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

•DAPR20,10 properties are investigated in photon beams with field sizes below 2 cm.•Influence of field size on photon spectrum and dosimetric parameters is evaluated.•Measurements by ionization chambers with different characteristics are compared.•Results on DAPR20,10 obtained with two different accelerators are reported.•Criteria to obtain a DAPR20,10 useful as small beam quality index are discussed. 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. 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. 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. 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.