PO53

Abstract

Purpose Permanent prostate brachytherapy with seeds represent a standard of care procedure for low to intermediate risk prostate cancer. It is known to provide high cure rates and disease-free survival with tolerable toxicity. One disadvantage is that the implant arrangement cannot be altered after implantation. However, it is known that seed-displacements against their implant location may occur during the treatment course. The scope of this work was to perform a comparative analysis of seed-displacements within the prostate until day 1 and day 30 after implantation. This aimed to assess geometric and dosimetric implant variations and to identify possibilities for corresponding stability enhancements. Materials and Methods Seed-displacements between intraoperative transrectal ultrasound (TRUS) (day 0 of brachytherapy), quality assurance computed tomography (CT) (day 1), and post-plan CT (day 30) were analyzed for 21 consecutive patients. The implant arrangement observed at day 1 and 30 was registered to the day 0 and day 1 implant, and a corresponding 1:1 seed assignment was performed using the Hungarian method. These procedures were done on a pure seed-only level, i.e. without resorting to patient anatomy. Seed-displacements were evaluated depending on strand-length and implant location within the prostate. Corresponding dosimetric effects were assessed. Correlations of implant variations with patient-specific factors as prostate volume (change), dosimetric effects, as well as number of used needles and seeds were evaluated. Results Seed-displacements in the immediate post-implant phase until day 1 of brachytherapy (median displacements: 3.9±3.4 mm) were stronger than in the time to post-plan CT (2.3±2.6 mm). Implant variations occurred enhanced along the cranial-caudal direction, i.e. along the implantation direction. Seeds in base and apex tended to move towards the prostate midzone in both examined time periods. No dependency of seed-displacements on seed strand-length was observed until day 30, but strands containing one (7.0±4.5 mm) or two (8.0±5.7 mm) seeds showed larger positional deviations than strands of higher lengths (up to 4.2±7.0 mm) from day 0 to day 1. D90 (dose that 90% of prostate receives) was with variations of 2±17 Gy more stable from day 1 to 30 than in the immediate post-implant phase (-18±10 Gy). Seed-displacements were correlated with both dosimetric variations as well as prostate volume changes and the number of implanted seeds and needles. Conclusions Seed-displacements were stronger in the immediate post-implant phase than from day 1 to 30. Based on our observations, this may result from uncertainties in the gold-standard TRUS-guided implantation process. Our findings suggest a high importance of achieving a dose coverage close to 100% during intra-operative treatment planning, to ensure sufficient prostate dose coverage even after corresponding coverage declines originating from edema or systematic uncertainties. Implantations in base and apex, the number of implanted seeds and needles, and the usage of single- and double-strands should be reduced where applicable. Furthermore, we are currently implementing an adaptive implantation workflow based on co-registered intraoperative TRUS and mobile CBCT imaging. While TRUS enables accurate contouring, CBCT serves for exact seed identification at multiple time points during implantation. This helps to adapt treatment planning to the location of already implanted seeds, aiming to ensure improved prostate dose coverage starting from day 1 of brachytherapy. Permanent prostate brachytherapy with seeds represent a standard of care procedure for low to intermediate risk prostate cancer. It is known to provide high cure rates and disease-free survival with tolerable toxicity. One disadvantage is that the implant arrangement cannot be altered after implantation. However, it is known that seed-displacements against their implant location may occur during the treatment course. The scope of this work was to perform a comparative analysis of seed-displacements within the prostate until day 1 and day 30 after implantation. This aimed to assess geometric and dosimetric implant variations and to identify possibilities for corresponding stability enhancements. Seed-displacements between intraoperative transrectal ultrasound (TRUS) (day 0 of brachytherapy), quality assurance computed tomography (CT) (day 1), and post-plan CT (day 30) were analyzed for 21 consecutive patients. The implant arrangement observed at day 1 and 30 was registered to the day 0 and day 1 implant, and a corresponding 1:1 seed assignment was performed using the Hungarian method. These procedures were done on a pure seed-only level, i.e. without resorting to patient anatomy. Seed-displacements were evaluated depending on strand-length and implant location within the prostate. Corresponding dosimetric effects were assessed. Correlations of implant variations with patient-specific factors as prostate volume (change), dosimetric effects, as well as number of used needles and seeds were evaluated. Seed-displacements in the immediate post-implant phase until day 1 of brachytherapy (median displacements: 3.9±3.4 mm) were stronger than in the time to post-plan CT (2.3±2.6 mm). Implant variations occurred enhanced along the cranial-caudal direction, i.e. along the implantation direction. Seeds in base and apex tended to move towards the prostate midzone in both examined time periods. No dependency of seed-displacements on seed strand-length was observed until day 30, but strands containing one (7.0±4.5 mm) or two (8.0±5.7 mm) seeds showed larger positional deviations than strands of higher lengths (up to 4.2±7.0 mm) from day 0 to day 1. D90 (dose that 90% of prostate receives) was with variations of 2±17 Gy more stable from day 1 to 30 than in the immediate post-implant phase (-18±10 Gy). Seed-displacements were correlated with both dosimetric variations as well as prostate volume changes and the number of implanted seeds and needles. Seed-displacements were stronger in the immediate post-implant phase than from day 1 to 30. Based on our observations, this may result from uncertainties in the gold-standard TRUS-guided implantation process. Our findings suggest a high importance of achieving a dose coverage close to 100% during intra-operative treatment planning, to ensure sufficient prostate dose coverage even after corresponding coverage declines originating from edema or systematic uncertainties. Implantations in base and apex, the number of implanted seeds and needles, and the usage of single- and double-strands should be reduced where applicable. Furthermore, we are currently implementing an adaptive implantation workflow based on co-registered intraoperative TRUS and mobile CBCT imaging. While TRUS enables accurate contouring, CBCT serves for exact seed identification at multiple time points during implantation. This helps to adapt treatment planning to the location of already implanted seeds, aiming to ensure improved prostate dose coverage starting from day 1 of brachytherapy.