Publications by authors named "Leon Dunn"

14 Publications

  • Page 1 of 1

A simple and efficient method to measure beam attenuation through a radiotherapy treatment couch and immobilization devices.

Australas Phys Eng Sci Med 2019 Dec 26;42(4):1183-1189. Epub 2019 Aug 26.

Discipline of Medical Radiations, RMIT University, Melbourne, Australia.

We propose a simple and efficient method to measure beam attenuation in one or two dimensions using an amorphous silicon electronic portal imaging device (a-Si EPID). The proposed method was validated against ionization chamber measurements. Beam attenuation through treatment couches (Varian Medical Systems) and immobilization devices (CIVCO Radiotherapy, USA) was examined. The dependency of beam attenuation on field size, photon energy, thickness of the couch, and the presence of a phantom were studied. Attenuation images were derived by computing the percentage difference between images obtained without and with a couch or immobilization devices determining the percentage of attenuation at the center and the mean attenuation. The beam attenuation measurements obtained with an a-Si EPID and an ionization chamber agreed to within ± 0.10 to 1.80%. No difference was noted between the center and mean of an attenuated image for a small field size of 5 × 5 cm, whereas a large field size of 15 × 15 cm exhibited differences of up to 1.13%. For an 18 MV beam, the a-Si EPID required additional build-up material for accurate assessment of beam attenuation. The a-Si EPID could measure differences in beam attenuation through an image guided radiotherapy (IGRT) couch regardless of the variabilities in couch thickness. Interestingly, the addition of a phantom reduced the magnitude of attenuation by approximately 1.20% for a field size of 15 × 15 cm. A simple method is proposed that provides the user with beam attenuation data in either 2D or 1D within a few minutes.
View Article and Find Full Text PDF

Download full-text PDF

Source
http://dx.doi.org/10.1007/s13246-019-00789-8DOI Listing
December 2019

Automated data mining of a plan-check database and example application.

J Appl Clin Med Phys 2018 Sep 29;19(5):739-748. Epub 2018 Jun 29.

Icon Cancer Centre - Richmond, Richmond, Melbourne, Vic, Australia.

Purpose: The aim of this work was to present the development and example application of an automated data mining software platform that preforms bulk analysis of results and patient data passing through the 3D plan and delivery QA system, Mobius3D.

Methods: Python, matlab, and Java were used to create an interface that reads JavaScript Object Notation (JSON) created for every approved Mobius3D pre-treatment plan-check. The aforementioned JSON files contain all the information for every pre-treatment QA check performed by Mobius3D, including all 3D dose, CT, structure set information, as well as all plan information and patient demographics. Two Graphical User Interfaces (GUIs) were created, the first is called Mobius3D-Database (M3D-DB) and presents the check results in both filterable tabular and graphical form. These data are presented for all patients and includes mean dose differences, 90% coverage, 3D gamma pass rate percentages, treatment sites, machine, beam energy, Multi-Leaf Collimator (MLC) mode, treatment planning system (TPS), plan names, approvers, dates and times. Group statistics and statistical process control levels are then calculated based on filter settings. The second GUI, called Mobius3D organ at risk (M3DOAR), analyzes dose-volume histogram data for all patients and all Organs-at-Risk (OAR). The design of the software is such that all treatment parameters and treatment site information are able to be filtered and sorted with the results, plots, and statistics updated.

Results: The M3D-DB software can summarize and filter large numbers of plan-checks from Mobius3D. The M3DOAR software is also able to analyze large amounts of dose-volume data for patient groups which may prove useful in clinical trials, where OAR doses for large numbers of patients can be compared and correlated. Target DVHs can also be analyzed en mass.

Conclusions: This work demonstrates a method to extract the large amount of treatment data for every patient that is stored by Mobius3D but not easily accessible. With scripting, it is possible to mine this data for research and clinical trials as well as patient and TPS QA.
View Article and Find Full Text PDF

Download full-text PDF

Source
http://dx.doi.org/10.1002/acm2.12396DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC6123163PMC
September 2018

Dosimetric end-to-end tests in a national audit of 3D conformal radiotherapy.

Phys Imaging Radiat Oncol 2018 Apr 24;6:5-11. Epub 2018 Apr 24.

Australian Clinical Dosimetry Service (ACDS), Australian Radiation Protection and National Safety Agency (ARPANSA), 619 Lower Plenty Road, Yallambie, VIC 3085, Australia.

Background And Purpose: Independent dosimetry audits improve quality and safety of radiation therapy. This work reports on design and findings of a comprehensive 3D conformal radiotherapy (3D-CRT) Level III audit.

