Seed distribution stability in permanent breast seed implant brachytherapy

PURPOSES In this study we aim to quantitatively evaluate the stability of implanted seeds in permanent breast seed implant (PBSI) brachytherapy and assess any impact on treatment quality. METHODS AND MATERIALS Sixty-seven consecutive patients who received PBSI treatment at BC Cancer Kelowna from 2013 to 2021 with post-implant CT images available were included in this study. For each patient, two sets of post-implant CT scans were retrospectively analyzed: Day0, obtained immediately after implant, and Day30, obtained approximately one month following implant. Seed distributions were quantified using the 90% isodose contour, outlier seed maximum spread, and number of seeds located in the seroma as well as seroma quadrants. These were then compared between Day0 and Day30. Post-implant dosimetry of target volumes as well as critical structures were compared. RESULTS The 90% isodose volume was found to decrease over time. All seeds remained in the breast region however the maximum spread of seeds increased in all directions from Day0 to Day30. All recorded target volume dosimetric parameters were, on average, lower on Day30 compared to Day0 but mean dosimetry levels still met clinical goals. Dose in critical structures was overall similar. CONCLUSIONS In this study, we quantitatively described the changes in seed distributions as well as dosimetry from Day0 to Day30 post PBSI procedure. We addressed concerns related to seed stability in breast tissue and provided clinical evidence on dosimetric efficacy of the PBSI technique.