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EP-1369 NEUTRON DOSE EVALUATION OF ELEKTA LINAC BY MCNP CODE AND COMPARISON WITH EXPERIMENTAL MEASUREMENTS
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EP-1367 EXPERIMENTAL COMPARISON OF VARIOUS DETECTORS FOR SMALL FIELD OUTPUT FACTORS MEASUREMENTS ON A TRUEBEAM ACCELERATOR C. Khamphan1, V. Bodez1, R. Garcia1, M.E. Alayrach Biarnés1, E. Jaegle1, A. Badey1 1 Institut Sainte Catherine, Radiotherapy, Avignon - Cedex 02, France Purpose/Objective: Development of Intensity Modulated Radiotherapy and Stereotactic Radiotherapy in recent years has lead to an increasing use of small beams. The dosimetry of small fields is challenging due to non equilibrium conditions, partial blocking of the beam source and non-negligible detector perturbations. It is commonly accepted that one can expect reduced accuracy for such fields compared to standard clinical dosimetry. The purpose of this work is to compare small field Output Factors measured with five different detectors on a TrueBeam STx accelerator (Varian Medical Systems). Materials and Methods: Measurements of output factors (i.e. Total scatter factor, Scp) were made on the 6MV beam of a TrueBeam STx accelerator for square field sizes of 1.0, 1.5, 2.0, 3.0, 4.0, 6.0, 8.0 and 10.0 cm. Isocentric measurements were made in water at 5cm depth using the IBA 'Blue Phantom' 3D scanning system. Data were acquired with five different detectors: two stereotactic ionization chambers (Exradin A16 Standard Imaging, CC04 IBA), a liquid ionization chamber (MicroLion 31018 PTW), a stereotactic unshielded diode (SFD IBA) and a shielded diode (PFD IBA). Measurements were normalized to a 10cm x 10cm reference field except for diodes which were referenced to an intermediate 4cm x 4cm field measured with the CC04 ionization chamber. This 'Daisy chain' strategy is used to mitigate the overestimation of output factors by silicon detectors. Standard deviation and Coefficient of Variation were calculated to characterize measurement dispersion of the five different detectors. Results: Results are presented in Figure 1 and Table 1. As expected, there is a significant discrepancy in measurements for 1cm square field. For the same field size, maximum deviation between the highest and the lowest value is about 9%. Coefficient of variation is 3.3% for 1cm square field, 1.5% for 1.5cm square field and ≤1% for larger field sizes.
Figure 1: Output factors measured with each of five detectors for a 6MV photon beam. The curves are referenced to the 10cm square field, except PFD and SFD diodes which are normalized with 'Daisy chaining' method. Table 1: Comparison of Output factors measured with five different detectors for a 6MV photon beam.
