Publications by authors named "Rick D Franich"

16 Publications

  • Page 1 of 1

Nanoparticle dose enhancement of synchrotron radiation in PRESAGE dosimeters.

J Synchrotron Radiat 2020 Nov 23;27(Pt 6):1590-1600. Epub 2020 Oct 23.

School of Health and Biomedical Sciences, RMIT University, Plenty Road, Bundoora, Victoria 3083, Australia.

The physical absorbed dose enhancement by the inclusion of gold and bismuth nanoparticles fabricated into water-equivalent PRESAGE dosimeters was investigated. Nanoparticle-loaded water-equivalent PRESAGE dosimeters were irradiated with superficial, synchrotron and megavoltage X-ray beams. The change in optical density of the dosimeters was measured using UV-Vis spectrophotometry pre- and post-irradiation using a wavelength of 630 nm. Dose enhancement was measured for 5 nm and 50 nm monodispersed gold nanoparticles, 5-50 nm polydispersed bismuth nanoparticles, and 80 nm monodispersed bismuth nanoparticles at concentrations from 0.25 mM to 2 mM. The dose enhancement was highest for the 95.3 keV mean energy synchrotron beam (16-32%) followed by the 150 kVp superficial beam (12-21%) then the 6 MV beam (2-5%). The bismuth nanoparticle-loaded dosimeters produced a larger dose enhancement than the gold nanoparticle-loaded dosimeters in the synchrotron beam for the same concentration. For the superficial and megavoltage beams the dose enhancement was similar for both species of nanoparticles. The dose enhancement increased with nanoparticle concentration in the dosimeters; however, there was no observed nanoparticle size dependence on the dose enhancement.
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http://dx.doi.org/10.1107/S1600577520012849DOI Listing
November 2020

Dose response and stability of water equivalent PRESAGE dosimeters for synchrotron radiation therapy dosimetry.

Phys Med Biol 2018 Dec 6;63(23):235027. Epub 2018 Dec 6.

Alfred Health Radiation Oncology, The Alfred, Melbourne, Victoria 3004, Australia. School of Health and Biomedical Sciences, RMIT University, Bundoora, Victoria 3083, Australia. Author to whom any correspondence should be addressed.

This research investigated the dose response and post-irradiation stability of water-equivalent PRESAGE dosimeters exposed to synchrotron radiation. Water-equivalent PRESAGE dosimeters were irradiated up to 1000 Gy in a synchrotron x-ray beam with a mean energy of 95.3 keV. The change in optical density was measured using UV/visible spectrophotometry pre- and post-irradiation using a wavelength of 630 nm. Dose response was found to be approximately linear from 0-200 Gy with saturation occurring above 300 Gy. The post-irradiation stability was determined by measuring the change in optical density at 10, 30, 60, 180, 420 min and 7, 21 and 33 d post-irradiation for three groups of dosimeters stored at different temperatures. Each group had two dosimeters irradiated at 50, 100, 200 and 300 Gy and each group was stored at a different temperature following irradiation: room temperature (22 °C), 4 °C and  -18 °C. The optimal time for readout of the dosimeters varied with the post-irradiation storage temperature. The room temperature group had an optimal time-to-readout of 10 min for maximum signal before fading, while the 4 °C group was reasonably stable from 90 min to 1 week. The  -18 °C group showed the least amount of ongoing post-irradiation development and fading with an optimal readout window from 30 min to 21 d. The intra-batch variation between the mean of each temperature control group was 4.2% at 10 min post-irradiation.
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http://dx.doi.org/10.1088/1361-6560/aaf1f5DOI Listing
December 2018

Water equivalent PRESAGE for synchrotron radiation therapy dosimetry.

Med Phys 2018 Mar 20;45(3):1255-1265. Epub 2018 Jan 20.

School of Health and Biomedical Sciences, RMIT University, Bundoora, Vic, 3083, Australia.

