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Abstract ID: 106 Performance evaluation of two dedicated radioprotective disks in breast intraoperative electron radiotherapy
Abstracts / Physica Medica 42 (2017) 1–50
stopping power (Leq), used here as a key physical quantity. For any step, the mean energy loss was simply defined as the product of the step with Leq. Proton inelastic collisions with electrons were added to GPUMCD, a GPU-based Monte Carlo dose calculation code. The proton continuous slowing-down was modelled with the Leq formalism. First, the dose and time impacts of dmax were studied within Geant4. Second, in voxelized geometries, GPUMCD was compared to Geant4 using a high accuracy simulation setup (dmax = 10 lm). The ionization processes alone were activated and the energy straggling was first switched off to validate alone the Leq formalism. The default settings (dmax = 1 mm) in Geant4 led to an error of up to 16.5% in the falloff region, up to 4.8% elsewhere and the computation times were inversely proportional to the maximal step length allowed. Dose differences between Geant4 and GPUMCD were smaller than 0.31% in the Bragg peak for the Leq formalism. GPUMCD 80% falloff positions (R80) matched Geant R80 within 1 lm. With the energy straggling, dose agreements were within 2.7% in the falloff, below 0.83% elsewhere and R80 positions matched within 100 lm. The overall computation times per million transported protons with GPUMCD were 31–173 ms. Under similar conditions, Geant4 computation times were 1.4–20 h. The Leq formalism allows larger steps while preserving the accuracy. It significantly accelerates Monte Carlo proton transport. The Leq formalism constitutes a promising variance reduction technique for computing proton dose distributions in a clinical context. http://dx.doi.org/10.1016/j.ejmp.2017.09.057
Abstract ID: 106 Performance evaluation of two dedicated radioprotective disks in breast intraoperative electron radiotherapy Shiva Ghasemi a,*, Hamid Reza Baghani b,c, Seyed Rabi Mahdavi d, Mohsen Bakhshandeh a, Nahid Nafissi e a
Radiation Technology Department, Shahid Beheshti University of Medical Sciences, Tehran, Iran b Radiation Medicine Department, Shahid Beheshti University, Tehran, Iran c Physics Department, Hakim Sabzevari University, Sabzevar, Iran d Medical Physics Department, Iran University of Medical Sciences, Tehran, Iran e Surgery Department, Iran University of Medical Sciences, Tehran, Iran ⇑ Presenting author. The aim of breast intraoperative electron radiotherapy is to deliver the prescribed dose to the tumor bed during surgery. In this method, sensitive organs such as pectoral muscles, heart and lungs may be exposed to radiation. Therefore, a radioprotective disk is commonly used to protect the underlying healthy tissues. In this study, the performance of two employed radioprotective disks for breast intraoperative radiotherapy in terms of transmission factor (TF) and backscatter factor (BSF) were compared and the optimum disk was introduced. TF and BSF of the disks understudy, first disk consisted of PTFEstainless steel and the second one consisted of PMMA-Copper, were determined through irradiating the disks by LIAC mobile accelerator inside the water phantom and Advanced Markus ion chamber dosimetry. According to the obtained results, the BSF values of second disk in energies of 6, 8, 10 and 12 MeV was 4.5%, 2.8%, 3.8% and 2.9% lower than the first disk, respectively. In addition, the TF values of second disk in energies of 6, 8, 10 and 12 MeV was also 100%, 20%, 60% and 71% lower than the first one, respectively. Based on the results, it can be concluded that the second radioprotective disk (consisted of PMMA and Copper) has the better pro-
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tecting performance and in addition to the dose uniformity inside the tumor bed, will minimize the received dose to the organs at risk. http://dx.doi.org/10.1016/j.ejmp.2017.09.058
