjens"

164 Publications

Comparison of quantitative and qualitative scoring approaches for radiation-induced pulmonary fibrosis as applied to a preliminary investigation into the efficacy of mesenchymal stem cell delivery methods in a rat model.

BJR Open 2021 5;2(1):20210006. Epub 2021 Jul 5.

Research Institute of the McGill University Healthcare Centre & Medical Physics Unit, CedarsCancer Centre, McGill University Healthcare Centre, Montreal, Canada.

Objectives: Compare a quantitative, algorithm-driven, and qualitative, pathologist-driven, scoring of radiation-induced pulmonary fibrosis (RIPF). And using these scoring models to derive preliminary comparisons on the effects of different mesenchymal stem cell (MSC) administration modalities in reducing RIPF.

Methods: 25 rats were randomized into 5 groups: non-irradiated control (CG), irradiated control (CR), intraperitoneally administered granulocyte-macrophage colony stimulating factor or GM-CSF (Drug), intravascularly administered MSC (IV), and intratracheally administered MSC (IT). All groups, except CG, received an 18 Gy conformal dose to the right lung. Drug, IV and IT groups were treated immediately after irradiation. After 24 weeks of observation, rats were euthanized, their lungs excised, fixed and stained with Masson's Trichrome. Samples were anonymized and RIPF was scored qualitatively by a certified pathologist and quantitatively using ImageScope. An analysis of association was conducted, and two binary classifiers trained to validate the integrity of both qualitative and quantitative scoring. Differences between the treatment groups, as assessed by the pathologist score, were then tested by variance component analysis and mixed models for differences in RIPF outcomes.

Results: There is agreement between qualitative and quantitative scoring for RIPF grades from 4 to 7. Both classifiers performed similarly on the testing set (AUC = 0.923) indicating accordance between the qualitative and quantitative scoring. For comparisons between MSC infusion modalities, the Drug group had better outcomes (mean pathologist scoring of 3.96), correlating with significantly better RIPF outcomes than IV [lower by 0.97, = 0.047, 95% CI = (0.013, 1.918)] and resulting in an improvement over CR [lower by 0.93, = 0.037, 95% CI = (0.062, 1.800].

Conclusion: Quantitative image analysis may help in the assessment of therapeutic interventions for RIPF and can serve as a scoring surrogate in differentiating between severe and mild cases of RIPF. Preliminary data demonstrate that the use of GM-CSF was best correlated with lower RIPF severity.

Advances In Knowledge: Quantitative image analysis can be a viable supplemental system of quality control and triaging in situations where pathologist work hours or resources are limited. The use of different MSC administration methods can result in different degrees of MSC efficacy and study outcomes.
View Article and Find Full Text PDF

Download full-text PDF

Source
http://dx.doi.org/10.1259/bjro.20210006DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC8320116PMC
July 2021

Investigating the impact of the CT Hounsfield unit range on radiomic feature stability using dual energy CT data.

Phys Med 2021 Aug 27;88:272-277. Epub 2021 Jul 27.

McGill University, Medical Physics Unit, Montreal, Canada.

Purpose: Radiomic texture calculation requires discretizing image intensities within the region-of-interest. FBN (fixed-bin-number), FBS (fixed-bin-size) and FBN and FBS with intensity equalization (FBNequal, FBSequal) are four discretization approaches. A crucial choice is the voxel intensity (Hounsfield units, or HU) binning range. We assessed the effect of this choice on radiomic features.

Methods: The dataset comprised 95 patients with head-and-neck squamous-cell-carcinoma. Dual energy CT data was reconstructed at 21 electron energies (40, 45,… 140 keV). Each of 94 texture features were calculated with 64 extraction parameters. All features were calculated five times: original choice, left shift (-10/-20 HU), right shift (+10/+20 HU). For each feature, Spearman correlation between nominal and four variants were calculated to determine feature stability. This was done for six texture feature types (GLCM, GLRLM, GLSZM, GLDZM, NGTDM, and NGLDM) separately. This analysis was repeated for the four binning algorithms. Effect of feature instability on predictive ability was studied for lymphadenopathy as endpoint.

Results: FBN and FBNequal algorithms showed good stability (correlation values consistently >0.9). For FBS and FBSequal algorithms, while median values exceeded 0.9, the 95% lower bound decreased as a function of energy, with poor performance over the entire spectrum. FBNequal was the most stable algorithm, and FBS the least.

Conclusions: We believe this is the first multi-energy systematic study of the impact of CT HU range used during intensity discretization for radiomic feature extraction. Future analyses should account for this source of uncertainty when evaluating the robustness of their radiomic signature.
View Article and Find Full Text PDF

Download full-text PDF

Source
http://dx.doi.org/10.1016/j.ejmp.2021.07.023DOI Listing
August 2021

The Rapidly-Developing Area of Radiocardiology: Principles, Complications and Applications of Radiotherapy on the Heart.

Can J Cardiol 2021 Jul 22. Epub 2021 Jul 22.

Department of Medicine and Research Center, Montreal Heart Institute and Université de Montréal; Cardiovascular Division, Department of Medicine, Brigham and Women's Hospital, Boston, Massachusetts.

Ventricular arrhythmias are the leading cause of sudden cardiac death. Current treatment strategies for VT, including antiarrhythmic drugs and catheter ablation, have limited efficacy in patients with structural heart disease. Non-invasive ablation with the use of externally applied radiation (cardiac radio-ablation) has emerged as a promising and novel approach to treating recurrent VTs. However, the heart is generally an "organ at risk" for radiation treatments, such that very little is known on the effects of radiotherapy on cardiac ultrastructure and electrophysiological properties. Furthermore, there has been limited interaction between the fields of cardiology and radiation oncology and physics. The advent of cardiac radio-ablation will undoubtedly increase interactions between cardiologists, cardiac electrophysiologists, radiation oncologists and physicists There is an important knowledge gap separating these specialties while scientific developments, technical optimization and improvements are dependent on intense multidisciplinary collaboration. This manuscript seeks to review the basic of radiation physics and biology for cardiovascular specialists in an effort to facilitate constructive scientific and clinical collaborations to improve patient outcomes.
View Article and Find Full Text PDF

Download full-text PDF

Source
http://dx.doi.org/10.1016/j.cjca.2021.07.011DOI Listing
July 2021

Determination of field output correction factors of radiophotoluminescence glass dosimeter and CC01 ionization chamber and validation against IAEA-AAPM TRS-483 code of practice.

Phys Med 2021 Aug 16;88:167-174. Epub 2021 Jul 16.

Medical Physics Unit, McGill University, Montreal, Québec H3G 1A4, Canada.

Purpose: To determine the field output correction factors of the radiophotoluminescence glass dosimeter (RPLGD) in parallel and perpendicular orientations with reference to CC01, the ionization chamber.

Methods: The dose to a small water volume and the sensitive volume of the RPLGD and the IBA-CC01 were determined for 6-MV, 100-cm SAD, 10-cm depth using egs_chamber user-code. The RPLGD in perpendicular and parallel orientations to the beam axis were studied. The field output correction factors of each detector for 0.5 × 0.5 to 10 × 10 cm field sizes were determined. These field output correction factors were validated by comparing field output factors against data determined from IAEA-AAPM TRS-483 code of practice.

Results: The field output correction factors of all detectors were within 5% for field sizes down to 0.8 × 0.8 cm. For 0.5 × 0.5 cm, the field output correction factors of CC01, RPLGD in perpendicular and parallel orientations differed from unity by 14%, 19%, and 5%, respectively. The percentage difference between field output factors determined using RPLGD and CC01 data, corrected using the field output correction factors determined in this work and measurements with CC01 data corrected using TRS-483, was less than 3% for all field sizes, except for the smallest field size of RPLGD in perpendicular orientation and the CC01.

Conclusions: The field output correction factors of RPLGD and CC01 are reported. The validation proves that RPLGD in parallel orientation combined with the field output correction factors is the most suitable for determining the field output factors for the smallest field used in this study.
View Article and Find Full Text PDF

Download full-text PDF

Source
http://dx.doi.org/10.1016/j.ejmp.2021.07.004DOI Listing
August 2021

Ion chamber and film-based quality assurance of mixed electron-photon radiation therapy.

