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 (DAPR) 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 (D) over a surface larger than the beam size. 6 MV and 10 MV photon beams with field diameters from 0.