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Modeling gan dosimetric response to therapeutic photon beam radiation
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Abstracts of the SFPM Annual Meeting 2013 / Physica Medica 29 (2013) e1–e46
and the GaN dosimetric system (p-value >5%) for the tested parameters. The system exhibited a linear dosimetric response with good reproducibility and repeatability for the measurements. For external radiotherapy tolerance limit of 2 mm for distance and of 2% for dose is verified on the central beam axis. Accuracy better than 5 mm for distance and 5% for dose is achieved on the field edges. Conclusion: GaN showed a good accuracy with ionization chamber for clinic application using homogenous and heterogeneous phantom. The correction factors were conformed to ESTRO and AAPM recommendations. References [1] Ismail A, Pittet P, Lu GN, Galvan JM, Giraud JY, Balosso J. In vivo dosimetric system based on Gallium Nitride radioluminescence. Radiat. Measur. 2011;46:1350–4487. [2] P. Pittet, A. Ismail, J. Ribouton, R. Wang, J.-M. Galvan, A. Chaikh, G.-N. Lu, P. Jalade, J.-Y. Giraud, J. Balosso, Fiber background rejection and crystal over-response compensation for GaN based in vivo dosimetry, Physica Medica (in press) http://dx.doi.org/ 10.1016/j.ejmp.2012.12.007. [3] Annemarie Bakai, Markus Alber, FridtjofNusslin. A revision of the c-evaluation concept for the comparison of dose distributions”. Phys. Med. Biol. 2003;48:3543–53. http://dx.doi.org/10.1016/j.ejmp.2013.08.134
129 MODELING GAN DOSIMETRIC RESPONSE TO THERAPEUTIC PHOTON BEAM RADIATION R. Wang, P. Pittet, J. Ribouton, G.N. Lu, A. Ahnesjö. INL CNRS UMR5270, Université Lyon 1, Villeurbanne, France, Service de Radiophysique et Radiovigilance, Centre Hospitalier Lyon Sud, Pierre-Bénite, France, Department of Oncology Radiology and Radiation Sciences, Uppsala University, Uppsala, Sweden Introduction: The use of Gallium Nitride (GaN), a direct-gap semiconductor, has been recently pro-posed as a radioluminescent transducer for real-time in-vivo dosimetric applications [1]. GaN has the following advantages: a lower e–h pair creation energy than conventional scintillators, a high prompt radioluminescence yield resulting from the high probability of band edge radiative recombination. Nevertheless, it shares the common drawback of silicon diodes or MOSFET dosimeters with an over response relative to water for low energy scattered photons. The aim of our study is to compensate for this dependence through modelling, which may lead us to improve the accuracy of GaN-based dosimetric systems. Our modeling work presented here is focused on GaN dosimetric response for therapeutic photo beam irradiation from medical linacs. Experimental: The dosimetric response of a GaN transducer was first modeled by combining large cavity theory and the Spencer– Attix small cavity theory respectively for the low and high energy components of the local spectrum according the approach proposed in [2]. The local spectra were calculated from fluence pencil kernels, and Monte Carlo simulations were performed to determine some key parameters included into the model. Results and discussion: The developed model was used to compute TMR curves of a implantable GaN-based probe. A good agreement with measurements is achieved which validates the modeling work. Furthermore, we used the model to calculate GaN response factor at different depths in water for 6 MV photon beam. The obtained results (not shown here) confirm that GaN response factor is proportional to the square field aperture (with a proportionality coefficient
which varies with depth). This linear relationship was experimentally observed in clinical conditions [3]. Acknowledgements The authors acknowledge the French National Research Agency (ANR-11-TECS-018) for research funding. References [1] Ismail A et al. In vivo dosimetric system based on Gallium Nitride radioluminescence. Radiat. Meas. 2011;46:1960–2. [2] Eklund K et al. Modeling silicon diode energy response factors for use in therapeutic photon beams. Phys. Med. Biol. 2009;54:6135–50. [3] Pittet P et al. Fiber background rejection and crystal overresponse compensation for GaN based in vivo dosimetry. Physica Medica 2013 (in press). http://dx.doi.org/10.1016/j.ejmp.2013.08.135
