Calculated relative biological effectiveness (RBE) for initial DNA double-strand breaks (DSB) from flattening filter and flattening filter-free 6 MV X-ray fields.

Objectives: We evaluated the radiobiological effectiveness based on the yields of DNA double-strand breaks (DSBs) of field induction with flattening filter (FF) and FF-free (FFF) photon beams.

Methods: We used the particle and heavy ion transport system (PHITS) and a water equivalent phantom (30 × 30 × 30 cm) to calculate the physical qualities of the dose-mean lineal energy (y) with 6 MV FF and FFF. The relative biological effectiveness based on the yields of DNA-DSBs (RBE) was calculated for standard radiation such as 220 kVp X-rays by using the estimating yields of SSBs and DSBs. The measurement points used to calculate the in-field y and RBE were located at a depth of 3, 5, and 10 cm in the water equivalent phantom on the central axis. Measurement points at 6, 8, and 10 cm in the lateral direction of each of the three depths from the central axis were set to calculate the out-of-field y and RBE.

Results: The RBE of FFF in-field was 1.7% higher than FF at each measurement depth. The RBE of FFF out-of-field was 1.9 to 6.4% higher than FF at each depth measurement point. As the distance to out-of-field increased, the RBE of FFF rose higher than those of FF. FFF has a larger RBE than FF based on the yields of DNA-DSBs as the distance to out-of-field increased.

Conclusions: The out-of-field radiobiological effect of FFF could thus be greater than that of FF since the spreading of the radiation dose out-of-field with FFF could be a concern compared to the FF.

Advances In Knowledge: The RBE of FFF of out-of-field might be larger than FF.