Purpose: Rotationally-symmetric dose distributions provided by brachytherapy sources deliver a high dose to the tumor, but often with poor target conformity resulting in dose spillage to the organs at risk (OAR). By incorporating dynamically-rotating metallic shields, intensity-modulated brachytherapy (IMBT) opens the possibility to deliver more conformal dose distributions. The goal of this study was to develop a novel MRI-compatible dynamic-shield IMBT applicator for the treatment of rectal cancer and compare the results with conventional high-dose-rate brachytherapy (HDR-BT) with and without static shielding.
Methods: For dynamic-shield IMBT, a single-grooved, MRI-compatible, 15 mm diameter tungsten shield with a 180° emission window was designed. A cylindrical intracavitary applicator (length 28 cm, diameter 2 cm) (Elekta Brachytherapy, Veenendaal, The Netherlands) with a central lumen that fits an 8 mm diameter tungsten rod for static shielding was also modeled. For both applicator models, four rectal cancer patients were retrospectively planned using an in-house Monte Carlo based treatment planning system called RapidBrachyMCTPS and a generic Ir-192 source. The plans were optimized using a fast mixed-integer optimization method. The prescribed dose was 3 fractions of 10 Gy. The resolution of the scoring grid was 3x3x3 mm³ and 10⁶ decay events were simulated.
Results: For the same target coverage, the contralateral healthy rectal wall D2cc, rectum D50 and pelvis D50 decreased on average by 38.93%, 18.82% and 18.75% when using dynamic-shield IMBT compared with HDR-BT with the static shield and by 53.04%, 68.35% and 36.62% compared with HDR-BT without the static shield. However, the inferior and superior dose spill regions D2cc were 11.14% and 9.57% greater compared with static-shielding HDR-BT. The treatment times were below 30 minutes for all cases.
Conclusion: The designed dynamically-rotating IMBT applicator for the treatment of rectal cancer shields the OAR more effectively with the possibility to increase the delivered dose to the tumor.
Funding Support, Disclosures, and Conflict of Interest: Funding support: Collaborative Health Research Program (Canadian Institutes of Health Research and Natural Sciences and Engineering Research Council of Canada, grant number 248480).