An in-vivo dosimetry procedure for Elekta step and shoot IMRT

Purpose: The aim of this work was to extend an in-vivo dosimetry ( IVD) method, previously developed by the authors for 3D-conformal radiotherapy, to step and shoot IMRT treatments for pelvic tumors delivered by Elekta linacs. Materials and methods: The algorithm is based on correlation functions to convert EPID transit signals into in-vivo dose values at the isocenter point, D-iso. The EPID images were obtained by the so-called "IMRT Dosimetric Weighting" mode as a superposition of many segment fields. This way each integral dosimetric image could be acquired in about 10 s after the end of beam delivery and could be processed while delivering the successive IMRT beams. A specific algorithm for Diso reconstruction especially featured for step and shoot IMRT was implemented using a fluence inhomogeneity index, FI, introduced to describe the degree of beam modulation with respect to open beams. A gamma-analysis of 2D-EPID images obtained day to day, resulted rapid enough to verify the plan delivery reproducibility. Results: Fifty clinical IMRT beams, planned for patients undergoing radiotherapy of pelvic tumors, were used to irradiate a homogeneous phantom. For each beam the agreement between the reconstructed dose, Diso, and the TPS computed dose, Diso, TPS, was well within 5%, while the mean ratio R = D-iso/D-iso, TPS resulted for 250 tests equal to 1.006 +/- 0.036. The same beams were checked in vivo, i.e. during patient treatment delivery, obtaining 500 tests whose average R ratio resulted equal to 1.011 +/- 0.042. The gamma-analysis of the EPID images with 5% 3 mm criteria supplied 85% of the tests with pass rates gamma mean <= 0.5 and P-gamma< 1 >= 90%. (c) 2013 Associazione Italiana di Fisica Medica. Published by Elsevier Ltd. All rights reserved.