Med Phys 2021 Jul 5. Epub 2021 Jul 5.

Department of Mathematics and Industrial Engineering, Polytechnique Montréal, Montreal, QC, Canada.

Purpose: In previous work, we demonstrated that mixed electron-photon radiation therapy (MBRT) produces treatment plans with improved normal tissue sparing and similar target coverage, when compared to photon-only plans. The purpose of this work was to validate the MBRT delivery process on a Varian TrueBeam accelerator and laying the groundwork for a patient-specific quality assurance (QA) protocol based on ion chamber point measurements and 2D film measurements.

Methods: MC beam models used to calculate the MBRT dose distributions of each modality (photons/electrons) were validated with a single-angle beam MBRT treatment plan delivered on a slab of Solid Water phantom with a film positioned at a depth of 2 cm. The measured film absorbed dose was compared to the calculated dose. To validate clinical deliveries, a polymethyl methacrylate (PMMA) cylinder was machined and holes were made to fit an ionization chamber. A complex MBRT plan involving a photon arc and three electron delivery angles was created with the aim of reproducing a clinically realistic dose distribution in typical soft tissue sarcoma tumours of the extremities. The treatment plan was delivered on the PMMA cylinder. Point measurements were taken with an Exradin A1SL chamber at two nominal depths: 1.4 cm and 2.1 cm. The plan was also delivered on a second identical phantom with an insert at 2 cm depth, where a film was placed. An existing EGSnrc user-code, SPRRZnrc, was modified to calculate the stopping power ratios between any materials in the same voxelized geometry used for dose calculation purposes. This modified code, called SPRXYZnrc, was used to calculate a correction factor, , accounting for the differences in electron fluence spectrum at the measurement point compared to that at reference conditions. The uncertainty associated with neglecting potential ionization chamber fluence perturbation correction factors using this approach was estimated.

Results: The film measurement from the Solid Water phantom treatment plan was in good agreement with the simulated dose distribution, with a gamma pass rate of 96.1% for a 3%/2 mm criteria. For the PMMA phantom delivery, for the same gamma criteria, the pass rate was 97.3%. The ion chamber measurements of the total delivered dose agreed with the MC-simulated dose within 2.1%. The beam quality correction factors amounted to, at most, a 4% correction on the ion chamber measurement. However, individual contribution of low electron energies proved difficult to precisely measure due to their steep dose gradients, with disagreements of up to 28% ± 15% at 2.1 cm depth (6 MeV). Ion chamber measurement procedure of electron beams was achieved in less than 5 min, and the entire validation process including phantom setup was performed in less than 30 min.

Conclusion: The agreement between measured and simulated MBRT doses indicates that the dose distributions obtained from the MBRT treatment planning algorithm are realistically achievable. The SPRXYZnrc MC code allowed for convenient calculations of simultaneously with the dose distributions, laying the groundwork for patient-specific QA protocol practical for clinical use. Further investigation is needed to establish the accuracy of our ionization chamber correction factors calculations at low electron energies.
View Article and Find Full Text PDF

Download full-text PDF

Source
http://dx.doi.org/10.1002/mp.15081DOI Listing
July 2021

Recommendations on the practice of calibration, dosimetry, and quality assurance for gamma stereotactic radiosurgery: Report of AAPM Task Group 178.

Med Phys 2021 Jul;48(7):e733-e770

San Diego Gamma Knife Center, La Jolla, CA, 92037, USA.

The American Association of Physicists in Medicine (AAPM) formed Task Group 178 (TG-178) to perform the following tasks: review in-phantom and in-air calibration protocols for gamma stereotactic radiosurgery (GSR), suggest a dose rate calibration protocol that can be successfully utilized with all gamma stereotactic radiosurgery (GSR) devices, and update quality assurance (QA) protocols in TG-42 (AAPM Report 54, 1995) for static GSR devices. The TG-178 report recommends a GSR dose rate calibration formalism and provides tabulated data to implement it for ionization chambers commonly used in GSR dosimetry. The report also describes routine mechanical, dosimetric, and safety checks for GSR devices, and provides treatment process quality assurance recommendations. Sample worksheets, checklists, and practical suggestions regarding some QA procedures are given in appendices. The overall goal of the report is to make recommendations that help standardize GSR physics practices and promote the safe implementation of GSR technologies.
View Article and Find Full Text PDF

Download full-text PDF

Source
http://dx.doi.org/10.1002/mp.14831DOI Listing
July 2021

Strategic Training in Transdisciplinary Radiation Science for the 21st Century (STARS21): 15-Year Evaluation of an Innovative Research Training Program.

Int J Radiat Oncol Biol Phys 2021 07 10;110(3):656-666. Epub 2021 Jan 10.

Department of Radiation Oncology, University of Toronto, Ontario, Canada; Radiation Medicine Program, Princess Margaret Cancer Centre, University Health Network, Toronto, Ontario, Canada. Electronic address:

Purpose: To evaluate the 15-year impact of a transdisciplinary research training program for graduate students, postdoctoral fellows, and clinical trainees focused on radiation science, entitled Strategic Training in Transdisciplinary Radiation Science for the 21st Century (STARS21) with a primary objective to build capacity in radiation research.

Methods And Materials: Alumni (n = 128) and mentors (n = 41) who participated in STARS21 between 2003 and 2018 were sent an anonymized online survey designed to evaluate the program. Twelve alumni and 7 mentors also volunteered to participate in semistructured interviews. The transcribed interviews were coded and analyzed using NVivo12-Pro software. Alumni employment and publications were assessed from program records and by web-based search queries.

Results: Alumni are located in 11 countries, and nearly 90% are employed in a research-oriented career and continue to publish in radiation medicine- or cancer-related fields. Of those invited, 46 alumni (36%) and 12 mentors (29%) completed the online survey. Approximately 87% of alumni valued interdisciplinary collaboration, and 80% indicated that STARS21 had encouraged them to pursue such collaborations. Alumni emphasized that STARS21 assisted their career development, and the majority of alumni and mentors would recommend STARS21 to other trainees (4.48 and 4.58, respectively; 5 = strongly agree). The time invested in the program was perceived by mentors as worthwhile for the knowledge and skills gained by trainees (4.67; 5 = strongly agree), and 64% of mentors indicated that these benefits were associated with improved trainee research productivity. From the alumni and mentor perspectives, the valuable skills acquired from STARS21 included scientific communication (85% and 83%, respectively) and networking (83% and 92%, respectively).

Conclusions: STARS21 is an innovative research training program that promotes interdisciplinary collaboration in radiation medicine research, which is valued by alumni and mentor respondents. Alumni can acquire important skill sets for career development, with a large proportion of alumni currently engaged in radiation research around the world.
View Article and Find Full Text PDF

Download full-text PDF

Source
http://dx.doi.org/10.1016/j.ijrobp.2021.01.010DOI Listing
July 2021

IAEA-AAPM TRS-483-based reference dosimetry of the new RefleXion biology-guided radiotherapy (BgRT) machine.

Med Phys 2021 Apr 4;48(4):1884-1892. Epub 2021 Mar 4.

Medical Physics Unit, McGill University, Montreal, Quebec, H4A 3J1, Canada.

Purpose: The purpose of this study is to provide data for the calibration of the recent biology-guided radiotherapy (BgRT) machine (Hayward, CA, USA) following the International Atomic Energy Agency (IAEA) and the American Association of Physicists in Medicine (AAPM) TRS-483 code of practice (COP) (Palmans et al. International Atomic Energy Agency, Vienna, 2017) and (Mirzakhanian et al. Med Phys, 2020).