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Conclusions: Results are clearly dependant on detectors and it appears difficult to know which is the suitable one. As expected, we observe that ionization chambers (CC04, Exradin) underestimate Scp due to lateral electronic disequilibrium, and diodes overestimate Scp due to the high atomic number of silicon. We plan to make further measurements with a Large Area Chamber (PTW 34070) which hopefully will help us to answer the question: which is the reference detector for which field size range? Achieving a satisfying accuracy could not be done with a decision at the local scale. As a reference dataset is not currently available for TrueBeam STx, it appears important to compare measurements with other customers. EP-1368 EVALUATION OF NEUTRON DOSE IN CENTRAL AXIS ABSORBED DOSE IN LINAC MACHINES BY TLD600 AND TLD700 DOSIMETERS H. Darestani1, H.A. Nedaie2, K.H. Mohammadi3, A. Shahvar1, E. Bayat4, N. Shahgholi1, M. Nazarnejad1 1 Islamic Azad University Sience and Research branch, Nuclear Engineering Department, Tehran, Iran Islamic Republic of 2 Imam Khomeini Hospital, Radiotherapy Oncology Department, Tehran, Iran Islamic Republic of 3 Amirkabir University of Technology, Nuclear Engineering Department, Tehran, Iran Islamic Republic of 4 Birjand University of Technology, Nuclear Engineering Department, Tehran, Iran Islamic Republic of Purpose/Objective: In spite of clinically useful photon and electron beams, high energy linacs produce secondary particles such as neutrons (photoneutron production). The neutrons have the important role during application of high energy photons in terms of protection and dose escalation. The dosimetry in a neutron-gamma mixed field like the linac field needs at least two detectors, one sensitive to gamma and the other sensitive to neutrons. TLD600 and TLD700 gamma sensitivities are roughly the same, while TLD600 is much more sensitive to thermal neutrons. Consequently, simultaneous use of TLD600 and TLD700 for dosimetry of a mixed field and discriminating the neutron and gamma component is the solution. In this study, the central axis neutron dose of three different linac machines (Varian, Siemens and Elekta) and the neutron dose in the vicinity of the Varian linac is measured by TLD600 and TLD700 dosimeters. Materials and Methods: First, TLDs were calibrated versus definite gamma and neutron doses. Photoneutron dose equivalent was calculated in central axis for 18 MV Varian and Elekta linacs and 15 MV Siemens linac on the phantom surface and depths of 1, 2, 3.3, 4, 5 and 6 cm. In order to determine neutron dose equivalent in the vicinity of the Varian 18 MV linac, TLDs were irradiated in the center of the 25 cm diameter polyethylene sphere in four points at the distance of 1 and 2 m of the isocentre in the patient plane. Results: The maximum photoneutron dose equivalent was observed at the depth of 4-5 cm, no photoneutron dose equivalent was obtained on the phantom surface and depths of less than 3 cm. The minimum and the maximum photoneutron doses were calculated for Siemens and Varian linac respectively. For three linacs of Siemens, Elekta and Varian, the maximum photoneutron doses were obtained equal to 15, 44 and 52 mSv.Gy-1 respectively. The neutron dose equivalent at the distance of 1 and 2 m of the isocentre on the patient couch was measured 2.2 and 0.75 mSv.Gy-1 respectively. Conclusions: The neutron dose equivalent along the beam axis was achieved up to ten times of the published data, whereas neutron dose measurement at the distance of 1 m of the isocentre on the patient couch had only the error of 4%. According to the results it seems that TLD600 and TLD700 pairs are not suitable dosimeters for neutron dosimetry inside the field due to high photon fluence, but they can be used for outside the field measurement with good accuracy. EP-1369 NEUTRON DOSE EVALUATION OF ELEKTA LINAC BY MCNP CODE AND COMPARISON WITH EXPERIMENTAL MEASUREMENTS N. Shahgholi1, H. Nedaie2, M. Sadeghi1, K. Mohammadi3, A. Shahvar1, E. Bayat4, H. Darestani1, M. Nazarnejad1 1 Science and Research unit Azad University, Nuclear Engineering Department, Tehran, Iran Islamic Republic of 2 Cancer Institute Tehran University, Radiotherapy Oncology Department, Tehran, Iran Islamic Republic of 3 Amirkabir Industrial University, Nuclear Engineering Department, Tehran, Iran Islamic Republic of 4 Birjand University, Physics department, Birjand, Iran Islamic Republic of