Materials And Methods: The audit was conducted as onsite audit using an anthropomorphic thorax phantom in an end-to-end test by the Australian Clinical Dosimetry Service (ACDS). Absolute dose point measurements were performed with Farmer-type ionization chambers. The audited treatment plans included open and half blocked fields, wedges and lung inhomogeneities. Audit results were determined as Pass Optimal Level (deviations within 3.3%), Pass Action Level (greater than 3.3% but within 5%) and Out of Tolerance (beyond 5%), as well as Reported Not Scored (RNS). The audit has been performed between July 2012 and January 2018 on 94 occasions, covering approximately 90% of all Australian facilities.

Results: The audit pass rate was 87% (53% optimal). Fifty recommendations were given, mainly related to planning system commissioning. Dose overestimation behind low density inhomogeneities by the analytical anisotropic algorithm (AAA) was identified across facilities and found to extend to beam setups which resemble a typical breast cancer treatment beam placement. RNS measurements inside lung showed a variation in the opposite direction: AAA under-dosed a target beyond lung and over-dosed the lung upstream and downstream of the target. Results also highlighted shortcomings of some superposition and convolution algorithms in modelling large angle wedges.

Conclusions: This audit showed that 3D-CRT dosimetry audits remain relevant and can identify fundamental global and local problems that also affect advanced treatments.
View Article and Find Full Text PDF

Download full-text PDF

Source
http://dx.doi.org/10.1016/j.phro.2018.03.006DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC7807562PMC
April 2018

A software platform for statistical evaluation of patient respiratory patterns in radiation therapy.

Phys Med 2017 Oct 22;42:135-140. Epub 2017 Sep 22.

Epworth Radiation Oncology, Epworth HealthCare, Australia. Electronic address:

Aim: The aim of this work was to design and evaluate a software tool for analysis of a patient's respiration, with the goal of optimizing the effectiveness of motion management techniques during radiotherapy imaging and treatment.

Materials And Methods: A software tool which analyses patient respiratory data files (.vxp files) created by the Varian Real-Time Position Management System (RPM) was developed to analyse patient respiratory data. The software, called RespAnalysis, was created in MATLAB and provides four modules, one each for determining respiration characteristics, providing breathing coaching (biofeedback training), comparing pre and post-training characteristics and performing a fraction-by-fraction assessment. The modules analyse respiratory traces to determine signal characteristics and specifically use a Sample Entropy algorithm as the key means to quantify breathing irregularity. Simulated respiratory signals, as well as 91 patient RPM traces were analysed with RespAnalysis to test the viability of using the Sample Entropy for predicting breathing regularity.

Results: Retrospective assessment of patient data demonstrated that the Sample Entropy metric was a predictor of periodic irregularity in respiration data, however, it was found to be insensitive to amplitude variation. Additional waveform statistics assessing the distribution of signal amplitudes over time coupled with Sample Entropy method were found to be useful in assessing breathing regularity.

Conclusions: The RespAnalysis software tool presented in this work uses the Sample Entropy method to analyse patient respiratory data recorded for motion management purposes in radiation therapy. This is applicable during treatment simulation and during subsequent treatment fractions, providing a way to quantify breathing irregularity, as well as assess the need for breathing coaching. It was demonstrated that the Sample Entropy metric was correlated to the irregularity of the patient's respiratory motion in terms of periodicity, whilst other metrics, such as percentage deviation of inhale/exhale peak positions provided insight into respiratory amplitude regularity.
View Article and Find Full Text PDF

Download full-text PDF

Source
http://dx.doi.org/10.1016/j.ejmp.2017.09.128DOI Listing
October 2017

A clinical database to assess action levels and tolerances for the ongoing use of Mobius3D.

J Appl Clin Med Phys 2017 Jan 30;18(1):59-65. Epub 2016 Nov 30.

Epworth Radiation Oncology Research Centre, Epworth HealthCare, Melbourne, VIC, Australia.

