EPID calibration method for transit in-vivo dosimetry

Abstract

The mostly used in-vivo dosimetry techniques require efforts for their implementation measurements, workload for the detector positioning and for the data analysis. The transit in-vivo dosimetry, performed by the Electronic Portal Imaging Device (EPID), avoids the problem of the detector positioning on the patient.Recently the present authors have developed an in-vivo dosimetry method based on correlation functions F(w,L), defined as the ratio between the transit signal, St (w,L), and the phantom mid-plane dose, Dm(w,L) as a function of the phantom water equivalent thickness, w, and of the field dimensions, L.This paper reports a method to determine the generalized correlation functions F(w,L) for the Varian a-Si EPIDs equipped with a commercial software that allows the reading of 0.01 Calibrated Unit per Monitor Unit when the EPID is positioned at the SAD=100 cm and irradiated by a field size 10x10cm2. These generalized correlation functions are generated for a reference linac with a calibration of 1 cGy/UM at the water depth of the maximum dose, dmax, positioned at the SAD, for a 10x10 cm2 field. This way the use of these functions avoids the measurements for the implementation of the in-vivo dosimetry method with the a-Si EPIDs of the same manufacturer. An example of these functions for the 15 MV photon beams is reported in the present paper.Three Varian a-Si EPIDs have been examined to verify their ability to be used as transit detectors to reconstruct the isocenter dose Diso in patient. The in-vivo dosimetry reconstruction adopted here supplies an accuracy well within 5% (2SD), and when Diso is compared with the Diso,TPS, computed by the TPS, a tolerance level up to ±6% has been adopted for different pathologies as pelvis, thorax and head tumors.KeywordsTransit in-vivo dosimetryEPID dosimetrydose reconstruction method