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A comparison of dosimetric properties of three solid state dosimetry systems for dosimetry audit in radiotherapy
Abstracts / Physica Medica 30 (2014) e45ee74
from approximately 32% of the given tumor dose for eyes and dropped rapidly to 0.3% for thyroid and to 0.02% for testes in the case of 10 year-old phantom. For the 5 year-old phantom, doses ranged from approximately 43% of the given tumor dose for eyes and 0.4% for thyroid to 0.035% for testes. Radiation risks of cancer incidence for seven organs (thyroid, lung, breast, liver, stomach, bladder and prostate) in the form of the lifetime attributable risk (LAR) were estimated using BEIR VII model. For IMRT, the highest risk per single fraction (2 Gy in tumour) was estimated for 5 year-old girls (50 and 34 cancers per 105 patients for breast and thyroid respectively). The highest organ risk in the case of boys was for lungs (11 cancers per 105 5 year old patients).
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The following dosimeters were used: thermoluminescent (TLD-700, MTS7, MTS-6 and MTS-N), radiophotoluminescent (GD-352M and GD302-M) and CR-39 track detectors. The results obtained within the irradiated volume show good agreement between prescribed and measured dose with RPLs (94.6Gy ± 2.4%). In Figure 1 preliminary results for TLDs and RPLs are shown for measurements outside the radiation field (beyond the range of proton beam). For photons, doses decrease from 22 mGy and 18 mGy at distance aproximately 13 cm from the inner surface of phantom, to 1 mGy and 0.5 mGy at distance aproximately 56 cm for MTS-7 and GD-352M respectively. Higher doses for TLDs compared with RPLs are observed due to higher sensitivity of TLDs to secondary neutrons in comparison to RPLs. A systematic decrease of the geequivalent neutron dose (Dn) with distance is observed. Dn is defined as the TL signal induced by neutrons in MTS-6 expressed in terms of the g-ray dose producing an identical TL signal. Dn varied from 130 mGy at 13 cm from the inner surface of the phantom to 2 mGy at 56 cm. The detailed analysis of results combined with future Monte Carlo calculations will give valuable information about doses from scattered radiation distant from the target volume.
Figure 1. Organ doses (per 2 Gy in target volume) for IMRT measured with RPLsĂ
COMPARISON OF PASSIVE DOSIMETERS FOR SECONDARY RADIATION MEASUREMENTS IN SCANNING PROTON RADIOTHERAPY Kne zevi c b, N. Adamek c, C. Algranati d, I. Ambrozova e, C. L. Stolarczyk a, Z. Domingo f, V. Dufek g, J. Farah h, F. Fellin d, M. Klodowska a, J. Kubancak e, M. c b, O. Ploc e, M. RomeroLiszka a, M. Majer b, V. Mares i, S. Miljani sito f, K. Schinner i, M. Schwarz d, S. Trinkl i, F. Trompier h, M. Expo Wielunski i, R. Harrison j, P. Olko a. a Institute of Nuclear Physics PAN, Radzikowskiego 152, 31-342 Krakow, Poland; b RuCer Boskovic Institute, Bijenicka c. 54, 10000 Zagreb, Croatia; c Faculty of Physics and Applied Computer Science AGH, Krakow, Poland; d Medical Physics Department, Trento Proton Therapy Center, Via Al Desert 14, 38123 Trento, Italy; e Department of Radiation Dosimetry, Nuclear Physics Institute, CZ-250 68 z, Czech Republic; f Departament de Física, Universitat Auto noma de Re Barcelona, E-08193 Bellaterra, Spain; g National Radiation Protection Institute, Bartoskova 28, 140 00 Prague, Czech Republic; h Institute for Radiological Protection and Nuclear Safety, Human Health Division, BP17, 92260 Fontenay-aux-Roses; France; i Helmholtz Zentrum München, €dter Landstraße 1, 85764 Institute of Radiation Protection, Ingolsta Neuherberg, Germany; j University of Newcastle upon Tyne, Tyne and Wear NE1 7RU, Newcastle upon Tyne, United Kingdom Proton therapy is used increasingly in cancer treatment because of the possibility of sparing healthy tissue close to the target volume. However, the interactions of protons with matter result in the production of secondary radiation comprised mostly of neutrons and gamma radiation. Unwanted doses, deposited distantly from the target volume, may lead to an increasing probability of late effects of radiotherapy including the generation of secondary cancers. The EURADOS WG9 measurement campaign is designed to investigate the secondary radiation generated by a scanning proton beam. Experiments were carried out in the IBA (230 MeV) active-scanning proton beam therapy facility in Trento, Italy. A volume of 10 x 10 x 10 cm3 inside the water phantom (60 x 30 x 30 cm3) was irradiated uniformly to a dose of 100 Gy. Depth dose distributions along the beam axis and profiles at various depths were measured.
