Quality assurance of measured depth dose for megavoltage photon beams

We have evaluated the use of a five-parameter analytical expression to perform quality assurance of measured fractional depth dose. Two of the parameters, attenuation and beam-hardening coefficients, are directly measured. The remaining three parameters are determined by fitting the product of measured depth doses anal the phantom-scatter factor, S-p(s), for various square field sizes (5 - 40 cm) in the region of electronic equilibrium. S-p(s) is the dose at a reference depth and field size s, normalized to the PQ x 10 cm(2) field size, excluding the head-scatter factor, H. It is obtained by the division of S-pc(c) and Pb(c), The data were fitted to a published analytic expression for the scatter-to-primary dose ratio, and modified to account for backscatter. We used a differential evolution algorithm for the minmax optimization of the relative errors. The electron disequilibrium factor, E; is also extrapoiated for all the field sizes. Ten photon beams were processed (6 and 18 MV, open and four solid wedges). Nine of the ten processed beams showed a maximum error of 1.8% with a mean error of 0.5%, One photon beam (6 MV, open) had a larger maximum error of 3.8%, Further investigation indicated that this error was caused by measurement errors for the 5 x 5 cm2 field. The maximum error was reduced to 1.8% when the depth dose was remeasured.