Real-time dose reconstruction for wedged photon beams: a generalized procedure

A practical and accurate generalized procedure to reconstruct the isocenter dose D(iso) for 3D conformal radiotherapy (3DCRT) has been developed for X-ray open beams supplied by linacs of different manufacturers and equipped with aSi electronic portal imaging devices (aSi EPIDs). This paper reports an extension of the method, to be applied at the wedged X-ray beams characterized by the wedge attenuation factor W(AF). Using water-equivalent solid phantoms (SPs) of different thicknesses, w, and photon square fields of sizes, L, the generalized midplane doses D(0)(W(AF), w/2,L) and generalized transit signals s(t)(0)(W(AF),w,L) by 38 beams of six different linacs were determined. The generalized data were fitted by surface equations and used together with the information of the ‘record & verify’ network of the centers. In this manner, for every beam, the D(iso) reconstruction was obtained in about 25 seconds after the treatment. To test the in vivo dosimetric procedure, six pelvic treatments that used conformed wedged beams were carried out with three linacs of different manufacturers. For every beam, the comparison between the reconstructed D(iso) and the D(iso,TPS) computed by the TPS, resulted in an acceptable tolerance level of ±5%, estimated for this kind of treatment. Generally the in vivo dosimetry methods that use EPIDs require: (i) a special effort for the dosimetric commissioning with SPs of different thicknesses, and (ii) extra time for the analysis of the EPID signals. The proposed procedure simplifies the commissioning step and supplies for Varian, Elekta, and Siemens linacs equipped with the aSi EPIDs a quasi-real time in vivo dosimetry for open and wedged 3DCRT fields.