Clinical application of flattening filter free beams for extracranial stereotactic radiotherapy

Purpose: To investigate the clinical application of flattening filter free (FFF) beams at maximum dose rate for stereotactic body radiotherapy (SBRT). Methods and materials: Patients with tumors in the lung or abdomen were subjected to SBRT using 6 MV FFF or 10 MV FFF beams. For each patient, three plans were calculated using 6 MV flattened, 6 MV FFF, and 10 MV FFF beams. Treatment times were recorded and analyzed, and tumor displacements were assessed by pre- and post-treatment cone beam computed tomography (CBCT). Results: Altogether, 26 patients (16 lung, 10 abdominal tumors) were treated. The average dose rate per patient ranged from 442 to 1860 MU/min. Beam-on time was on average 1.6 min (1SD = 0.6 min), with the total treatment times recorded at 18.5 min (1SD = 3.5 min). The time advantage of using FFF beams was dose-dependent and started at 4 Gy for 6 MV FFF and at 10 Gy for 10 MV FFF beams. The average of the tumor displacements during treatment was 2.0 mm (1SD = 1.0 mm). Conclusions: SBRT using FFF beams is time efficient and associated with excellent patient stability. According to Van Herk's formula, ITV-PTV margins of 6 mm are sufficient in our patient cohort. Further studies are necessary to assess clinical outcome and toxicity. (C) 2013 Elsevier Ireland Ltd. All rights reserved. Radiotherapy and Oncology 106 (2013) 255-259