130 EBT3 FILM DOSIMETRY: DOSE DISTRIBUTION AROUND CAVITIES R. Garcia, M.E. Alayrach, V. Bodez, C. Khamphan, E. Jaegle, A. Badey. Institut Sainte Catherine, Avignon, France Introduction: The quality of the dosimetric calculation is directly related to computerized calculation model and its operating parameters of the irradiation. The toughest conditions affect areas containing air cavities such as sinuses. Outside the Monte Carlo method, no other model correctly reproduces the dose distribution. The film dosimetry combined with the use of an anthropomorphic phantom help to assess the exact isodoses positions. Material and methods: A head phantom, dedicated to this study is voluntary cut in coronal planes passing through the air area cavities that were hollowed out. The film is constructed with EBT3 sensitive layer sandwiched between two plastic films. The shapes of the cavities are laser cut so as to accurately reproduce the contours. Laser cutting allows, at the same time to weld the two edges of the two plastic films and thus to maintain the sensitive layer in normal.The evaluation method was to compare dose distributions without cutting and with cutting, respect to the calculation, for a fixed rectangular irradiation (dimension 10 12 cm2) and a modulated irradiation arc therapy. Results: Laser cutting edge creates a film of constant quality. Soldering is uniform and effective. Comparison of dose distributions using the gamma index does not provide a useful analysis because of the very important differences. The comparison with profiles provides an information on the actual dose in the first few millimeters at the edge of the cavities. The uncut part of the film contributes to change the dose measurement in the first millimeters. So, cutting optimizes the measurement conditions. Comparison with the calculations brings provisionally useful information for potential clinical judgment. Conclusion: The method of laser cutting the EBT3 film is effective. The measurement is accurate and reflects the dose distribution at the edge of the cavities. Perspectives: Measurements of cut films will be compared to a Monte Carlo calculation. http://dx.doi.org/10.1016/j.ejmp.2013.08.136
Abstracts of the SFPM Annual Meeting 2013 / Physica Medica 29 (2013) e1–e46
and the GaN dosimetric system (p-value >5%) for the tested parameters. The system exhibited a linear dosimetric response with good reproducibility and repeatability for the measurements. For external radiotherapy tolerance limit of 2 mm for distance and of 2% for dose is verified on the central beam axis. Accuracy better than 5 mm for distance and 5% for dose is achieved on the field edges. Conclusion: GaN showed a good accuracy with ionization chamber for clinic application using homogenous and heterogeneous phantom. The correction factors were conformed to ESTRO and AAPM recommendations. References [1] Ismail A, Pittet P, Lu GN, Galvan JM, Giraud JY, Balosso J. In vivo dosimetric system based on Gallium Nitride radioluminescence. Radiat. Measur. 2011;46:1350–4487. [2] P. Pittet, A. Ismail, J. Ribouton, R. Wang, J.-M. Galvan, A. Chaikh, G.-N. Lu, P. Jalade, J.-Y. Giraud, J. Balosso, Fiber background rejection and crystal over-response compensation for GaN based in vivo dosimetry, Physica Medica (in press) http://dx.doi.org/ 10.1016/j.ejmp.2012.12.007. [3] Annemarie Bakai, Markus Alber, FridtjofNusslin. A revision of the c-evaluation concept for the comparison of dose distributions”. Phys. Med. Biol. 2003;48:3543–53. http://dx.doi.org/10.1016/j.ejmp.2013.08.134
129 MODELING GAN DOSIMETRIC RESPONSE TO THERAPEUTIC PHOTON BEAM RADIATION R. Wang, P. Pittet, J. Ribouton, G.N. Lu, A. Ahnesjö. INL CNRS UMR5270, Université Lyon 1, Villeurbanne, France, Service de Radiophysique et Radiovigilance, Centre Hospitalier Lyon Sud, Pierre-Bénite, France, Department of Oncology Radiology and Radiation Sciences, Uppsala University, Uppsala, Sweden Introduction: The use of Gallium Nitride (GaN), a direct-gap semiconductor, has been recently pro-posed as a radioluminescent transducer for real-time in-vivo dosimetric applications [1]. GaN has the following advantages: a lower e–h pair creation energy than conventional scintillators, a high prompt radioluminescence yield