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Purpose/Objective: Medical linacs, besides the clinically high energy electron and photon beams (>8MV), produce other secondary particles such as neutrons which increased the delivered dose. For dosimetry in a neutron-gamma mixed field, use of at least two detectors one sensitive to gamma and the other more sensitive to neutrons are necessary. TLD600 and TLD700 gamma sensitivities are almost the same, while TLD600 is much more sensitive to thermal neutrons. Consequently simultaneous employment of TLD600 and TLD700 for neutron dose assessment in linac field is suitable. Application of simulation methods also could help to assess the dose and contribution of each. In this study the neutron dose at 10 and 18 MV Elekta linac was obtained by using of TLD600 and TLD700 as well as Monte Carlo simulation. Materials and Methods: For neutron dose assessment in 20× 20 cm2 field, TLDs were calibrated at first. Gamma calibration was performed with standard 60Co source and 18 MV linac and neutron calibration was done with 241Am-Be neutron source. MCNPX code was used for simulation of accelerator head and phantom and calculated neutron dose equivalent was compared with measurement data. Results: Neutron dose equivalent at 18 MV was measured by using TLDs on the phantom surface and depths of 1, 2, 3.3, 4, 5 and 6 cm. Neutron dose at depths of less than 3.3cm was zero and maximized at the depth of 4 cm (44.39 mSvGy-1), whereas calculation resulted in the maximum of 2.32 mSvGy-1 at the same depth. Neutron dose at 10 MV was measured by using TLDs on the phantom surface and depths of 1, 2, 2.5, 3.3, 4 and 5 cm. No photoneutron dose was observed at depths of less than 3.3cm and the maximum was at 4cm equal to 5.44 mSvGy-1, however, the simulation showed the maximum of 0.077mSvGy-1 at the same depth. Conclusions: The comparison between measured photoneutron dose and simulated data along the beam axis in different depths shows that the measurement data were much more than the simulation data, so it seems that TLD600 and TLD700 pairs are not suitable dosimeters for neutron dosimetry in central axis due to high photon flux, whereas MCNPX Monte Carlo techniques still remain a valuable tool for photonuclear dose studies. EP-1370 SMOOTHING AND CALCULATION OF CALIBRATION CURVE INDEPENDENTLY OF TOMOTHERAPY PLANNING SYSTEM 1 1 1 J.M. Verde Velasco , J.A. Ramos Pacho , S. Garcia Repiso , C. Martin Rincon1, E. De Sena Espinel1, M.E. Perez Alvarez1, J.M. Delgado Aparicio1 1 Hospital Universitario, Medical Physics and Radiation Protection, Salamanca, Spain Purpose/Objective: Tomotherapy Hi-Art radiation unit consists of a linear accelerator that can rotate around the treatment couch while it is moving at constant speed, i.e., it works like a CT unit but with a treatment end. This unit has a binary MLC of 64 leafs moving continuously during irradiation time. These and other characteristics allow a helical IMRT treatment. For each patient a quality control is performed, namely DQA (Delivery Quality Assurance), by which a phantom is irradiated with a treatment plan while dose measurements are done, and then compared with the dose calculation of the patient plan on the phantom. In order to get those measures two ionization chambers and a radiochromic film are used. To use the latter correctly and to be able to compare measures, a calibration curve is needed. This is found by means of a set of measurements using an ionization chamber and a piece of film simultaneously, for different irradiation times, i.e., for different dose levels. At this point, this set of values is introduced in Film Analyzer v.1.1.2.6 software (Tomotherapy Inc.) and a calibration file is obtained, which is used by TPS. However, the small amount of values used to obtain this curve and the lack of knowledge about how does the TPS interpolates this set of points have caused that an external processing of data have been done, trying to improve the calculation process