- Email: [email protected]
1113 poster ACTIVE ON-LINE NEUTRON MONITOR FOR HIGH MEGAVOLTAGE RADIOTHERAPY
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D OSE MEASUREMENT: N OVEL DETECTORS
Results: The leakage radiation from a pair of abutment leaves was focused on. Measurement value of cross-leaf leakage showed the rhythmicity which were composed of a big wave and a small wave in the profile, which was due to the cross-sectional MLC which have Tongue and Groove design. A pair of abutment leaves having Tongue and Groove design produced two types of inter-leakage. The component of leakage through a single narrow slit was approximated by Gauss function. From the obtained calculation value, the ratio of inter-leakage 1 and inter-leakage 2 to reference value were 0.389 % and 0.118 %, respectively. The sum of component of the intra-leakage and the leaf scatter was approximated by Linear function. The contribution of the intra-leakage and the leaf scatter in reference value was about 0.964%. This indicates that the approximate calculation in this study was very sufficient, because RMS value between measurement values and calculation values was 0.012. Conclusions: The components of leakage radiation from MLCs having Tongue and Groove design includes two types of Inter-leakage, a Intraleakage and a leaf scatter. A Tongue and Groove design made the component of leakage radiation complicated and reduced the component of a Interleakage, but increased the component of a Intra-leakage and a leaf scatter. 1111 poster THE QUANTITATIVE ASSESSMENT OF MAYNEORD FACTOR AT DIFFERENT ENERGY, FIELD SIZE AND SOURCE TO SKIN DISTANCE USING MONTE CARLO SIMULATION AND EXPERIMENTAL MEASUREMENT S. A. Vaezzadeh1 , M. Allahverdi2 , H. A. Nedaie3 1 T EHRAN U NIVERSITY OF M EDICAL S CIENCES, Tehran, Iran Islamic Republic of 2 T EHRAN U NIVERSITY OF M EDICAL S CIENCES R ADIATION O N, Tehran, Iran Islamic Republic of 3 T EHRAN U NIVERSITY OF M EDICAL S CIENCES, Cancer Institute, Tehran, Iran Islamic Republic of
Purpose: When two source-to-skin distances (SSD) use for treatment, the correction of Percent depth doses (PDD) from one SSD to another consist of two components; the inverse square law correction component, as represented by Mayneord factor (FM) and the Scatter correction component, which is often ignored. In this study, the accuracy of FM quantitatively was evaluated using Monte Carlo (MC) simulations. Materials: MC simulations of Varian clinac 2100CD using MCNP code were implemented. Based on PDDs, FM at two energies (6MV and 18MV), four field sizes (10x10, 20x20, 30x30 and 40x40 cm2) and three SSDs (80, 100 and 110 cm) were calculated and analyzed for each combination. The correspondent experimental measurements were performed using PTW Dosimeter (0.125 ionization chamber) in the water tank (50x50x70 cm3). Finally, FM was calculated by its analytical formula for those same fields. Results: FM results from simulations, experimental measurements and analytical formula calculations were analyzed and differences were quantified for each case. For the case of 80 cm vs. 100 cm SSDs, Mean differences between calculation and measurement were 2% and 3% while mean differences between simulation and measurement were 0.7% and 1% for 6MV and 18MV respectively. For 110 cm vs. 100 cm SSDs, Mean differences between calculation and measurement were and Mean differences between simulation and measurement were 0.7% and 1.0% whilst mean differences between simulation and measurement were 1.1% and 1.5% for 6MV and 18MV respectively. Conclusions: Because MC simulations have lower differences than calculations with measurements for all cases, it could give more accurate result for calculation of dose at the desired point. Since overall uncertainty in the delivery of absorbed dose to the target volume is 5%, Scatter correction component should be considered to calculate accurate dose at a specified point especially at 6MV.
