PO23 Presentation Time: 7:40 AM: Dosimetric Effects of Model-Based TG-186 Vaginal Cylinder Treatment Plans

Abstract

<h3>Purpose</h3> The vaginal cuff is the main location of relapse after curative surgery for early-stage endometrial cancer. Adjuvant vaginal cylinder brachytherapy is increasingly used to decrease the risk of postoperative recurrence due to low vaginal cuff recurrence rates and low gastrointestinal toxicity. Current clinical treatment planning follows TG-43 guidance, which assumes that all material is water. Cylinder composition can vary substantially from water equivalent materials, highlighting the importance of investigations into the dosimetric impact of cylinder composition. In the present study, we evaluated dosimetric characteristics of plans utilizing TG-186, which provides a model-based calculation approach to account for heterogeneities in cylinder configuration. <h3>Materials and Methods</h3> Ten graphically planned vaginal cylinder plans were retrospectively evaluated in this study. Each patient was fitted with an Elekta cylinder (part# 084350) comprised primarily of polysulfone (PSU, 1.29g/cc) and secondarily of stainless steel (8.00g/cc) for the stem and rings. Cylinder diameters ranged from 2.5cm to 3.5cm, and treatment lengths from 4.0cm to 7.5cm. Dose prescription was 7Gy/fx x 3fx at 5mm depth for the most proximal 2cm of the cylinder and at cylinder surface for the remainder of the treatment length. Treatment planning was conducted in Oncentra Brachy 4.5.3. The original TG-43 plans (which assume a water-equivalent material, 1.00g/cc) were recalculated using Oncentra's Advanced Collapsed-Cone Engine (ACE) according to TG-186's guidance to account for differences in density. Dwell times were unchanged, so heterogeneity correction accounts for any dosimetric changes. Points placed at the middle of the 5mm depth portion and the middle of the surface depth portion - prescription dose for TG-43 - were compared, as were rectum points. In addition, all isodose lines were converted to contours and compared for the following metrics: percent volume change, Hausdorff distance (HD), mean distance to agreement (MDA), and Dice coefficient. A paired Wilcoxon signed-rank test assessed for significance. <h3>Results</h3> Recalculated TG-186 plans were found to have consistently and statistically significantly (p<0.05) less dose: 5mm depth: -10.5% +/- 2.4%, surface: -15.9% +/- 3.5%, and rectum: -12.1% +/- 3.4% average dose reduction. Compared to TG-43, the TG-186 100% isodose lines as contours show an average volume decrease of 22.2% +/- 2.5%, HD 4.6mm +/- 1.2mm, MDA 1.5mm +/- 0.3mm, and Dice 0.87 +/- 0.02. The high dose region as defined by 150% of prescription (25.7% volume decrease, HD 4.4mm, MDA 1.3mm) and the low dose region of 50% prescription (19.5% volume decrease, HD 8.2mm, MDA 1.9mm) show similar trends. <h3>Conclusions</h3> Ten TG-43 graphical vaginal cylinder plans were recalculated using TG-186 to account for the material composition of the cylinders. ACE calculations show that the dose delivered to target prescription points and to OARs may be less than originally planned, although the impact on treatment efficacy is presently unclear. Further studies on other cylinder models will be useful to evaluate the generalizability of these findings, as well as any potential effect on treatment.