ESTRO 2021 Abstract Book

S230

ESTRO 2021

Dose profiles of a collimated beam with a circular aperture diameter of 15 mm were measured along and perpendicular to the beam direction. Measurements were performed in single energy layer beams with a range in water of 25 mm and SOBP using Advanced Markus (AM) ionization chamber, Micro Diamond (MD), Dosimetry Diode (DD) detectors (T34045, T60019 and T60020 from PTW, Germany) and Gafchromic EBT3 films (Ashland Inc, USA). Gate9.0/Geant4.10.6 MC simulations were carried out using a detailed description of the entire beamline to determine the mass stopping power ratios water to material (s w,m ), the mean energy to produce an ion pair in air (w air ) and the beam quality correction factor g Q,Q0 for EBT3 films at each point of the measurement. Only models for proton beams were available for the latter two corrections (Grosswendt and Baek 1989, Resch et al. 2020). Results At the Bragg peak the dose reading from the MD was 2% and 1% higher than the AM (Fig 1) for the proton and carbon beam, respectively. After applying the s w,m and w air correction deviations were below 1% for the MD in the Bragg peak of the single energy as well as the SOBP. The DD over-responded – when considering for the s w,Si increase with depth - in the peak by about 2% and 20% for protons and carbon ions, respectively.

Fig. 1: Uncorrected depth dose profiles of a single energy proton (upper) and carbon (lower) beam. Error bars represent one standard deviation of 3 repeated measurements while points without error bars were only measured once. Volume averaging effects were the dominant factor deteriorating the lateral profile measurements using the solid detectors causing systematic biases in the order of 0.3 mm in determining the lateral penumbra parameter, i.e., the distance between the 80% and 20% dose level. EBT3 films, however, did not suffer from spatial resolution, but from more pronounced beam quality effects also referred to as LET quenching. Yet, g Q,Q0 were negligible on the lateral profiles causing an increase of the penumbra parameter 80-20% in the order of less than 0.05 mm. Conclusion Beam quality effects have a notable effect on dosimetry of ion beams in the energy regime relevant for ocular treatments with its main dependence in beam direction but a minor one perpendicular to it. In this study, the AM and the MD were suitable for depth dose measurements, while the DD could rather be used for relative dose measurements in lateral direction only. PH-0321 Determination of the effective point of measurement of a ionization chamber in light-ion beams S. Barna 1 , A. Resch 1 , M. Puchalska 2 , D. Georg 1 , H. Palmans 3,4 1 Medical University of Vienna, Department of Radiation Oncology, Vienna, Austria; 2 Technical University of Vienna, Atominstitute, Vienna, Austria; 3 MedAustron Ion Therapy Centre, Medical Physics, Wiener Neustadt, Austria; 4 National Physical Laboratory, Medical Physics, Vienna, Austria Purpose or Objective For plane-parallel ionization chambers (IC), the effective point of measurement (Peff) is well known and located at the center of the inner surface of the entrance window. For cylindrical IC, Peff is shifted from the chamber’s center towards the beam source. According to IAEA TRS-398, this shift can be calculated with