not proportional to absorbed dose. Building on existing research carried out at the NPL, we aim to create a tissue-equivalent detector primarily for microdosimetry of protons and light ions in order to better understand the biological effect of different treatment modalities. The detector is based on the Inductive Superconducting Transition-Edge Detector (ISTED) designed at the NPL, where we intend to extend the technology’s current application in low energy photon detection to the dosimetry of protons and ions. Work has been carried out by Kellerer et al. to develop a method for using microdosimetric spectra to predict the relative biological effectiveness (RBE) of ions. It is our intention to use microdosimetric spectra measured with our detector to calculate RBE. The device is at present in the design and initial production phase and Monte Carlo simulations are used to aid this design process. Materials: Simulations of microdosmetric spectra were performed with Geant4 version 4.9.3. For electromagnetic interactions, low-energy models were used for all particles using the default ICRU Report 49 stopping power parameterisation for protons. For non-elastic nuclear interactions the Bertini Intra-nuclear Cascade models were used. The phantom was made of Perspex containing a spherical detection region of 3 mm diameter composed of low-pressure tissue equivalent propane gas equivalent to a 1μm sphere of tissue. The secondary production range cut was set to 30 nm in the absorber giving an energy threshold of 100 eV for electrons/photons. Simulations of 4 MeV protons and 194 MeV/u carbon ions have been carried out to determine the final absorber dimensions and material. The results of the carbon ion simulations were compared with measurements recorded by Gerlach et al. using a TEPC. Results: The calculated spectra compare qualitatively well with the data from Gerlach et al. although a higher proportion of low energy interactions were found in the simulation. Simulations of potential absorber geometries using a 4 MeV proton beam show microdosimetric spectra peaking at about 10 keV/μm with a width of 10 keV/μm. For absorbers of 15 35 μm total thickness split into 0.1 μm layers, a small increase in the number of interactions above 15 keV/μm was found as a function of depth. Conclusions: Geant4 simulations of microdosimetric spectra were performed to aid the design of the microcalorimeter. Following these simulations a range of detectors sized between ~25 μm and 40 μm square are now in production. Whilst the larger size reduces the sensitivity it simplifies the production. In future more detailed simulations of the exact detector geometry as produced will be performed to assess corrections to measured microdosimetric spectra. 1113 poster ACTIVE ON-LINE NEUTRON MONITOR FOR HIGH MEGAVOLTAGE RADIOTHERAPY F. Gomez1 , F. Sanchez-Doblado2 , J. Marin-Muñoz3 , X. L. González Soto4 , G. A. Araceli4 , M. Zapata-Criado1 1 U NIVERSIDADE DE S ANTIAGO DE C OMPOSTELA, Física de Partículas, Santiago de Compostela, Spain 2 U NIVERSIDAD S EVILLA - FACULDAD M EDICINA, Sevilla, Spain 3 CIEMAT, Sensors and electronics, Madrid, Spain 4 U NIVERSIDADE DE S ANTIAGO DE C OMPOSTELA, Santiago de Compostela, Spain
Purpose: High megavoltage radiotherapy modalities have the undesired outcome of an additional neutron dose delivered to the patient. Due to the large scattered photon fluence and its pulsed nature it has been difficult to consider an active system as a real alternative to passive methods of neutron detection inside radiotherapy room. From the research of Single Event Upset in digital devices it has appeared a LET discriminating methodology that overcomes the mixed photon-neutron radiation field problems. Several articles have been published demonstrating the reliability and performance of this technique [1, 2, 3]. Several problems of the first version of the instrument have been addressed and considered to produce a new planar version with mapping capabilities and faster readout, while keeping the sensitivity of previous version.
Dose measurement: Novel detectors
1. F. G, A. Iglesias and F. Shez-Doblado 2010 Phys. Med. Biol. 55 1025
1112 poster
3. C. Domingo, F. G, F. Shez-Doblado, G.H. Hartmann, K. Amgarou, M.J. GarcFust.T. Romero, R. Bttger, R. Nolte, F. Wissmann, A. Zimbal, H. Schuhmacher 2010 Rad. Meas. 45 1513-1517
A NOVEL DETECTOR FOR THE MEASUREMENT OF MICRODOSIMETRIC SPECTRA FOR PROTONS AND LIGHT IONS S. Galer1 2 , H. Palmans1 , D. Shipley1 , L. Hao1 , A. Nisbet3 , K. Kirkby2 1 N ATIONAL P HYSICAL L ABORATORY, Teddington, United Kingdom 2 U NIVERSITY OF S URREY, Guildford, United Kingdom 3 S T. L UKE ’ S C ANCER C ENTRE R OYAL S URREY C OUNTY H OSP, Guildford, United Kingdom
Purpose: The Bureau International des Poids et Mesures has recommended to define a new quantity to account for the biological effect of ionising radiation used in radiotherapy as the biological effect of proton and ion beams is
2. F. G, F. Shez-Doblado, A. Iglesias, C. Domingo 2010, Rad. Meas. 45 1532-1535
Materials: The detector is based on COTS (Components Off the Shelf) using 128 SRAM with 512 kiB size (8 bit words) (Figure 1). The system is based on the detection of memory upsets in the array that is managed through an embedded microprocessor. The system has serial readout and represents the second generation of the device under patent US 2010/0258732 A1. Figure 1 Caption: New version of the planar neutron monitor showing the 128 SRAM array.
