Quality assurance of measured depth dose for megavoltage photon beams

The authors have evaluated the use of a 5-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 3 parameters are determined by fitting the product of measured depth doses and 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 10× 10 cm² field size, excluding the head-scatter factor, H. It is obtained by the division of S_pc(c) and H(c). The data were fitted to a published analytic expression for the scatter-to-primary dose ratio, and modified to account for backscatter. The authors used a differential evolution algorithm for the minmax optimization of the relative errors. The electron disequilibrium factor, E, is also extrapolated for all the field sizes. Ten photon beams were processed (6 and 18 MV, open and 4 solid wedges). Nine of the 10 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 5×5 2 field. The maximum error was reduced to 1.8% cm when the depth dose was remeasured