Room: Exhibit Hall | Forum 7
Purpose: To examine how the normalization method affects target coverage for horizontally and vertically oriented Leipzig-style surface applicators in skin HDR treatments.
Methods: Plans were generated in Varianâ€™s BrachyVision planning system for both variations of the Varian Leipzig-style surface applicator and for each collimation insert including 30mm, 35mm, 40mm, and 45mm. Cylindrical contours were generated to represent the maximum target volume treatable for a given insert and depth. Target diameters ranged from 10mm to 25mm (providing a 10mm radial margin for a given insert), and depths included 2mm, 3mm, and 4mm. Each plan was normalized via three
methods: central axis at depth, target edge at depth, and D95% receiving prescription dose. Target minimum dose, maximum dose, and D95% values were analyzed for each combination of applicator, insert, depth, and normalization method.
Results: For the horizontally oriented applicator, normalizing to central axis results in a target minimum dose and D95% as low as 56% and 64%, respectively. Normalizing to the target edge results in a hot spot as high as 280%. Setting D95% to 100% results in target minimum and maximum doses of 87% and 243%, respectively. For the vertically oriented applicator, normalizing to central axis results in a hot spot as high as 177%. Normalizing to the target edge results in a minimum dose and D95% as low as 81% and 89%, respectively. Setting D95% to 100% results in target minimum and maximum doses of 85% and 160%, respectively.
Conclusion: Normalizing to central axis results in under-coverage of the target with the horizontal applicator and a hot spot with the vertical applicator. Normalizing to target edge results in a hot spot with the horizontal applicator and under-coverage with the vertical applicator. Using a volume normalization may provide adequate target coverage while reducing the severity of any hot spots.