D OSE MEASUREMENT: N OVEL DETECTORS
Results: Several previous works within the NEUTOR collaboration [4] have proved that (once calibrated) the detector readout could be used for the estimation of patient neutron exposure at least for each treatment plan class (such us pelvis, head & neck, etc). Figure 2 was obtained dividing the dose measured through PADC detectors inside an anthropomorphic phantom normalized to the digital detector readout placed inside the radiotherapy room. As it can be appreciated in the Figure 2, the dose to detector readout ratio is quite independent (up to 15%) from the linac energy and room size.Figure 2 Caption: Neutron dose at different points in an anthropomorphic phantom normalized to the digital detector readout for different clinical facilities.
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to possess a 15% sensitivity variation when exposed to doserates varying from 0.012cGy to 0.12cGy. A recent study has also shown that parallel-plate chambers, which are the reference dosimeter for electron beams, exhibit significant depth and energy dependence in electron beams2 , especially for low energy electrons. In light of these facts, there is a need to revisit the question of silicon diode response in electron beams. Materials: We used a single scanning water equivalent plastic scintillation detector (PSD) mounted on a high-precision (0.1 mm reproducibility) scanning tank to measure depth-dose curves in 6, 12 and 18 MeV electron beams from a Varian Clinac iX. The PSD dosimeter used in this study consists in a 1 mm diameter and 2 mm long plastic scintillating fiber coupled to a clear non scintillating fiber. The scintillation photons are output onto color charge coupled device camera (CCD). The prototype is described in detail in Ref. 2. We also measured depth-dose curves using three types of diodes: IBA dosimetry EFD and SFD and PTW 60012 unshielded. The diodes reading was corrected for known doserate dependencies. We assumed, based on previous studies2 , that the PSD was water equivalent and did not exhibit any significant dose, doserate or energy dependencies. The PSD was then considered to be the reference depth-dose, i.e. to be free of artifacts for the subsequent analysis. We calculated the ratio of PSD to diode dose for each measurement depth and energy by dividing the normalized depth-doses. This ratio is termed the "relative perturbation factor" in accordance with ion chamber dosimetry practice. Results: Figure 1 shows the relative perturbation factors for the EFD, SFD and PTW diodes for 6 MeV. Also shown are Monte-Carlo results from the Wang and Rogers paper3 . The response of all three diodes differs from that of the PSD to various degrees for the buildup region and the Bremmstrahlung tail. All diodes tend to overestimate the Bremmstrahlung component. All diodes furnish adequate depth-dose curves from dref to close to R50. However, the relative perturbation factor is significantly higher than predicted by Monte-Carlo, especially at large depths (> R50) and low energies.
1. F. Shez-Doblado et al., "Neutron-induced second cancer risk estimation in patients under radiotherapy by means of an online in-vivo dosimeter" 11th symposium on neutron and ion dosimetry, iThemba Laboratory for Accelerator-Based Sciences, Cape Town, South Africa 12-16 October 2009
Conclusions: The diodes tested exhibit little perturbation for depths between dref to close to R50. However, the perturbation increases rapidly for depths greater than R50. 1. M McEwen and C Ross, "Unexpected Doserate Dependencies of Diodes for Beam Scanning", Med. Phys. 35,pp. 2920-2920 (2008).
