SU-F-T-607: Study of Dose Falloff Slope in RapidArc Planning of Lung SBRT

Abstract

Purpose: SBRT of lung tumor results in steep dose fall-off outside the PTV to limit the normal tissue dose. RTOG report 0915 recommended metrics for plan quality measure, CI, D2cm, R50% and HDloc were evaluated. This study was aimed at ascertaining moderate to low dose spillage using 5 mm ring structures outside the PTV. Methods: In this retrospective study, 41 clinical RapidArc lung SBRT plans (Eclipse TPS ver 8.9) previously treated at our institution in a TrueBeam STx linac were analyzed. The delivered plans were re-planed using either 2 or 3 complete or partial arcs in coplanar or non-coplanar arrangement with upto ±15 degree couch angle. Seven concentric 5 mm wide rings were created outside the PTV for evaluation of dose fall-off based on mean, maximum doses as a function of distance. Mean and maximum percent dose fall-off in the rings (% dose/mm) were compared between the delivered and research plans. The differences in the mean and maximum percent doses between coplanar and non-coplanar arc plans were studied for any significant differences. Results: Mean dose falloff in the rings outside the PTV followed an exponential curve. For all the patients, interestingly the slope of maximum dose falloff increased at the intersection of 1st and 2nd ring and monotonically fell at outer rings. Altering coplanar to non-coplanar arc arrangements had a minimal impact on the CI as well as D2cm, R50% values (p-value > 0.05). Conclusion: Slope of dose falloff from the PTV surface could serve as an additional metric. Coplanar arc plan had slightly steep dose falloff outside the PTV than non-coplanar arcs, but the differences are statistically insignificant.