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Deriving electron contamination characteristics using Monte Carlo beam data
Abstracts/Physica Medica 31 (2015) S1–S14
measurement, in order for the small fields to be characterized properly. Ideally, these detectors should not alter the electronic equilibrium. Detectors that are currently available differ in physical density, which will alter the electronic equilibrium. As a result the total scatter factors (TSFs) for small fields will depend on the detector used. The aim was to determine the measured TSFs for detectors with different physical densities. Materials and Methods: ELEKTA AgilityTM linear accelerator, Scanditronix Wellhofer water scanning phantom, IBA cc01, Scanditronix EFG3G electron diode, and PTW 60019 microDiamond were used to measure TSFs. TSFs were measured for 1 × 1, 2 × 2, 3 × 3, 4 × 4 and 5 × 5 cm2 square field sizes, with the 10 × 10 cm2 used as a reference field, at a reference depth of 10 cm. The measurements were repeated at 90, 100 and 110 cm SSDs. Results: Differences between the TSFs increase as the field size decreases, with the microdiamond TSFs being higher compared to the CC01 and the diode. The change in SSD shows significant differences in TSFs measured below 2 × 2 cm2 field sizes, the larger fields do not show significant SSD dependence. Conclusion: Results show that all three detectors respond differently at smaller fields. The change in TSFs with SSD for smaller field sizes can be attributed to a decrease in source occlusion and improvement in lateral electron equilibrium. Different detectors should be employed when measuring TSFs. Keywords: Total scatter factor, cc01, Diode, Microdiamond, Small field dosimetry O.39 A COMPARISON OF 6 MV AND 18 MV CANCER OF THE CERVIX VOLUMETRIC MODULATED ARC THERAPY (VMAT) TREATMENT PLANS FOR LARGE PATIENT THICKNESS S. Shambira *, G.L. Lazarus. Addington Hospital, 16 Erskine Terrace, Durban, South Africa Introduction: Cancer of the cervix (CACX) patients with large thicknesses require treatment with higher energy X-rays. Not much research is found for volumetric modulated arc therapy (VMAT) so this study compares 6 MV with 18 MV for VMAT plans for these patients to determine if using higher energies is advantageous. Materials and Methods: CT images of 15 CACX patients of thickness ≥24 cm were clinically contoured. VMAT plans were generated using an Eclipse TPS version 8.6 for both 6 MV and 18 MV for each patient. Plan evaluations were done using dose color-wash and point dose tools for dose coverage verification. The dose to anatomical sites were compared to ICRU 83 (target), QUANTEC (bladder and rectum), and EMAMI (femoral heads). Monitor units (MUs) were also compared. Results: The patient thicknesses ranged between 24.0 and 29.2 cm (average: 26.8 cm). No significant differences were observed for mean, maximum, nearmax (D2), median (D50) and near-min absorbed doses (D98) for the PTV. There was, however, an average increase of 6% in volume of the rectum receiving more than 5000 cGy with 18 MV. Rectal and bladder doses were still well below recommended tolerances. 6 MV had 1.5% higher maximum dose to bladder than 18 MV. Although both femoral heads obeyed Emami recommendations, there was no noticeable trend. The volume that received less than 5000 cGy of the dose was almost the same for the right femoral head. For the left femoral head, 18 MV had 10% less volume that received less than 5000 cGy compared to 6 MV. About 17% reduction in MUs was observed using 18 MV. Conclusion: Using high energy photons for VMAT of CACX patients of large thickness seems to have no significant advantage except for improving delivery efficiency. Keywords: VMAT, Cervix, Large patient thickness O.40 IGART WITH FRACTIONATION COMPENSATION-BASED SIMULTANEOUS INTEGRATED BOOST OF NODAL POSITIVE CERVIX CANCER W. Shaw *, F.H.J. O’Reilly. Department of Medical Physics, University of the Free State, Bloemfontein, South Africa Introduction: The collective contribution of external beam image guided adaptive Radiotherapy (IGART) and image guided adaptive Brachytherapy
S11
(IGABT) to achieve local control and distant metastasis free survival in cervix cancer is immense. Both have significant organ at risk (OAR) sparing and tumour dose escalation capabilities. We investigated a fractionation compensation (COMP) planning method with equivalent uniform dose (EUD)based IGART optimization to escalate lymph node (LN) dose in a simultaneous integrated boost (SIB). Materials and Methods: SIB IMRT plans were produced for 10 patients utilizing a margin of the day (MOD) treatment execution concept in combination with an on-line re-planning approach. 10 CT datasets were used to represent various phases of a 25 fraction treatment schedule. Dose accumulation was performed by EUD summation and compared to dose volume histogram (DVH) parameter variations. COMP was applied to escalate positive LN dose beyond 60 Gy without violating OAR total dose constraints. This was achieved by allowing variations in the daily OAR constraint criteria. Results: An MOD approach alone led to tumour under-dosage in some cases, while OAR doses were kept in check. When no re-planning was performed, primary tumour EUDs were lower by more than 5% in 4 patients. These under-dosages could not be recovered without on-line re-planning. The application of fractionation compensation eliminated these suboptimal doses in such mobile environments while SIB could be applied at the same toxicity levels as non-boosted plans. The EUD optimization controlled typical DVH objective/constraint parameters effectively for both the tumour and OARs, especially those close to the boost volume, like the duodenum in the case of a para-aortic boost. Conclusion: The dose accumulation properties of the EUD provide a platform for simple dose escalation techniques of tumour volumes located in a very mobile environment. In combination with fractionation compensation, tumour dose could potentially be escalated to small volumes beyond 60 Gy. Keywords: IGART, SIB, Fractionation compensation, EUD, Planning study