Purpose: Synchrotron Radiation Therapy techniques are currently being trialed and commissioned at synchrotrons around the world. The patient treatment planning systems (TPS) developed for these treatments use simulated data of the synchrotron x-ray beam to produce the dosimetry in the treatment plan. The purpose of this study was to investigate a water equivalent PRESAGE dosimeter capable of 3D dosimetry over an energy range suitable for synchrotron x-ray beams.

Methods: Water equivalent PRESAGE dosimeters were fabricated with a radiological effective atomic number similar to water over an energy range of 10 keV to 10 MeV. The dosimeters were irradiated at various energies, scanned using optical CT (OCT) scanning and compared to ion chamber measurements. Percentage depth dose and beam profiles of the synchrotron beam were compared to Monte Carlo (MC) model simulations.

Results: The PDD profiles of the water equivalent PRESAGE agreed with ion chamber measurements and MC calculations within 2% for all keV energies investigated. The PRESAGE also showed good agreement to the MC model for depths below 5 mm of the synchrotron beam where ion chamber data do not exist. The spatial resolution of the OCT was not sufficient to accurately measure the penumbra of the synchrotron beams compared to MC calculations or EBT3 film; however, the water equivalent PRESAGE was able to verify dose profile characteristics of the MC model.

Conclusions: The radiological response of a water equivalent PRESAGE dosimeter has been validated for synchrotron x-ray beam energies along with the ability to independently verify dose distributions of a MC model.
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http://dx.doi.org/10.1002/mp.12745DOI Listing
March 2018

Commissioning of a PTW 34070 large-area plane-parallel ionization chamber for small field megavoltage photon dosimetry.

J Appl Clin Med Phys 2017 Nov 4;18(6):206-217. Epub 2017 Oct 4.

School of Science, RMIT University, Melbourne, Vic., Australia.

Purpose: This study investigates a large-area plane-parallel ionization chamber (LAC) for measurements of dose-area product in water (DAP ) in megavoltage (MV) photon fields.

Methods: Uniformity of electrode separation of the LAC (PTW34070 Bragg Peak Chamber, sensitive volume diameter: 8.16 cm) was measured using high-resolution microCT. Signal dependence on angle α of beam incidence for square 6 MV fields of side length s = 20 cm and 1 cm was measured in air. Polarity and recombination effects were characterized in 6, 10, and 18 MV photons fields. To assess the lateral setup tolerance, scanned LAC profiles of a 1 × 1 cm field were acquired. A 6 MV calibration coefficient, N , was determined in a field collimated by a 5 cm diameter stereotactic cone with known DAP . Additional calibrations in 10 × 10 cm fields at 6, 10, and 18 MV were performed.

Results: Electrode separation is uniform and agrees with specifications. Volume-averaging leads to a signal increase proportional to ~1/cos(α) in small fields. Correction factors for polarity and recombination range between 0.9986 to 0.9996 and 1.0007 to 1.0024, respectively. Off-axis displacement by up to 0.5 cm did not change the measured signal in a 1 × 1 cm field. N was 163.7 mGy cm nC and differs by +3.0% from the coefficient derived in the 10 × 10 cm 6 MV field. Response in 10 and 18 MV fields increased by 1.0% and 2.7% compared to 6 MV.

Conclusions: The LAC requires only small correction factors for DAP measurements and shows little energy dependence. Lateral setup errors of 0.5 cm are tolerated in 1 × 1 cm fields, but beam incidence must be kept as close to normal as possible. Calibration in 10 × 10 fields is not recommended because of the LAC's over-response. The accuracy of relative point-dose measurements in the field's periphery is an important limiting factor for the accuracy of DAP measurements.
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http://dx.doi.org/10.1002/acm2.12185DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC5689907PMC
November 2017

An integrated system for clinical treatment verification of HDR prostate brachytherapy combining source tracking with pretreatment imaging.

Brachytherapy 2018 Jan - Feb;17(1):111-121. Epub 2017 Sep 22.

School of Science, RMIT University, Melbourne, VIC, Australia.