Fig 1. The decrease of secondary radiation doses with lateral distance from the field edge (at 18.5 cm from the inner surface of the phantom) measured with different passive dosimeters in water phantom.
A COMPARISON OF DOSIMETRIC PROPERTIES OF THREE SOLID STATE DOSIMETRY SYSTEMS FOR DOSIMETRY AUDIT IN RADIOTHERAPY P. Grochowska a, J. Izewska a, H. Mizuno a, J.F. Aguirre b, G. Azangwe a, P. Bera a, A. Meghzifene a. a International Atomic Energy Agency, Dosimetry and Medical Radiation Physics Section, Vienna, Austria; b U.T.M.D. Anderson Cancer Center, Radiation Physics, Houston, USA Background: The purpose of this study is to investigate and compare characteristics of three solid state dosimetry systems in order to determine working parameters and corrections needed for remote dosimetry audits of high energy photon and electron beams. The following systems were investigated: a thermo luminescent dosimetry (TLD) system (TLD-100, PCL3 reader), an optically stimulated luminescent dosimetry (OSLD) system (nanoDots, microStar reader) and a radiophoto luminescent dosimetry (RPLD) system (GD 302M, FGD-1000 reader). Materials and Methods: Prior to performing tests, the read-out procedure for each dosimetry system was optimized to achieve the best reproducibility of the measured signal. For OSLDs and RPLDs corrections for sensitivity of individual dosimeters were determined. Numerous tests were performed in order to describe the dosimetric characteristics: reproducibility, signal depletion (RPLD, OSLD), dose response non-linearity, fading effect and energy dependence. Results: All three systems tested show adequate reproducibility of about 1% for a group of dosimeters irradiated and read several times. Signal loss for OSLDs and RPLDs during repeatable read-out is minimal and can be neglected during the standard reading procedure. Correction for dose response non-linearity is needed in 1.5 e 2.5 Gy range for OSLDs and TLDs,
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Abstracts / Physica Medica 30 (2014) e45ee74
whereas for RPLDs the response is linear with dose. Fading effect was checked and results show that for TLDs and OSLDs signal stabilizes in around 10 and 2 days after irradiation, respectively. For RPLDs, when the preheating treatment is applied, the signal stabilizes and there is no further change. Energy corrections for a range of photon beams (6 - 18 MV) and electron beams (6 - 20 MeV) relative to a Co-60 beam, are of the order of 3- 5% for all three detectors for highest beam energies. Discussion: All three systems, especially TLD and RPLD systems need very careful handling procedures in order to achieve good reproducibility. The advantage of RPLDs and OSLDs is that the reading process can be repeated if necessary. RPLD does not need a non-linearity and fading corrections. Assuming that all corrections are properly applied, all three systems can be used for dosimetry audit in radiotherapy. DOSE CALCULATION ACCURACY AT PRESENCE OF DENTAL IMPLANTS ON UNCORRECTED AND METAL ARTIFACT REDUCED COMPUTED TOMOGRAPHY DATA Manuel Maerz, Pia Mittermair, Oliver Koelbl, Barbara Dobler. Department of Radiotherapy, RegensburgUniversity Medical Center, Germany Background: Dose calculation in modern treatment planning is based on Computed Tomography (CT) data of the patient. Metallic dental implants cause severe streaking artifacts, which inhibit the correct representation of shape and density of the metal and the surrounding tissue.We will present the influence of dental implants, metal artifacts and the benefit of the metal artifact reduction algorithm IFS (iterative frequency split) on dose calculation accuracy depending on treatment technique and dose calculation algorithm. Materials and Methods: The study is conducted on cylindrical phantoms consisting of water equivalent material surrounding dental implant materialand other heterogeneities (e.g. air, muscle) in various geometric arrangements. Computed Tomography images of the phantoms are generated and corrected using the IFS algorithm. Several plans are irradiated to the phantoms equipped with Gafchromic EBT 3 films: five beams IMRT (Intensity Modulated Radiation Therapy), nine beams IMRT und dual arc VMAT (Volumetric Modulated Arc Therapy). The measured dose distributions are compared to