Skin dose estimation using virtual structures for Contura Multi-Lumen Balloon breast brachytherapy

Purpose: To propose a workflow that uses ultrasound (US)-measured skin–balloon distances and virtual structure creations in the treatment planning system to evaluate the maximum skin dose for patients treated with Contura Multi-Lumen Balloon applicators. Methods and Materials: Twenty-three patients were analyzed in this study. CT and US were used to investigate the interfractional skin–balloon distance variations. Virtual structures were created on the planning CT to predict the maximum skin doses. Fitted curves and its equation can be obtained from the skin–balloon distance vs. maximum skin dose plot using virtual structure information. The fidelity of US-measured skin distance and the skin dose prediction using virtual structures were assessed. Results: The differences between CT- and US-measured skin–balloon distances values had an average of −0.5 ± 1.1 mm (95% confidence interval [CI] = −1.0 to 0.1 mm). Using virtual structure created on CT, the average difference between the predicted and the actual dose overlay maximum skin dose was −1.7% (95% CI = −3.0 to −0.4%). Furthermore, when applying the US-measured skin distance values in the virtual structure trendline equation, the differences between predicted and actual maximum skin dose had an average of 0.7 ± 6.4% (95% CI = −2.3% to 3.7%). Conclusions: It is possible to use US to observe interfraction skin–balloon distance variation to replace CT acquisition. With the proposed workflow, based on the creation of virtual structures defined on the planning CT- and US-measured skin–balloon distances, the maximum skin doses can be reasonably estimated.