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

Abstract : Purpose: To investigate the feasibility of using the ratio of dose-area product at 20 cm and 10 cm water depths (DAPR(20,10)) 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 (D-w) 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 DAPR(20,10) properties. Aspects relevant to DAPR(20,10) measurement were explored using large-area plane-parallel ionization chambers with different diameters. Results: DAPR(20,10) 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 DAPR(20,10) on the measurement setup and the surface over which D-w is integrated. For a given setup, DAPR(20,10) 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. DAPR(20,10) 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 D-w. Conclusions: For a specific measurement setup and integration area, DAPR(20,10) proved suitable to specify the beam quality of small photon beams for the selection of energy-dependent dosimetric parameters.
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Maria Pimpinella, Claudio Caporali, Antonio Stefano Guerra, Luca Silvi, Vanessa de Coste, et al.. Feasibility of using a dose-area product ratio as beam quality specifier for photon beams with small field sizes. Physica Medica, Elsevier, 2018, 45, pp.106 - 116. ⟨10.1016/j.ejmp.2017.12.012⟩. ⟨cea-01765177⟩

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