Use of Flattening Filter Free Photon Beams for Off-Axis Targets in Conformal Arc Stereotactic Body Radiation Therapy

Dynamic conformal arc therapy (DCAT) and FFF beams are commonly adopted for efficient conformal dose delivery in SBRT. Off-axis geometry (OAG) may be necessary to obtain full gantry rotation without collision, which has been shown to be beneficial for peripheral targets using flattened beams. We investigated dose distributions in OAG using FFF, and the effect of mechanical rotation induced uncertainty.Sphere targets (2, 4, and 6 cm diameter) were placed at three locations (central axis, 3 cm, and 6 cm off-axis) in a representative patient CT set. DCAT plans were obtained for 6X, 6FFF, 10X, and 10FFF. Homogeneity index (HI), conformality indices (CI), and beam on time (BOT) were calculated. Mechanical rotation induced uncertainty was evaluated using five SBRT patient plans with laterally located tumors. For each, a plan was generated using FFF beams for OAG and CAG. Each was replanned to account for one degree collimator/couch rotation errors during delivery. Prescription isodose coverage, CI, and lung dose were evaluated.HI and CI values were similar for flattened and unflattened beams; however, 6FFF provided slightly better values than 10FFF in OAG. For all plans HI and CI were acceptable with the maximum difference between flattened and FFF beams being 0.1. FFF beams showed better conformality for low doses and small targets. Variation due to rotational error for isodose coverage, CI, and lung dose was generally smaller for CAG compared to OAG, with some of these comparisons reaching statistical significance. However, variations in dose distributions for either technique were small and not clinically significant.FFF beams showed acceptable dose distributions in OAG. 10FFF provides more dramatic BOT reduction, but generally provides less favorable dosimetric indices compared to 6FFF in OAG. Mechanical rotation induced uncertainty had an increased effect for OAG compared to CAG; however, variations for either treatment technique were minimal.