In radiation therapy, calculation of dose within the patient contains inherent uncertainties, inaccuracies, limitations, and the potential for random error. Thus, point dose-independent verification of such calculations is a well-established process, with published data to support the setting of both action levels and tolerances. Mobius3D takes this process one step further with a full independent calculation of patient dose and comparisons of clinical parameters such as mean target dose and voxel-by-voxel gamma analysis. There is currently no published data to directly inform tolerance levels for such parameters, and therefore this work presents a database of 1000 Mobius3D results to fill this gap. The data are tested for normality using a normal probability plot and found to fit this distribution for three sub groups of data; Eclipse, iPlan and the treatment site Lung. The mean (μ) and standard deviation (σ) of these sub groups is used to set action levels and tolerances at μ ± 2σ and μ ± 3σ, respectively. A global (3%, 3 mm) gamma tolerance is set at 88.5%. The mean target dose tolerance for Eclipse data is the narrowest at ± 3%, whilst iPlan and Lung have a range of -5.0 to 2.2% and -1.8 to 5.0%, respectively. With these limits in place, future results failing the action level or tolerance will fall within the worst 5% and 1% of historical results and an informed decision can be made regarding remedial action prior to treatment.
View Article and Find Full Text PDF

Download full-text PDF

Source
http://dx.doi.org/10.1002/acm2.12009DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC5689886PMC
January 2017

National dosimetric audit network finds discrepancies in AAA lung inhomogeneity corrections.

Phys Med 2015 Jul 23;31(5):435-41. Epub 2015 Apr 23.

Australian Clinical Dosimetry Service, Yallambie, Victoria 3085, Australia; School of Applied Science, RMIT University, Melbourne, Australia.

This work presents the Australian Clinical Dosimetry Service's (ACDS) findings of an investigation of systematic discrepancies between treatment planning system (TPS) calculated and measured audit doses. Specifically, a comparison between the Anisotropic Analytic Algorithm (AAA) and other common dose-calculation algorithms in regions downstream (≥2cm) from low-density material in anthropomorphic and slab phantom geometries is presented. Two measurement setups involving rectilinear slab-phantoms (ACDS Level II audit) and anthropomorphic geometries (ACDS Level III audit) were used in conjunction with ion chamber (planar 2D array and Farmer-type) measurements. Measured doses were compared to calculated doses for a variety of cases, with and without the presence of inhomogeneities and beam-modifiers in 71 audits. Results demonstrate a systematic AAA underdose with an average discrepancy of 2.9 ± 1.2% when the AAA algorithm is implemented in regions distal from lung-tissue interfaces, when lateral beams are used with anthropomorphic phantoms. This systemic discrepancy was found for all Level III audits of facilities using the AAA algorithm. This discrepancy is not seen when identical measurements are compared for other common dose-calculation algorithms (average discrepancy -0.4 ± 1.7%), including the Acuros XB algorithm also available with the Eclipse TPS. For slab phantom geometries (Level II audits), with similar measurement points downstream from inhomogeneities this discrepancy is also not seen.
View Article and Find Full Text PDF

Download full-text PDF

Source
http://dx.doi.org/10.1016/j.ejmp.2015.04.002DOI Listing
July 2015

Long term OSLD reader stability in the ACDS level one audit.

Australas Phys Eng Sci Med 2015 Mar 14;38(1):151-6. Epub 2014 Dec 14.

Australian Clinical Dosimetry Service, Yallambie, VIC, 3085, Australia,

The Australian Clinical Dosimetry Service (ACDS) has demonstrated the capacity to perform a basic dosimetry audit on all radiotherapy clinics across Australia. During the ACDS's three and a half year trial the majority of the audits were performed using optically stimulated luminescence dosimeters (OSLD) mailed to facilities for exposure to a reference dose, and then returned to the ACDS for analysis. This technical note investigates the stability of the readout process under the large workload of the national dosimetry audit. The OSLD readout uncertainty contributes to the uncertainty of several terms of the dose calculation equation and is a major source of uncertainty in the audit. The standard deviation of four OSLD readouts was initially established at 0.6 %. Measurements over 13 audit batches--each batch containing 200-400 OSLDs--showed variability (0.5-0.9 %) in the readout standard deviation. These shifts have not yet necessitated a change to the audit scoring levels. However, a standard deviation in OSLD readouts greater than 0.9 % will change the audit scoring levels. We identified mechanical wear on the OSLD readout adapter as a cause of variability in readout uncertainty, however, we cannot rule out other causes. Additionally we observed large fluctuations in the distribution of element correction factors (ECF) for OSLD batches. We conclude that the variability in the width of the ECF distribution from one batch to another is not caused by variability in readout uncertainty, but rather by variations in the OSLD stock.
View Article and Find Full Text PDF

Download full-text PDF

Source
http://dx.doi.org/10.1007/s13246-014-0320-7DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC4445253PMC
March 2015

A 2D ion chamber array audit of wedged and asymmetric fields in an inhomogeneous lung phantom.