Purpose: High-dose-rate (HDR) prostate brachytherapy treatment is usually delivered in one or a few large dose fractions. Poor execution of a planned treatment could have significant clinical impact, as high doses are delivered in seconds, and mistakes in an individual fraction cannot be easily rectified. Given that most potential errors in HDR brachytherapy ultimately lead to a geographical miss, a more direct approach to verification of correct treatment delivery is to directly monitor the position of the source throughout the treatment. In this work, we report on the clinical implementation of our treatment verification system that uniquely combines the 2D source-tracking capability with 2D pretreatment imaging, using a single flat panel detector (FPD).

Methods And Materials: The clinical brachytherapy treatment couch was modified to allow integration of the FPD into the couch. This enabled the patient to be set up in the brachytherapy bunker in a position that closely matched that at treatment planning imaging. An anteroposterior image was acquired of the patient immediately before treatment delivery and was assessed by the Radiation Oncologist online, to reestablish the positions of the catheters relative to the prostate. Assessment of catheter positions was performed in the left-right and superior-inferior directions along the entire catheter length and throughout the treatment volume. Source tracking was then performed during treatment delivery, and the measured position of the source dwells were directly compared to the treatment plan for verification.

Results: The treatment verification system was integrated into the clinical environment without significant change to workflow. Two patient cases are presented in this work to provide clinical examples of this system, which is now in routine use for all patient treatments in our clinic. The catheter positions were visualized relative to the prostate, immediately before treatment delivery. For one of the patient cases presented in this work, they agreed with the treatment plan on average by 1.5 mm and were identifiable as a predominantly inferior shift. The source tracking was performed during treatment delivery, and for the same case, the mean deviation from the planned dwell positions was 1.9 mm (max = 4.9 mm) for 280 positions across all catheters.

Conclusion: We have implemented our noninvasive treatment verification system based on an FPD in the clinical environment. The device is integrated into a patient treatment couch, and the process is now included in the routine clinical treatment procedure with minor impact on workflow. The system which combines both 2D pretreatment imaging and HDR 2D source tracking provides a range of information that can be used for comprehensive treatment verification. The system has the potential to meaningfully improve safety standards by allowing widespread adoption of routine treatment verification in HDR brachytherapy.
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http://dx.doi.org/10.1016/j.brachy.2017.08.004DOI Listing
July 2018

3D catheter reconstruction in HDR prostate brachytherapy for pre-treatment verification using a flat panel detector.

Phys Med 2017 Jul 16;39:121-131. Epub 2017 Jun 16.

School of Science, RMIT University, Melbourne 3000, VIC, Australia.

Purpose: High dose rate prostate brachytherapy is a widely-practiced treatment, delivering large conformal doses in relatively few treatment fractions. Inter- and intra-fraction catheter displacements have been reported. Unrecognized displacement can have a significant impact on dosimetry. Knowledge of the implant geometry at the time of treatment is important for ensuring safe and effective treatment. In this work we demonstrate a method to reconstruct the catheter positions pre-treatment, using a 'shift' imaging technique, and perform registration with the treatment plan for verification relative to the prostate.

Methods: Two oblique 'shift' images were acquired of a phantom containing brachytherapy catheters, representing the patient immediately pre-treatment. Using a back projection approach, the catheter paths were reconstructed in 3D and registered with the planned catheter paths. The robustness of the reconstruction and registration process was investigated as a function of phantom rotation. Catheter displacement detection was performed and compared to known applied displacements.

Results: Reconstruction of the implant geometry in 3D immediately prior to treatment was achieved. A mean reconstruction uncertainty of 0.8mm was determined for all catheters with a mean registration uncertainty of 0.5mm. A catheter displacement detection threshold of 2.2mm was demonstrated. Catheter displacements were all detected to within 0.5mm of the applied displacements.