Abstract ID: 108 Study on conformal proton therapy using multileaf collimated beams without tumour-specific range compensators via flat dose-layer stacking Wencheng Shao *, Xiaobin Tang, Changran Geng, Diyun Shu, Chunhui Gong, Yao Ai, Xudong Zhang Nanjing University of Aeronautics and Astronautics, Department of Nuclear Science and Engineering, Nanjing, China ⇑ Presenting author. Purpose. In traditional conformal proton therapy (CPT), proximal dose conformity of tumours is sacrificed for achieving distal dose conformity due to tumour-specific range compensators. This study investigated whether CPT can be realized without tumour-specific range compensators using multileaf-collimated proton beams, and whether range-compensator-free CPT (RCF-CPT) can improve the proximal dose conformity without sacrificing distal conformity. Methods. First, geometric configurations of a virtual multileafcollimator (MLC) and three tumour cases (brain, liver, and prostate) were adopted during Geant4 geometry setup. Second, conformal proton radiation fields were generated for tumour CT slices based on the predesigned MLC using Geant4. Third, flat dose-layers corresponding to these tumour slices were produced using dose patching method in which MLC-collimated subbeams can be utilized to modulate the dose uniformity of these flat dose-layers. Forth, these flat dose-layers were stacked and integrated throughout the tumours for obtaining sufficient tumour dose coverages. Furthermore, dosimetric comparisons between the RCF-CPT and traditional CPT were performed to detail the dosimetric advantages of RCF-CPT. Results. For the three tumour cases, the tumours can be sufficiently covered by 95% relative doses through RCF-CPT, and the maximum tumour doses were smaller than 110% relative doses. Approximately, the proximal doses of RCF-CPT were controlled to 60% relative doses, and 95% dose lines fit with the tumour profiles at proximal tumour regions. Conclusions. Compared with traditional CPT, RCF-CPT can highly enhance the proximal dose conformity of tumours without sacrificing distal conformity. Moreover, the workflows of CPT can be largely simplified based on the range-compensator-free characteristic of RCF-CPT. http://dx.doi.org/10.1016/j.ejmp.2017.09.059
Abstract ID: 113 Accurate extraction of tissues parameters for Monte Carlo simulations using multi-energy CT Arthur Lalonde a,*, Hugo Bouchard a,b a
Universite de Montreal, Departement de Physique, Montreal, Canada Centre de Recherche du Centre Hospitalier de l’Universite de Montreal (CRCHUM), Montreal, Canada ⇑ Presenting author. b
Purpose. Robust tissue characterization is essential for accurate dose calculation[1,2]. In this work, we present a novel method called Bayesian eigentissue decomposition (BETD) [3] to extract Monte Carlo inputs from computed tomography (CT) data having an arbitrary number of energies. Method. Principal component analysis is applied on a reference dataset of human tissues to define eigentissues which are used as
stopping power (Leq), used here as a key physical quantity. For any step, the mean energy loss was simply defined as the product of the step with Leq. Proton inelastic collisions with electrons were added to GPUMCD, a GPU-based Monte Carlo dose calculation code. The proton continuous slowing-down was modelled with the Leq formalism. First, the dose and time impacts of dmax were studied within Geant4. Second, in voxelized geometries, GPUMCD was compared to Geant4 using a high accuracy simulation setup (dmax = 10 lm). The ionization processes alone were activated and the energy straggling was first switched off to validate alone the Leq formalism. The default settings (dmax = 1 mm) in Geant4 led to an error of up to 16.5% in the falloff region, up to 4.8% elsewhere and the computation times were inversely proportional to the maximal step length allowed. Dose differences between Geant4 and GPUMCD were smaller than 0.31% in the Bragg peak for the Leq formalism. GPUMCD 80% falloff positions (R80) matched Geant R80 within 1 lm. With the energy straggling, dose agreements were within 2.7% in the falloff, below 0.83% elsewhere and R80 positions matched within 100 lm. The overall computation times per million transported protons with GPUMCD were 31–173 ms. Under similar conditions, Geant4 computation times were 1.4–20 h. The Leq formalism allows larger steps while preserving the accuracy. It significantly accelerates Monte Carlo proton transport. The Leq formalism constitutes a promising variance reduction technique for computing proton dose distributions in a clinical context. http://dx.doi.org/10.1016/j.ejmp.2017.09.057
Abstract ID: 106 Performance evaluation of two dedicated radioprotective disks in breast intraoperative electron radiotherapy Shiva Ghasemi a,*, Hamid Reza Baghani b,c, Seyed Rabi Mahdavi d, Mohsen Bakhshandeh a, Nahid Nafissi e a
