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

Phys Med

U.O.C. di Fisica Sanitaria, Università Cattolica del S. Cuore, Roma, Italy; Istituto di Fisica, Università Cattolica del S. Cuore, Roma, Italy; Istituto Nazionale di Fisica Nucleare (INFN), Sezione di Roma Tre, Roma, Italy. Electronic address:

Published: June 2014

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, Diso. 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 γ-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 = Diso/Diso,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 γ-analysis of the EPID images with 5% 3 mm criteria supplied 85% of the tests with pass rates γ(mean) ≤ 0.5 and P(γ<1) ≥ 90%.

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http://dx.doi.org/10.1016/j.ejmp.2013.11.005DOI Listing

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