Methods: In RefleXion BgRT machine, reference dosimetry was performed using two methodologies described in TRS-483 and (Mirzakhanian et al. Med Phys, 2020) In the first approach (Approach 1), the generic beam quality correction factor was calculated using an accurate Monte Carlo (MC) model of the beam and of six ionization chamber types. The is a beam quality factor that corrects (absorbed dose to water calibration coefficient in a calibration beam quality ) for the differences between the response of the chamber in the conventional reference calibration field with beam quality at the standards laboratory and the response of the chamber in the user's A field with beam quality . Field A represents the reference calibration field that does not fulfill msr conditions. In the second approach (Approach 2), a square equivalent field size was determined for field A of and . Knowing the equivalent field size, the beam quality specifier for the hypothetical field size was derived. This was used to calculate the beam quality correction factor analytically for the six chamber types using the TRS-398. (Andreo et al. Int Atom Energy Agency 420, 2001) Here, TRS-398 was used instead of TRS-483 since the beam quality correction values for the chambers used in this study are not tabulated in TRS-483. The accuracy of Approach 2 is studied in comparison to Approach 1.

Results: Among the chambers, the PTW 31010 had the largest correction due to the volume averaging effect. The smallest-volume chamber (IBA CC01) had the smallest correction followed by the other microchambers Exradin-A14 and -A14SL. The equivalent square fields sizes were found to be 3.6 cm and 4.8 cm for the and field sizes, respectively. The beam quality correction factors calculated using the two approaches were within 0.27% for all chambers except IBA CC01. The latter chamber has an electrode made of steel and the differences between the correction calculated using the two approaches was the largest, that is, 0.5%.

Conclusions: In this study, we provided the values as a function of the beam quality specifier at the RefleXion BgRT setup ( and ) for six chamber types. We suggest using the first approach for calibration of the RefleXion BgRT machine. However, if the MC correction is not available for a user's detector, the user can use the second approach for estimating the beam quality correction factor to sufficient accuracy (0.3%) provided the chamber electrode is not made of high Z material.
View Article and Find Full Text PDF

Download full-text PDF

Source
http://dx.doi.org/10.1002/mp.14631DOI Listing
April 2021

Monte Carlo calculation of the relative TG-43 dosimetry parameters for the INTRABEAM electronic brachytherapy source.

Phys Med Biol 2020 12 22;65(24):245041. Epub 2020 Dec 22.

Medical Physics Unit, McGill University and Cedars Cancer Center, Montreal, Canada.

The INTRABEAM system (Carl Zeiss Meditec AG, Jena, Germany) is an electronic brachytherapy (eBT) device designed for intraoperative radiotherapy applications. To date, the INTRABEAM x-ray source has not been characterized according to the AAPM TG-43 specifications for brachytherapy sources. This restricts its modelling in commercial treatment planning systems (TPSs), with the consequence that the doses to organs at risk are unknown. The aim of this work is to characterize the INTRABEAM source according to the TG-43 brachytherapy dosimetry protocol. The dose distribution in water around the source was determined with Monte Carlo (MC) calculations. For the validation of the MC model, depth dose calculations along the source longitudinal axis were compared with measurements using a soft x-ray ionization chamber (PTW 34013) and two synthetic diamond detectors (microDiamond PTW TN60019). In our results, the measurements in water agreed with the MC model calculations within uncertainties. The use of the microDiamond detector yielded better agreement with MC calculations, within estimated uncertainties, compared to the ionization chamber at points of steeper dose gradients. The radial dose function showed a steep fall-off close to the INTRABEAM source ([Formula: see text]10 mm) with a gradient higher than that of commonly used brachytherapy radionuclides (Ir, I and Pd), with values of 2.510, 1.645 and 1.232 at 4, 6 and 8 mm, respectively. The radial dose function partially flattens at larger distances with a fall-off comparable to that of the Xoft Axxent® (iCAD, Inc., Nashua, NH) eBT system. The simulated 2D polar anisotropy close to the bare probe walls showed deviations from unity of up to 55% at 10 mm and 155°. This work presents the MC calculated TG-43 parameters for the INTRABEAM, which constitute the necessary data for the characterization of the source as required by a TPS used in clinical dose calculations.
View Article and Find Full Text PDF

Download full-text PDF

Source
http://dx.doi.org/10.1088/1361-6560/abc6f1DOI Listing
December 2020

Positional and angular tracking of HDR Ir source for brachytherapy quality assurance using radiochromic film dosimetry.

Med Phys 2020 Dec 2;47(12):6122-6139. Epub 2020 Nov 2.

Department of Radiation Oncology, Dana Farber/Brigham and Women's Cancer Center, Harvard Medical School, Boston, MA, 02115, USA.

Purpose: To quantify and verify the dosimetric impact of high-dose rate (HDR) source positional uncertainty in brachytherapy, and to introduce a model for three-dimensional (3D) position tracking of the HDR source based on a two-dimensional (2D) measurement. This model has been utilized for the development of a comprehensive source quality assurance (QA) method using radiochromic film (RCF) dosimetry including assessment of different digitization uncertainties.

Methods: An algorithm was developed and verified to generate 2D dose maps of the mHDR-V2 Ir source (Elekta, Veenendaal, Netherlands) based on the AAPM TG-43 formalism. The limits of the dosimetric error associated with source (0.9 mm diameter) positional uncertainty were evaluated and experimentally verified with EBT3 film measurements for 6F (2.0 mm diameter) and 4F (1.3 mm diameter) size catheters at the surface (4F, 6F) and 10 mm further (4F only). To quantify this uncertainty, a source tracking model was developed to incorporate the unique geometric features of all isodose lines (IDLs) within any given 2D dose map away from the source. The tracking model normalized the dose map to its maximum, then quantified the IDLs using blob analysis based on features such as area, perimeter, weighted centroid, elliptic orientation, and circularity. The Pearson correlation coefficients (PCCs) between these features and source coordinates (x, y, z, θ , θ ) were calculated. To experimentally verify the accuracy of the tracking model, EBT3 film pieces were positioned within a Solid Water® (SW) phantom above and below the source and they were exposed simultaneously.

Results: The maximum measured dosimetric variations on the 6F and 4F catheter surfaces were 39.8% and 36.1%, respectively. At 10 mm further, the variation reduced to 2.6% for the 4F catheter which is in agreement with the calculations. The source center (x, y) was strongly correlated with the low IDL-weighted centroid (PCC = 0.99), while the distance to source (z) was correlated with the IDL areas (PCC = 0.96) and perimeters (PCC = 0.99). The source orientation θ was correlated with the difference between high and low IDL-weighted centroids (PCC = 0.98), while θ was correlated with the elliptic orientation of the 60-90% IDLs (PCC = 0.97) for a maximum distance of z = 5 mm. Beyond 5 mm, IDL circularity was significant, therefore limiting the determination of θ (PCC ≤ 0.48). The measured positional errors from the film sets above and below the source indicated a source position at the bottom of the catheter (-0.24 ± 0.07 mm).

Conclusions: Isodose line features of a 2D dose map away from the HDR source can reveal its spatial coordinates. RCF was shown to be a suitable dosimeter for source tracking and dosimetry. This technique offers a novel source QA method and has the potential to be used for QA of commercial and customized applicators.
View Article and Find Full Text PDF

Download full-text PDF

Source
http://dx.doi.org/10.1002/mp.14540DOI Listing
December 2020

On a local (de-)trapping model for highly doped Pr radioluminescent and persistent luminescent nanoparticles.

Nanoscale 2020 Oct;12(40):20759-20766

Concordia University Centre for NanoScience Research, 7141 Rue Sherbrooke Ouest, Montreal, QC H4B 1R6, Canada.

Trivalent praseodymium exhibits a wide range of luminescent phenomena when doped into a variety of different materials. Herein, radioluminescent NaLuF4:20%Pr3+ nanoparticles are studied. Four different samples of this composition were prepared ranging from 400-70 nm in size. Kinetic studies of radioluminescence as a function of X-ray irradiation time revealed a decrease in the emissions originating from the 1S0 level, due to the formation or optical activation of defects during excitation, and a simultaneous increase in the visible emissions resulting from the lower optical levels. Thermoluminescence measurements elucidated that a local de-trapping mechanism was responsible for the increase in steady state emission and persistent luminescence originating from the lower optical levels. The results and mechanism described through this study serve to provide a novel nanoparticle composition with versatile luminescent properties and provides experimental evidence in favor of a local trapping model.
View Article and Find Full Text PDF

Download full-text PDF

Source
http://dx.doi.org/10.1039/d0nr06577cDOI Listing
October 2020

Step-size effect on calculated photon and electron beam Cherenkov-to-dose conversion factors.