calculations on corrected and uncorrected CT data using the dose calculation algorithms Pencil Beam and Collapsed Cone, implemented in Oncentra External Beam v.4.3 and the Monte Carlo simulation XVMC. The Pencil Beam and XVMC algorithms report dose to water, whereas Collapsed Cone reports dose to media. To compare film measurements to Collapsed Cone calculations the film measurements which are calibrated to dose to water are rescaled to dose to media. Inaccuracies caused by artifacts or not adequately corrected CT images and inaccuracies caused by incorrect modeled transmission of radiation through metal implants are separated. Results: Depending on the dose calculation algorithm and the treatment technique artifacts lead to inaccuracies in dose calculation that can be reduced by application of the IFS algorithms. The accuracy of the calculation on the IFS corrected data and the improvement with respect to the uncorrected data depends on the dose calculation algorithm and the composition of material in the phantom. Conclusion: Metal artifact reduction leads to an improvement in accuracy of dose calculations. The application of a metal artifact reduction algorithm is recommended to reduce dose uncertainties. Acknowledgments:The work was supported by the Wilhelm Sander Foundation. TEST, VALIDATION AND UPGRADE OF THE MD ANDERSON ANALYTICAL MODEL PREDICTING SECONDARY NEUTRON RADIATION IN PROTON THERAPY FACILITIES rault c, F. J. Farah a, A. Bonfrate a, A. De Oliveira b, S. Delacroix b, J. He Martinetti a, S. Piau d, F. Trompier a, I. Vabre d, I. Clairand a. a Institute for Radiological Protection and Nuclear Safety (IRSN), Human Health Division, 31 ave de la Division Leclerc, 92260 Fontenay-aux-Roses; France; b Institut Curie e Centre de Protonth erapie d’Orsay (ICPO), Campus universitaire ^timent 101, 91898 Orsay, France; c Centre Antoine Lacassagne (CAL) ba Cyclotron biom edical, 227 avenue de la Lanterne, 06200 Nice, France; d Institut de Physique Nucl eaire d’Orsay (IPNO), Service de dosim etrie, ^timent 104, 91406 Orsay, France Campus universitaire ba
Purpose: This study follows the MD Anderson approach* to build an analytical model predicting leakage neutrons within the local 75 MeV ocular proton therapy facility. Its main goal is to test, validate and upgrade the model to clinically relevant configurations. Methods: Using Monte Carlo (MC) calculations, neutron ambient dose equivalents, H*(10), were simulated at different positions inside the treatment room while considering a closed final collimator and pristine Bragg peak delivery as per the MD Anderson method. Using this data, a facility specific analytical model was developed and tested. Starting from H*(10) values at isocentre, this model attempts to reproduce the neutron decrease with axial and lateral distance to isocentre while separately accounting for the contribution of intranuclear cascade, evaporation, epithermal and thermal neutrons. To validate the model, simulated H*(10) values were considered as well as experimental measurements previously unavailable at MD Anderson. The model was also expended in the vertical direction to enable a full 3D mapping of H*(10) inside the treatment room. Results: The work first proved that it is possible to build ones’ own analytical model following the MD Anderson approach which however requires a MC model of the local proton therapy facility. Validation showed that the analytical model efficiently reproduced simulated H*(10) values with a maximum difference below 10%. In addition, it succeeded in predicting measured H*(10) values with differences <40%. The highest differences were found at the closest and farthest positions from isocenter where the analytical model failed to faithfully reproduce the high variations in neutron fluence and energy. The differences remain however acceptable when taking into account the non negligible measurement/ simulation uncertainties and considering the end use of this model, i.e. radiation protection. Finally, the model was successfully extended to predict neutrons in the vertical direction with respect to the beam line as patients are in the upright seated position during ocular treatments (differences <20% on simulations and <45% on measurements). Conclusion: Analytical models represent a promising solution that substitutes for time-consuming MC calculations. Further studies remain necessary to upgrade the model and account for beam modulation, collimation and patient-induced neutron fluctuations. rez-Andújar A, Zhang R and Newhauser W. Monte Carlo and analytical *Pe model predictions of leakage neutron exposures from passively scattered proton therapy. Med. Phys. 40, 1714-25 (2013) DOSE PERTURBATION CLOSE TO HIGH Z MATERIALS IN MV FIELDS John Kalef-Ezra, Konstantina Karava. Medical University of Ioannina, Ioannina 45110, Greece