in DQA procedures. Materials and Methods: First of all, a static procedure is required, with a 5 x 5 cm2 field and 4, 8, 12, 15, 18, 20, 23, 25, 30, 35, 40 and 60 seconds long, each one using a twelfth piece of a previously divided radiochromic sheet. An irradiation of 20 s is done before irradiating these pieces, with two ionization chambers in the isocenter and 10 cm below, respectively. This way the dose to the sheet can be related with the dose to the second chamber when the first chamber has been replaced by the sheet. Then those twelve measures are done and, according to recommendations, sheets are scanned after 24 hours approximately, using an Epson Expression 10000 XL scanner. Usually the next step is to run the Film Analyzer
ESTRO 31
software and create a calibration file, using the mean pixel value taken from a central ROI of 10 x 10 pixels. However, as it has been indicated, these values are plotted with a spreadsheet, and then fit to a polynomial of third degree. To reduce noise of scanning process, a median-mean filter is applied. Now it can be obtained more values using the polynomial function and therefore a more accurate calibration curve, which is used in DQA calculations. The pixel values found with Film Analyzer software were compared with those found with ImageJ v.1.44p. Results: A significant improvement in DQAs has been observed by means of gamma index calculation since this new calibration curve is used. Moreover the correct matching between results obtained with Film Analyzer and ImageJ software has been checked. Conclusions: The results demonstrate the TPS needs a higher number of points to perform an acceptable calibration curve. EP-1371 COMPARISON AND EVALUATION OF RAPID-ARC PATIENT SPECIFIC QUALITY ASSURANCE USING ARCCHECK & PORTAL DOSIMETRY. M. Ramachandran1, T. Richardson1, R. Chauhan1, V. Patel1, S. Chaib Rassou1 1 American Hospital Dubai, Radiotherapy, Dubai, United Arab Emirates Purpose/Objective: To compare and evaluate quality assurance tests for RapidArc plans using ArcCheck and Portal Dosimetry. Materials and Methods: Varian Trilogy Linear accelerator, ArcCheck QA device, Sun nuclear patient QA software, Electronic portal imaging device, Portal dosimetry software. Patient specific quality assurance test for RapidArc plans is mandatory as a pre-treatment quality assurance test. Quality assurance in RapidArc is performed by different methods, including solid state Arc detectors, portal dosimetry and ion chamber based methods. In our department we perform two different quality assurances for all our RapidArc plans. The first test is using ArcCheck quality assurance device with SNC patient QA software and the second test is using Varian Electronic Portal Imaging Devices (EPID) with Portal dosimetry software. In this work, we have compared and evaluated the results of the above mentioned quality assurance test for seven RapidArc patient plans. Gamma criteria of 3% dose difference and 3mm Distance to Agreement (DTA) was used to evaluate the quality assurance test results in both the methods. Results: The results of both the methods showed good agreement with the 3% or 3mm gamma criteria, portal dosimetry showed 98.7% agreement with a standard deviation of 1.1, ArcCheck results showed 98.0% agreement with a standard deviation of 1.7. Conclusions: The two quality assurance methods showed good agreement and there is no significant difference between them, we have compared and evaluated the spatial resolution, type of the detectors, and dependence on photon energy and easy execution of the two tests and presented in this poster presentation. EP-1372 OPTIMIZATION OF BACKSCATTERED SHIELD TO IMPROVE THE DOSE DISTRIBUTION IN BREAST IORT TREATMENT F. Lucio1, E. Calamia1, A. Boriano1, A. Melano2 1 Azienda Ospedaliera S. Croce e Carle, Medical Physics, Cuneo, Italy 2 Azienda Ospedaliera S. Croce e Carle, Radiotherapy Department, Cuneo, Italy Purpose/Objective: In the present work have been analyzed different materials to homogenize the dose distribution in IORT treatment of breast cancer. Materials and Methods: We were studied 3 different shielding materials copper, lead and steel with different thicknesses. The measurements were performed using calibrated films