gms | German Medical Science

A model has been developed to calculate radiochromic lm response to ion irradiation. This model is based on photon film response and film saturation by high local energy deposition computed by Monte Carlo (MC) simulation. Validation has been performed with 95 MeV/u ¹²C beam and shows error less than 2% for homogeneous media. In this study we show some preliminary results for validation in heterogenous media.

Material and methods: We have developped a model to calculate the optical density of Gafchromic MD-55-V2 lms [Frisson T, et al. 2009]. With photon irradiation the lm response is a logarithmic function where the transmittance of the lm is inversely proportional to the dose deposit in the lm. Photon irradiations were carried out at the Centre Léon Berard using an Elekta 6 MV beam linear accelerator under clinical reference conditions. However the response of the lm for carbon irradiation is less sensitive than photon irradiation. This is due to the local saturation of the lm and the excess energy deposited does not contribute to darkening the lm. An e ective dose (Deff ) is calculated as a function of the local linear energy transfer of the particles obtained by MC simulation. The optical density is obtained by applying the photon calibration to the Deff . Ion irradiations were done at GANIL (Caen, France). A PMMA triangle and other cubic material lung equivalent and cortical bone equivalent were irradiated with a 95 MeV/u ¹²C beam at several doses (10 to 60 Gy). Films were installed behind each material. We then obtained the deposition of the whole Bragg curve in each lm. Optical density was measured with about 5% accuracy. Simulations

were done with the GATE software based on Geant4.

Results: Figure 1 [Fig. 1] shows result for an irradiation at 60 Gy. We illustrate the net optical density as a function of the dose. On the left, using the photon calibration. A signi cant di erence is observed between data and simulated response. On the right, we show the result of the response of the lm using the model proposed by [Frisson T, et al. 2009] taking into account the local saturation due to the ion tracks. We have then applied this model to an irradiation of lms in a heterogeneous con guration. Results are shown in Figure 2 [Fig. 2] for lm irradiated behind a lung equivalent material (left) and then cortical bone equivalent material (right).

Conclusion: Preliminary results show good agreement between measurement and simulation in heterogeneous media. The relative error between data and simulation was less than 2% except in the region behind the Bragg peak where the error could reach 5% which is maybe due to fragmentation process. To improve the accuracy of our model, we plan to perform other experiments with proton irradiation and higher energy of ¹²C beam to operate in the conditions of medical hadrontherapy.