Physics
Department,
Background: Inhomegeneities in the human body result in electron disequilibrium close to the interfaces. The purpose of this study was the assessment of the dose perturbation in tissues of low atomic number, Z, before an interface with a high Z / high density medium during irradiations with MV beams. Methods and Materials: Dose measurements in PMMA were carried out using H810 and MD-55 radiochromic films close to aluminum, copper and lead interfaces irradiated with either 60Co gamma rays or 6 MV X-rays. Results: The dose enhancement in PMMA due to the presence of the high Z inhomogeneity increases with increasing Z of the inhomogeneity (~Z1/2) and its thickness up to an energy-dependent saturation thickness and decreases with increasing distance from the interface. For example 17%, 37% and 109% dose increases were found at 4 mm distance from adequately thick Al, Cu and Pb layers irradiated with 6 MV X-rays, dropping to about 5%, 6% and 7% at 7 mm distance, respectively. Higher dose increases were found at the 4 mm distance from the interfaces of the three media irradiated with 60Co beams, 23%, 56% and 138%, respectively, dropping to 2%, 5% and 9% at 2.3 mm distance from the interface. On the other hand, the dose enhancement decreases with increasing irradiation angle. Both the field size and the depth of the inhomogeneity (studied only at depths larger than the one that corresponds to the maximum build-up) have marginal effect on the dose perturbation in radiation fields of large size. Empirical equations were derived, which correlate with the irradiation conditions the dose perturbation in soft tissues at short distances from high Z inhomogeneities, such as cortical bone, implanted devices and contrast media.
from approximately 32% of the given tumor dose for eyes and dropped rapidly to 0.3% for thyroid and to 0.02% for testes in the case of 10 year-old phantom. For the 5 year-old phantom, doses ranged from approximately 43% of the given tumor dose for eyes and 0.4% for thyroid to 0.035% for testes. Radiation risks of cancer incidence for seven organs (thyroid, lung, breast, liver, stomach, bladder and prostate) in the form of the lifetime attributable risk (LAR) were estimated using BEIR VII model. For IMRT, the highest risk per single fraction (2 Gy in tumour) was estimated for 5 year-old girls (50 and 34 cancers per 105 patients for breast and thyroid respectively). The highest organ risk in the case of boys was for lungs (11 cancers per 105 5 year old patients).
e65
The following dosimeters were used: thermoluminescent (TLD-700, MTS7, MTS-6 and MTS-N), radiophotoluminescent (GD-352M and GD302-M) and CR-39 track detectors. The results obtained within the irradiated volume show good agreement between prescribed and measured dose with RPLs (94.6Gy ± 2.4%). In Figure 1 preliminary results for TLDs and RPLs are shown for measurements outside the radiation field (beyond the range of proton beam). For photons, doses decrease from 22 mGy and 18 mGy at distance aproximately 13 cm from the inner surface of phantom, to 1 mGy and 0.5 mGy at distance aproximately 56 cm for MTS-7 and GD-352M respectively. Higher doses for TLDs compared with RPLs are observed due to higher sensitivity of TLDs to secondary neutrons in comparison to RPLs. A systematic decrease of the geequivalent neutron dose (Dn) with distance is observed. Dn is defined as the TL signal induced by neutrons in MTS-6 expressed in terms of the g-ray dose producing an identical TL signal. Dn varied from 130 mGy at 13 cm from the inner surface of the phantom to 2 mGy at 56 cm. The detailed analysis of results combined with future Monte Carlo calculations will give valuable information about doses from scattered radiation distant from the target volume.