Phys Med 2020 Oct 8;78:32-37. Epub 2020 Sep 8.

Medical Physics Unit, McGill University, Montreal, QC H4A 3J1, Canada.

Purpose: Previous work presented and validated in-water Cherenkov emission (CE)-based radiotherapy dosimetry. Condensed history Monte Carlo (MC)-calculated electron beam CE-to-dose conversion with <4π CE detection, however, could exhibit step-size dependence. This work presents a physics update and numerical study of this step-size dependence in photon and electron beams, elucidates the CE generation physics, and guides further research.

Methods: The CE-to-dose conversion, k, is calculated for photons (6X, 15X) and electrons (6E, 20E) on-axis in-water with: θ±δθ∈{90°±90°(4π),90°±5°,45°±45°,90°±45°}, 10 cm equivalent square, 100 cm SSD, 1cm voxel radius and beam-dependent length. Relative deviation from single-scattering (SS) simulation is evaluated on maximum fractional electron step energy loss ESTEPE∈0.01-0.25. Standard uncertainties (k=1, 10histories) are reported. A simplified method considering only the straight step direction is also implemented.

Results: No significant step-size effect (>0.1%) was observed for dose and all k, except for surface dosimetry at 90°±5° (-1.6%±0.5%, 20E), which is not recommended. Electron SS deviation uncertainties (k=1), otherwise, varied from <0.2% overall to <0.1% with large apertures. Photon uncertainties varied from <1.1% overall to <0.2% non-superficially with large apertures. The simplified straight-step method exhibited overall greater deviation from SS, most notably -2.8%±0.1% (6E) and -2.5%±0.4% (20E) superficially with 90°±45°, and -1.4%±0.3% (6X) and -0.6%±0.2% (15X) non-superficially with 90°±5° for ESTEPE∈[0.10,0.25].

Conclusions: We demonstrate step-size independence of newly-implemented correction in EGSnrc directional Cherenkov calculations. This advances clinical CE-based dosimetry and is useful for the general Monte Carlo community.
View Article and Find Full Text PDF

Download full-text PDF

Source
http://dx.doi.org/10.1016/j.ejmp.2020.08.015DOI Listing
October 2020

Extending the IAEA-AAPM TRS-483 methodology for radiation therapy machines with field sizes down to 10 × 2 cm.

Med Phys 2020 Oct 20;47(10):5209-5221. Epub 2020 Aug 20.

Medical Physics Unit, McGill University, Montreal, QC, H4A 3J1, Canada.

Purpose: The purpose of this study is to provide a calibration methodology for radiation therapy machines where the closest field to the conventional reference field may not meet the lateral charged particle equilibrium (LCPE) condition of the machine-specific reference (msr) field. We provided two methodologies by extending the International Atomic Energy Agency (IAEA) and the American Association of Physicists in Medicine (AAPM) TRS-483 code of practice (COP) (Palmans et al. TRS-483: Dosimetry of small static fields used in external beam radiotherapy: an international code of practice for reference and relative dose determination; 2017) methodology for the calibration of radiation therapy machines with 6 MV flattening filter free (FFF) beam and with field sizes down to 10 ×  2 cm .

Methods: Two methods of calibration were provided following the TRS-483. In calibration Method I, the generic correction factors were calculated using Monte Carlo (MC) for seven detectors and rectangular physical field sizes ranging from 10 × 2 cm to 10 × 10 cm . In calibration Method II, we extended the methodology in TRS-483 for deriving the equivalent square msr field sizes for rectangular field sizes down to 10 × 2 cm . The beam quality specifier for a hypothetical 10 × 10 cm field was derived by extending the methodology provided in the TRS-483. Since the beam quality correction values for the conventional reference field ( ) tabulated in TRS-483 are provided only for large reference chambers, we calculated the values analytically for our beam quality specifier and chambers used, using interaction data in TRS-398 (Andreo, et al. TRS-398: Absorbed dose determination in external beam radiotherapy: an international code of practice for dosimetry based on standards of absorbed dose to water; 2001).

Results: The correction values calculated using the first method for chambers with an electrode made of C552 almost did not vary across the different field sizes studied (within 0.1%) while it varied by 1.6% for IBA CC01 with electrode made of steel. Extending the equivalent field and beam quality specifier determination methodology of TRS-483 resulted in a maximum error of 1.3% on the beam quality specifier for the 2 × 2 cm field size. However, this had a negligible impact on the values (less than 0.1%). For chambers with C552 and Al electrode material, the correction factors determined using the two methods of calibration were in agreement to within 0.5%. However, for the chambers with electrode made of higher atomic number (Z), the difference between the two methodologies could be as large as 1.5%. It was shown that this difference can be reduced to less than 0.5% if central electrode perturbation effects and values introduced in TRS-483 were taken into account.

Conclusions: In this study, applying the correction values calculated using the calibration Method I to the chamber reading improved the consistency on an absorbed dose determination from 0.5% to 0.1% standard deviation (except for the Exradin A16). For this reason we recommend using calibration Method I. If the values are not available for the user's detector, calibration Method II can be used to predict the correction factors. However, the second methodology should not be used for chambers with electrode made of high-Z material unless the electrode perturbation effects and values are taken into account.
View Article and Find Full Text PDF

Download full-text PDF

Source
http://dx.doi.org/10.1002/mp.14325DOI Listing
October 2020

Clinical Implication of Dosimetry Formalisms for Electronic Low-Energy Photon Intraoperative Radiation Therapy.

Pract Radiat Oncol 2021 Jan-Feb;11(1):e114-e121. Epub 2020 Aug 11.

Medical Physics Unit, McGill University, Montreal, QC, Canada.

Purpose: Intraoperative radiation therapy (IORT) using the INTRABEAM, a miniature x-ray source, has shown to be effective in treating breast cancer. However, recent investigations have suggested a significant deviation between the reported and delivered doses. In this work, the dose delivered by INTRABEAM in the TARGIT breast protocol was investigated, along with the dose from the Xoft Axxent, another source used in breast IORT.

Methods And Materials: The absorbed dose from the INTRABEAM was determined from ionization chamber measurements using: (a) the manufacturer-recommended formula (Zeiss V4.0 method), (b) a Monte Carlo calculated chamber conversion factor (C method), and (c) the formula consistent with the TARGIT breast protocol (TARGIT method). The dose from the Xoft Axxent was determined from ionization chamber measurements using the Zeiss V4.0 method and calculated using the American Association of Physicists in Medicine TG-43 formalism.

Results: For a nominal TARGIT prescription of 20 Gy, the dose at the INTRABEAM applicator surface ranged from 25.2 to 31.7 Gy according to the C method for the largest (5 cm) and smallest (1.5 cm) diameter applicator, respectively. The Zeiss V4.0 method results were 7% to 10% lower (23.2 to 28.6 Gy). At 1 cm depth, the C and Zeiss V4.0 absorbed doses were also larger than those predicted by the TARGIT method. The dose at 1 cm depth from the Xoft Axxent for a surface dose of 20 Gy was slightly less than INTRABEAM (3%-7% compared with C method). An exception was for the 3 cm applicator, where the Xoft dose was appreciably lower (31%).

Conclusions: The doses delivered in the TARGIT breast protocol with INTRABEAM were significantly greater than the prescribed 20 Gy and depended on the size of spherical applicator used. Breast IORT treatments with the Xoft Axxent received less dose compared with TARGIT INTRABEAM, which could have implications for studies comparing clinical outcomes between the 2 devices.
View Article and Find Full Text PDF

Download full-text PDF

Source
http://dx.doi.org/10.1016/j.prro.2020.07.005DOI Listing
August 2021

Large-scale dosimetric assessment of Monte Carlo recalculated doses for lung robotic stereotactic body radiation therapy.

Phys Med 2020 Aug 20;76:7-15. Epub 2020 Jun 20.

Medical Physics Unit, McGill University and Cedars Cancer Center, 1001 Boulevard Décarie, Montréal, QC H4A 3J1, Canada.