EBT2, positioned parallel to the beam axis, with electrons energies 6, 9 and 12 MeV produced with a mobile accelerator Mobetron 1000J. Then, for the same energy, it is estimated the thickness of plexiglas required to reduce the amount of backscattered radiation and meets the ICRU criteria. Results: The PDD without shield performed with radiochromic film and Markus ionization chamber showed a correspondence between the curves obtained with the two systems at least until the bremsstrahlung region. This has allowed us the use of EBT2 as a reference dosimeter for the subsequent measures of backscattered radiation. The evaluation has been made as the difference between PDD with shield at depth R90 and those without shield. This were in steel, independent for all energies used between thickness 3 and 4 mm, lower than 15% of maximum dose; for copper instead
EP-1367 EXPERIMENTAL COMPARISON OF VARIOUS DETECTORS FOR SMALL FIELD OUTPUT FACTORS MEASUREMENTS ON A TRUEBEAM ACCELERATOR C. Khamphan1, V. Bodez1, R. Garcia1, M.E. Alayrach Biarnés1, E. Jaegle1, A. Badey1 1 Institut Sainte Catherine, Radiotherapy, Avignon - Cedex 02, France Purpose/Objective: Development of Intensity Modulated Radiotherapy and Stereotactic Radiotherapy in recent years has lead to an increasing use of small beams. The dosimetry of small fields is challenging due to non equilibrium conditions, partial blocking of the beam source and non-negligible detector perturbations. It is commonly accepted that one can expect reduced accuracy for such fields compared to standard clinical dosimetry. The purpose of this work is to compare small field Output Factors measured with five different detectors on a TrueBeam STx accelerator (Varian Medical Systems). Materials and Methods: Measurements of output factors (i.e. Total scatter factor, Scp) were made on the 6MV beam of a TrueBeam STx accelerator for square field sizes of 1.0, 1.5, 2.0, 3.0, 4.0, 6.0, 8.0 and 10.0 cm. Isocentric measurements were made in water at 5cm depth using the IBA 'Blue Phantom' 3D scanning system. Data were acquired with five different detectors: two stereotactic ionization chambers (Exradin A16 Standard Imaging, CC04 IBA), a liquid ionization chamber (MicroLion 31018 PTW), a stereotactic unshielded diode (SFD IBA) and a shielded diode (PFD IBA). Measurements were normalized to a 10cm x 10cm reference field except for diodes which were referenced to an intermediate 4cm x 4cm field measured with the CC04 ionization chamber. This 'Daisy chain' strategy is used to mitigate the overestimation of output factors by silicon detectors. Standard deviation and Coefficient of Variation were calculated to characterize measurement dispersion of the five different detectors. Results: Results are presented in Figure 1 and Table 1. As expected, there is a significant discrepancy in measurements for 1cm square field. For the same field size, maximum deviation between the highest and the lowest value is about 9%. Coefficient of variation is 3.3% for 1cm square field, 1.5% for 1.5cm square field and ≤1% for larger field sizes.
Figure 1: Output factors measured with each of five detectors for a 6MV photon beam. The curves are referenced to the 10cm square field, except PFD and SFD diodes which are normalized with 'Daisy chaining' method. Table 1: Comparison of Output factors measured with five different detectors for a 6MV photon beam.
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Conclusions: Results are clearly dependant on detectors and it appears difficult to know which is the suitable one. As expected, we observe that ionization chambers (CC04, Exradin) underestimate Scp due to lateral electronic disequilibrium, and diodes overestimate Scp due to the high atomic number of silicon. We plan to make further measurements with a Large Area Chamber (PTW 34070) which hopefully will help us to answer the question: which is the reference detector for which field size range? Achieving a satisfying accuracy could not be done with a decision at the local scale. As a reference dataset is not currently available for TrueBeam STx, it appears important to compare measurements with other customers. EP-1368 EVALUATION OF NEUTRON DOSE IN CENTRAL AXIS ABSORBED DOSE IN LINAC MACHINES BY