title: aSi-EPID transit signal calibration for dynamic beams: a needful step for the IMRT in vivo dosimetry.
abstract: This work reports a method based on correlation functions to convert EPID transit signals into in vivo dose values at the isocenter point, D iso, of dynamic IMRT beams supplied by Varian linac. Dose reconstruction for intensity-modulated beams required significant corrections of EPID response, due to the X-ray component transmitted through multileaf collimator. The algorithm was formulated using a set of simulated IMRT beams. The beams were parameterized by means of a fluence inhomogeneity index, FI, introduced to describe the degree of beam modulation with respect to open beams. This way, all dosimetric parameters involved in D iso reconstruction algorithm, such as the correlation functions, the correction factor for EPID to phantom distance and the modulated tissue maximum ratios, were determined as a function of the FI index. Clinical IMRT beams were used to irradiate a homogeneous phantom, and for each beam, the agreement between the reconstructed dose, D iso, and the dose computed by TPS, D iso,TPS, was well within 5 %. Moreover, the average ratios, R, between the D iso, and D iso,TPS, resulted equal to 1.002 ± 0.030. Thirty-five IMRT fields of 5 different patients undergoing radiotherapy for head-neck tumors were tested and the results were displayed on a computer screen after 2 min from the end of the treatment. However, 350 in vivo tests supplied an average ratio R equal to 1.004 ± 0.040. The in vivo dosimetry procedure here presented is among the objectives of a National Project financially supported by the Istituto Nazionale di Fisica Nucleare for the development of in vivo dosimetry procedures (Piermattei et al. in Nucl Instrum Methods Phys Res B 274:42-50, 2012) connected to the Record-Verify system of the radiotherapy center.
authors.affiliation: U.O.C di Fisica Sanitaria, Università Cattolica del Sacro Cuore, Rome, Italy.
authors.forename: Francesca
authors.initials: F
authors.lastname: Greco
authors.forename: Angelo
authors.initials: A
authors.lastname: Piermattei
authors.forename: Luigi
authors.initials: L
authors.lastname: Azario
authors.forename: Lorenzo
authors.initials: L
authors.lastname: Placidi
authors.forename: Savino
authors.initials: S
authors.lastname: Cilla
authors.forename: Rocchina
authors.initials: R
authors.lastname: Caivano
authors.forename: Vincenzo
authors.initials: V
authors.lastname: Fusco
authors.forename: Andrea
authors.initials: A
authors.lastname: Fidanzio
mesh_terms: D000465:Algorithms; D002138:Calibration; D006258:Head and Neck Neoplasms / Q000532:radiotherapy; D006801:Humans; D019047:Phantoms, Imaging; D011879:Radiotherapy Dosage / Q000592:standards*; D011880:Radiotherapy Planning, Computer-Assisted / Q000295:instrumentation / Q000379:methods*; D012815:Signal Processing, Computer-Assisted