Figure 1. Organ doses (per 2 Gy in target volume) for IMRT measured with RPLsĂ
COMPARISON OF PASSIVE DOSIMETERS FOR SECONDARY RADIATION MEASUREMENTS IN SCANNING PROTON RADIOTHERAPY Kne zevi c b, N. Adamek c, C. Algranati d, I. Ambrozova e, C. L. Stolarczyk a, Z. Domingo f, V. Dufek g, J. Farah h, F. Fellin d, M. Klodowska a, J. Kubancak e, M. c b, O. Ploc e, M. RomeroLiszka a, M. Majer b, V. Mares i, S. Miljani sito f, K. Schinner i, M. Schwarz d, S. Trinkl i, F. Trompier h, M. Expo Wielunski i, R. Harrison j, P. Olko a. a Institute of Nuclear Physics PAN, Radzikowskiego 152, 31-342 Krakow, Poland; b RuCer Boskovic Institute, Bijenicka c. 54, 10000 Zagreb, Croatia; c Faculty of Physics and Applied Computer Science AGH, Krakow, Poland; d Medical Physics Department, Trento Proton Therapy Center, Via Al Desert 14, 38123 Trento, Italy; e Department of Radiation Dosimetry, Nuclear Physics Institute, CZ-250 68 z, Czech Republic; f Departament de Física, Universitat Auto noma de Re Barcelona, E-08193 Bellaterra, Spain; g National Radiation Protection Institute, Bartoskova 28, 140 00 Prague, Czech Republic; h Institute for Radiological Protection and Nuclear Safety, Human Health Division, BP17, 92260 Fontenay-aux-Roses; France; i Helmholtz Zentrum München, €dter Landstraße 1, 85764 Institute of Radiation Protection, Ingolsta Neuherberg, Germany; j University of Newcastle upon Tyne, Tyne and Wear NE1 7RU, Newcastle upon Tyne, United Kingdom Proton therapy is used increasingly in cancer treatment because of the possibility of sparing healthy tissue close to the target volume. However, the interactions of protons with matter result in the production of secondary radiation comprised mostly of neutrons and gamma radiation. Unwanted doses, deposited distantly from the target volume, may lead to an increasing probability of late effects of radiotherapy including the generation of secondary cancers. The EURADOS WG9 measurement campaign is designed to investigate the secondary radiation generated by a scanning proton beam. Experiments were carried out in the IBA (230 MeV) active-scanning proton beam therapy facility in Trento, Italy. A volume of 10 x 10 x 10 cm3 inside the water phantom (60 x 30 x 30 cm3) was irradiated uniformly to a dose of 100 Gy. Depth dose distributions along the beam axis and profiles at various depths were measured.
Fig 1. The decrease of secondary radiation doses with lateral distance from the field edge (at 18.5 cm from the inner surface of the phantom) measured with different passive dosimeters in water phantom.
A COMPARISON OF DOSIMETRIC PROPERTIES OF THREE SOLID STATE DOSIMETRY SYSTEMS FOR DOSIMETRY AUDIT IN RADIOTHERAPY P. Grochowska a, J. Izewska a, H. Mizuno a, J.F. Aguirre b, G. Azangwe a, P. Bera a, A. Meghzifene a. a International Atomic Energy Agency, Dosimetry and Medical Radiation Physics Section, Vienna, Austria; b U.T.M.D. Anderson Cancer Center, Radiation Physics, Houston, USA Background: The purpose of this study is to investigate and compare characteristics of three solid state dosimetry systems in order to determine working parameters and corrections needed for remote dosimetry audits of high energy photon and electron beams. The following systems were investigated: a thermo luminescent dosimetry (TLD) system (TLD-100, PCL3 reader), an optically stimulated luminescent dosimetry (OSLD) system (nanoDots, microStar reader) and a radiophoto luminescent dosimetry (RPLD) system (GD 302M, FGD-1000 reader). Materials and Methods: Prior to performing tests, the read-out procedure for each dosimetry system was optimized to achieve the best reproducibility of the measured signal. For OSLDs and RPLDs corrections for sensitivity of individual dosimeters were determined. Numerous tests were performed in order to describe the dosimetric characteristics: reproducibility, signal depletion (RPLD, OSLD), dose response non-linearity, fading effect and energy dependence. Results: All three systems tested show adequate reproducibility of about 1% for a group of dosimeters irradiated and read several times. Signal loss for OSLDs and RPLDs during repeatable read-out is minimal and can be neglected during the standard reading procedure. Correction for dose response non-linearity is needed in 1.5 e 2.5 Gy range for OSLDs and TLDs,
e66
Abstracts / Physica Medica 30 (2014) e45ee74
