ESTRO 37 Abstract book

S955

ESTRO 37

EP-1778 Long-term dose-response of Gafchromic EBT 2 film irradiated by proton beam and γ- rays. D. Borowicz 1,2 , J. Malicki 1,3 , G. Mitsyn 2 1 Greater Poland Cancer Centre, Dept. of Medical Physics, Poznań, Poland 2 Joint Institute for Nuclear Research, DLNP, Dubna, Russian Federation 3 Poznan University of Medical Sciences, Department Electroradiology, Poznań, Poland Purpose or Objective This study presents the long-term dose-response of dosimetric film after irradiation by γ-rays and proton beam with different energies. Material and Methods The EBT 2 film produced by Gafchromic was used in this work. The detector was irradiated by proton beam produced by a synchrocyclotron and by γ- rays. We used four energies of proton beam to our study: 110 MeV, 145 MeV, 180 MeV and 215 MeV. The sheet of the EBT 2 film was cut into small pieces-squares of 20mm x 20mm with perpetuated orientation for each square. The small films were irradiated perpendicularly to the central axis beam (CAX) with different doses from 0.0 Gy to 9.0 Gy at intervals of 1.0 Gy. After 24 h, the small film were scanned for the first time by a flatbed Epson Perfection V750-M Pro Scanner (transparent 48-bit mode with 300 dpi resolution). ImageJ software was used to analyse the darkening of the film after irradiation. For each small film the pixel value (PV) was read at the red canal of the RGB bit map The response of the EBT 2 film to irradiation was articulated by net optical density (netOD) calculated from the PV. The small pieces of film were scanned several times over 5 months. Results The results of this study show that the time after irradiation affects on the dose-response of the EBT 2 film. The darkening of irradiated film changes and the netOD increases with time after irradiation. The observed changes are not big. However, we observed that boost of netOD is more evident for high doses of irradiation, e.g. 8 Gy, than for low doses, e.g. 2 Gy (Fig.1).

detector. The length of time after irradiation does not change the correlation between different types of irradiation and energies. EP-1779 Comparison between different acquisition protocols in cone beam CT-based dose calculation D. Sánchez-Artuñedo 1 , C. Delgado-Soler 1 1 Hospital Universitari Vall d'Hebron, Servei de Física i Protecció Radiològica, Barcelona, Spain Purpose or Objective Cone beam CT (CBCT) images in radiotherapy are mainly used to assess patient setup during treatment. Varian On- Board Imager (OBI) includes default imaging protocols for head, thorax and pelvis to ease its implementation. Adaptive radiotherapy and recently developed concepts as “dose of the day” try to exploit the potential for CBCT-based dose calculation. In any case, variability in HU due to using a CBCT can lead to differences in calculations. This study aims to evaluate differences in CT-based and CBCT-based dose calculation for the default Varian protocols. Material and Methods Commercial CT phantom, Catphan®504, was CT scanned (pCT). In module CTP404 the materials of interest were outlined in the Eclipse treatment planning system v.11.0.13 (Varian Medical Systems) using CT ranger with the nominal HU-range and a cylindrical central volume (PTV) was drawn with a diameter of 5.25 cm. Two plans were created with eight 6 MV fields, each field aligned with one material. First plan was calculated with AAA and the other with AcurosXB algorithm. For five Varian OBI CBCT default protocols (standard-dose head, high-quality head, low-dose head, pelvis and low-dose thorax) five CBCT were acquired. Each CBCT was rigid-registered to pCT and structures were copied into it creating a set of 25 planning CBCTs (pCBCT). Plans were copied into pCBCT and calculated with AAA algorithm (pCBCTAAA) and AcurosXB (pCBCTAcu) maintaining the number of monitor units fixed and using the CT HU to electronic density table, generating two sets of 25 plans. As dose differences between AAA and AcurosXB are well known, and AAA calculates dose to water while AcurosXB dose to medium, every pCBCT was compared only with its correspondent pCT. Differences in the average dose for the structures outlined between pCBCT and pCT were analyzed. Results Graph1 and Graph 2 show dose differences between pCT and pCBCTAAA and pCBCTAcu respectively for all the structures studied. Maximum average difference observed was in PTV (-1.3% ± 0.1%) in a pCBCTAcu acquired with the thorax protocol. The maximum absolute difference of 2% was found in Acrylic and Air cavity in a pCBCTAcu also acquired with the thorax protocol. No statistically significant differences were found between average dose differences observed in pCBCTAAA and pCBCTAcu. Taking into account all the structures studied, pelvis protocol slightly increased average dose both for AAA (0.2 %, p=0.005) and AcurosXB (0.2%, p=0.01). On the contrary, in low-dose thorax, high-quality head and low- dose head average dose tended to decrease (-0.5%, p<0.001). Average dose differences were not statistically significant in standard dose head.

Figure. 1 Long-term dose-response of the EBT2 film to irradiation: 180 MeV proton beam and γ-rays. Conclusion There is no significant difference in netOD between γ-ray and proton beam irradiation. The energy of the proton beam also does not affect to the response of the film