Does the apex optimization line matter for single-channel vaginal cylinder brachytherapy planning?

The objective of this study is to test the impact of the use of the apex optimization line for new vaginal cylinder (VC) applicators. New single channel VC applicators (Varian) that have different top thicknesses but the same diameters as the old VC applicators (2.0 cm diameter, 2.3, 2.6, 3.0, and 3.5 cm) were compared using phantom studies. Old VC applicator plans without the apex optimization line were also compared to the plans with an apex optimization line. The apex doses were monitored at 5 mm depth doses (eight points) where a prescription dose (Rx) of 6 Gy was prescribed. VC surface doses (eight points) were also analyzed. The new VC applicator plans without apex optimization line presented significantly lower 5-mm depth doses over the Rx (on average -31 ± 7%, P < 0.00001) due to thicker VC tops (3.4 ± 1.1 mm thicker with the range of 1.2-4.4 mm) than the old VC applicators. Old VC applicator plans also showed a statistically significant reduction (P < 0.00001) due to the Ir-192 source anisotropic effect at the apex region, but the percent reduction over the Rx was only -7 ± 9%. However, by adding the apex optimization line to the new VC applicator plans, the plans improved 5-mm depth doses (-7 ± 9% over Rx) that were not statistically different from old VC applicator plans (P = 0.923), along with apex VC surface doses (-22 ± 10% over old VC vs -46 ± 7% without using apex optimization line). The use of the apex optimization line is important in order to avoid significant additional cold doses (-24 ± 2%) at the prescription depth (5 mm) of the apex, specifically for the new VC applicators that have thicker tops. A template-based vaginal cylinder planning reduced the intra- and inter-planner variations of manual generation of apex optimization line, along with treatment time.