Med Phys 2014 Oct;41(10):101712

Australian Clinical Dosimetry Service, Yallambie, Victoria 3085, Australia and School of Applied Science, RMIT University, Melbourne 3000, Australia.

Purpose: The Australian Clinical Dosimetry Service (ACDS) has implemented a new method of a nonreference condition Level II type dosimetric audit of radiotherapy services to increase measurement accuracy and patient safety within Australia. The aim of this work is to describe the methodology, tolerances, and outcomes from the new audit.

Methods: The ACDS Level II audit measures the dose delivered in 2D planes using an ionization chamber based array positioned at multiple depths. Measurements are made in rectilinear homogeneous and inhomogeneous phantoms composed of slabs of solid water and lung. Computer generated computed tomography data sets of the rectilinear phantoms are supplied to the facility prior to audit for planning of a range of cases including reference fields, asymmetric fields, and wedged fields. The audit assesses 3D planning with 6 MV photons with a static (zero degree) gantry. Scoring is performed using local dose differences between the planned and measured dose within 80% of the field width. The overall audit result is determined by the maximum dose difference over all scoring points, cases, and planes. Pass (Optimal Level) is defined as maximum dose difference ≤3.3%, Pass (Action Level) is ≤5.0%, and Fail (Out of Tolerance) is >5.0%.

Results: At close of 2013, the ACDS had performed 24 Level II audits. 63% of the audits passed, 33% failed, and the remaining audit was not assessable. Of the 15 audits that passed, 3 were at Pass (Action Level). The high fail rate is largely due to a systemic issue with modeling asymmetric 60° wedges which caused a delivered overdose of 5%-8%.

Conclusions: The ACDS has implemented a nonreference condition Level II type audit, based on ion chamber 2D array measurements in an inhomogeneous slab phantom. The powerful diagnostic ability of this audit has allowed the ACDS to rigorously test the treatment planning systems implemented in Australian radiotherapy facilities. Recommendations from audits have led to facilities modifying clinical practice and changing planning protocols.
View Article and Find Full Text PDF

Download full-text PDF

Source
http://dx.doi.org/10.1118/1.4896097DOI Listing
October 2014

Angular dependence of the response of the nanoDot OSLD system for measurements at depth in clinical megavoltage beams.

Med Phys 2014 Jun;41(6):061712

Australian Clinical Dosimetry Service, 619 Lower Plenty Road, Yallambie, VIC 3085, Australia and School of Applied Sciences, Royal Melbourne Institute of Technology (RMIT) University, GPO Box 2476, Melbourne, VIC 3000, Australia.

Purpose: The purpose of this investigation was to assess the angular dependence of a commercial optically stimulated luminescence dosimeter (OSLD) dosimetry system in MV x-ray beams at depths beyond d(max) and to find ways to mitigate this dependence for measurements in phantoms.

Methods: Two special holders were designed which allow a dosimeter to be rotated around the center of its sensitive volume. The dosimeter's sensitive volume is a disk, 5 mm in diameter and 0.2 mm thick. The first holder rotates the disk in the traditional way. It positions the disk perpendicular to the beam (gantry pointing to the floor) in the initial position (0°). When the holder is rotated the angle of the disk towards the beam increases until the disk is parallel with the beam ("edge on," 90°). This is referred to as Setup 1. The second holder offers a new, alternative measurement position. It positions the disk parallel to the beam for all angles while rotating around its center (Setup 2). Measurements with five to ten dosimeters per point were carried out for 6 MV at 3 and 10 cm depth. Monte Carlo simulations using GEANT4 were performed to simulate the response of the active detector material for several angles. Detector and housing were simulated in detail based on microCT data and communications with the manufacturer. Various material compositions and an all-water geometry were considered.

Results: For the traditional Setup 1 the response of the OSLD dropped on average by 1.4% ± 0.7% (measurement) and 2.1% ± 0.3% (Monte Carlo simulation) for the 90° orientation compared to 0°. Monte Carlo simulations also showed a strong dependence of the effect on the composition of the sensitive layer. Assuming the layer to completely consist of the active material (Al2O3) results in a 7% drop in response for 90° compared to 0°. Assuming the layer to be completely water, results in a flat response within the simulation uncertainty of about 1%. For the new Setup 2, measurements and Monte Carlo simulations found the angular dependence of the dosimeter to be below 1% and within the measurement uncertainty.

