The work reports the first results of a National Project financed by the National Institute of Nuclear Physics for the development of in-vivo dosimetric procedures in radiotherapy

A National Project for the in-vivo dosimetry in radiotherapy: first results A.Piermattei 1 , L. Azario 1 , S. Cilla 2 , A.Fidanzio 1 , F.Greco 1 , M.T.Russo 3 , S. Zucca 4 1 Istituto di Fisica e Unità Operativa di Fisica Sanitaria, Università Cattolica del S. Cuore, Roma 2 Unità Operativa di Fisica Sanitaria, Fondazione per la Ricerca e Cura ‘Giovanni Paolo II’, Campobasso 3 Unità Operative di Radioterapia e Fisica Sanitaria, Ospedale Belcolle, Viterbo 4 Unità Operativa di Fisica Sanitaria, Presidio Oncologico Businco, Cagliari The work reports the first results of a National Project financed by the National Institute of Nuclear Physics for the development of in-vivo dosimetric procedures in radiotherapy. The first objective has been the development of a generalized procedure for the in-vivo dose reconstruction at the isocenter point, Diso , for 3D conformal radiotherapy treatments, with open and wedged x-ray beams supplied by linacs of different manufacturers and equipped with aSi EPID. This way the commissioning procedure is greatly simplified and applicable at Elekta, Siemens and Varian linacs and the results of the in-vivo tests are reported in quasi-real time. The method is based on two calibration procedures for x-ray beams and aSi EPIDs. In particular, generalized mid-plane doses in solid water phantom D 0 , and transit signal s 0 t values were obtained by 19 open and 38 wedged beams of 8 different linacs. Generalized ratios F°= st°/ D° were fitted by surface equations and together with other parameters were used by the algorithm to convert the integral transit signals St, in reconstructed doses Diso, to compare with the Diso,TPS (computed by the TPS) using a tolerance level of 5% for pelvic, breast, head tumors and lung cancer treatments. Moreover the dedicated software supplied for each beam, the γ-analysis for a 2D comparison between portal images, using alert criteria γmean >0.5 and Pγ >1 >10%. The generalized in-vivo dosimetry procedure has been adopted by 8 centers that used different linacs . The results of about 5000 tests showed that about 85% of the tests were well-within tolerance levels. The dose discrepancies were due to patient's set-up misalignment, presence of attenuating media in the field, patient's morphological changes, no correct wedge TPS implementation, no correct CT number calibration, dose output variations, and CT simulator laser misalignments. The results were supplied in quasi-real time because the dedicated software used the information of the 'record and verify' network of the centers. Consequently, the extra-time needed to obtain the results after the dose delivery was about 20 seconds per beam. At present, the second objective of the National Project is the application of the same method at IMRT beams. The same algorithm developed for open beams can be utilized for the IMRT step-shoot and slidewindow beams, adopting corrections that take into account (i) the effect of the radiation transmitted through the MLC on the integral signal St and (ii) the effect of the fluence inhomogeneity on the generalized ratios F 0 . However, the preliminary in-vivo results obtained for step-shoot and slide-window IMRT with different linacs confirm that it is possible (i) to reduce strongly the commissioning time and (ii) to obtain in a quasi-real time the results of tests. The success of this step could encourage future developments of the project for modulated arc-radiotherapy in-vivo dosimetry.