Radiation Technology Department, Shahid Beheshti University of Medical Sciences, Tehran, Iran b Radiation Medicine Department, Shahid Beheshti University, Tehran, Iran c Physics Department, Hakim Sabzevari University, Sabzevar, Iran d Medical Physics Department, Iran University of Medical Sciences, Tehran, Iran e Surgery Department, Iran University of Medical Sciences, Tehran, Iran ⇑ Presenting author. The aim of breast intraoperative electron radiotherapy is to deliver the prescribed dose to the tumor bed during surgery. In this method, sensitive organs such as pectoral muscles, heart and lungs may be exposed to radiation. Therefore, a radioprotective disk is commonly used to protect the underlying healthy tissues. In this study, the performance of two employed radioprotective disks for breast intraoperative radiotherapy in terms of transmission factor (TF) and backscatter factor (BSF) were compared and the optimum disk was introduced. TF and BSF of the disks understudy, first disk consisted of PTFEstainless steel and the second one consisted of PMMA-Copper, were determined through irradiating the disks by LIAC mobile accelerator inside the water phantom and Advanced Markus ion chamber dosimetry. According to the obtained results, the BSF values of second disk in energies of 6, 8, 10 and 12 MeV was 4.5%, 2.8%, 3.8% and 2.9% lower than the first disk, respectively. In addition, the TF values of second disk in energies of 6, 8, 10 and 12 MeV was also 100%, 20%, 60% and 71% lower than the first one, respectively. Based on the results, it can be concluded that the second radioprotective disk (consisted of PMMA and Copper) has the better pro-
23
tecting performance and in addition to the dose uniformity inside the tumor bed, will minimize the received dose to the organs at risk. http://dx.doi.org/10.1016/j.ejmp.2017.09.058
Abstract ID: 108 Study on conformal proton therapy using multileaf collimated beams without tumour-specific range compensators via flat dose-layer stacking Wencheng Shao *, Xiaobin Tang, Changran Geng, Diyun Shu, Chunhui Gong, Yao Ai, Xudong Zhang Nanjing University of Aeronautics and Astronautics, Department of Nuclear Science and Engineering, Nanjing, China ⇑ Presenting author. Purpose. In traditional conformal proton therapy (CPT), proximal dose conformity of tumours is sacrificed for achieving distal dose conformity due to tumour-specific range compensators. This study investigated whether CPT can be realized without tumour-specific range compensators using multileaf-collimated proton beams, and whether range-compensator-free CPT (RCF-CPT) can improve the proximal dose conformity without sacrificing distal conformity. Methods. First, geometric configurations of a virtual multileafcollimator (MLC) and three tumour cases (brain, liver, and prostate) were adopted during Geant4 geometry setup. Second, conformal proton radiation fields were generated for tumour CT slices based on the predesigned MLC using Geant4. Third, flat dose-layers corresponding to these tumour slices were produced using dose patching method in which MLC-collimated subbeams can be utilized to modulate the dose uniformity of these flat dose-layers. Forth, these flat dose-layers were stacked and integrated throughout the tumours for obtaining sufficient tumour dose coverages. Furthermore, dosimetric comparisons between the RCF-CPT and traditional CPT were performed to detail the dosimetric advantages of RCF-CPT. Results. For the three tumour cases, the tumours can be sufficiently covered by 95% relative doses through RCF-CPT, and the maximum tumour doses were smaller than 110% relative doses. Approximately, the proximal doses of RCF-CPT were controlled to 60% relative doses, and 95% dose lines fit with the tumour profiles at proximal tumour regions. Conclusions. Compared with traditional CPT, RCF-CPT can highly enhance the proximal dose conformity of tumours without sacrificing distal conformity. Moreover, the workflows of CPT can be largely simplified based on the range-compensator-free characteristic of RCF-CPT. http://dx.doi.org/10.1016/j.ejmp.2017.09.059
Abstract ID: 113 Accurate extraction of tissues parameters for Monte Carlo simulations using multi-energy CT Arthur Lalonde a,*, Hugo Bouchard a,b a
Universite de Montreal, Departement de Physique, Montreal, Canada Centre de Recherche du Centre Hospitalier de l’Universite de Montreal (CRCHUM), Montreal, Canada ⇑ Presenting author. b
Purpose. Robust tissue characterization is essential for accurate dose calculation[1,2]. In this work, we present a novel method called Bayesian eigentissue decomposition (BETD) [3] to extract Monte Carlo inputs from computed tomography (CT) data having an arbitrary number of energies. Method. Principal component analysis is applied on a reference dataset of human tissues to define eigentissues which are used as