whereas for RPLDs the response is linear with dose. Fading effect was checked and results show that for TLDs and OSLDs signal stabilizes in around 10 and 2 days after irradiation, respectively. For RPLDs, when the preheating treatment is applied, the signal stabilizes and there is no further change. Energy corrections for a range of photon beams (6 - 18 MV) and electron beams (6 - 20 MeV) relative to a Co-60 beam, are of the order of 3- 5% for all three detectors for highest beam energies. Discussion: All three systems, especially TLD and RPLD systems need very careful handling procedures in order to achieve good reproducibility. The advantage of RPLDs and OSLDs is that the reading process can be repeated if necessary. RPLD does not need a non-linearity and fading corrections. Assuming that all corrections are properly applied, all three systems can be used for dosimetry audit in radiotherapy. DOSE CALCULATION ACCURACY AT PRESENCE OF DENTAL IMPLANTS ON UNCORRECTED AND METAL ARTIFACT REDUCED COMPUTED TOMOGRAPHY DATA Manuel Maerz, Pia Mittermair, Oliver Koelbl, Barbara Dobler. Department of Radiotherapy, RegensburgUniversity Medical Center, Germany Background: Dose calculation in modern treatment planning is based on Computed Tomography (CT) data of the patient. Metallic dental implants cause severe streaking artifacts, which inhibit the correct representation of shape and density of the metal and the surrounding tissue.We will present the influence of dental implants, metal artifacts and the benefit of the metal artifact reduction algorithm IFS (iterative frequency split) on dose calculation accuracy depending on treatment technique and dose calculation algorithm. Materials and Methods: The study is conducted on cylindrical phantoms consisting of water equivalent material surrounding dental implant materialand other heterogeneities (e.g. air, muscle) in various geometric arrangements. Computed Tomography images of the phantoms are generated and corrected using the IFS algorithm. Several plans are irradiated to the phantoms equipped with Gafchromic EBT 3 films: five beams IMRT (Intensity Modulated Radiation Therapy), nine beams IMRT und dual arc VMAT (Volumetric Modulated Arc Therapy). The measured dose distributions are compared to calculations on corrected and uncorrected CT data using the dose calculation algorithms Pencil Beam and Collapsed Cone, implemented in Oncentra External Beam v.4.3 and the Monte Carlo simulation XVMC. The Pencil Beam and XVMC algorithms report dose to water, whereas Collapsed Cone reports dose to media. To compare film measurements to Collapsed Cone calculations the film measurements which are calibrated to dose to water are rescaled to dose to media. Inaccuracies caused by artifacts or not adequately corrected CT images and inaccuracies caused by incorrect modeled transmission of radiation through metal implants are separated. Results: Depending on the dose calculation algorithm and the treatment technique artifacts lead to inaccuracies in dose calculation that can be reduced by application of the IFS algorithms. The accuracy of the calculation on the IFS corrected data and the improvement with respect to the uncorrected data depends on the dose calculation algorithm and the composition of material in the phantom. Conclusion: Metal artifact reduction leads to an improvement in accuracy of dose calculations. The application of a metal artifact reduction algorithm is recommended to reduce dose uncertainties. Acknowledgments:The work was supported by the Wilhelm Sander Foundation. TEST, VALIDATION AND UPGRADE OF THE MD ANDERSON ANALYTICAL MODEL PREDICTING SECONDARY NEUTRON RADIATION IN PROTON THERAPY FACILITIES rault c, F. J. Farah a, A. Bonfrate a, A. De Oliveira b, S. Delacroix b, J. He Martinetti a, S. Piau d, F. Trompier a, I. Vabre d, I. Clairand a. a Institute for Radiological Protection and Nuclear Safety (IRSN), Human Health Division, 31 ave de la Division Leclerc, 92260 Fontenay-aux-Roses; France; b Institut Curie e Centre de Protonth erapie d’Orsay (ICPO), Campus universitaire ^timent 101, 91898 Orsay, France; c Centre Antoine Lacassagne (CAL) ba Cyclotron biom edical, 227 avenue de la Lanterne, 06200 Nice, France; d Institut de Physique Nucl eaire d’Orsay (IPNO), Service de dosim etrie, ^timent 104, 91406 Orsay, France Campus universitaire ba