Owing to its short computation time and simplicity, the Ray-Tracing algorithm (RAT) has long been used to calculate dose distributions for the CyberKnife system. However, it is known that RAT fails to fully account for tissue heterogeneity and is therefore inaccurate in the lung. The aim of this study is to make a dosimetric assessment of 219 non-small cell lung cancer CyberKnife plans by recalculating their dose distributions using an independent Monte Carlo (MC) method. For plans initially calculated by RAT without heterogeneity corrections, target coverage was found to be significantly compromised when considering MC doses. Only 35.4% of plans were found to comply to their prescription doses. If the normal tissue dose limits were respected in the treatment planning dose, the MC recalculated dose did not exceed these limits in over 97% of the plans. Comparison of RAT and recalculated-MC doses confirmed the overestimation of RAT doses observed in previous studies. An inverse correlation between the RAT/MC dose ratio and the target size was also found to be statistically significant (p<10), consistent with other studies. In addition, the inaccuracy and variability in target coverage incurred from dose calculations using RAT without heterogeneity corrections was demonstrated. On average, no clinically relevant differences were observed between MC-calculated dose-to-water and dose-to-medium for all tissues investigated (⩽1%). Patients receiving a dose D larger than 119 Gy in EQD2 (or ≈52 Gy in 3 fractions) as recalculated by MC were observed to have significantly superior loco-regional progression-free survival rates (p=0.02) with a hazard ratio of 3.45 (95%CI: 1.14-10.5).
View Article and Find Full Text PDF

Download full-text PDF

Source
http://dx.doi.org/10.1016/j.ejmp.2020.06.006DOI Listing
August 2020

Simultaneous trajectory generation and volumetric modulated arc therapy optimization.

Med Phys 2020 Jul 27;47(7):3078-3090. Epub 2020 Apr 27.

Medical Physics Unit, McGill University & Research Institute of the McGill University Health Centre, Montréal, QC, H4A 3J1, Canada.

Purpose: Trajectory-based treatment planning involves the combination of a gantry-couch trajectory with volumetric modulated arc therapy (VMAT) treatment plan optimization. This work presents the implementation of an optimization methodology that generates a trajectory simultaneous with treatment plan optimization (simTr-VMAT).

Methods: The optimization algorithm is based on the column generation approach, in which a treatment plan is iteratively constructed through the solution of a subproblem called the "pricing problem." The property of the pricing problem to rank candidate apertures based on their associated price is leveraged to select an optimal aperture while simultaneously determining the trajectory path. A progressively increasing gantry-couch grid resolution is used to provide an initial coarse sampling of the angular solution space while maintaining fine control point spacing with the final treatment plan. The trajectory optimization was applied and compared to coplanar VMAT treatment plans for a lung patient, a glioblastoma patient, and a prostate patient. Algorithm validation was performed through the generation of 5000 random trajectories and optimization using column generation VMAT for each patient case, representing the solution space for the trajectory optimization problem. The simTr-VMAT trajectories were compared against these random trajectories based on a quality metric that prefers trajectories with few control points and low objective function value over long, inefficient trajectories.

Results: For the lung patient, the simTr-VMAT plan resulted in a decrease of the mean dose of 1.5 and 1.0 Gy to the heart and ipsilateral lung, respectively. For the glioblastoma patient, the simTr-VMAT plan resulted in improved planning target volume coverage with a decrease in mean dose to the eyes, lens, nose, and contralateral temporal lobe between 2 and 7 Gy. The prostate patient showed no clinically relevant dosimetric improvement. The simTr-VMAT treatment plans ranked at the 99.6, 96.3, and 99.4 percentiles compared to the distribution of randomly generated trajectories for the lung, glioblastoma, and prostate patients, respectively.

Conclusion: The simTr-VMAT optimization methodology resulted in treatment plans with equivalent or improved dosimetric outcomes compared to coplanar VMAT treatment plans, with the trajectories resulting from the optimization ranking among the optimal trajectories for each patient case.
View Article and Find Full Text PDF

Download full-text PDF

Source
http://dx.doi.org/10.1002/mp.14155DOI Listing
July 2020

Monte Carlo and water calorimetric determination of kilovoltage beam radiotherapy ionization chamber correction factors.

Phys Med Biol 2020 05 11;65(10):105001. Epub 2020 May 11.

Medical Physics Unit, McGill University, Cedars Cancer Centre, Montréal, QC, H4A 3J1, Canada.

The in-phantom calibration method for radiotherapy kilovoltage x-ray beams requires ionization chamber correction factors. The overall ionization chamber correction factor accounts for changes in the chamber response due to the displacement of water by the chamber cavity and wall, the presence of the stem and the change in incident photon energy and angular distribution in the phantom to that in air. A waterproof sheath, if required, is accounted for in a sheath correction factor. The aim of this study is to determine chamber correction factors through Monte Carlo (MC) simulations and water calorimetry measurements. Correction factors are determined for the PTW TM30013, NE2571, IBA FC65-G, IBA FC65-P and Exradin A12 ionization chambers. They are compared to experimental values obtained at the German national metrology institute Physikalisch-Technische Bundesanstalt (PTB) with their water calorimetry-based absorbed dose to water primary standard and at other national metrological institutes. An uncertainty analysis considers the contributions to the uncertainty on the chamber correction factors from the field size, photon cross sections, photon fluence spectra and chamber wall and central electrode dimensions. The MC calculated chamber correction factors are within 2.2% of unity with a standard uncertainty of 0.3%. For the 50 kV and 100 - 140 kV radiation beam qualities, the calculated correction factors deviate from the measured correction factors (with a standard uncertainty of 1%) by up to 2.6%. The calculated chamber correction factors for the PTW TM30013 and Exradin A12 are consistent with those derived from the BIPM kilovoltage primary standard. The inconsistencies between the calculated and experimental chamber correction factors indicate the need to further investigate the accuracy of kilovoltage absorbed dose to water primary standards and the use of MC simulations to determine kilovoltage beam chamber correction factors.
View Article and Find Full Text PDF

Download full-text PDF

Source
http://dx.doi.org/10.1088/1361-6560/ab82e7DOI Listing
May 2020

Trajectory-based VMAT for cranial targets with delivery at shortened SAD.

Med Phys 2020 Jul 27;47(7):3103-3112. Epub 2020 Apr 27.

Medical Physics Unit, McGill University & Research Institute of the McGill University Health Centre, Montréal, QC, H4A 3J1, Canada.

Introduction: Trajectory-based volumetric modulated arc therapy (tr-VMAT) treatment plans enable the option for noncoplanar delivery yielding steeper dose gradients and increased sparing of critical structures compared to conventional treatment plans. The addition of translational couch motion to shorten the effective source-to-axis distance (SAD) may result in improved delivery precision and an increased effective dose rate. In this work, tr-VMAT treatment plans using a noncoplanar "baseball stitch" trajectory were implemented, applied to patients presented with cranial targets, and compared to the clinical treatment plans.

Methods: A treatment planning workflow was implemented: (a) beamlet doses were calculated for control points defined along a baseball stitch trajectory using a collapsed-cone convolution-superposition algorithm; (b) VMAT treatment plans were optimized using the column generation approach; (c) a final dose distribution was calculated in Varian Eclipse using the analytical anisotropic algorithm by importing the optimized treatment plan parameters. Tr-VMAT plans were optimized for ten patients presented with cranial targets at both standard and shortened SAD, and compared to the clinical treatment plans through isodose distributions, dose-volume histograms, and dosimetric indices. The control point specifications of the optimized tr-VMAT plans were used to estimate the delivery time.

Results: The optimized tr-VMAT plans with both shortened and standard SAD delivery yielded a comparable plan quality to the clinical treatment plans. A statistically significant benefit was observed for dose gradient index and monitor unit efficiency for shortened SAD tr-VMAT plans, while improved target volume conformity was observed for the clinical treatment plan (P ≤ 0.05). A clear dosimetric benefit was not demonstrated between tr-VMAT delivery at shortened SAD compared to standard SAD, but shortened SAD delivery yielded a fraction size-dependent reduction in the estimated delivery time.