Conclusion: This technique is robust and sensitive to assess catheter displacements throughout the implant volume. This approach provides a method to detect, in 3D, changes in catheter positions relative to the prostate. The method has sufficient sensitivity to enable clinically significant decisions immediately prior to treatment delivery.
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http://dx.doi.org/10.1016/j.ejmp.2017.06.008DOI Listing
July 2017

Activation of hip prostheses in high energy radiotherapy and resultant dose to nearby tissue.

J Appl Clin Med Phys 2017 Mar 27;18(2):100-105. Epub 2017 Feb 27.

School of Science, RMIT University, Melbourne, Victoria, Australia.

High energy radiotherapy can produce contaminant neutrons through the photonuclear effect. Patients receiving external beam radiation therapy to the pelvis may have high-density hip prostheses. Metallic materials such as those in hip prostheses, often have high cross-sections for neutron interaction. In this study, Thackray (UK) prosthetic hips have been irradiated by 18 MV radiotherapy beams to evaluate the additional dose to patients from the activation products. Hips were irradiated in- and out-of field at various distances from the beam isocenter to assess activation caused in-field by photo-activation, and neutron activation which occurs both in and out-of-field. NaI(Tl) scintillator detectors were used to measure the subsequent gamma-ray emissions and their half-lives. High sensitivity Mg, Cu, P doped LiF thermoluminescence dosimeter chips (TLD-100H) were used to measure the subsequent dose at the surface of a prosthesis over the 12 h following an in-field irradiation of 10,000 MU to a hip prosthesis located at the beam isocenter in a water phantom. Fe, Mn, and V were identified within the hip following irradiation by radiotherapy beams. The dose measured at the surface of a prosthesis following irradiation in a water phantom was 0.20 mGy over 12 h. The dose at the surface of prostheses irradiated to 200 MU was below the limit of detection (0.05 mGy) of the TLD100H. Prosthetic hips are activated by incident photons and neutrons in high energy radiotherapy, however, the dose resulting from activation is very small.
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http://dx.doi.org/10.1002/acm2.12058DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC5689951PMC
March 2017

A method for verification of treatment delivery in HDR prostate brachytherapy using a flat panel detector for both imaging and source tracking.

Med Phys 2016 May;43(5):2435

Alfred Health Radiation Oncology, The Alfred Hospital, Melbourne, VIC 3004, Australia and School of Science, RMIT University, Melbourne, VIC 3000, Australia.

Purpose: Verification of high dose rate (HDR) brachytherapy treatment delivery is an important step, but is generally difficult to achieve. A technique is required to monitor the treatment as it is delivered, allowing comparison with the treatment plan and error detection. In this work, we demonstrate a method for monitoring the treatment as it is delivered and directly comparing the delivered treatment with the treatment plan in the clinical workspace. This treatment verification system is based on a flat panel detector (FPD) used for both pre-treatment imaging and source tracking.

Methods: A phantom study was conducted to establish the resolution and precision of the system. A pretreatment radiograph of a phantom containing brachytherapy catheters is acquired and registration between the measurement and treatment planning system (TPS) is performed using implanted fiducial markers. The measured catheter paths immediately prior to treatment were then compared with the plan. During treatment delivery, the position of the (192)Ir source is determined at each dwell position by measuring the exit radiation with the FPD and directly compared to the planned source dwell positions.

Results: The registration between the two corresponding sets of fiducial markers in the TPS and radiograph yielded a registration error (residual) of 1.0 mm. The measured catheter paths agreed with the planned catheter paths on average to within 0.5 mm. The source positions measured with the FPD matched the planned source positions for all dwells on average within 0.6 mm (s.d. 0.3, min. 0.1, max. 1.4 mm).

Conclusions: We have demonstrated a method for directly comparing the treatment plan with the delivered treatment that can be easily implemented in the clinical workspace. Pretreatment imaging was performed, enabling visualization of the implant before treatment delivery and identification of possible catheter displacement. Treatment delivery verification was performed by measuring the source position as each dwell was delivered. This approach using a FPD for imaging and source tracking provides a noninvasive method of acquiring extensive information for verification in HDR prostate brachytherapy.
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http://dx.doi.org/10.1118/1.4946820DOI Listing
May 2016

Asymmetric breast dose in coronary angiography.

