A National project for in vivo dosimetry procedures in radiotherapy: First results

The paper reports the results of a National project financed by the Istituto Nazionale di Fisica Nucleare (INFN) for the development of in vivo dosimetric procedures in radiotherapy. In particular, a generalized procedure for the in vivo reconstruction of the isocenter dose, Diso, has been developed for 3D conformal radiotherapy treatments with open and wedged X-ray beams supplied by linacs of different manufacturers and equipped with aSi Electronic Portal Imaging Devices (EPIDs). In this way, the commissioning procedure is very simplified and applicable to Elekta, Siemens and Varian linacs. The method here reported is based on measurements in solid-water phantoms of different thicknesses, w, irradiated by square field sizes, L. Generalized mid-plane doses, D0, and transit signal by EPIDs, s0t , obtained by 19 open and 38 wedged beams of 8 different linacs, were determined taking into account X-ray beam and EPID calibrations. Generalized ratios F0 = s0t /D0 for open and wedged beams were fitted by surface equations and used by a dedicated software, for the Diso reconstruction. Moreover, for each beam the software supplied a set of transit signal profiles crossing the beam central axis to test the beam irradiation reproducibility. The tolerance level of the comparison between the Diso and the dose computed by the TPS, Diso,TPS, was estimated 5%. The generalized in vivo dosimetry procedure was adopted by 3 centers that used different linacs. The results of 480 tests showed that in absence of errors, the comparison between Diso and Diso,TPS resulted well-within the tolerance level. The presence of errors was detected in 10% of the tests and were due to essentially an incorrect set-up, presence of an attenuator on the beams and patient morphological changes. Moreover, the dedicated software used the information of the Record and Verify system of the centres and consequently the extra-time needed to obtain, for each beam, the Diso reconstruction after the dose delivery, was about 20 s.