Room: Exhibit Hall
Purpose: We investigate treatment plan quality for lung SBRT cases with multiple lesions. We compare combined single-isocenter approach versus separate multiple-isocenter approach, evaluating resultant target conformality, and low-dose spread.
Methods: Retrospective plan evaluation of fifty-three patients with multiple lesion lung tumors, treated to 45-60 Gy in 3-5 fractions. PTV was contoured as ITV plus 3mm margin for 44 cases, 5mm for 9 cases. Patients were treated with both 3D conformal arcs and VMAT, using coplanar and non-coplanar arc geometry. Target volumes, relative lesion locations, and shape were features collected for each case. Plans were generated using both single-isocenter approach and multiple-isocenter approach for various clusters of lesions, depending on relative spatial distribution of lesions. To standardize plans for comparison, dose was normalized to deliver 95% of the prescription dose to 95% of the PTV. Plan comparison evaluated conformity index, total volume irradiated to the 95%, 75%, 50% and 25% isodose levels, and the percent of lung receiving â‰¥25% of the Rx.
Results: The mean GTV was 13.9 cc. The mean PTV equivalent spherical radius (RPTV) was 1.80 cm. The volume of the PTV was found to be predictive of each isodose volume. The R95%, R75%, R50%, and R25%, were estimated to be 1.13, 1.39, 1.78, and 2.64 times larger than the RPTV. The choice of single-isocenter versus separate multiple-isocenter planning was found to have a statistically significant difference for target conformality and low-dose spread.
Conclusion: SBRT for multiple lesions can be delivered with a high degree of conformity using either single-iso or multiple-isocenter arcs. While total treatment time is reduced using single-iso approach, lesion separation distance and target volumes greatly determine the appropriateness of using either approach.