J Appl Clin Med Phys 2016 03 8;17(2):532-541. Epub 2016 Mar 8.

Austin Health; RMIT University.

The purpose of this study was to demonstrate asymmetric radiation dose distribution to the breasts in coronary angiography. Gafchromic XR-QA2 film was used as an area dosimeter to capture the asymmetric dose distribution to the breasts at various tissue depths in an anthropomorphic phantom. A selection of tube angulations were used under a controlled experiment and during a mock coronary angiography procedure. The Gafchromic XR-QA2 film was able to confirm the asymmetric distribution of radiation dose to the breast and provide a normalized breast dose value. The right breast received the majority of dose for most of the tube angulations in the controlled experiment. However the left breast received the most radiation dose during the mock procedure. Asymmetric dose distribution to the breasts is normally not observed if Monte Carlo based simulations are performed because individual breast dose calculations are not available. The application of a typical coronary angiogram determined in the experiment showed the normalized left breast dose is 0.16 mGy/ Gy.cm2 and the right breast dose is 0.08 mGy/ Gy.cm2.
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http://dx.doi.org/10.1120/jacmp.v17i2.5746DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC5874947PMC
March 2016

High resolution 3D imaging of synchrotron generated microbeams.

Med Phys 2015 Dec;42(12):6973-86

School of Medical Sciences, RMIT University, Bundoora, Victoria 3083, Australia.

Purpose: Microbeam radiation therapy (MRT) techniques are under investigation at synchrotrons worldwide. Favourable outcomes from animal and cell culture studies have proven the efficacy of MRT. The aim of MRT researchers currently is to progress to human clinical trials in the near future. The purpose of this study was to demonstrate the high resolution and 3D imaging of synchrotron generated microbeams in PRESAGE® dosimeters using laser fluorescence confocal microscopy.

Methods: Water equivalent PRESAGE® dosimeters were fabricated and irradiated with microbeams on the Imaging and Medical Beamline at the Australian Synchrotron. Microbeam arrays comprised of microbeams 25-50 μm wide with 200 or 400 μm peak-to-peak spacing were delivered as single, cross-fire, multidirectional, and interspersed arrays. Imaging of the dosimeters was performed using a nikon a1 laser fluorescence confocal microscope.

Results: The spatial fractionation of the MRT beams was clearly visible in 2D and up to 9 mm in depth. Individual microbeams were easily resolved with the full width at half maximum of microbeams measured on images with resolutions of as low as 0.09 μm/pixel. Profiles obtained demonstrated the change of the peak-to-valley dose ratio for interspersed MRT microbeam arrays and subtle variations in the sample positioning by the sample stage goniometer were measured.

Conclusions: Laser fluorescence confocal microscopy of MRT irradiated PRESAGE® dosimeters has been validated in this study as a high resolution imaging tool for the independent spatial and geometrical verification of MRT beam delivery.
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http://dx.doi.org/10.1118/1.4935410DOI Listing
December 2015

The Importance of Quasi-4D Path-Integrated Dose Accumulation for More Accurate Risk Estimation in Stereotactic Liver Radiotherapy.

Technol Cancer Res Treat 2016 06 20;15(3):428-36. Epub 2015 May 20.

School of Applied Sciences and Health Innovations Research Institute, RMIT University, Melbourne, Australia.