Conclusions: The new planar version of the neutron monitor will solve problems arising from self-neutron absorption and will provide a faster and more reliable device for monitoring the neutron production in radiotherapy facilities. 1114 poster DEPTH-DEPENDENT RESPONSE OF SILICON DIODES IN ELECTRON BEAMS F. Lacroix1 , M. Guillot2 , M. McEwen3 , L. Beaulieu4 1 CHUM - C AMPUS N OTRE -DAME, Radio-oncologie, Montréal, Canada 2 H OTEL -D IEU DE Q UEBEC CHUQ, Radio-oncologie, Montreal, Canada 3 N ATIONAL R ESEARCH C OUNCIL, Ionizing Radiation Standards, Ottawa, ON, Canada 4 C ENTRE H OSPITALIER U NIVERSITAIRE DE Q UÉBEC, L’H ÔTEL -D IEU DE Q UÉBEC, Radio-oncologie, Quebec, Canada
Purpose: It is commonly assumed, based on qualitative comparisons to ion chamber measurements, that silicon diode response is independent of depth in electron beams. However, the AAPM TG-25 did point out that the stopping power ratio decreases by up to 6% with decreasing electron energy below 5 MeV. This is likely to affect depth dose curves at depth where the electron energy drops. It has also been shown recently that silicon diodes exhibit doserate dependencies1 ; for example, the PTW 60012 electron diode was shown
2. F. Lacroix, M. Guillot, M. McEwen, C. Cojocaru, L. Gingras, A. S. Beddar, and L. Beaulieu, "Extraction of depth-dependent perturbation factors for parallel-plate chambers in electron beams using a plastic scintillation detector", Med. Phys. 37, 4331 (2010). 3. L. L. W. Wang and D. W. O. Rogers, "Monte Carlo study of Si diode response in electron beams", Med. Phys. 34, 1734 (2007)
1115 poster DETERMINATION OF CORRECTION FACTORS IN KILOVOLTAGE ENERGY RANGE OF THE ONEDOSETM SYSTEM A. Lo Bosco1 , M. D. Falco2 , G. Rinaldi3 , L. Strigari4 , M. D’Andrea5 , F. Quagliani5 , R. Santoni6 1 TOR V ERGATA U NIVERSITY, Department of Physics, Rome, Italy 2 TOR V ERGATA U NIVERSITY G ENERAL H OSPITAL, Department of Diagnostic Imaging, Molecular Imaging, Interventional Radiology and Radiotherapy, Rome, Italy 3 T EMA S INERGIE, Faenza, Italy 4 R EGINA E LENA C ANCER I NSTITUTE, Roma, Italy 5 N ATIONAL C ANCER I NSTITUTE R EGINA E LENA, Laboratory of Medical Physics and Expert Systems, Rome, Italy 6 P OLICLINICO TOR V ERGATA, Roma, Italy
D OSE MEASUREMENT: N OVEL DETECTORS
Results: The leakage radiation from a pair of abutment leaves was focused on. Measurement value of cross-leaf leakage showed the rhythmicity which were composed of a big wave and a small wave in the profile, which was due to the cross-sectional MLC which have Tongue and Groove design. A pair of abutment leaves having Tongue and Groove design produced two types of inter-leakage. The component of leakage through a single narrow slit was approximated by Gauss function. From the obtained calculation value, the ratio of inter-leakage 1 and inter-leakage 2 to reference value were 0.389 % and 0.118 %, respectively. The sum of component of the intra-leakage and the leaf scatter was approximated by Linear function. The contribution of the intra-leakage and the leaf scatter in reference value was about 0.964%. This indicates that the approximate calculation in this study was very sufficient, because RMS value between measurement values and calculation values was 0.012. Conclusions: The components of leakage radiation from MLCs having Tongue and Groove design includes two types of Inter-leakage, a Intraleakage and a leaf scatter. A Tongue and Groove design made the component of leakage radiation complicated and reduced the component of a Interleakage, but increased the component of a Intra-leakage and a leaf scatter. 1111 poster THE QUANTITATIVE ASSESSMENT OF MAYNEORD FACTOR AT DIFFERENT ENERGY, FIELD SIZE AND SOURCE TO SKIN DISTANCE USING MONTE CARLO SIMULATION AND EXPERIMENTAL MEASUREMENT S. A. Vaezzadeh1 , M. Allahverdi2 , H. A. Nedaie3 1 T EHRAN U NIVERSITY OF M EDICAL S CIENCES, Tehran, Iran Islamic Republic of 2 T EHRAN U NIVERSITY OF M EDICAL S CIENCES R ADIATION O N, Tehran, Iran Islamic Republic of 3 T EHRAN U NIVERSITY OF M EDICAL S CIENCES, Cancer Institute, Tehran, Iran Islamic Republic of
Purpose: When two source-to-skin distances (SSD) use for treatment, the correction of Percent depth doses (PDD) from one SSD to another consist of two components; the inverse square law correction component, as represented by Mayneord factor (FM) and the Scatter correction component, which is often ignored. In this study, the accuracy of FM quantitatively was evaluated using Monte Carlo (MC) simulations. Materials: MC simulations of Varian clinac 2100CD using MCNP code were implemented. Based on PDDs, FM at two energies (6MV and 18MV), four field sizes (10x10, 20x20, 30x30 and 40x40 cm2) and three SSDs (80, 100 and 110 cm) were calculated and analyzed for each combination. The correspondent experimental measurements were performed using PTW Dosimeter (0.125 ionization chamber) in the water tank (50x50x70 cm3). Finally, FM was calculated by its analytical formula for those same fields. Results: FM results from simulations, experimental