Purpose: To explore the relationship between applicator surface dose and 5 mm depth dose and to optimize both locations simultaneously for three most used cylinder sizes (2.5, 3.0 and 3.5 cm in diameter) in treating patients with endometrial adenocarcinoma.
Methods: Total 216 plans were created (9 dose levels x 4 plans/level x 3 Cylinder size x 2 patients/size): if the prescription is on the surface, dose will be optimized on the surface alone or both. In the later plan, surface points will be given a higher priority (100 ~ 150%) than points at 5 mm lines (60% ~ 70%). The same strategy is applied to dose prescribed at 5 mm depth. We use VEGO TG-43 Volume Optimization (Varian Eclipse) for planning and single channel to treat patients.
Results: A dose table between surface and 5 mm depth and its fifth order polynomial mapping function were established for each applicator size, so any prescribed dose at one site can find the prescription dose on the other for optimization at both locations. For dose prescribed at 5 mm, the maximum surface dose can be as high as 30% and 40% on the surface, and 5% and 12% high at 5 mm w/o surface dose optimization. For dose prescribed on the surface, the maximum doses are similar (10% ~ 15%) w/o 5 mm dose optimization, but the addition of 5 mm dose optimization can slightly improve the minimum dose (~5%) on both locations.
Conclusion: 1. Proposed three dose tables and their mapping functions between surface dose and their corresponding 5mm depth doses for three different applicator sizes; 2. Proposed a new method to optimize doses at both locations simultaneously; 3. Reduce the maximum surface dose from 30 - 40% to 10% or so if changing prescribed dose from 5 mm to applicator surface.