Strategies to Expand Viable Treatment Area in Tungsten Shell Applicators for Surface Brachytherapy

Abstract

For surface brachytherapy, tungsten shell applicators are generally used to treat small, round skin lesions with a single, central source dwell position. The applicator consists of an enclosed air cavity within a tungsten shell and a horizontal source channel which extends only partially through the air cavity. The purpose of this work was to explore the use of multiple dwells and applicator positions to treat larger targets, as well as the possible uncertainties that could compromise these treatments. Due to the stem design, separate deliveries were required to create a delivery geometry with a dwell on each side of the inner shell. Thus, three different treatment strategies were examined: one application with a single dwell position (129.4 cm), one application with two dwell positions (130 and 128.5 cm), and two applications each with a single dwell position (128.5 cm). The two application plans consisted of a single dwell followed by a second application with the applicator rotated 180° parallel to the phantom surface. 80% and 90% isodose distributions at 3 mm depth were calculated for a 6 Gy prescription using a commercially available model-based dose calculation algorithm. To quantify the impact of setup uncertainties in rotation angle for two-application plans, the V100%, D95% and minimum dose (Dmin) were measured after angular offsets up to 10° were manually inserted. For the 90% isodose line, in-stem (cross-plane) dimensions increased from 9.4 (9.7) mm to 16.6 (11.3) mm or 21.5 (12.0) mm when using multiple dwells or applications, respectively. With multiple applications, the in-stem isodose line width was more than doubled (229%) compared to the conventional single dwell technique. The cross-plane isodose line width increased by 24%. For the 80% isodose line, in-stem (cross-plane) dimensions increased from 14.6 (15.3) mm to 23.5 (17.5) mm or 27.7 (18.2) mm when using multiple dwells or applications, respectively. For multiple applications, the 5° angular offset maintained a V100% of 89.3%, with the D95% remaining above 91%, and Dmin of 87%. The conventional single dwell technique in tungsten shell applicators can be significantly improved by utilizing a second application or a second dwell. While two-application plans produced the largest dose distributions, these plans are dependent on angular precision to within ∼5°. Compared with the other techniques, a single application with two dwells may provide a compromise between setup uncertainties and increased dose coverage. Redesigning the applicator such that the stem extends entirely through the shell would allow for multiple dwells without angular setup uncertainties.Tabled 1Abstract 2764; TableOffset AngleV100% (%)D95% (%)Dmin (%)0° (Reference)--92.289.42°96.492.188.93°93.892.088.34°92.191.988.45°89.391.587.010°76.689.383.5 Open table in a new tab