SU-E-T-479: Skin Dose from Flattening Filter Free Beams: A Monte Carlo Investigation.

PURPOSE Flattening filter free (FFF) beams in radiotherapy have advantages such as shorter treatment delivery time and lower out-of-field dose compared with conventional flattened beams. This study investigates in detail the skin dose induced by FFF beams from a TrueBeam accelerator (Varian Medical Systems, Palo Alto, CA) using Monte Carlo method. METHODS Phase space files generated using real geometry of a TrueBeam accelerator above the jaws, were used as the input radiation source files in beam simulation for various field sizes using BEAMnrc. Phase space files for various field sizes were generated at the phantom surface. DOSXYZnrc was used for dose calculations in phantom and in patient using the generated phase space files as source input files. RESULTS The calculated percentage depth dose curves and profiles in water agreed with measurements within ± 2% for the high dose region and ±2 mm in the penumbra. The peak fluence of a 6 MV FFF beam with the same electron beam incident on the target is about 3 times that of a flattened beam . The mean energy of a 6 MV FFF beam is 0.92-0.95 MeV while it is 1.18-1.30 MeV for the flattened beam. Due to the mean energy difference, the dose in a 6 MV FFF beam is about 6% (of the maximum dose, or 12% of local dose) higher at depth of 1 mm compared with a flattened beam. CONCLUSIONS Due to the lower mean photon energy, in an FFF beam the surface (skin) dose is slightly higher compared to the conventional flattened beam of the same field size.