Implementation of a Clarkson summation algorithm for dose verification in MLC-based IMRT

Dose verification by ion chamber measurements Is a time-consuming quality assurance (QA) process for intensity modulation radiotherapy (IMRT). A Clarkson summation algorithm was investigated as an alternative to calculate the dose to the isocenter of IMRT treatments. Scatter contributions to the isocenter dose were calculated using a method similar to that of the Clarkson calculation for the central axis dose of an irregular field. The independent dose calculation was performed using the leaf sequences generated by a commercial treatment planning system (Corvus, NOMOS Corporation, Sewickley, PA) and the patient geometry obtained from CT-simulations. The isocenter dose was decomposed into contributions from the surrounding finite-size beamlets. Each beamlet contribution was calculated by differentiating the phantom scatter data for open fields. The result for a spherically shaped target agreed with the Corvus calculation to within 3%. Special corrections were needed for those cases in which the isocenter was at a low dose point under the shadow of the MLC. This Clarkson-type calculation was found to overestimate the dose when the beams pass through a large air volume, since no tissue heterogeneity was considered in the algorithm. In such cases the dose calculation is performed using a homogeneous phantom plan with the same fields and compared with the Corvus calculation for the same phantom