TLD600 AND TLD700 DOSIMETERS H. Darestani1, H.A. Nedaie2, K.H. Mohammadi3, A. Shahvar1, E. Bayat4, N. Shahgholi1, M. Nazarnejad1 1 Islamic Azad University Sience and Research branch, Nuclear Engineering Department, Tehran, Iran Islamic Republic of 2 Imam Khomeini Hospital, Radiotherapy Oncology Department, Tehran, Iran Islamic Republic of 3 Amirkabir University of Technology, Nuclear Engineering Department, Tehran, Iran Islamic Republic of 4 Birjand University of Technology, Nuclear Engineering Department, Tehran, Iran Islamic Republic of Purpose/Objective: In spite of clinically useful photon and electron beams, high energy linacs produce secondary particles such as neutrons (photoneutron production). The neutrons have the important role during application of high energy photons in terms of protection and dose escalation. The dosimetry in a neutron-gamma mixed field like the linac field needs at least two detectors, one sensitive to gamma and the other sensitive to neutrons. TLD600 and TLD700 gamma sensitivities are roughly the same, while TLD600 is much more sensitive to thermal neutrons. Consequently, simultaneous use of TLD600 and TLD700 for dosimetry of a mixed field and discriminating the neutron and gamma component is the solution. In this study, the central axis neutron dose of three different linac machines (Varian, Siemens and Elekta) and the neutron dose in the vicinity of the Varian linac is measured by TLD600 and TLD700 dosimeters. Materials and Methods: First, TLDs were calibrated versus definite gamma and neutron doses. Photoneutron dose equivalent was calculated in central axis for 18 MV Varian and Elekta linacs and 15 MV Siemens linac on the phantom surface and depths of 1, 2, 3.3, 4, 5 and 6 cm. In order to determine neutron dose equivalent in the vicinity of the Varian 18 MV linac, TLDs were irradiated in the center of the 25 cm diameter polyethylene sphere in four points at the distance of 1 and 2 m of the isocentre in the patient plane. Results: The maximum photoneutron dose equivalent was observed at the depth of 4-5 cm, no photoneutron dose equivalent was obtained on the phantom surface and depths of less than 3 cm. The minimum and the maximum photoneutron doses were calculated for Siemens and Varian linac respectively. For three linacs of Siemens, Elekta and Varian, the maximum photoneutron doses were obtained equal to 15, 44 and 52 mSv.Gy-1 respectively. The neutron dose equivalent at the distance of 1 and 2 m of the isocentre on the patient couch was measured 2.2 and 0.75 mSv.Gy-1 respectively. Conclusions: The neutron dose equivalent along the beam axis was achieved up to ten times of the published data, whereas neutron dose measurement at the distance of 1 m of the isocentre on the patient couch had only the error of 4%. According to the results it seems that TLD600 and TLD700 pairs are not suitable dosimeters for neutron dosimetry inside the field due to high photon fluence, but they can be used for outside the field measurement with good accuracy. EP-1369 NEUTRON DOSE EVALUATION OF ELEKTA LINAC BY MCNP CODE AND COMPARISON WITH EXPERIMENTAL MEASUREMENTS N. Shahgholi1, H. Nedaie2, M. Sadeghi1, K. Mohammadi3, A. Shahvar1, E. Bayat4, H. Darestani1, M. Nazarnejad1 1 Science and Research unit Azad University, Nuclear Engineering Department, Tehran, Iran Islamic Republic of 2 Cancer Institute Tehran University, Radiotherapy Oncology Department, Tehran, Iran Islamic Republic of 3 Amirkabir Industrial University, Nuclear Engineering Department, Tehran, Iran Islamic Republic of 4 Birjand University, Physics department, Birjand, Iran Islamic Republic of
