ESTRO 2022 - Abstract Book

S731

Abstract book

ESTRO 2022

Results Figure 1 shows the relative dose distribution within the sensitive volume of the SNC125c chamber as a function of the pencil beam position. The distribution is normalized to the dose at the position (x,y) = (0,0) without magnetic field. As can be seen, the dose response within the air-filled cavity for B = 0T is more or less homogeneous, the central electrode and especially the guard ring shows a clear overresponse due to electrons released by photon interactions within these high-Z components. In the presence of an external B-field, the electron trajectories are changed due to the Lorentz force, resulting in non-symmetric dose distributions within the cavity, i.e. the effective point of measurement is shifted according to the magnetic field directions. The perturbation function of the ionization chamber for the central electrode, stem and wall in presence and absence of magnetic field were studied either. Figure 1: Spatial resolved relative dose within the sensitive volume of the SNC 125c ionization chamber as a function of the pencil beam position.

Conclusion In this study the spatial resolved 2D-dose response of the SNC 125c ion chamber was determined via Monte Carlo simulations with and without external magnetic fields B . The results may help to understand the different perturbation corrections and the shift of the effective point of measurement in ion chambers due to external magnetic fields B .

PD-0812 Validation of a high resolution solid state detector array for SBRT/SRS patient specific QA

J. Smeulders 1 , T. Gevaert 1 , S. Robbe 1 , T. Everaert 1 , A. Gutierrez 1 , A. Bom 1 , M. Boussaer 1 , M. De Ridder 1

1 UZ Brussel, Radiotherapy, Brussel, Belgium

Purpose or Objective The myQA ® SRS device (IBA, Brussels, Belgium), a recently released high resolution detector array, was validated as a patient specific quality assurance (PSQA) tool for stereotactic radiosurgery (SRS) and stereotactic body radiation therapy (SBRT) by comparing PSQA field measurements with TPS calculations and radiochromic film. Materials and Methods Data from 35 SBRT and 15 SRS fields were collected using the myQA ® SRS detector and dedicated phantom. The corresponding PSQA plans were delivered by a Truebeam STx (Varian, , USA) with 6 MV flattening filter free photon beams. The plans were selected to reflect a clinically relevant range of lesion sizes with GTV volumes ranging from 0.25 to 2.74 cc and 0.11 to 0.2 cc for SBRT and SRS cases, respectively. All SBRT plans were delivered using VMAT, while SRS plans were conformal arc therapy fields. Radiochromic film measurements were performed for ten SBRT and SRS fields each. TPS dose calculations were performed using Eclipse v15.6 (AAA) and Elements v2.0 (Pencil Beam) for SBRT and SRS plans respectively. The measurements were evaluated in the myQA software (Figure 1) using absolute gamma analysis with a criterion of 3% dose difference and 1 mm distance-to-agreement.

Figure 1: From left to right respectively, the dose distribution of film, myQA measurement and gamma map of film to detector measurement comparison for an ARC of a SRS plan of a PTV with a volume of 0.76cc. Results The mean gamma passing rate of the myQA ® SRS measurements in comparison to film was 96.5% and 97.8% for SBRT and SRS fields, respectively. Only two fields out of 20 did not exceed a 95% passing rate, both being VMAT fields. Comparisons of myQA ® SRS measurements against TPS dose calculations yielded a mean gamma passing rate of 97.6% for SRBT cases and