Feasibility of using a dose-area product ratio as beam quality specifier for photon beams with small field sizes.

PURPOSE: To investigate the feasibility of using the ratio of dose-area product at 20 cm and 10 cm water depths (DAPR20,10) as a beam quality specifier for radiotherapy photon beams with field diameter below 2 cm. METHODS: Dose-area product was determined as the integral of absorbed dose to water (Dw) over a surface larger than the beam size. 6 MV and 10 MV photon beams with field diameters from 0.75 cm to 2 cm were considered. Monte Carlo (MC) simulations were performed to calculate energy-dependent dosimetric parameters and to study the DAPR20,10 properties. Aspects relevant to DAPR20,10 measurement were explored using large-area plane-parallel ionization chambers with different diameters. RESULTS: DAPR20,10 was nearly independent of field size in line with the small differences among the corresponding mean beam energies. Both MC and experimental results showed a dependence of DAPR20,10 on the measurement setup and the surface over which Dw is integrated. For a given setup, DAPR20,10 values obtained using ionization chambers with different air-cavity diameters agreed with one another within 0.4%, after the application of MC correction factors accounting for effects due to the chamber size. DAPR20,10 differences among the small field sizes were within 1% and sensitivity to the beam energy resulted similar to that of established beam quality specifiers based on the point measurement of Dw. CONCLUSIONS: For a specific measurement setup and integration area, DAPR20,10 proved suitable to specify the beam quality of small photon beams for the selection of energy-dependent dosimetric parameters.