Purpose: To evaluate the impact of distant shields on dose distributions for a recently developed delivery system (AIM-Brachy) that can enable intensity modulated brachytherapy (IMBT) for prostate cancer. The system simultaneously rotates up to 20 platinum shields, which dynamically direct the radiation emitted from a custom-designed Yb-169 source to spare organs at risk.
Methods: The IMBT plans were generated for 12 prostate cases using an in-house column generation-based optimizer coupled to a Geant4-based dose calculation engine, RapidBrachyMC. These plans were calculated by using pre-generated dose distributions which only consider attenuation from the local shield. Dose distributions were then recalculated by placing the distant shields in the patient geometry in addition to the local shield. Distant shields were placed with the same orientation as the local shield for each dwell position/shield angle combination, which is a feature of the system design. Simulations times were reported for an 18 Ci Yb-169 source.
Results: The presence of the distant shields reduced the PTV D90, urethra D10, bladder D2cc and rectum D2cc on average by 7.5%, 6.5%, 11.0% and 11.9%, respectively. Median local dose differences within the PTV, urethra, bladder and rectum were 19.5%, 18.8%, 38.3% and 32.9%, respectively. Dose differences were as high as ~30% in voxels adjacent to shields. Local dose differences were generally larger outside the PTV than inside the PTV. The delivery times required to achieve the same PTV D90 were 18.6Â±4.0 min and 20.0Â±4.3 min with intershield attenuation turned off and on, respectively.
Conclusion: The intershield attenuation can have a significant impact on calculated dose distributions and should be taken into account for accurate IMBT treatment planning in cases where multiple high-Z shields are introduced in the patient geometry.
Funding Support, Disclosures, and Conflict of Interest: This work was supported by the Collaborative Health Research Projects (grant number 523394-18) and Natural Sciences and Engineering Research Council of Canada (grant number 241018). G.F. acknowledges support by the Natural Sciences and Engineering Research Council of Canada.