Intrafraction organ deformation may be accounted for by inclusion of temporal information in dose calculation models. In this article, we demonstrate a quasi-4-dimensional method for improved risk estimation. Conventional 3-dimensional and quasi-4-dimensional calculations employing dose warping for dose accumulation were undertaken for patients with liver metastases planned for 42 Gy in 6 fractions of stereotactic body radiotherapy. Normal tissue complication probabilities and stochastic risks for radiation-induced carcinogenesis and cardiac complications were evaluated for healthy peripheral structures. Hypothetical assessments of other commonly employed dose/fractionation schedules on normal tissue complication probability estimates were explored. Conventional 3-dimensional dose computation may result in significant under- or overestimation of doses to organ at risk. For instance, doses differ (on average) by 17% (σ = 14%) for the left kidney, by 14% (σ = 7%) for the right kidney, by 7% (σ = 9%) for the large bowel, and by 10% (σ = 14%) for the duodenum. Discrepancies in the excess relative risk range up to about 30%. The 3-dimensional approach was shown to result in cardiac complication risks underestimated by >20%. For liver stereotactic body radiotherapy, we have shown that conventional 3-dimensional dose calculation may significantly over-/underestimate dose to organ at risk (90%-120% of the 4-dimensional estimate for the mean dose and 20%-150% for D2%). Providing dose estimates that most closely represent the actual dose delivered will provide valuable information to improve our understanding of the dose response for partial volume irradiation using hypofractionated schedules. Excess relative risks of radiocarcinogenesis were shown to range up to approximately excess relative risk = 4 and the prediction thereof depends greatly on the use of either 3-dimensional or 4-dimensional methods (with corresponding results differing by tens of percent).
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http://dx.doi.org/10.1177/1533034615584120DOI Listing
June 2016

Evaluation of dosimetric misrepresentations from 3D conventional planning of liver SBRT using 4D deformable dose integration.

J Appl Clin Med Phys 2014 Nov 8;15(6):4978. Epub 2014 Nov 8.

Radiation Oncology Victoria and RMIT University.

The purpose of this study is to evaluate dosimetric errors in 3D conventional plan- ning of stereotactic body radiotherapy (SBRT) by using a 4D deformable image registration (DIR)-based dose-warping and integration technique. Respiratory- correlated 4D CT image sets with 10 phases were acquired for four consecutive patients with five liver tumors. Average intensity projection (AIP) images were used to generate 3D conventional plans of SBRT. Quasi-4D path-integrated dose accumulation was performed over all 10 phases using dose-warping techniques based on DIR. This result was compared to the conventional plan in order to evalu- ate the appropriateness of 3D (static) dose calculations. In addition, we consider whether organ dose metrics derived from contours defined on the average intensity projection (AIP), or on a reference phase, provide the better approximation of the 4D values. The impact of using fewer (< 10) phases was also explored. The AIP- based 3D planning approach overestimated doses to targets by 1.4% to 8.7% (mean 4.2%) and underestimated dose to normal liver by up to 8% (mean -5.5%; range -2.3% to -8.0%), compared to the 4D methodology. The homogeneity of the dose distribution was overestimated when using conventional 3D calculations by up to 24%. OAR doses estimated by 3D planning were, on average, within 10% of the 4D calculations; however, differences of up to 100% were observed. Four-dimensional dose calculation using 3 phases gave a reasonable approximation of that calculated from the full 10 phases for all patients, which is potentially useful from a workload perspective. 4D evaluation showed that conventional 3D planning on an AIP can significantly overestimate target dose (ITV and GTV+5mm), underestimate normal liver dose, and overestimate dose homogeneity. Implementing nonadaptive quasi- 4D dose calculation can highlight the potential limitation of 3D conventional SBRT planning and the resultant misrepresentations of dose in some regions affected by motion and deformation. Where the 4D approach is unavailable, contouring on the full expiration phase may yield more accurate dose calculations, most relevant in the case of the healthy liver, but the absolute dose differences are in general small for the other healthy organs. The technique has the potential to quantify under- and over-dosage and improve treatment plan evaluation, retrospective plan analysis, and clinical outcome correlation. 
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http://dx.doi.org/10.1120/jacmp.v15i6.4978DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC5711129PMC
November 2014

The influence of the dwell time deviation constraint (DTDC) parameter on dosimetry with IPSA optimisation for HDR prostate brachytherapy.

Australas Phys Eng Sci Med 2015 Mar 7;38(1):55-61. Epub 2014 Dec 7.