Conclusion: The implementation of "baseball stitch" tr-VMAT treatment plans to patients presented with cranial targets demonstrated comparable plan quality to clinical treatment plans. The delivery at shortened SAD produced a fraction size-dependent decrease in estimated delivery time.
View Article and Find Full Text PDF

Download full-text PDF

Source
http://dx.doi.org/10.1002/mp.14151DOI Listing
July 2020

On the relativistic impulse approximation for the calculation of Compton scattering cross sections and photon interaction coefficients used in kV dosimetry.

Phys Med Biol 2020 06 17;65(12):125010. Epub 2020 Jun 17.

Department of Physics and Medical Physics Unit, McGill University, Montréal, Québec, H3A 2T8, Canada.

We calculate differential and integrated cross sections for the Compton interaction as well as mass attenuation ([Formula: see text]), mass energy-transfer ([Formula: see text]), and mass energy-absorption ([Formula: see text]) coefficients, within the relativistic impulse approximation (RIA) using Compton profiles (CPs) obtained from unrestricted Hartree-Fock electron densities. We investigate the impact of using molecular as opposed to atomic CPs on dosimetric photon interaction coefficients for air, water and graphite, and compare our cross sections to the simpler Waller-Hartree (WH) and Klein-Nishina (KN) formalisms. We find that differences in [Formula: see text] and [Formula: see text] resulting from the choice of CPs within the RIA are small relative to the differences between the RIA, WH, and KN calculations. Surprisingly, although the WH binding corrections seem accurate when considering [Formula: see text], there are significant discrepancies between the WH and RIA results when we look at [Formula: see text]. The WH theory can differ substantially from the predictions of KN and the RIA in the tens of keV range (e.g. 6%-10% at 20 keV), when Compton scattering becomes the dominant interaction mechanism. For lower energies, the disagreement further grows to about one order of magnitude at 1 keV. However, since the photoelectric effect transfers more energy than the Compton interaction in the tens of keV range and below, the differences in the total [Formula: see text] values resulting from the choice of Compton models (KN, WH, or RIA) are not larger than 0.4%, and the differences between WH and the other two theories are no longer prominent.
View Article and Find Full Text PDF

Download full-text PDF

Source
http://dx.doi.org/10.1088/1361-6560/ab8108DOI Listing
June 2020

Comparing local control and distant metastasis in NSCLC patients between CyberKnife and conventional SBRT.

Radiother Oncol 2020 03 7;144:201-208. Epub 2020 Feb 7.

Medical Physics Unit, McGill University and Cedars Cancer Center, Montréal, Canada.

Background And Purpose: Previous literature suggests that the dose proximally outside the PTV could have an impact on the incidence of distant metastasis (DM) after SBRT in stage I NSCLC patients. We investigated this observation (along with local failure) in deliveries made by different treatment modalities: robotic mounted linac SBRT (CyberKnife) vs conventional SBRT (VMAT/CRT).

Materials And Methods: This study included 422 stage I NSCLC patients from 2 institutions who received SBRT: 217 treated conventionally and 205 with CyberKnife. The dose behavior outside the PTV of both sub-cohorts were compared by analyzing the mean dose in continuous shells extending 1, 2, 3, …, 100 mm from the PTV. Kaplan-Meier analysis was performed between the two sub-cohorts with respect to DM-free survival and local progression-free survival. A multivariable Cox proportional hazards model was fitted to the combined cohort (n = 422) with respect to DM incidence and local failure.

Results: The shell-averaged dose fall-off beyond the PTV was found to be significantly more modest in CyberKnife plans than in conventional SBRT plans. In a 30 mm shell around the PTV, the mean dose delivered with CyberKnife (38.1 Gy) is significantly larger than with VMAT/CRT (22.8 Gy, p<10). For 95% of CyberKnife plans, this region receives a mean dose larger than the 21 Gy threshold dose discovered in our previous study. In contrast, this occurs for only 75% of VMAT/CRT plans. The DM-free survival of the entire CyberKnife cohort is superior to that of the 25% of VMAT/CRT patients receiving less than the threshold dose (VMAT/CRT), with a hazard ratio of 5.3 (95% CI: 3.0-9.3, p<10). The 2 year DM-free survival rates were 87% (95% CI: 81%-91%) and 44% (95% CI: 28%-58%) for CyberKnife and the below-threshold dose conventional cohorts, respectively. A multivariable analysis of the combined cohort resulted in the confirmation that threshold dose was a significant predictor of DM(HR = 0.28, 95% CI: 0.15-0.55, p<10) when adjusted for other clinical factors. CyberKnife was also found to be superior to the entire VMAT/CRT with respect to local control (HR = 3.44, CI: 1.6-7.3). The 2-year local progression-free survival rates for the CyberKnife cohort and the VMAT/CRT cohort were 96% (95% CI: 92%-98%) and 88% (95% CI: 82%-92%) respectively.

Conclusions: In standard-of-care CyberKnife treatments, dose distributions that aid distant control are achieved 95% of the time. Although similar doses could be physically achieved by conventional SBRT, this is not always the case with current prescription practices, resulting in worse DM outcomes for 25% of conventional SBRT patients. Furthermore, CyberKnife was found to provide superior local control compared to VMAT/CRT.
View Article and Find Full Text PDF

Download full-text PDF

Source
http://dx.doi.org/10.1016/j.radonc.2020.01.017DOI Listing
March 2020

Overlooked pitfalls in multi-class machine learning classification in radiation oncology and how to avoid them.

Phys Med 2020 Feb 25;70:96-100. Epub 2020 Jan 25.

McGill University, Medical Physics Unit, Montreal, QC, Canada.

In radiation oncology, Machine Learning classification publications are typically related to two outcome classes, e.g. the presence or absence of distant metastasis. However, multi-class classification problems also have great clinical relevance, e.g., predicting the grade of a treatment complication following lung irradiation. This work comprised two studies aimed at making work in this domain less prone to statistical blindsides. In multi-class classification, AUC is not defined, whereas correlation coefficients are. It may seem like solely quoting the correlation coefficient value (in lieu of the AUC value) is a suitable choice. In the first study, we illustrated using Monte Carlo (MC) models why this choice is misleading. We also considered the special case where the multiple classes are not ordinal, but nominal, and explained why Pearson or Spearman correlation coefficients are not only providing incomplete information but are actually meaningless. The second study concerned surrogate biomarkers for a clinical endpoint, which have purported benefits including potential for early assessment, being inexpensive, and being non-invasive. Using a MC experiment, we showed how conclusions derived from surrogate markers can be misleading. The simulated endpoint was radiation toxicity (scale of 0-5). The surrogate marker was the true toxicity grade plus a noise term. Five patient cohorts were simulated, including one control. Two of the cohorts were designed to have a statistically significant difference in toxicity. Under 1000 repeated experiments using the biomarker, these two cohorts were often found to be statistically indistinguishable, with the fraction of such occurrences rising with the level of noise.
View Article and Find Full Text PDF

Download full-text PDF

Source
http://dx.doi.org/10.1016/j.ejmp.2020.01.009DOI Listing
February 2020

Experimental validation of recommended msr-correction factors for the calibration of Leksell Gamma Knife Icon unit following IAEA TRS-483.

Phys Med Biol 2020 03 11;65(6):065003. Epub 2020 Mar 11.

Medical Physics Unit, McGill University, Montreal, QC, H4A 3J1, Canada. Author to whom any correspondence should be addressed.

