Comparison of surface dose delivered by 7MV-unflattened and 6MV-flattened photon beams.

Abstract

The purpose of this study is to determine the central-axis dose in the buildup region and the surface dose delivered by a 6MV flattened photon beam (6MV-FB) and a higher energy unflattened (7MV-FFF) therapeutic photon beam for different-sized square fields with open fields and modifying filters. The beams are produced by a Siemens Artiste linear accelerator with a NACP-02 ionization chamber and the dose is measured by using GafChromic film and two different, commonly used, dosimeters: a p-type photon semiconductor dosimeter (PFD) and a cylindrical ionization chamber (CC13). The results indicate that the surface dose increases linearly with FS for both open and wedged fields for the 6MV-FB and 7MV-FFF beams. The surface dose delivered by the 7MV-UFB beam is consistent with that delivered by the 6MV-FB beam for field sizes up to 10cm×10cm, after which the surface dose decreases. The buildup dose for the 7MV-UFB beam is slightly less than that for the 6MV-FB beam for field sizes ranging from 5cm×5cm to 15cm×15cm. For both the 6MV-FB and 7MV-FFF beams, the measured surface dose clearly increases with increasing field size, regardless of the detector used in the measurement. The surface dose measured with the PFD dosimeter and the NACP-02 and CC13 chambers differ significantly from the results obtained when using GafChromic film. The 7MV-FFF beam results in a slightly smaller surface dose in the buildup region compared with the 6MV-FB beam. The surface dose delivered by the higher energy 7MV-FFF beam is less than that delivered by the energy-unmatched FFF beam in previously published works.