26 Dosimetric comparison of dynamic conformal arc therapy with flattened beams and flattening filter free beams for SBRT lung cancer

Introduction The aim of this study was to compare dosimetric gain using dynamic conformal arc (DCA) with flattened filter (FF) and flattening filter free (FFF) beam for lung stereotactic body radiation therapy (SBRT). Methods and materials In our institution, SBRT lung cancer patients were treated with prescribed dose 60 Gy in 4 or 8 fractions. In Pinnacle 9.10 were planned two partial DCA’s for peripheral tumors and three to four partial DCA’s for central region tumors to help pull dose of the OARs (heart, esophagus, spinal cord and ribs). Treatments were delivered with 6 MV beams and dose rate 600 MU, for TrueBeam STx accelerator. Fifty patients who underwent lung SBRT treatment with DCA with FF were retrospectively analyzed using 6 MV FFF beams with dose rate of 1400 MU. We were observing the planning target volume (PTV) coverage, sparing of organ at risks (OARs) and time of treatment delivery. Results The PTV coverage (D98% > 95%) were similar, 98.3% vs 98% for FF and FFF, with 1887 MU’s vs 1860 MU’s, respectively. Dosimetric gain between FF and FFF for max dose to the heart was 1 Gy vs 0.87 Gy (p 0.0001). Max dose for spinal cord was 8.84 Gy vs 8.38 Gy (p 0.0001). Max dose for esophagus was 9.76 Gy vs 8.94 Gy (p 0.0001). Max dose for ribs was 34.16 Gy vs 31.89 Gy (p 0.0001). Time of treatment delivery was 4.50 min vs 2.43 min (p = 0.0006), for FF and FFF, respectively. Conclusion Treatment delivery with FFF DCA allows better sparing of OARs for SBRT of lung cancer patients. Shorter treatment time with FFF DCA for SBRT of lung cancer patients can reduce potentially discomfort or patient motion.