Flattening filter free beam energy selection and its impact in multitarget intracranial stereotactic radiosurgery treatments

Extensive work has been done on the characteristics, dosimetry, and efficacy of flattening filter free (FFF) beams in radiosurgery. However, no study has addressed the dosimetric impact of FFF beam energy selection on treatment plan quality. This study aims to present a systematic dosimetric comparison of plan quality between 10 FFF vs 6 FFF beams in intracranial stereotactic radiosurgery (SRS) treatments using volumetric modulated arc therapy (VMAT). The dosimetric evaluation is based on radiation therapy oncology group (RTOG) dose conformity (CIRTOG) and gradient (GIRTOG) indices, and irradiated normal brain tissue volume. Thirty-five VMAT-based intracranial SRS treatments to multiple brain metastases using a 2.5 mm multileaf collimator (MLC) and 10 MV FFF beam were replanned with a 6 MV FFF and same MLC. The replans incorporated the same arc arrangement, planning target volume (PTV) and organs at risk structures, PTV coverage and prescription isodose normalization. The 6 MV FFF had a sharper dose fall-off compared to the 10 MV FFF (GIRTOG-10 FFF = 4.70, GIRTOG-6 FFF = 4.56; p < 0.05) and comparable conformity index (CIRTOG-10 FFF = 1.11, CIRTOG-6 FFF = 1.10; p = 0.9). On average, the irradiated normal brain tissue volume was 11% lower with 6 MV FFF compared to 10 MV FFF (p < 0.05). However, this difference was diminished for large target volumes and increased number of targets treated. The main dosimetric improvement of a 6 MV FFF over a 10 MV FFF beam is the sharper dose fall-off which directly correlates with less normal brain tissue volume irradiation.