WE‐A‐108‐06: Dynamic Modulated Brachytherapy for Cervical Cancer

Abstract

Purpose: We propose two new intra‐uterine tandem designs that are capable of creating non‐isotropic 192‐Ir dose distributions and give unprecedented dose conformality for treatment of cervical cancer. Methods: The first tandem design, MC6, has 6 peripheral holes of 1‐mm diameter, and the in‐between‐space filled with tungsten (18.0 g/cc), wrapped in delrin (1.41 g/cc). The second design, DMBT, has 19 holes of 1‐mm diameter, each occupied by gold wires (19.3 g/cc) that can slide in and out (with motors), and the in‐between‐space filled with silicon (1.14 g/cc). MCNPX Monte Carlo was used to simulate the resulting non‐isotropic dose distributions. An in‐house developed HDR brachytherapy planning platform, with intensity modulated planning capability using Simulated Annealing and Constrained‐Gradient Optimization algorithms, was used to plan 36 patient cases and compare with the clinically treated, conventional T&O applicator‐based plans. For the proposed tandem designs, the plans were optimized with the same ovoids in place, as the conventional T&O plans. Results: All 36 plans were prescribed 6Gy and normalized to D90=6Gy and V100=95%. Generally, the plan qualities were markedly better using MC6 and DMBT, with DMBT usually performing the best overall. Mean 2cc/0.1cc doses to the bladder were 9.14±0.78/9.32± 0.80Gy, 6.87±0.57/7.05±0.59Gy, and 6.61±0.51/6.74±0.52Gy, for T&O, MC6, and DMBT, respectively. For the rectum, they were 5.78± 0.69/5.92±0.70Gy, 5.17±0.52/5.30±0.52Gy, and 4.92±0.50/5.04± 0.50Gy. For the sigmoid, they were 5.23±0.58/5.34±0.59Gy, 5.04± 0.56/5.21±0.57Gy, and 4.86±0.51/5.03±0.52Gy. The most improvement was in the bladder dose and this was due to the horseshoe‐like wrapping of the bladder around the CTV, thus benefitting the most with the dose conformation enhancements achieved by the proposed designs. The CTV dose heterogeneity index (DHI) was 2.43±0.01, 2.30±0.01, and 2.26±0.01, respectively. Conclusions: We have shown two tandem designs that advance the conformality of image‐guided cervix HDR, in congruence with the current trend of 3D image based planning to maximize the therapeutic ratio.