ORIGINAL ARTICLE A generalized calibration procedure for in vivo transit dosimetry using siemens electronic portal imaging devices Andrea Fidanzio • Francesca Greco • Laura Gargiulo • Savino Cilla • Domenico Sabatino • Massimo Cappiello • Cinzia Di Felice • Elisabetta Di Castro • Luigi Azario • Mariateresa Russo • Luciano Pompei • Guido D’Onofrio • Angelo Piermattei Received: 9 March 2010 / Accepted: 9 October 2010 / Published online: 4 November 2010 Ó International Federation for Medical and Biological Engineering 2010 Abstract A practical and accurate generalized in vivo dosimetry procedure has been implemented for Siemens linacs supplying 6, 10, and 15 MV photon beams, equipped with aSi electronic portal imaging devices (EPIDs). The in vivo dosimetry method makes use of correlation ratios between EPID transit signal, s t 0 (TPR,w,L), and phantom mid-plane dose, D 0 (TPR,w,L), as functions of phantom thickness, w, square field dimensions, L, and tissue-phan- tom ratio TPR 20,10 . The s t 0 (TPR,w,L) and D 0 (TPR,w,L) values were defined to be independent of the EPID sensi- tivity and monitor unit calibration, while their dependence on TPR 20,10 was investigated to determine a set of gen- eralized correlation ratios to be used for beams with TPR 20,10 falling in the examined range. This way, other radiotherapy centers can use the method with no need to locally perform the whole set of measurements in solid water phantoms, required to implement it. Tolerance levels for 3D conformal treatments, ranging between ±5 and ±6% according to tumor type and location, were estimated for comparison purposes between reconstructed isocenter dose, D iso , and treatment planning system (TPS) computed dose D iso,TPS . Finally a dedicated software, interfaceable with record and verify (R&V) systems used in the centers, was developed to obtain in vivo dosimetry results in less than 2 min after beam delivery. Keywords aSi EPID  In vivo dosimetry  Radiotherapy 1 Introduction Electronic portal imaging devices (EPIDs) have been originally designed and developed for the purpose of visual inspection of patient set-up during radiotherapy sessions. However, their use has been increasingly exten- ded to obtain dosimetric information for pre-treatment verification [31, 34, 35] and in vivo dosimetry [14, 19, 21, 22]. In particular, the complexity of some radiotherapy techniques requires verifications of dose delivered to the patient during treatment, to point out possible discrepan- cies between reconstructed doses and expected doses, caused by inaccuracies/errors during pre-treatment and treatment stages. The most used in vivo dosimetry method is based on the use a couple of diodes, positioned at the beam entrance and exit on patient’s surface, respectively, to verify the patient’s mid-plane dose, D m , along the beam central axis. However, the use of diodes requires (i) periodic recali- brations; (ii) accurate positioning on patient for every gantry angle; (iii) corrections for photon fluence perturba- tion in patient; (iv) corrections for temperature, angle of A. Fidanzio (&)  F. Greco  L. Gargiulo  L. Azario  A. Piermattei Istituto di Fisica, Universita ` Cattolica del Sacro Cuore, Largo F. Vito 1, 00168 Rome, Italy e-mail: andrea.fidanzio@rm.unicatt.it S. Cilla  D. Sabatino  A. Piermattei U.O. di Fisica Sanitaria, Centro di Ricerca e Formazione ad Alta Tecnologia nelle Scienze Biomediche, Universita ` Cattolica del Sacro Cuore, Campobasso, Italy M. Cappiello  C. Di Felice  E. Di Castro U.O. di Fisica Sanitaria, Universita’ La Sapienza, Rome, Italy M. Russo  L. Pompei U.O. di Radioterapia, Ospedale Belcolle, Viterbo, Italy G. D’Onofrio U.O. di Fisica Sanitaria, Ospedale Spirito Santo, Pescara, Italy 123 Med Biol Eng Comput (2011) 49:373–383 DOI 10.1007/s11517-010-0699-6