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

Abstract

Aim

The purpose of this study is to determine the central-axis dose in the buildup region and the surface dose delivered by a 6[[ce:hsp sp="0.25"/]]MV flattened photon beam (6[[ce:hsp sp="0.25"/]]MV-FB) and a higher energy unflattened (7[[ce:hsp sp="0.25"/]]MV-FFF) therapeutic photon beam for different-sized square fields with open fields and modifying filters.

Materials and methods

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).

Results

The results indicate that the surface dose increases linearly with FS for both open and wedged fields for the 6[[ce:hsp sp="0.25"/]]MV-FB and 7[[ce:hsp sp="0.25"/]]MV-FFF beams. The surface dose delivered by the 7[[ce:hsp sp="0.25"/]]MV-UFB beam is consistent with that delivered by the 6[[ce:hsp sp="0.25"/]]MV-FB beam for field sizes up to 10[[ce:hsp sp="0.25"/]]cm[[ce:hsp sp="0.25"/]]×[[ce:hsp sp="0.25"/]]10[[ce:hsp sp="0.25"/]]cm, after which the surface dose decreases. The buildup dose for the 7[[ce:hsp sp="0.25"/]]MV-UFB beam is slightly less than that for the 6[[ce:hsp sp="0.25"/]]MV-FB beam for field sizes ranging from 5[[ce:hsp sp="0.25"/]]cm[[ce:hsp sp="0.25"/]]×[[ce:hsp sp="0.25"/]]5[[ce:hsp sp="0.25"/]]cm to 15[[ce:hsp sp="0.25"/]]cm[[ce:hsp sp="0.25"/]]×[[ce:hsp sp="0.25"/]]15[[ce:hsp sp="0.25"/]]cm. For both the 6[[ce:hsp sp="0.25"/]]MV-FB and 7[[ce:hsp sp="0.25"/]]MV-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 7[[ce:hsp sp="0.25"/]]MV-FFF beam results in a slightly smaller surface dose in the buildup region compared with the 6[[ce:hsp sp="0.25"/]]MV-FB beam.

Conclusions

The surface dose delivered by the higher energy 7[[ce:hsp sp="0.25"/]]MV-FFF beam is less than that delivered by the energy-unmatched FFF beam in previously published works.