title: Quasi real time in vivo dosimetry for VMAT.
abstract: OBJECTIVE
  Results about the feasibility of a method for quasi real time in vivo dosimetry (IVD) at the isocenter point for volumetric modulated arc therapy (VMAT) are here reported. The method is based on correlations between the EPID signal and the dose on the beam central axis. Moreover, the γ-analysis of EPID images was adopted to verify off-axis reproducibility of fractionated plan delivery.
  
  
  METHODS
  An algorithm to reconstruct in vivo the isocenter dose, D(iso), for RapidArc treatments has been developed. 20 VMAT plans, optimized with two opposite arcs, for prostate, pancreas, and head treatments have been delivered by a Varian linac both to a conic PMMA phantom with elliptical section and to patients. The ratios R between reconstructed D(iso) and the planned doses were determined for phantom and patient irradiations adopting an acceptance criterion of ±5%. In total, 40 phantom checks and 400 patient checks were analyzed. Moreover, 3% and 3 mm criteria were adopted for portal image γ-analysis to assess patient irradiation reproducibility.
  
  
  RESULTS
  The average ratio R, between reconstructed and planned doses for the PMMA phantom irradiations was equal to 1.007 ± 0.024. When the IVD method was applied to the 20 patients, the average R ratio was equal to 1.003 ± 0.017 and 96% of the tests were within the acceptance criteria. The portal image γ-analysis supplied 88% of the tests within the pass rates γ(mean) ≤ 0.4 and P(γ<1) ≥ 98%. All the warnings were understood comparing the CT and the cone beam CT images and in one case a patient's setup error was detected and corrected for the successive fractions.
  
  
  CONCLUSIONS
  This preliminary experience suggests that the method is able to detect dosimetric errors in quasi real time at the end of the therapy session. The authors intend to extend this procedure to other pathologies with the integration of in-room imaging verification by cone beam CT.
authors.affiliation: Istituto di Fisica, Università Cattolica del Sacro Cuore, Rome 00168, Italy; U.O.C di Fisica Sanitaria, Università Cattolica del Sacro Cuore, Rome 00168, Italy; and Istituto Nazionale di Fisica Nucleare (INFN), Sezione di Roma Tre, Rome 00146, Italy.
authors.forename: A
authors.initials: A
authors.lastname: Fidanzio
authors.affiliation: U.O.C di Fisica Sanitaria, Università Cattolica del Sacro Cuore, Rome 00168, Italy.
authors.forename: A
authors.initials: A
authors.lastname: Porcelli
authors.affiliation: Istituto di Fisica, Università Cattolica del Sacro Cuore, Rome 00168, Italy; U.O.C di Fisica Sanitaria, Università Cattolica del Sacro Cuore, Rome 00168, Italy; and Istituto Nazionale di Fisica Nucleare (INFN), Sezione di Roma Tre, Rome 00146, Italy.
authors.forename: L
authors.initials: L
authors.lastname: Azario
authors.affiliation: U.O.C di Fisica Sanitaria, Università Cattolica del Sacro Cuore, Rome 00168, Italy and Istituto Nazionale di Fisica Nucleare (INFN), Sezione di Roma Tre, Rome 00146, Italy.
authors.forename: F
authors.initials: F
authors.lastname: Greco
authors.affiliation: U.O di Fisica Sanitaria, Fondazione di Ricerca e Cura Giovanni Paolo II, Campobasso 86100, Italy and Istituto Nazionale di Fisica Nucleare (INFN), Sezione di Roma Tre, Rome 00146, Italy.
authors.forename: S
authors.initials: S
authors.lastname: Cilla
authors.affiliation: Istituto di Fisica, Università Cattolica del Sacro Cuore, Rome 00168, Italy.
authors.forename: M
authors.initials: M
authors.lastname: Grusio
authors.affiliation: U.O.C di Radioterapia, Università Cattolica del Sacro Cuore, Rome 00168, Italy.
authors.forename: M
authors.initials: M
authors.lastname: Balducci
authors.affiliation: U.O.C di Radioterapia, Università Cattolica del Sacro Cuore, Rome 00168, Italy.
authors.forename: V
authors.initials: V
authors.lastname: Valentini
authors.affiliation: Istituto di Fisica, Università Cattolica del Sacro Cuore, Rome 00168, Italy; U.O.C di Fisica Sanitaria, Università Cattolica del Sacro Cuore, Rome 00168, Italy; and Istituto Nazionale di Fisica Nucleare (INFN), Sezione di Roma Tre, Rome 00168, Italy.
authors.forename: A
authors.initials: A
authors.lastname: Piermattei
mesh_terms: D000008:Abdominal Neoplasms / Q000000981:diagnostic imaging / Q000532:radiotherapy; D000465:Algorithms; D054893:Cone-Beam Computed Tomography; D005240:Feasibility Studies; D006258:Head and Neck Neoplasms / Q000000981:diagnostic imaging / Q000532:radiotherapy; D006801:Humans; D008297:Male; D008954:Models, Biological; D010190:Pancreatic Neoplasms / Q000000981:diagnostic imaging / Q000532:radiotherapy; D010315:Particle Accelerators; D019047:Phantoms, Imaging; D011471:Prostatic Neoplasms / Q000000981:diagnostic imaging / Q000532:radiotherapy; D011874:Radiometry / Q000295:instrumentation / Q000379:methods*; D011879:Radiotherapy Dosage; D011880:Radiotherapy Planning, Computer-Assisted / Q000295:instrumentation / Q000379:methods; D050397:Radiotherapy, Intensity-Modulated / Q000295:instrumentation / Q000379:methods*; D013997:Time Factors; D014057:Tomography, X-Ray Computed