Purpose: This study follows the MD Anderson approach* to build an analytical model predicting leakage neutrons within the local 75 MeV ocular proton therapy facility. Its main goal is to test, validate and upgrade the model to clinically relevant configurations. Methods: Using Monte Carlo (MC) calculations, neutron ambient dose equivalents, H*(10), were simulated at different positions inside the treatment room while considering a closed final collimator and pristine Bragg peak delivery as per the MD Anderson method. Using this data, a facility specific analytical model was developed and tested. Starting from H*(10) values at isocentre, this model attempts to reproduce the neutron decrease with axial and lateral distance to isocentre while separately accounting for the contribution of intranuclear cascade, evaporation, epithermal and thermal neutrons. To validate the model, simulated H*(10) values were considered as well as experimental measurements previously unavailable at MD Anderson. The model was also expended in the vertical direction to enable a full 3D mapping of H*(10) inside the treatment room. Results: The work first proved that it is possible to build ones’ own analytical model following the MD Anderson approach which however requires a MC model of the local proton therapy facility. Validation showed that the analytical model efficiently reproduced simulated H*(10) values with a maximum difference below 10%. In addition, it succeeded in predicting measured H*(10) values with differences <40%. The highest differences were found at the closest and farthest positions from isocenter where the analytical model failed to faithfully reproduce the high variations in neutron fluence and energy. The differences remain however acceptable when taking into account the non negligible measurement/ simulation uncertainties and considering the end use of this model, i.e. radiation protection. Finally, the model was successfully extended to predict neutrons in the vertical direction with respect to the beam line as patients are in the upright seated position during ocular treatments (differences <20% on simulations and <45% on measurements). Conclusion: Analytical models represent a promising solution that substitutes for time-consuming MC calculations. Further studies remain necessary to upgrade the model and account for beam modulation, collimation and patient-induced neutron fluctuations. rez-Andújar A, Zhang R and Newhauser W. Monte Carlo and analytical *Pe model predictions of leakage neutron exposures from passively scattered proton therapy. Med. Phys. 40, 1714-25 (2013) DOSE PERTURBATION CLOSE TO HIGH Z MATERIALS IN MV FIELDS John Kalef-Ezra, Konstantina Karava. Medical University of Ioannina, Ioannina 45110, Greece
Physics
Department,
Background: Inhomegeneities in the human body result in electron disequilibrium close to the interfaces. The purpose of this study was the assessment of the dose perturbation in tissues of low atomic number, Z, before an interface with a high Z / high density medium during irradiations with MV beams. Methods and Materials: Dose measurements in PMMA were carried out using H810 and MD-55 radiochromic films close to aluminum, copper and lead interfaces irradiated with either 60Co gamma rays or 6 MV X-rays. Results: The dose enhancement in PMMA due to the presence of the high Z inhomogeneity increases with increasing Z of the inhomogeneity (~Z1/2) and its thickness up to an energy-dependent saturation thickness and decreases with increasing distance from the interface. For example 17%, 37% and 109% dose increases were found at 4 mm distance from adequately thick Al, Cu and Pb layers irradiated with 6 MV X-rays, dropping to about 5%, 6% and 7% at 7 mm distance, respectively. Higher dose increases were found at the 4 mm distance from the interfaces of the three media irradiated with 60Co beams, 23%, 56% and 138%, respectively, dropping to 2%, 5% and 9% at 2.3 mm distance from the interface. On the other hand, the dose enhancement decreases with increasing irradiation angle. Both the field size and the depth of the inhomogeneity (studied only at depths larger than the one that corresponds to the maximum build-up) have marginal effect on the dose perturbation in radiation fields of large size. Empirical equations were derived, which correlate with the irradiation conditions the dose perturbation in soft tissues at short distances from high Z inhomogeneities, such as cortical bone, implanted devices and contrast media.