title: An in-vivo dosimetry procedure for Elekta step and shoot IMRT.
abstract: OBJECTIVE
  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.
  
  
  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%.
authors.affiliation: U.O. di Fisica Sanitaria, Fondazione di Ricerca e Cura Giovanni Paolo II, Campobasso, Italy; Istituto Nazionale di Fisica Nucleare (INFN), Sezione di Roma Tre, Roma, Italy.
authors.forename: S
authors.initials: S
authors.lastname: Cilla
authors.affiliation: 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.
authors.forename: L
authors.initials: L
authors.lastname: Azario
authors.affiliation: U.O.C. di Fisica Sanitaria, Università Cattolica del S. Cuore, Roma, Italy; Istituto Nazionale di Fisica Nucleare (INFN), Sezione di Roma Tre, Roma, Italy.
authors.forename: F
authors.initials: F
authors.lastname: Greco
authors.affiliation: 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.
authors.forename: A
authors.initials: A
authors.lastname: Fidanzio
authors.affiliation: U.O.C. di Fisica Sanitaria, Università Cattolica del S. Cuore, Roma, Italy.
authors.forename: A
authors.initials: A
authors.lastname: Porcelli
authors.affiliation: Istituto di Fisica, Università Cattolica del S. Cuore, Roma, Italy.
authors.forename: M
authors.initials: M
authors.lastname: Grusio
authors.affiliation: U.O.C. di Radioterapia, Fondazione di Ricerca e Cura Giovanni Paolo II, Campobasso, Italy.
authors.forename: G
authors.initials: G
authors.lastname: Macchia
authors.affiliation: U.O.C. di Radioterapia, Fondazione di Ricerca e Cura Giovanni Paolo II, Campobasso, Italy.
authors.forename: A G
authors.initials: AG
authors.lastname: Morganti
authors.affiliation: U.O. di Fisica Sanitaria, Fondazione di Ricerca e Cura Giovanni Paolo II, Campobasso, Italy.
authors.forename: D
authors.initials: D
authors.lastname: Meluccio
authors.affiliation: 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: a.piermattei@rm.unicatt.it.
authors.forename: A
authors.initials: A
authors.lastname: Piermattei
mesh_terms: D006801:Humans; D010315:Particle Accelerators; D010386:Pelvic Neoplasms / Q000532:radiotherapy; D019047:Phantoms, Imaging; D011874:Radiometry / Q000379:methods*; D011880:Radiotherapy Planning, Computer-Assisted; D050397:Radiotherapy, Intensity-Modulated / Q000295:instrumentation*
keywords: Dosimetry; Electronic portal imaging device; IMRT in-vivo dosimetry; Radiotherapy