Currently, the American Association of Physicists in Medicine (AAPM) TG-21 is the conventional protocol currently used for the calibration of the Leksell Gamma Knife (LGK) (despite the publication of the AAPM TG-51 protocol). However, this protocol is based on the air-kerma standards requiring an elaborate conversion process resulting in an increase in the possibility of errors in the clinic. The International Atomic Energy Agency (IAEA) Technical Reports Series (TRS)-483 Code of Practice provides new recommendations on the dosimetry of small static fields and correction factor data for the calibration of the LGK unit. The purpose of this study is to experimentally validate previously calculated [Formula: see text] factors for the calibration of the LGK Perfexion/Icon unit in the context of the TRS-483 protocol. An experimental comparison between three protocols (TG-51, TG-21 and TRS-483 with the aforementioned correction factors) for calibration of the LGK unit is provided. Dose-rate measurements were performed on a LGK Icon unit using three ionization chambers and three phantoms with different orientations of the chambers with respect to the LGK unit. The dose rate was determined following the three calibration protocols. The standard deviation on the mean dose rate over all phantoms and chambers in different orientations determined using TG-51, TG-21 and TRS-483 protocols were 0.9%, 0.5% and 0.4%, respectively. The mean dose rate calculated using TG-51 protocol was 1.6% and 1.2% lower comparing to the TG-21 and TRS-483 protocols respectively. Applying the [Formula: see text] values calculated in Mirzakhanian et al (2018) to the measured dose rates in LGK unit for all chambers and phantoms resulted in dose rates that are consistent to within 0.4%. The TRS-483 protocol improves the consistency of the results especially when the chamber was positioned in different orientations with respect to the LGK (from 1.6% when using TG-51 or TG-21 protocols to 0.2% when using TRS-483 protocol) since the other protocols do not correct for the different chamber orientations.
View Article and Find Full Text PDF

Download full-text PDF

Source
http://dx.doi.org/10.1088/1361-6560/ab6953DOI Listing
March 2020

Absolute dosimetry of a 1.5 T MR-guided accelerator-based high-energy photon beam in water and solid phantoms using Aerrow.

Med Phys 2020 Mar 23;47(3):1291-1304. Epub 2020 Jan 23.

Medical Physics Unit, McGill University, Montréal, QC, H4A 3J1, Canada.

Purpose: In this work, the fabrication, operation, and evaluation of a probe-format graphite calorimeter - herein referred to as Aerrow - as an absolute clinical dosimeter of high-energy photon beams while in the presence of a B = 1.5 T magnetic field is described. Comparable to a cylindrical ionization chamber (IC) in terms of utility and usability, Aerrow has been developed for the purpose of accurately measuring absorbed dose to water in the clinic with a minimum disruption to the existing clinical workflow. To our knowledge, this is the first reported application of graphite calorimetry to magnetic resonance imaging (MRI)-guided radiotherapy.

Methods: Based on a previously numerically optimized and experimentally validated design, an Aerrow prototype capable of isothermal operation was constructed in-house. Graphite-to-water dose conversions as well as magnetic field perturbation factors were calculated using Monte Carlo, while heat transfer and mass impurity corrections and uncertainties were assessed analytically. Reference dose measurements were performed in the absence and presence of a B = 1.5 T magnetic field using Aerrow in the 7 MV FFF photon beam of an Elekta MRI-linac and were directly compared to the results obtained using two calibrated reference-class IC types. The feasibility of performing solid phantom-based dosimetry with Aerrow and the possible influence of clearance gaps is also investigated by performing reference-type dosimetry measurements for multiple rotational positions of the detector and comparing the results to those obtained in water.

Results: In the absence of the B-field, as well as in the parallel orientation while in the presence of the B-field, the absorbed dose to water measured using Aerrow was found to agree within combined uncertainties with those derived from TG-51 using calibrated reference-class ICs. Statistically significant differences on the order of (2-4)%, however, were observed when measuring absorbed dose to water using the ICs in the perpendicular orientation in the presence of the B-field. Aerrow had a peak-to-peak response of about 0.5% when rotated within the solid phantom regardless of whether the B-field was present or not.

Conclusions: This work describes the successful use of Aerrow as a straightforward means of measuring absolute dose to water for large high-energy photon fields in the presence of a 1.5 T B-field to a greater accuracy than currently achievable with ICs. The detector-phantom air gap does not appear to significantly influence the response of Aerrow in absolute terms, nor does it contribute to its rotational dependence. This work suggests that the accurate use of solid phantoms for absolute point dose measurement is possible with Aerrow.
View Article and Find Full Text PDF

Download full-text PDF

Source
http://dx.doi.org/10.1002/mp.13968DOI Listing
March 2020

Novel knowledge-based treatment planning model for hypofractionated radiotherapy of prostate cancer patients.

Phys Med 2020 Jan 6;69:36-43. Epub 2019 Dec 6.

McGill University Health Centre, Montreal, QC, Canada.

Purpose: To demonstrate the strength of an innovative knowledge-based model-building method for radiotherapy planning using hypofractionated, multi-target prostate patients.

Material And Methods: An initial RapidPlan model was trained using 48 patients who received 60 Gy to prostate (PTV60) and 44 Gy to pelvic nodes (PTV44) in 20 fractions. To improve the model's goodness-of-fit, an intermediate model was generated using the dose-volume histograms of best-spared organs-at-risk (OARs) of the initial model. Using the intermediate model and manual tweaking, all 48 cases were re-planned. The final model, trained using these re-plans, was validated on 50 additional patients. The validated final model was used to determine any planning advantage of using three arcs instead of two on 16 VMAT cases and tested on 25 additional cases to determine efficacy for single-PTV (PTV60-only) treatment planning.

Results: For model validation, PTV V of 99.9% was obtained by both clinical and knowledge-based planning. D was lower for model plans: by 1.23 Gy (PTV60, CI = [1.00, 1.45]), and by 2.44 Gy (PTV44, CI = [1.72, 3.16]). OAR sparing was superior for knowledge-based planning: ΔD = 3.70 Gy (bladder, CI = [2.83, 4.57]), and 3.22 Gy (rectum, CI = [2.48, 3.95]); ΔD = 1.17 Gy (bowel bag, CI = [0.64, 1.69]), and 4.78 Gy (femoral heads, CI = [3.90, 5.66]). Using three arcs instead of two, improvements in OAR sparing and PTV coverage were statistically significant, but of magnitudes < 1 Gy. The model failed at reliable DVH predictions for single PTV plans.

Conclusions: Our knowledge-based model delivers efficient, consistent plans with excellent PTV coverage and improved OAR sparing compared to clinical plans.
View Article and Find Full Text PDF

Download full-text PDF

Source
http://dx.doi.org/10.1016/j.ejmp.2019.11.023DOI Listing
January 2020

Proton beam therapy should remain in the public domain.

CMAJ 2019 11;191(46):E1284

Director, Medical Physics Unit, McGill University, Montréal, Que.

View Article and Find Full Text PDF

Download full-text PDF

Source
http://dx.doi.org/10.1503/cmaj.73445DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC6861142PMC
November 2019

Beam modeling and beam model commissioning for Monte Carlo dose calculation-based radiation therapy treatment planning: Report of AAPM Task Group 157.

Med Phys 2020 Jan 19;47(1):e1-e18. Epub 2019 Nov 19.

RaySearch Laboratories AB, SE-103 65, Stockholm, Sweden.

Dose calculation plays an important role in the accuracy of radiotherapy treatment planning and beam delivery. The Monte Carlo (MC) method is capable of achieving the highest accuracy in radiotherapy dose calculation and has been implemented in many commercial systems for radiotherapy treatment planning. The objective of this task group was to assist clinical physicists with the potentially complex task of acceptance testing and commissioning MC-based treatment planning systems (TPS) for photon and electron beam dose calculations. This report provides an overview on the general approach of clinical implementation and testing of MC-based TPS with a specific focus on models of clinical photon and electron beams. Different types of beam models are described including those that utilize MC simulation of the treatment head and those that rely on analytical methods and measurements. The trade-off between accuracy and efficiency in the various source-modeling approaches is discussed together with guidelines for acceptance testing of MC-based TPS from the clinical standpoint. Specific recommendations are given on methods and practical procedures to commission clinical beam models for MC-based TPS.
View Article and Find Full Text PDF

Download full-text PDF

Source
http://dx.doi.org/10.1002/mp.13898DOI Listing
January 2020

Monte Carlo calculated kilovoltage x-ray arc therapy plans for three lung cancer patients.

Biomed Phys Eng Express 2019 Nov 1;5(6). Epub 2019 Nov 1.

Department of Physics and Astronomy, University of Victoria, PO Box 1700 ST CSC, Victoria, BC V8W 2Y2, Canada.