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Purpose/Objective: Medical linacs, besides the clinically high energy electron and photon beams (>8MV), produce other secondary particles such as neutrons which increased the delivered dose. For dosimetry in a neutron-gamma mixed field, use of at least two detectors one sensitive to gamma and the other more sensitive to neutrons are necessary. TLD600 and TLD700 gamma sensitivities are almost the same, while TLD600 is much more sensitive to thermal neutrons. Consequently simultaneous employment of TLD600 and TLD700 for neutron dose assessment in linac field is suitable. Application of simulation methods also could help to assess the dose and contribution of each. In this study the neutron dose at 10 and 18 MV Elekta linac was obtained by using of TLD600 and TLD700 as well as Monte Carlo simulation. Materials and Methods: For neutron dose assessment in 20× 20 cm2 field, TLDs were calibrated at first. Gamma calibration was performed with standard 60Co source and 18 MV linac and neutron calibration was done with 241Am-Be neutron source. MCNPX code was used for simulation of accelerator head and phantom and calculated neutron dose equivalent was compared with measurement data. Results: Neutron dose equivalent at 18 MV was measured by using TLDs on the phantom surface and depths of 1, 2, 3.3, 4, 5 and 6 cm. Neutron dose at depths of less than 3.3cm was zero and maximized at the depth of 4 cm (44.39 mSvGy-1), whereas calculation resulted in the maximum of 2.32 mSvGy-1 at the same depth. Neutron dose at 10 MV was measured by using TLDs on the phantom surface and depths of 1, 2, 2.5, 3.3, 4 and 5 cm. No photoneutron dose was observed at depths of less than 3.3cm and the maximum was at 4cm equal to 5.44 mSvGy-1, however, the simulation showed the maximum of 0.077mSvGy-1 at the same depth. Conclusions: The comparison between measured photoneutron dose and simulated data along the beam axis in different depths shows that the measurement data were much more than the simulation data, so it seems that TLD600 and TLD700 pairs are not suitable dosimeters for neutron dosimetry in central axis due to high photon flux, whereas MCNPX Monte Carlo techniques still remain a valuable tool for photonuclear dose studies. EP-1370 SMOOTHING AND CALCULATION OF CALIBRATION CURVE INDEPENDENTLY OF TOMOTHERAPY PLANNING SYSTEM 1 1 1 J.M. Verde Velasco , J.A. Ramos Pacho , S. Garcia Repiso , C. Martin Rincon1, E. De Sena Espinel1, M.E. Perez Alvarez1, J.M. Delgado Aparicio1 1 Hospital Universitario, Medical Physics and Radiation Protection, Salamanca, Spain Purpose/Objective: Tomotherapy Hi-Art radiation unit consists of a linear accelerator that can rotate around the treatment couch while it is moving at constant speed, i.e., it works like a CT unit but with a treatment end. This unit has a binary MLC of 64 leafs moving continuously during irradiation time. These and other characteristics allow a helical IMRT treatment. For each patient a quality control is performed, namely DQA (Delivery Quality Assurance), by which a phantom is irradiated with a treatment plan while dose measurements are done, and then compared with the dose calculation of the patient plan on the phantom. In order to get those measures two ionization chambers and a radiochromic film are used. To use the latter correctly and to be able to compare measures, a calibration curve is needed. This is found by means of a set of measurements using an ionization chamber and a piece of film simultaneously, for different irradiation times, i.e., for different dose levels. At this point, this set of values is introduced in Film Analyzer v.1.1.2.6 software (Tomotherapy Inc.) and a calibration file is obtained, which is used by TPS. However, the small amount of values used to obtain this curve and the lack of knowledge about how does the TPS interpolates this set of points have caused that an external processing of data have been done, trying to improve the calculation process in DQA procedures. Materials and Methods: First of all, a static procedure is required, with a 5 x 5 cm2 field and 4, 8, 12, 15, 18, 20, 23, 25, 30, 35, 40 and 60 seconds long, each one using a twelfth piece of a previously divided radiochromic sheet. An irradiation of 20 s is done before irradiating these pieces, with two ionization chambers in the isocenter and 10 cm below, respectively. This way the dose to the sheet can be related with the dose to the second chamber when the first chamber has been replaced by the sheet. Then those twelve measures are done and, according to recommendations, sheets are scanned after 24 hours approximately, using an Epson Expression 10000 XL scanner. Usually the next step is to run the Film Analyzer
ESTRO 31