resulting from the high probability of band edge radiative recombination. Nevertheless, it shares the common drawback of silicon diodes or MOSFET dosimeters with an over response relative to water for low energy scattered photons. The aim of our study is to compensate for this dependence through modelling, which may lead us to improve the accuracy of GaN-based dosimetric systems. Our modeling work presented here is focused on GaN dosimetric response for therapeutic photo beam irradiation from medical linacs. Experimental: The dosimetric response of a GaN transducer was first modeled by combining large cavity theory and the Spencer– Attix small cavity theory respectively for the low and high energy components of the local spectrum according the approach proposed in [2]. The local spectra were calculated from fluence pencil kernels, and Monte Carlo simulations were performed to determine some key parameters included into the model. Results and discussion: The developed model was used to compute TMR curves of a implantable GaN-based probe. A good agreement with measurements is achieved which validates the modeling work. Furthermore, we used the model to calculate GaN response factor at different depths in water for 6 MV photon beam. The obtained results (not shown here) confirm that GaN response factor is proportional to the square field aperture (with a proportionality coefficient
which varies with depth). This linear relationship was experimentally observed in clinical conditions [3]. Acknowledgements The authors acknowledge the French National Research Agency (ANR-11-TECS-018) for research funding. References [1] Ismail A et al. In vivo dosimetric system based on Gallium Nitride radioluminescence. Radiat. Meas. 2011;46:1960–2. [2] Eklund K et al. Modeling silicon diode energy response factors for use in therapeutic photon beams. Phys. Med. Biol. 2009;54:6135–50. [3] Pittet P et al. Fiber background rejection and crystal overresponse compensation for GaN based in vivo dosimetry. Physica Medica 2013 (in press). http://dx.doi.org/10.1016/j.ejmp.2013.08.135
130 EBT3 FILM DOSIMETRY: DOSE DISTRIBUTION AROUND CAVITIES R. Garcia, M.E. Alayrach, V. Bodez, C. Khamphan, E. Jaegle, A. Badey. Institut Sainte Catherine, Avignon, France Introduction: The quality of the dosimetric calculation is directly related to computerized calculation model and its operating parameters of the irradiation. The toughest conditions affect areas containing air cavities such as sinuses. Outside the Monte Carlo method, no other model correctly reproduces the dose distribution. The film dosimetry combined with the use of an anthropomorphic phantom help to assess the exact isodoses positions. Material and methods: A head phantom, dedicated to this study is voluntary cut in coronal planes passing through the air area cavities that were hollowed out. The film is constructed with EBT3 sensitive layer sandwiched between two plastic films. The shapes of the cavities are laser cut so as to accurately reproduce the contours. Laser cutting allows, at the same time to weld the two edges of the two plastic films and thus to maintain the sensitive layer in normal.The evaluation method was to compare dose distributions without cutting and with cutting, respect to the calculation, for a fixed rectangular irradiation (dimension 10 12 cm2) and a modulated irradiation arc therapy. Results: Laser cutting edge creates a film of constant quality. Soldering is uniform and effective. Comparison of dose distributions using the gamma index does not provide a useful analysis because of the very important differences. The comparison with profiles provides an information on the actual dose in the first few millimeters at the edge of the cavities. The uncut part of the film contributes to change the dose measurement in the first millimeters. So, cutting optimizes the measurement conditions. Comparison with the calculations brings provisionally useful information for potential clinical judgment. Conclusion: The method of laser cutting the EBT3 film is effective. The measurement is accurate and reflects the dose distribution at the edge of the cavities. Perspectives: Measurements of cut films will be compared to a Monte Carlo calculation. http://dx.doi.org/10.1016/j.ejmp.2013.08.136