William Buckland Radiation Oncology, The Alfred Hospital, Commercial Road, Melbourne, 3004, Australia,

To investigate how the dwell time deviation constraint (DTDC) parameter, applied to inverse planning by simulated annealing (IPSA) optimisation limits large dwell times from occurring in each catheter and to characterise the effect on the resulting dosimetry for prostate high dose rate (HDR) brachytherapy treatment plans. An unconstrained IPSA optimised treatment plan, using the Oncentra Brachytherapy treatment planning system (version 4.3, Nucletron an Elekta company, Elekta AB, Stockholm, Sweden), was generated for 20 consecutive HDR prostate brachytherapy patients, with the DTDC set to zero. Successive constrained optimisation plans were also created for each patient by increasing the DTDC parameter by 0.2, up to a maximum value of 1.0. We defined a "plan modulation index", to characterise the change of dwell time modulation as the DTDC parameter was increased. We calculated the dose volume histogram indices for the PTV (D90, V100, V150, V200%) and urethra (D10%) to characterise the effect on the resulting dosimetry. The average PTV D90% decreases as the DTDC is applied, on average by only 1.5 %, for a DTDC = 0.4. The measures of high dose regions in the PTV, V150 and V200%, increase on average by less than 5 and 2 % respectively. The net effect of DTDC on the modulation of dwell times has been characterised by the introduction of the plan modulation index. DTDC applied during IPSA optimisation of HDR prostate brachytherapy plans reduce the occurrence of large isolated dwell times within individual catheters. The mechanism by which DTDC works has been described and its effect on the modulation of dwell times has been characterised. The authors recommend using a DTDC parameter no greater than 0.4 to obtain a plan with dwell time modulation comparable to a geometric optimised plan. This yielded on average a 1.5 % decrease in PTV coverage and an acceptable increase in V150%, without compromising the urethral dose.
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http://dx.doi.org/10.1007/s13246-014-0317-2DOI Listing
March 2015

The influence of field size on stopping-power ratios in- and out-of-field: quantitative data for the BrainLAB m3 micro-multileaf collimator.

J Appl Clin Med Phys 2012 Nov 8;13(6):4019. Epub 2012 Nov 8.

School of Applied Sciences and Health Innovations Research Institute, RMIT University, Melbourne, Australia.

The objective of this work is to quantify the systematic errors introduced by the common assumption of invariant secondary electron spectra with changing field sizes, as relevant to stereotactic radiotherapy and other treatment modes incorporating small beam segments delivered with a linac-based stereotactic unit. The EGSnrc/BEAMnrc Monte Carlo radiation transport code was used to construct a dosimetrically-matched model of a Varian 600C linear accelerator with mounted BrainLAB micro-multileaf collimator. Stopping-power ratios were calculated for field sizes ranging from 6 × 6 mm2 up to the maximum (98 × 98 mm2), and differences between these and the reference field were computed. Quantitative stopping power data for the BrainLAB micro-multileaf collimator has been compiled. Field size dependent differences to reference conditions increase with decreasing field size and increasing depth, but remain a fraction of a percent for all field sizes studied. However, for dosimetry outside the primary field, errors induced by the assumption of invariant electron spectra can be greater than 1%, increasing with field size. It is also shown that simplification of the Spencer-Attix formulation by ignoring secondary electrons below the cutoff kinetic energy applied to the integration results in underestimation of stopping-power ratios of about 0.3% (and is independent of field size and depth). This work is the first to quantify stopping powers from a BrainLAB micro-multileaf collimator. Many earlier studies model simplified beams, ignoring collimator scatter, which is shown to significantly influence the spectrum. Importantly, we have confirmed that the assumption of unchanging electron spectra with varying field sizes is justifiable when performing (typical) in-field dosimetry of stereotactic fields. Clinicians and physicists undertaking precise out-of-field measurements for the purposes of risk estimation, ought to be aware that the more pronounced spectral variation results in stopping powers (and hence doses) that differ more than for in-field dosimetry.
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http://dx.doi.org/10.1120/jacmp.v13i6.4019DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC5718545PMC
November 2012

A contemporary review of stereotactic radiotherapy: inherent dosimetric complexities and the potential for detriment.