software and create a calibration file, using the mean pixel value taken from a central ROI of 10 x 10 pixels. However, as it has been indicated, these values are plotted with a spreadsheet, and then fit to a polynomial of third degree. To reduce noise of scanning process, a median-mean filter is applied. Now it can be obtained more values using the polynomial function and therefore a more accurate calibration curve, which is used in DQA calculations. The pixel values found with Film Analyzer software were compared with those found with ImageJ v.1.44p. Results: A significant improvement in DQAs has been observed by means of gamma index calculation since this new calibration curve is used. Moreover the correct matching between results obtained with Film Analyzer and ImageJ software has been checked. Conclusions: The results demonstrate the TPS needs a higher number of points to perform an acceptable calibration curve. EP-1371 COMPARISON AND EVALUATION OF RAPID-ARC PATIENT SPECIFIC QUALITY ASSURANCE USING ARCCHECK & PORTAL DOSIMETRY. M. Ramachandran1, T. Richardson1, R. Chauhan1, V. Patel1, S. Chaib Rassou1 1 American Hospital Dubai, Radiotherapy, Dubai, United Arab Emirates Purpose/Objective: To compare and evaluate quality assurance tests for RapidArc plans using ArcCheck and Portal Dosimetry. Materials and Methods: Varian Trilogy Linear accelerator, ArcCheck QA device, Sun nuclear patient QA software, Electronic portal imaging device, Portal dosimetry software. Patient specific quality assurance test for RapidArc plans is mandatory as a pre-treatment quality assurance test. Quality assurance in RapidArc is performed by different methods, including solid state Arc detectors, portal dosimetry and ion chamber based methods. In our department we perform two different quality assurances for all our RapidArc plans. The first test is using ArcCheck quality assurance device with SNC patient QA software and the second test is using Varian Electronic Portal Imaging Devices (EPID) with Portal dosimetry software. In this work, we have compared and evaluated the results of the above mentioned quality assurance test for seven RapidArc patient plans. Gamma criteria of 3% dose difference and 3mm Distance to Agreement (DTA) was used to evaluate the quality assurance test results in both the methods. Results: The results of both the methods showed good agreement with the 3% or 3mm gamma criteria, portal dosimetry showed 98.7% agreement with a standard deviation of 1.1, ArcCheck results showed 98.0% agreement with a standard deviation of 1.7. Conclusions: The two quality assurance methods showed good agreement and there is no significant difference between them, we have compared and evaluated the spatial resolution, type of the detectors, and dependence on photon energy and easy execution of the two tests and presented in this poster presentation. EP-1372 OPTIMIZATION OF BACKSCATTERED SHIELD TO IMPROVE THE DOSE DISTRIBUTION IN BREAST IORT TREATMENT F. Lucio1, E. Calamia1, A. Boriano1, A. Melano2 1 Azienda Ospedaliera S. Croce e Carle, Medical Physics, Cuneo, Italy 2 Azienda Ospedaliera S. Croce e Carle, Radiotherapy Department, Cuneo, Italy Purpose/Objective: In the present work have been analyzed different materials to homogenize the dose distribution in IORT treatment of breast cancer. Materials and Methods: We were studied 3 different shielding materials copper, lead and steel with different thicknesses. The measurements were performed using calibrated films EBT2, positioned parallel to the beam axis, with electrons energies 6, 9 and 12 MeV produced with a mobile accelerator Mobetron 1000J. Then, for the same energy, it is estimated the thickness of plexiglas required to reduce the amount of backscattered radiation and meets the ICRU criteria. Results: The PDD without shield performed with radiochromic film and Markus ionization chamber showed a correspondence between the curves obtained with the two systems at least until the bremsstrahlung region. This has allowed us the use of EBT2 as a reference dosimeter for the subsequent measures of backscattered radiation. The evaluation has been made as the difference between PDD with shield at depth R90 and those without shield. This were in steel, independent for all energies used between thickness 3 and 4 mm, lower than 15% of maximum dose; for copper instead