measurements and analytical formula calculations were analyzed and differences were quantified for each case. For the case of 80 cm vs. 100 cm SSDs, Mean differences between calculation and measurement were 2% and 3% while mean differences between simulation and measurement were 0.7% and 1% for 6MV and 18MV respectively. For 110 cm vs. 100 cm SSDs, Mean differences between calculation and measurement were and Mean differences between simulation and measurement were 0.7% and 1.0% whilst mean differences between simulation and measurement were 1.1% and 1.5% for 6MV and 18MV respectively. Conclusions: Because MC simulations have lower differences than calculations with measurements for all cases, it could give more accurate result for calculation of dose at the desired point. Since overall uncertainty in the delivery of absorbed dose to the target volume is 5%, Scatter correction component should be considered to calculate accurate dose at a specified point especially at 6MV.
not proportional to absorbed dose. Building on existing research carried out at the NPL, we aim to create a tissue-equivalent detector primarily for microdosimetry of protons and light ions in order to better understand the biological effect of different treatment modalities. The detector is based on the Inductive Superconducting Transition-Edge Detector (ISTED) designed at the NPL, where we intend to extend the technology’s current application in low energy photon detection to the dosimetry of protons and ions. Work has been carried out by Kellerer et al. to develop a method for using microdosimetric spectra to predict the relative biological effectiveness (RBE) of ions. It is our intention to use microdosimetric spectra measured with our detector to calculate RBE. The device is at present in the design and initial production phase and Monte Carlo simulations are used to aid this design process. Materials: Simulations of microdosmetric spectra were performed with Geant4 version 4.9.3. For electromagnetic interactions, low-energy models were used for all particles using the default ICRU Report 49 stopping power parameterisation for protons. For non-elastic nuclear interactions the Bertini Intra-nuclear Cascade models were used. The phantom was made of Perspex containing a spherical detection region of 3 mm diameter composed of low-pressure tissue equivalent propane gas equivalent to a 1μm sphere of tissue. The secondary production range cut was set to 30 nm in the absorber giving an energy threshold of 100 eV for electrons/photons. Simulations of 4 MeV protons and 194 MeV/u carbon ions have been carried out to determine the final absorber dimensions and material. The results of the carbon ion simulations were compared with measurements recorded by Gerlach et al. using a TEPC. Results: The calculated spectra compare qualitatively well with the data from Gerlach et al. although a higher proportion of low energy interactions were found in the simulation. Simulations of potential absorber geometries using a 4 MeV proton beam show microdosimetric spectra peaking at about 10 keV/μm with a width of 10 keV/μm. For absorbers of 15 35 μm total thickness split into 0.1 μm layers, a small increase in the number of interactions above 15 keV/μm was found as a function of depth. Conclusions: Geant4 simulations of microdosimetric spectra were performed to aid the design of the microcalorimeter. Following these simulations a range of detectors sized between ~25 μm and 40 μm square are now in production. Whilst the larger size reduces the sensitivity it simplifies the production. In future more detailed simulations of the exact detector geometry as produced will be performed to assess corrections to measured microdosimetric spectra. 1113 poster ACTIVE ON-LINE NEUTRON MONITOR FOR HIGH MEGAVOLTAGE RADIOTHERAPY F. Gomez1 , F. Sanchez-Doblado2 , J. Marin-Muñoz3 , X. L. González Soto4 , G. A. Araceli4 , M. Zapata-Criado1 1 U NIVERSIDADE DE S ANTIAGO DE C OMPOSTELA, Física de Partículas, Santiago de Compostela, Spain 2 U NIVERSIDAD S EVILLA - FACULDAD M EDICINA, Sevilla, Spain 3 CIEMAT, Sensors and electronics, Madrid, Spain 4 U NIVERSIDADE DE S ANTIAGO DE C OMPOSTELA, Santiago de Compostela, Spain
Purpose: High megavoltage radiotherapy modalities have the undesired outcome of an additional neutron dose delivered to the patient. Due to the large scattered photon fluence and its pulsed nature it has been difficult to consider an active system as a real alternative to passive methods of neutron detection inside radiotherapy room. From the research of Single Event Upset in digital devices it has appeared a LET discriminating methodology that overcomes the mixed photon-neutron radiation field problems. Several articles have been published demonstrating the reliability and performance of this technique [1, 2, 3]. Several problems of the first version of the instrument have been addressed and considered to produce a new planar version with mapping capabilities and faster readout, while keeping the sensitivity of previous version.