Conclusions: The dosimeter system exhibits a small angular dependence of approximately 2% which needs to be considered for measurements involving other than normal incident beams angles. This applies in particular to clinical in vivo measurements where the orientation of the dosimeter is dictated by clinical circumstances and cannot be optimized as otherwise suggested here. When measuring in a phantom, the proposed new setup should be considered. It changes the orientation of the dosimeter so that a coplanar beam arrangement always hits the disk shaped detector material from the thin side and thereby reduces the angular dependence of the response to within the measurement uncertainty of about 1%. This improvement makes the dosimeter more attractive for clinical measurements with multiple coplanar beams in phantoms, as the overall measurement uncertainty is reduced. Similarly, phantom based postal audits can transition from the traditional TLD to the more accurate and convenient OSLD.
View Article and Find Full Text PDF

Download full-text PDF

Source
http://dx.doi.org/10.1118/1.4875698DOI Listing
June 2014

Remote auditing of radiotherapy facilities using optically stimulated luminescence dosimeters.

Med Phys 2014 Mar;41(3):032102

Australian Clinical Dosimetry Service, Yallambie, Victoria 3085, Australia and School of Applied Science, RMIT University, Melbourne 3000, Australia.

Purpose: On 1 July 2012, the Australian Clinical Dosimetry Service (ACDS) released its Optically Stimulated Luminescent Dosimeter (OSLD) Level I audit, replacing the previous TLD based audit. The aim of this work is to present the results from this new service and the complete uncertainty analysis on which the audit tolerances are based.

Methods: The audit release was preceded by a rigorous evaluation of the InLight® nanoDot OSLD system from Landauer (Landauer, Inc., Glenwood, IL). Energy dependence, signal fading from multiple irradiations, batch variation, reader variation, and dose response factors were identified and quantified for each individual OSLD. The detectors are mailed to the facility in small PMMA blocks, based on the design of the existing Radiological Physics Centre audit. Modeling and measurement were used to determine a factor that could convert the dose measured in the PMMA block, to dose in water for the facility's reference conditions. This factor is dependent on the beam spectrum. The TPR20,10 was used as the beam quality index to determine the specific block factor for a beam being audited. The audit tolerance was defined using a rigorous uncertainty calculation. The audit outcome is then determined using a scientifically based two tiered action level approach. Audit outcomes within two standard deviations were defined as Pass (Optimal Level), within three standard deviations as Pass (Action Level), and outside of three standard deviations the outcome is Fail (Out of Tolerance).

Results: To-date the ACDS has audited 108 photon beams with TLD and 162 photon beams with OSLD. The TLD audit results had an average deviation from ACDS of 0.0% and a standard deviation of 1.8%. The OSLD audit results had an average deviation of -0.2% and a standard deviation of 1.4%. The relative combined standard uncertainty was calculated to be 1.3% (1σ). Pass (Optimal Level) was reduced to ≤2.6% (2σ), and Fail (Out of Tolerance) was reduced to >3.9% (3σ) for the new OSLD audit. Previously with the TLD audit the Pass (Optimal Level) and Fail (Out of Tolerance) were set at ≤4.0% (2σ) and >6.0% (3σ).

Conclusions: The calculated standard uncertainty of 1.3% at one standard deviation is consistent with the measured standard deviation of 1.4% from the audits and confirming the suitability of the uncertainty budget derived audit tolerances. The OSLD audit shows greater accuracy than the previous TLD audit, justifying the reduction in audit tolerances. In the TLD audit, all outcomes were Pass (Optimal Level) suggesting that the tolerances were too conservative. In the OSLD audit 94% of the audits have resulted in Pass (Optimal level) and 6% of the audits have resulted in Pass (Action Level). All Pass (Action level) results have been resolved with a repeat OSLD audit, or an on-site ion chamber measurement.
View Article and Find Full Text PDF

Download full-text PDF

Source
http://dx.doi.org/10.1118/1.4865786DOI Listing
March 2014

Validation of a 4D-PET maximum intensity projection for delineation of an internal target volume.

Int J Radiat Oncol Biol Phys 2013 Jul 16;86(4):749-54. Epub 2013 Apr 16.

Centre for Molecular Imaging, Peter MacCallum Cancer Centre, East Melbourne, Victoria, Australia.

Purpose: The delineation of internal target volumes (ITVs) in radiation therapy of lung tumors is currently performed by use of either free-breathing (FB) (18)F-fluorodeoxyglucose-positron emission tomography-computed tomography (FDG-PET/CT) or 4-dimensional (4D)-CT maximum intensity projection (MIP). In this report we validate the use of 4D-PET-MIP for the delineation of target volumes in both a phantom and in patients.