O.41 DERIVING ELECTRON CONTAMINATION CHARACTERISTICS USING MONTE CARLO BEAM DATA C. Smit *, F.C.P. du Plessis. Department of Medical Physics, University of the Free State, Bloemfontein, South Africa Introduction: Electron contamination in photon beams contributes to surface dose in the build-up region of a percentage depth–dose (PDD) curve. The purpose of this study was to extract an electron contamination energy spectrum for an Elekta Precise Linac, based on pure photon and measured clinical beam PDD data, and to include this as an additional source in DOSXYZnrc’s isource 4. Materials and Methods: A pure photon beam was simulated for the Linac using isource 4 in the DOSXYZnrc Monte Carlo (MC) code. PDD data were extracted afterwards for a range of field sizes (FS). These simulated dose data were compared to actual measured dose PDD data, with the data normalized at 10 cm depth. The resulting PDD data resembled the electron contamination depth–dose. Since the dose fall-off is a strictly decreasing function, a method was adopted to derive the contamination electron spectrum. Afterwards this spectrum was used in a DOSXYZnrc MC simulation run to verify that the original electron depth–dose could be replicated. Results: Various square aperture FS for 6, 8 and 15 megavolt (MV) photon beams were modelled, simulated and compared to their respective actual measured PDD data. As FS increased, simulated pure photon depth–dose profiles shifted deeper, thus requiring electron contamination to increase the surface dose. The percentage of electron weight increased with an increase in FS. For an FS of 15 × 15 cm2, the percentage electron weight is 0.1%, 0.2% and 0.4% for 6, 8 and 15 MV beams respectively. Conclusion: From the PDD results obtained, an additional electron contamination source was added to the photon source model so that simulation and measured PDD data could match within 2%/2 mm gamma-index criteria. The improved source model could assure more accurate simulations of surface doses. Keywords: Electron energy spectrum, Monte Carlo, photon beams, Percentage depth–dose curve, Gamma-index criteria
measurement, in order for the small fields to be characterized properly. Ideally, these detectors should not alter the electronic equilibrium. Detectors that are currently available differ in physical density, which will alter the electronic equilibrium. As a result the total scatter factors (TSFs) for small fields will depend on the detector used. The aim was to determine the measured TSFs for detectors with different physical densities. Materials and Methods: ELEKTA AgilityTM linear accelerator, Scanditronix Wellhofer water scanning phantom, IBA cc01, Scanditronix EFG3G electron diode, and PTW 60019 microDiamond were used to measure TSFs. TSFs were measured for 1 × 1, 2 × 2, 3 × 3, 4 × 4 and 5 × 5 cm2 square field sizes, with the 10 × 10 cm2 used as a reference field, at a reference depth of 10 cm. The measurements were repeated at 90, 100 and 110 cm SSDs. Results: Differences between the TSFs increase as the field size decreases, with the microdiamond TSFs being higher compared to the CC01 and the diode. The change in SSD shows significant differences in TSFs measured below 2 × 2 cm2 field sizes, the larger fields do not show significant SSD dependence. Conclusion: Results show that all three detectors respond differently at smaller fields. The change in TSFs with SSD for smaller field sizes can be attributed to a decrease in source occlusion and improvement in lateral electron equilibrium. Different detectors should be employed when measuring TSFs. Keywords: Total scatter factor, cc01, Diode, Microdiamond, Small field dosimetry O.39 A COMPARISON OF 6 MV AND 18 MV CANCER OF THE CERVIX VOLUMETRIC MODULATED ARC THERAPY (VMAT) TREATMENT PLANS FOR LARGE PATIENT THICKNESS S. Shambira *, G.L. Lazarus. Addington Hospital, 16 Erskine Terrace, Durban, South Africa Introduction: Cancer of the cervix (CACX) patients with large thicknesses require treatment with higher energy X-rays. Not much research is found for volumetric modulated arc therapy (VMAT) so this study compares 6 MV with 18 MV for VMAT plans for these patients to determine if using higher energies is advantageous. Materials and Methods: CT images of 15 CACX patients of thickness ≥24 cm were clinically contoured. VMAT plans were generated using an Eclipse TPS version 8.6 for both 6 MV and 18 MV for each patient. Plan evaluations were done using dose color-wash and point dose tools for dose coverage verification. The dose to anatomical sites were compared to ICRU 83 (target), QUANTEC (bladder and rectum), and EMAMI (femoral heads). Monitor units (MUs) were also compared. Results: The patient thicknesses ranged between 24.0 and 29.2 cm (average: 26.8 cm). No significant differences were observed for mean, maximum, nearmax (D2), median (D50) and near-min absorbed doses (D98) for the PTV. There was, however, an average increase of 6% in volume of the rectum receiving more than 5000 cGy with 18 MV. Rectal and bladder doses were still well below recommended tolerances. 