Dose measurement: Novel detectors
1. F. G, A. Iglesias and F. Shez-Doblado 2010 Phys. Med. Biol. 55 1025
1112 poster
3. C. Domingo, F. G, F. Shez-Doblado, G.H. Hartmann, K. Amgarou, M.J. GarcFust.T. Romero, R. Bttger, R. Nolte, F. Wissmann, A. Zimbal, H. Schuhmacher 2010 Rad. Meas. 45 1513-1517
A NOVEL DETECTOR FOR THE MEASUREMENT OF MICRODOSIMETRIC SPECTRA FOR PROTONS AND LIGHT IONS S. Galer1 2 , H. Palmans1 , D. Shipley1 , L. Hao1 , A. Nisbet3 , K. Kirkby2 1 N ATIONAL P HYSICAL L ABORATORY, Teddington, United Kingdom 2 U NIVERSITY OF S URREY, Guildford, United Kingdom 3 S T. L UKE ’ S C ANCER C ENTRE R OYAL S URREY C OUNTY H OSP, Guildford, United Kingdom
Purpose: The Bureau International des Poids et Mesures has recommended to define a new quantity to account for the biological effect of ionising radiation used in radiotherapy as the biological effect of proton and ion beams is
2. F. G, F. Shez-Doblado, A. Iglesias, C. Domingo 2010, Rad. Meas. 45 1532-1535
Materials: The detector is based on COTS (Components Off the Shelf) using 128 SRAM with 512 kiB size (8 bit words) (Figure 1). The system is based on the detection of memory upsets in the array that is managed through an embedded microprocessor. The system has serial readout and represents the second generation of the device under patent US 2010/0258732 A1. Figure 1 Caption: New version of the planar neutron monitor showing the 128 SRAM array.
D OSE MEASUREMENT: N OVEL DETECTORS
Results: Several previous works within the NEUTOR collaboration [4] have proved that (once calibrated) the detector readout could be used for the estimation of patient neutron exposure at least for each treatment plan class (such us pelvis, head & neck, etc). Figure 2 was obtained dividing the dose measured through PADC detectors inside an anthropomorphic phantom normalized to the digital detector readout placed inside the radiotherapy room. As it can be appreciated in the Figure 2, the dose to detector readout ratio is quite independent (up to 15%) from the linac energy and room size.Figure 2 Caption: Neutron dose at different points in an anthropomorphic phantom normalized to the digital detector readout for different clinical facilities.
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to possess a 15% sensitivity variation when exposed to doserates varying from 0.012cGy to 0.12cGy. A recent study has also shown that parallel-plate chambers, which are the reference dosimeter for electron beams, exhibit significant depth and energy dependence in electron beams2 , especially for low energy electrons. In light of these facts, there is a need to revisit the question of silicon diode response in electron beams. Materials: We used a single scanning water equivalent plastic scintillation detector (PSD) mounted on a high-precision (0.1 mm reproducibility) scanning tank to measure depth-dose curves in 6, 12 and 18 MeV electron beams from a Varian Clinac iX. The PSD dosimeter used in this study consists in a 1 mm diameter and 2 mm long plastic scintillating fiber coupled to a clear non scintillating fiber. The scintillation photons are output onto color charge coupled device camera (CCD). The prototype is described in detail in Ref. 2. We also measured depth-dose curves using three types of diodes: IBA dosimetry EFD and SFD and PTW 60012 unshielded. The diodes reading was corrected for known doserate dependencies. We assumed, based on previous studies2 , that the PSD was water equivalent and did not exhibit any significant dose, doserate or energy dependencies. The PSD was then considered to be the reference depth-dose, i.e. to be free of artifacts for the subsequent analysis. We calculated the ratio of PSD to diode dose for each measurement depth and energy by dividing the normalized depth-doses. This ratio is termed the "relative perturbation factor" in accordance with ion chamber dosimetry practice. Results: Figure 1 shows the relative perturbation factors for the EFD, SFD and PTW diodes for 6 MeV. Also shown are Monte-Carlo results from the Wang and Rogers paper3 . The response of all three diodes differs from that of the PSD to various degrees for the buildup region and the Bremmstrahlung tail. All diodes tend to overestimate the Bremmstrahlung component. All diodes furnish adequate depth-dose curves from dref to close to R50. However, the relative perturbation factor is significantly higher than predicted by Monte-Carlo, especially at large depths (> R50) and low energies.