Methods And Materials: A phantom with 3 hollow spheres was prepared surrounded by air then water. The spheres and water background were filled with a mixture of (18)F and radiographic contrast medium. A 4D-PET/CT scan was performed of the phantom while moving in 4 different breathing patterns using a programmable motion device. Nine patients with an FDG-avid lung tumor who underwent FB and 4D-PET/CT and >5 mm of tumor motion were included for analysis. The 3 spheres and patient lesions were contoured by 2 contouring methods (40% of maximum and PET edge) on the FB-PET, FB-CT, 4D-PET, 4D-PET-MIP, and 4D-CT-MIP. The concordance between the different contoured volumes was calculated using a Dice coefficient (DC). The difference in lung tumor volumes between FB-PET and 4D-PET volumes was also measured.

Results: The average DC in the phantom using 40% and PET edge, respectively, was lowest for FB-PET/CT (DCAir = 0.72/0.67, DCBackground 0.63/0.62) and highest for 4D-PET/CT-MIP (DCAir = 0.84/0.83, DCBackground = 0.78/0.73). The average DC in the 9 patients using 40% and PET edge, respectively, was also lowest for FB-PET/CT (DC = 0.45/0.44) and highest for 4D-PET/CT-MIP (DC = 0.72/0.73). In the 9 lesions, the target volumes of the FB-PET using 40% and PET edge, respectively, were on average 40% and 45% smaller than the 4D-PET-MIP.

Conclusion: A 4D-PET-MIP produces volumes with the highest concordance with 4D-CT-MIP across multiple breathing patterns and lesion sizes in both a phantom and among patients. Freebreathing PET/CT consistently underestimates ITV when compared with 4D PET/CT for a lesion affected by respiration.
View Article and Find Full Text PDF

Download full-text PDF

Source
http://dx.doi.org/10.1016/j.ijrobp.2013.02.030DOI Listing
July 2013

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.
View Article and Find Full Text PDF

Download full-text PDF

Source
http://dx.doi.org/10.1120/jacmp.v13i6.4019DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC5718545PMC
November 2012

The Australian Clinical Dosimetry Service: a commentary on the first 18 months.

Australas Phys Eng Sci Med 2012 Dec 28;35(4):407-11. Epub 2012 Sep 28.

Australian Clinical Dosimetry Service, ARPANSA, Australia.

View Article and Find Full Text PDF

Download full-text PDF

Source
http://dx.doi.org/10.1007/s13246-012-0161-1DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC3562435PMC
December 2012

Determination of peripheral underdosage at the lung-tumor interface using Monte Carlo radiation transport calculations.

Med Dosim 2012 30;37(1):61-6. Epub 2011 Jun 30.

School of Applied Sciences, College of Science, Engineering and Health, RMIT University, Melbourne, Victoria, Australia.

Prediction of dose distributions in close proximity to interfaces is difficult. In the context of radiotherapy of lung tumors, this may affect the minimum dose received by lesions and is particularly important when prescribing dose to covering isodoses. The objective of this work is to quantify underdosage in key regions around a hypothetical target using Monte Carlo dose calculation methods, and to develop a factor for clinical estimation of such underdosage. A systematic set of calculations are undertaken using 2 Monte Carlo radiation transport codes (egsnrc and geant4). Discrepancies in dose are determined for a number of parameters, including beam energy, tumor size, field size, and distance from chest wall. Calculations were performed for 1-mm³ regions at proximal, distal, and lateral aspects of a spherical tumor, determined for a 6-MV and a 15-MV photon beam. The simulations indicate regions of tumor underdose at the tumor-lung interface. Results are presented as ratios of the dose at key peripheral regions to the dose at the center of the tumor, a point at which the treatment planning system (TPS) predicts the dose more reliably. Comparison with TPS data (pencil-beam convolution) indicates such underdosage would not have been predicted accurately in the clinic. We define a dose reduction factor (DRF) as the average of the dose in the periphery in the 6 cardinal directions divided by the central dose in the target, the mean of which is 0.97 and 0.95 for a 6-MV and 15-MV beam, respectively. The DRF can assist clinicians in the estimation of the magnitude of potential discrepancies between prescribed and delivered dose distributions as a function of tumor size and location. Calculation for a systematic set of "generic" tumors allows application to many classes of patient case, and is particularly useful for interpreting clinical trial data.
View Article and Find Full Text PDF

Download full-text PDF

Source
http://dx.doi.org/10.1016/j.meddos.2011.01.002DOI Listing
June 2012
-->