Dosimetric Impact of Applicator Misalignment, Source Mispositioning and Dwell Time Errors in High-Dose Rate (HDR) Brachytherapy

Background: High-dose-rate (HDR) brachytherapy delivers focused radiation to tumors using radioactive sources like Ir-192, commonly used in prostate, breast, gynecologic, and skin cancers. It serves as a primary treatment or a boost after external beam radiation therapy. Precise applicator placement, accurate source positioning, and appropriate dwell times are critical for effective treatment. While imaging confirms planned source and applicator locations, real-time validation during treatment remains a challenge. Purpose: This study evaluates the dosimetric effects of HDR brachytherapy applicator misalignment, source mispositioning, and dwell time errors on the target and organs at risk (OARs) to propose strategies for minimizing these issues. Methods: Gynecologic HDR treatments were planned using the Henschke applicator and BrachyVision. Plans were analyzed in RadCalc, where applicator misalignment, source mispositioning, and dwell time errors were simulated. Volumetric doses to the target and OARs were compared between BrachyVision and RadCalc. Results: Applicator misalignment in all directions similarly affected target coverage, with lateral misalignments exceeding 3 mm significantly impacting the small bowel. Anterior and posterior misalignments predominantly affected the bladder and rectum, with 5 mm movements having the most significant bowel impact. Superior and inferior misalignments, interpreted as source mispositioning, caused the largest rectal dose deviations at displacements above 3 mm. Dwell time errors impacted all OARs, most notably the sigmoid, and reduced target coverage by up to 6 percent per second. Conclusions: Minimizing applicator misalignment, source mispositioning, and dwell time errors is crucial for accurate HDR brachytherapy dose delivery and adherence to dose constraints. The observed reductions in target coverage may compromise local tumor control, while increased dose to organs at risk, particularly the small bowel, bladder, and rectum, may increase the risk of radiation-related toxicity. These findings highlight the importance of accurate treatment delivery and support the future development of real-time verification techniques in HDR brachytherapy.