Room: Exhibit Hall
Purpose: To compare two commercially available HDR skin brachytherapy applicators with different source orientations to determine their dosimetric and/or clinical differences.
Methods: Two Leipzig-style applicators from Varian Medical Systems, one with a vertically-oriented source (at 15mm offset) and the other horizontally-oriented, were tested with identical collimators (30mm, 35mm, 40mm and 45mm diameter circular inserts). Profiles were taken in solid water with gafchromic film at 2mm, 3mm, and 4mm depths. These were analyzed and compared in ImageJ at multiple azimuthal angles. Percent Depth Dose (PDD) curves for both applicators were generated from AcurosBV and analyzed in Excel. Typical delivery times to achieve the selected prescription dose were compared by using an off-axis, to-depth normalization for both the horizontal and vertical applicators. The resulting metric used was a ratio of the treatment times for vertical to horizontal for each of the insert sizes.
Results: Dose heterogeneity across the tumor base for the vertical applicator varied a maximum of 119.0% across all inserts while the dose across the tumor base for the horizontal applicator varied from 111.5% to 171.0% at 3mm depth for the 30mm and 45mm inserts respectively. The central axis PDD for a 3mm depth normalization on central axis yielded a surface dose of 145.6% for the horizontal applicator vs 131.9% surface dose for the vertical. The vertical applicator increased treatment times by a factor of 1.7, 1.9, 2.2, and 2.9 over the horizontal for the 30mm, 35mm, 40mm and 45mm diameter inserts respectively.
Conclusion: The horizontally oriented skin applicator resulted in shorter treatment times, however compared with the vertically oriented applicator, it had significantly greater dose heterogeneity across a generic tumor volume. For instance, with a tumor edge normalization at 3mm depth, the horizontal source applicator could result in a 249.0% hotspot at the skin surface central axis.