: The intent of this work was to evaluate the ability of our 200 kV kilovoltage arc therapy (KVAT) system to treat realistic lung tumors without exceeding dose constraints to organs-at-risk (OAR).: Monte Carlo (MC) methods and the McO optimization framework generated and inversely optimized KVAT treatment plans for 3 SABR lung cancer patients. The KVAT system was designed to treat deep-seated lesions with kilovoltage photons. KVAT delivers dose to roughly spherical PTVs and therefore non-spherical PTVs were divided into spherical sub-volumes. A prescription dose of 12 Gy/fx × 4 fractions was planned to 90% of the PTV volume. KVAT plans were compared to VMC++ calculated, 6 MV stereotactic ablative radiotherapy (SABR) treatment plans. Dose distributions, dose volume histograms, gradient index (GI), planned mean doses and plan treatment times were calculated. Dose constraints for organs-at-risk (OAR) were taken from RTOG 101.: All plans, with the exception of the rib dose calculated in one of the KVAT plans for a peripheral lesion, were within dose-constraints. In general, KVAT plans had higher planned doses to OARs. KVAT GI values were 5.7, 7.2 and 8.9 and SABR values were 4.6, 4.1, and 4.7 for patient 1, 2 and 3, respectively. KVAT plan treatment times were 49, 65 and 17 min for patients 1, 2 and 3, respectively.: Inverse optimization and MC methods demonstrated the ability of KVAT to produce treatment plans without exceeding TG 101 dose constraints to OARs for 2 out of 3 investigated lung cancer patients.
View Article and Find Full Text PDF

Download full-text PDF

Source
http://dx.doi.org/10.1088/2057-1976/ab4dc5DOI Listing
November 2019

PD-1/PD-L1 Immune Checkpoint Inhibition with Radiation in Bladder Cancer: and Abscopal Effects.

Mol Cancer Ther 2020 01 18;19(1):211-220. Epub 2019 Sep 18.

Urologic Oncology Research Program, Research Institute of the McGill University Health Center, Montreal, Quebec, Canada.

The combination of radiation with immune checkpoint inhibitors was reported in some cancers to have synergic effects both locally and distally. Our aim was to assess this combined therapy on both radiated and nonradiated bladder tumors and to characterize the immune landscape within the tumor microenvironment. Murine bladder cancer cells (MB49) were injected subcutaneously in both flanks of C57BL/6 mice. Mice were randomly assigned to the following treatments: placebo, anti-PD-L1 (four intraperitoneal injections over 2 weeks), radiation to right flank (10 Gy in two fractions), or radiation+anti-PD-L1. Tumor digestion, flow cytometry, and qPCR were performed. Log-rank analysis was used for statistical significance. Radiation+anti-PD-L1 group demonstrated statistically significant slower tumor growth rate both in the radiated and nonirradiated tumors ( < 0.001). Survival curves demonstrated superior survival in the combination group compared with each treatment alone ( = 0.02). Flow cytometry showed increased infiltration of immunosuppressive cells as well as CTL in the radiation and combination groups ( = 0.04). Ratio of immunosuppressive cells to CTL shifted in favor of cytotoxic activity in the combination arm ( < 0.001). The qPCR analysis revealed downregulation of immunosuppressive genes (, and ), as well as upregulation of markers of CTL activation (, , and ) within both the radiated and distant tumors within the combination group. Combining radiation with immune checkpoint inhibitor provided better response in the radiated tumors and also the distant tumors along with a shift within the tumor microenvironment favoring cytotoxic activity. These findings demonstrate a possible abscopal effect in urothelial carcinoma with combination therapy.
View Article and Find Full Text PDF

Download full-text PDF

Source
http://dx.doi.org/10.1158/1535-7163.MCT-18-0986DOI Listing
January 2020

Dose-response linearization in radiochromic film dosimetry based on multichannel normalized pixel value with an integrated spectral correction for scanner response variations.

Med Phys 2019 Nov 26;46(11):5336-5349. Epub 2019 Sep 26.

Department of Radiation Oncology, Dana Farber/Brigham and Women's Cancer Center, Harvard Medical School, Boston, MA, 02115, USA.

Purpose: To introduce a model that reproducibly linearizes the response from radiochromic film (RCF) dosimetry systems at extended dose range. To introduce a correction method, generated from the same scanned images, which corrects for scanner temporal response variation and scanner bed inhomogeneity.

Methods: Six calibration curves were established for different lot numbers of EBT3 GAFCHROMIC™ film model based on four EPSON scanners [10000XL (2 units), 11000XL, 12000XL] at three different centers. These films were calibrated in terms of absorbed dose to water based on TG51 protocol or TRS398 with dose ranges up to 40 Gy. The film response was defined in terms of a proposed normalized pixel value ( ) as a summation of first-order equations based on information from red, green, and blue channels. The fitting parameters of these equations are chosen in a way that makes the film response equal to dose at the time of calibration. An integrated set of correction factors (one per color channel) was also introduced. These factors account for the spatial and temporal changes in scanning states during calibration and measurements. The combination of and this "fingerprint" correction formed the basis of this new protocol and it was tested against net optical density ( ) single-channel dosimetry in terms of accuracy, precision, scanner response variability, scanner bed inhomogeneity, noise, and long-term stability.

Results: Incorporating multichannel features (RGB) into the normalized pixel value produced linear response to absorbed dose (slope of 1) in all six RCF dosimetry systems considered in this study. The "fingerprint" correction factors of each of these six systems displayed unique patterns at the time of calibration. The application of to all of these six systems could achieve a level of accuracy of ± 2.0% in the dose range of interest within modeled uncertainty level of 2.0%-3.0% depending on the dose level. Consistent positioning of control and measurement film pieces and integrating the multichannel correction into the response function formalism mitigated possible scanner response variations of as much as ± 10% at lower doses and scanner bed inhomogeneity of ± 8% to the established level of uncertainty at the time of calibration. The system was also able to maintain the same level of accuracy after 3 and 6 months post calibration.

Conclusions: Combining response linearity with the integrated correction for scanner response variation lead to a sustainable and practical RCF dosimetry system that mitigated systematic response shifts and it has the potential to reduce errors in reporting relative information from the film response.
View Article and Find Full Text PDF

Download full-text PDF

Source
http://dx.doi.org/10.1002/mp.13818DOI Listing
November 2019

Dose measurements nearby low energy electronic brachytherapy sources using radiochromic film.

Phys Med 2019 Aug 22;64:40-44. Epub 2019 Jun 22.

Medical Physics Unit, McGill University, Montréal, Québec, Canada; Department of Radiation Oncology, Montreal General Hospital, Montréal, Québec, Canada.

Purpose: We investigate the effect of the GafChromic™ film EBT3 model absorbed dose energy response when used for dose measurements around low-energy photon sources. Monte Carlo based correction procedure in synergy with appropriate calibration curves was shown to provide more accurate absorbed dose (either relative or absolute). An assessment was made of possible dose errors that might be encountered if such energy dependent response is ignored.

Methods: We measured PDDs in water from a Xoft 50 kVp source using EBT3 film, and compared to PDD measurements acquired with a PTW-TN34013 parallel-plate ionization chamber. For the x-ray source, we simulated spectra using the EGSnrc (BEAMnrc) Monte Carlo code, and calculated Half Value Layer (HVL) at different distances from the source in water. Measurement strips of EBT3 film were positioned at distances of 2-6 cm from the Xoft source in a water phantom using a custom-made holder and irradiated simultaneously.

Results: Our results show that film calibration curves obtained at beam qualities near the effective energy of the Xoft 50 kVp source in water lead to variation in absorbed dose energy dependence of the response of around 5%. However, if the calibration curve was established in an MV beam quality, the error in absorbed dose could be as large as 20%.

Conclusion: Accurate dose measurements using radiochromic films at low photon energies require that the radiochromic film dosimetry system be calibrated at appropriate corresponding low energies, as large absorbed dose errors are expected when film calibration is performed in MV beam qualities.
View Article and Find Full Text PDF

Download full-text PDF

Source
http://dx.doi.org/10.1016/j.ejmp.2019.05.017DOI Listing
August 2019
-->