TOPAS-based 4D Monte Carlo simulation of transit dose in esophageal HDR brachytherapy: a phantom study

<i>Objective.</i>Intraluminal high-dose-rate (HDR) brachytherapy is a well-established treatment modality for esophageal cancer, where a radioactive source moves through a catheter to deliver dose at discrete dwell positions. However, during the source's transit phase non-negligible dose may be delivered to surrounding healthy tissues. This study utilizes the time-dependent (4D) Monte Carlo (MC) simulation approach to quantify the transit dose to organs at risks (OARs) during esophageal brachytherapy using the TOPAS tool.<i>Approach.</i>A simplified yet anatomically representative male anthropomorphic phantom was modeled in TOPAS, incorporating key anatomical structures including the esophagus, trachea, spinal cord, pharynx, and other OARs. The motion of an Ir-192 HDR source was dynamically simulated through a brachytherapy catheter, with source acceleration and deceleration explicitly modeled to reflect clinical delivery. Focusing on a 6 cm tumor in the upper esophagus, absorbed doses were calculated under varying conditions: constant versus variable source speeds and differing numbers of dwell points, to evaluate the dosimetric impact of source motion.<i>Main results.</i>Compared to static simulations that neglect source movement, the inclusion of realistic source motion led to a substantial increase in dose to the tumor itself, up to 31%. Transit dose to OARs along the catheter path and those adjacent to the esophagus, such as the pre-pharyngeal region and pharynx, reached up to 11.7% of the tumor dose under variable speed profiles. These results reveal significant underestimation in static dose calculations, highlighting the importance of accounting for source motion.<i>Significance.</i>This study underscores the dosimetric importance of accounting for source transit in HDR brachytherapy and demonstrates the value of TOPAS's time-feature for conducting 4D MC simulations of realistic source motion. The findings indicate that transit dose can contribute substantially to overall dose distributions and should be considered in treatment planning, particularly for tumors situated in anatomically complex or radiosensitive regions.