ESTRO 2025 - Abstract Book

S2711

Physics - Dose calculation algorithms

ESTRO 2025

4264

Digital Poster mGLAaS: modified GeneraL Algorithm for a-Si detectors for EPID reading-to-dose conversion Antonella Fogliata 1,2 , Jack B Liver 2 , Giacomo Reggiori 1 , Luca Cozzi 1 1 Radiosurgery and Radiotherapy Department, IRCCS-Humanitas Research Hospital, Milan, Italy. 2 Medical Physics, Radiqa Developments SA, Bellinzona, Switzerland Purpose/Objective: The modified GLAaS algorithm (mGLAaS) for EPID reading-to-dose conversion was proven stable over many similar detectors using a single global algorithm configuration instead of a detector-dependent one [1]. The assessment of a global configuration was determined only for one of the commercially available detectors, the Portal Vision aS1200, available on most of the current Varian linacs. This work aims to evaluate the generalization of the mGLAaS for the other available EPID panels: the Varian Portal Vision aS1000 (not available on new machines), and the Elekta iViewGT installed on c-arm Elekta linacs. Material/Methods: The mGLAaS converts the EPID reading into absorbed dose in water at the depth of d max , typically at isocentric distance. It evaluates the amorphous silicon detector response in the different regions of each single component (segment/control point) of modulated fields: the primary radiation, whose response depends on the field size, and the transmitted radiation, with a significantly different response with respect to the primary beam due to the over response of silicon detectors to the low energy radiation. The core of the mGLAaS for the primary radiation is summarized in the two formulae: OR(FS) = A 2 (ln(FS+1)) 2 + A 1 (ln(FS+1)) + A 0 , representing the output ratio between the test and reference field size readings, and RtD=OF(FS) / OR(FS) , representing the reading-to-dose conversion, where the OF is the output factor measured with the ion chamber. The transmission is determined as the ratio between the constant terms of the OR(FS) from the ion chamber and the EPID measurements. The acquisitions of a set of field sizes ranging from 3x3 to 30x30 cm 2 , with MU from 10 to 50 were performed on 5 aS1200 (Varian), on 5 aS1000 (Varian), and on 5 iViewGT (Elekta). Detector-specific and global OR(FS) parameters’ fit were obtained. The dose differences between the ion chamber and EPID were evaluated for all the acquisitions, uncorrected and corrected with the specific or global RtD conversion. Results: In the plots, the OR as a function of the ln(FS[cm]+1) is presented for all the single detectors, unified for the same detector type. The global fit is determined from those data.