Flattening filter free Stereotactic radiosurgery for brain metastases using dynamic conformal arcs: 6 MV or 10 MV?

Introduction: Stereotactic radiosurgery (SRS) has proven itself as an effective toot in the treatment of intracranial lesions. Image-guided high dose single fraction treatments have the potential to deliver ablative doses to tumours; however, treatment times can be long. Flattening filter free (FFF) beams are available on most modern linacs and offer a higher dose rate compared to conventional flattened beams which should reduce treatment times. This study aimed to compare 6 MV FFF and 10 MV FFF to a 6 MV flattened beam for single fraction dynamic conformal arc SRS for a Varian Truebeam linac. Materials and methods: In total, 21 individual clinical treatment plans for 21 brain metastases treated with 6 MV were retrospectively replanned using both 6 MV FFF and 10 MV FFF. Plan quality and efficiency metrics were evaluated by analysing dose coverage, dose conformity, dose gradients, dose to normal brain, beam-on-time (BOT), treatment time and monitor units. Results: FFF resulted in a significant reduction in median BOT for both 6 MV FFF (57.9%; p <0.001) and 10 MV FFF (76.3%; p <0.001) which led to reductions in treatment times of 16.8 and 21.5% respectively. However, 6 MV FFF showed superior normal brain dose sparing (p < 0.001) and dose gradient (p < 0.001) compared to 10 MV FFF. No differences were observed for conformity. Conclusion: 6 MV FFF offers a significant reduction in average treatment time compared to 6 MV (3.7 minutes; p = 0.002) while maintaining plan quality.