1. F. Shez-Doblado et al., "Neutron-induced second cancer risk estimation in patients under radiotherapy by means of an online in-vivo dosimeter" 11th symposium on neutron and ion dosimetry, iThemba Laboratory for Accelerator-Based Sciences, Cape Town, South Africa 12-16 October 2009
Conclusions: The diodes tested exhibit little perturbation for depths between dref to close to R50. However, the perturbation increases rapidly for depths greater than R50. 1. M McEwen and C Ross, "Unexpected Doserate Dependencies of Diodes for Beam Scanning", Med. Phys. 35,pp. 2920-2920 (2008).
Conclusions: The new planar version of the neutron monitor will solve problems arising from self-neutron absorption and will provide a faster and more reliable device for monitoring the neutron production in radiotherapy facilities. 1114 poster DEPTH-DEPENDENT RESPONSE OF SILICON DIODES IN ELECTRON BEAMS F. Lacroix1 , M. Guillot2 , M. McEwen3 , L. Beaulieu4 1 CHUM - C AMPUS N OTRE -DAME, Radio-oncologie, Montréal, Canada 2 H OTEL -D IEU DE Q UEBEC CHUQ, Radio-oncologie, Montreal, Canada 3 N ATIONAL R ESEARCH C OUNCIL, Ionizing Radiation Standards, Ottawa, ON, Canada 4 C ENTRE H OSPITALIER U NIVERSITAIRE DE Q UÉBEC, L’H ÔTEL -D IEU DE Q UÉBEC, Radio-oncologie, Quebec, Canada
Purpose: It is commonly assumed, based on qualitative comparisons to ion chamber measurements, that silicon diode response is independent of depth in electron beams. However, the AAPM TG-25 did point out that the stopping power ratio decreases by up to 6% with decreasing electron energy below 5 MeV. This is likely to affect depth dose curves at depth where the electron energy drops. It has also been shown recently that silicon diodes exhibit doserate dependencies1 ; for example, the PTW 60012 electron diode was shown
2. F. Lacroix, M. Guillot, M. McEwen, C. Cojocaru, L. Gingras, A. S. Beddar, and L. Beaulieu, "Extraction of depth-dependent perturbation factors for parallel-plate chambers in electron beams using a plastic scintillation detector", Med. Phys. 37, 4331 (2010). 3. L. L. W. Wang and D. W. O. Rogers, "Monte Carlo study of Si diode response in electron beams", Med. Phys. 34, 1734 (2007)
1115 poster DETERMINATION OF CORRECTION FACTORS IN KILOVOLTAGE ENERGY RANGE OF THE ONEDOSETM SYSTEM A. Lo Bosco1 , M. D. Falco2 , G. Rinaldi3 , L. Strigari4 , M. D’Andrea5 , F. Quagliani5 , R. Santoni6 1 TOR V ERGATA U NIVERSITY, Department of Physics, Rome, Italy 2 TOR V ERGATA U NIVERSITY G ENERAL H OSPITAL, Department of Diagnostic Imaging, Molecular Imaging, Interventional Radiology and Radiotherapy, Rome, Italy 3 T EMA S INERGIE, Faenza, Italy 4 R EGINA E LENA C ANCER I NSTITUTE, Roma, Italy 5 N ATIONAL C ANCER I NSTITUTE R EGINA E LENA, Laboratory of Medical Physics and Expert Systems, Rome, Italy 6 P OLICLINICO TOR V ERGATA, Roma, Italy