6 MV had 1.5% higher maximum dose to bladder than 18 MV. Although both femoral heads obeyed Emami recommendations, there was no noticeable trend. The volume that received less than 5000 cGy of the dose was almost the same for the right femoral head. For the left femoral head, 18 MV had 10% less volume that received less than 5000 cGy compared to 6 MV. About 17% reduction in MUs was observed using 18 MV. Conclusion: Using high energy photons for VMAT of CACX patients of large thickness seems to have no significant advantage except for improving delivery efficiency. Keywords: VMAT, Cervix, Large patient thickness O.40 IGART WITH FRACTIONATION COMPENSATION-BASED SIMULTANEOUS INTEGRATED BOOST OF NODAL POSITIVE CERVIX CANCER W. Shaw *, F.H.J. O’Reilly. Department of Medical Physics, University of the Free State, Bloemfontein, South Africa Introduction: The collective contribution of external beam image guided adaptive Radiotherapy (IGART) and image guided adaptive Brachytherapy
S11
(IGABT) to achieve local control and distant metastasis free survival in cervix cancer is immense. Both have significant organ at risk (OAR) sparing and tumour dose escalation capabilities. We investigated a fractionation compensation (COMP) planning method with equivalent uniform dose (EUD)based IGART optimization to escalate lymph node (LN) dose in a simultaneous integrated boost (SIB). Materials and Methods: SIB IMRT plans were produced for 10 patients utilizing a margin of the day (MOD) treatment execution concept in combination with an on-line re-planning approach. 10 CT datasets were used to represent various phases of a 25 fraction treatment schedule. Dose accumulation was performed by EUD summation and compared to dose volume histogram (DVH) parameter variations. COMP was applied to escalate positive LN dose beyond 60 Gy without violating OAR total dose constraints. This was achieved by allowing variations in the daily OAR constraint criteria. Results: An MOD approach alone led to tumour under-dosage in some cases, while OAR doses were kept in check. When no re-planning was performed, primary tumour EUDs were lower by more than 5% in 4 patients. These under-dosages could not be recovered without on-line re-planning. The application of fractionation compensation eliminated these suboptimal doses in such mobile environments while SIB could be applied at the same toxicity levels as non-boosted plans. The EUD optimization controlled typical DVH objective/constraint parameters effectively for both the tumour and OARs, especially those close to the boost volume, like the duodenum in the case of a para-aortic boost. Conclusion: The dose accumulation properties of the EUD provide a platform for simple dose escalation techniques of tumour volumes located in a very mobile environment. In combination with fractionation compensation, tumour dose could potentially be escalated to small volumes beyond 60 Gy. Keywords: IGART, SIB, Fractionation compensation, EUD, Planning study
O.41 DERIVING ELECTRON CONTAMINATION CHARACTERISTICS USING MONTE CARLO BEAM DATA C. Smit *, F.C.P. du Plessis. Department of Medical Physics, University of the Free State, Bloemfontein, South Africa Introduction: Electron contamination in photon beams contributes to surface dose in the build-up region of a percentage depth–dose (PDD) curve. The purpose of this study was to extract an electron contamination energy spectrum for an Elekta Precise Linac, based on pure photon and measured clinical beam PDD data, and to include this as an additional source in DOSXYZnrc’s isource 4. Materials and Methods: A pure photon beam was simulated for the Linac using isource 4 in the DOSXYZnrc Monte Carlo (MC) code. PDD data were extracted afterwards for a range of field sizes (FS). These simulated dose data were compared to actual measured dose PDD data, with the data normalized at 10 cm depth. The resulting PDD data resembled the electron contamination depth–dose. Since the dose fall-off is a strictly decreasing function, a method was adopted to derive the contamination electron spectrum. Afterwards this spectrum was used in a DOSXYZnrc MC simulation run to verify that the original electron depth–dose could be replicated. Results: Various square aperture FS for 6, 8 and 15 megavolt (MV) photon beams were modelled, simulated and compared to their respective actual measured PDD data. As FS increased, simulated pure photon depth–dose profiles shifted deeper, thus requiring electron contamination to increase the surface dose. The percentage of electron weight increased with an increase in FS. For an FS of 15 × 15 cm2, the percentage electron weight is 0.1%, 0.2% and 0.4% for 6, 8 and 15 MV beams respectively. Conclusion: From the PDD results obtained, an additional electron contamination source was added to the photon source model so that simulation and measured PDD data could match within 2%/2 mm gamma-index criteria. The improved source model could assure more accurate simulations of surface doses. Keywords: Electron energy spectrum, Monte Carlo, photon beams, Percentage depth–dose curve, Gamma-index criteria