Comparison of skin dose of flattening filter and flattening filter-free beam in volumetric modulated arc therapy treatment plan for head and neck cancer

The study aims to investigate and compare the skin dose in the volumetric modulated arc therapy (VMAT) plan for head and neck cancer using a 6 MV flat-filtered (FF) photon beam and the flattening filter-free (FFF) beam. The computed tomography simulation data of 31 treated head and neck cancer patients was used to re-plan VMAT on Eclipse v13.6 software with the 6 MV FF and 6 MV FFF photon beams of TrueBeam STx Linac. The above plans were quantitatively compared the skin dose including Dmean, Dmax and surface volumes received the same dose level. All the plans have to meet the conditions of irradiating actual patients. In Dmean, the skin dose was significantly lower for the FFF beams than the FF beams in surface thicknesses of 2 mm and 4 mm. The skin dose was significantly higher for the FFF beams than the FF beams with a surface thickness of 1 mm. There was no statistical significance in Dmax and other surface thicknesses of Dmean. The skin surface volumes that received the same dose on the 6 MV FFF VMAT plans are smaller than those on the 6 MV FF plans at different surface thicknesses from 1 mm to 5 mm and various dose levels from 5 Gy to 60 Gy. Comparing the 6 MV FF and FFF VMAT radiotherapy plans for H&N cancer revealed that the skin dose was significantly lower for the FFF beams in surface thicknesses of 2 mm and 4 mm compared to the FF beams. The skin surface volume receiving the same dose (5-60 Gy) was less when the 6 MV FFF photon beam was utilized as opposed to the FF beam.