Acta Oncol 2011 May 3;50(4):483-508. Epub 2011 Feb 3.

School of Applied Sciences, RMIT University, Melbourne, Victoria, Australia.

Objective: The advantages of highly localised, conformal treatments achievable with stereotactic radiotherapy (SRT) are increasingly being extended to extracranial sites as stereotactic body radiotherapy with advancements in imaging and beam collimation. One of the challenges in stereotactic treatment lies in the significant complexities associated with small field dosimetry and dose calculation. This review provides a comprehensive overview of the complexities associated with stereotactic radiotherapy and the potential for detriment.

Methods: This study is based on a comprehensive review of literature accessible via PubMed and other sources, covering stereotactic radiotherapy, small-field dosimetry and dose calculation.

Findings: Several key issues were identified in the literature. They pertain to dose prescription, dose measurement and dose calculation within and beyond the treatment field. Field-edge regions and penumbrae occupy a significant portion of the total field size. Spectral and dosimetric characteristics are difficult to determine and are compounded by effects of tissue inhomogeneity. Measurement of small-fields is made difficult by detector volume averaging and energy response. Available dosimeters are compared, and emphasis is given to gel dosimetry which offers the greatest potential for three-dimensional small-field dosimetry. The limitations of treatment planning system algorithms as applied to small-fields (particularly in the presence of heterogeneities) is explained, and a review of Monte Carlo dose calculation is provided, including simplified treatment planning implementations. Not incorporated into treatment planning, there is evidence that far from the primary field, doses to patients (and corresponding risks of radiocarcinogenesis) from leakage/scatter in SRT are similar to large fields.

Conclusions: Improved knowledge of dosimetric issues is essential to the accurate measurement and calculation of dose as well as the interpretation and assessment of planned and delivered treatments. This review highlights such issues and the potential benefit that may be gained from Monte Carlo dose calculation and verification via three-dimensional dosimetric methods (such as gel dosimetry) being introduced into routine clinical practice.
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http://dx.doi.org/10.3109/0284186X.2010.551665DOI Listing
May 2011

Assessment of out-of-field doses in radiotherapy of brain lesions in children.

Int J Radiat Oncol Biol Phys 2011 Mar 21;79(3):927-33. Epub 2010 Aug 21.

School of Applied Sciences, RMIT University, Melbourne, Vic, Australia.

Purpose: To characterize the out-of-field doses in pediatric radiotherapy and to identify simple methods by which out-of-field dose might be minimized, with a view to reducing the risk of secondary cancers.

Methods And Materials: With the aim of characterizing the peripheral doses under different treatment conditions, the dose measurements in an anthropomorphic child phantom were taken in various organs and critical structures outside the primary field using thermoluminescent dosimetry. The doses from a Varian 600C and Varian Trilogy linear accelerator, both at 6 MV, were investigated.

Results: Larger field sizes have been shown to result in greater peripheral doses close to the primary beam, with the difference becoming less significant at large distances, indicating that most of out-of-field doses result from head leakage and collimator scatter>40 cm from the primary field. The use of lead shields has been shown to reduce the absorbed dose resulting from leakage. Aligning the craniocaudal axis of the patient with the x-plane of the collimator resulted in a dose reduction of 40%, for both machines. Out-of-field doses from the Varian Trilogy were shown to be approximately 40% greater than those from the 600C linear accelerator, despite being operated at the same energy.

Conclusion: Out-of-field doses to pediatric patients can be minimized by using simple treatment options, such as using the single-energy mode linear accelerator rather than the multimode, orienting the couch and collimator such that the patient lies along the x-plane and avoiding fields directed along the trunk of the body.
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http://dx.doi.org/10.1016/j.ijrobp.2010.04.064DOI Listing
March 2011
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