ESTRO 2021 Abstract Book

S231

ESTRO 2021

0.75·rcyl for heavy ions with rcyl being the radius of the cavity, while for proton beams no recommendation is made. In this study, we have experimentally determined Peff for the 0.125 cm3 Semiflex (PTW-31010, PTW Freiburg, Germany) IC in both proton and carbon ion beams. Materials and Methods The first method consisted of simultaneous irradiation of a plane-parallel IC (Advanced Markus, PTW-34045, PTW Freiburg, Germany) and the Semiflex, both fixed inside an in-house developed holder for biological experiments, with an energy-modulated beam resulting in a spread-out Bragg peak (see Fig. 1).

Fig. 1: (left) Schematics of the relative position of the IC in the first method. The plane-parallel and cylindrical IC were placed inside chamber slides, aligned by the outer wall of the sensitive volume and not by their Peff (red dot). (right) Spread-out Bragg peak with measured positions in a carbon ion beam. Each position was measured three times, mean values are shown. In the second method, a single-energy beam was used and both IC were positioned successively at the same measurement depth (see Fig. 2). For both approaches, the shift of the distal edge between the two IC at different reference points (R80, R50 and inflection point) was used to calculate Peff of the Semiflex. Both methods were applied in carbon ion beams and only the latter was applied in proton beams.

Fig. 2: (left) Second method: Schematics of the position of the plane-parallel and cylindrical IC at two different time points. They were aligned in respect to their (assumed) Peff (red dot). (right) Pristine Bragg peak with measured positions for carbon ions as well as protons. Each position was measured three times, mean values are shown. Results Both methods yield similar Peff for carbon ions, 0.881·rcyl and 0.845·rcyl respectively. Given the cavity radius of 2.75 mm, the difference between the two results for carbon ions is only 0.1 mm and shows very good agreement. The difference between the obtained Peff and recommended value of 0.75·rcyl is 0.4 and 0.3 mm respectively, which is within the order of magnitude of the positioning uncertainty. In the proton beam, a Peff of 0.918 rcyl was obtained. Conclusion For both light-ion beams, our measurements yield a Peff for the Semiflex that is shifted further from the cavity center as recommended by TRS-398 for heavy ions, with the shift for proton beams being even larger than for carbon ion beams. The accurate knowledge of Peff is particularly important for measurements in proximity to high dose gradients such as in the distal fall-off, which is of high relevance for radiobiological or detector response experiments due to its high linear energy transfer. PH-0322 Application of the TRS-483 protocol for the determination of field output factors R. Rodríguez Romero 1 , A. López Corella 1 , M. Pinto Monedero 1 , P. Sánchez Rubio 1 , A. Montes Uruén 1 , J. Martínez Ortega 1 1 Hospital Universitario Puerta de Hierro Majadahonda, Servicio de Radiofísica y Protección Radiológica, Majadahonda, Spain