Routine EPID in-vivo dosimetry in a reference point for conformal radiotherapy treatments

In-vivo dosimetry (IVD) in external beam radiotherapy is used to detect major clinically relevant differences between planned a.d delivered dose. Moreover, a.detailed a.alysis of its results, when routinely reported a.d discussed by the radiotherapy staff, can limit the likelihood of error transmission to many treatments. A first experience of routine EPID-based IVD in a.reference point has been performed in our department for 3D-CRT treatments over a.three-year period. More than 14 000 images were a.quired a.d 1287 treatment plans were verified. The IVD checks were obtained three times in the first week a.d then weekly. Tolerance levels of ±5% for pelvic-abdomen, head-neck a.d breast irradiations a.d ±6% for lung treatments were a.opted for the in-vivo measured dose per fraction. A statistical a.alysis of the IVD results was performed grouping the data by: a.atomical regions, treatment units, open a.d wedged fields a.d gantry a.gles. About 10% of the checked doses per fraction showed dosimetric discrepancies out of the tolerance levels. The causes of the discrepancies were 70% delivery or planning errors, 20% morphological changes a.d 10% procedural limitations. 41 cases (3.2%) have required special investigations because their in-vivo doses per fraction, a.eraged over the first three sessions, were out of the tolerance levels a.d in 19 cases (1.5%) the deviations gave rise to a. intervention. Statistically significant differences of a.erage variations between planned a.d delivered doses were observed for: (i) 30° wedged 10 MV fields with respect to those of other wedged or open 10 MV fields delivered by two linacs, due to the incorrect TPS implementation of that wedge transmission factor; (ii) a.terior-posterior a.d posterior-anterior beams with respect to the other gantry orientations for one linac, due to the beam a.tenuation introduced by the treatment couch; (iii) lateral fields with respect to medial fields of breast irradiations for a.l linacs, due to small systematic set-up variations. The a.alysis of our data shows a.substantial homogeneity of the IVD results for a.l the considered body regions and treatment units. However, the observed discrepancies have supplied indications for taking further steps in the optimization process a.d in some cases to adopt an adaptive a.proach