3228 - Dosimetric Impact of Intentional Proximal Seminal Vesicle Targeting In Low-Dose-Rate I-125 Prostate Brachytherapy Monotherapy

Purpose/Objective(s):

Low-dose-rate (LDR) prostate brachytherapy delivered as monotherapy remains an appropriate definitive option for selected patients with unfavorable intermediate-risk prostate cancer (UIR-PCa). However, prostate-only implantation delivers limited incidental dose beyond the prostate base, potentially resulting in undercoverage of the proximal seminal vesicles (SV-prox), a region at risk for microscopic extension. We evaluated the dosimetric impact of intentional SV-prox targeting and the incremental technical burden associated with this approach during LDR I-125 prostate brachytherapy.

Materials/Methods:

Fifty-five consecutive patients with UIR-PCa were treated with LDR I-125 prostate brachytherapy using stranded seeds (prescription dose 144 Gy), incorporating intentional coverage of the proximal 1 cm of the seminal vesicles (SV+). Post-implant dosimetry was performed using computed tomography obtained 30 days after implantation. Primary variables of interest were SV-prox dosimetric coverage and technical burden. Dosimetric variables included standard dose–volume histogram (DVH) parameters for the prostate, urethra, rectum, and SV-prox, which was contoured and analyzed as an independent target volume. Technical burden was assessed using the number of additional seeds required for SV-prox coverage beyond the prostate implant. SV-prox dosimetric parameters were compared with a contemporaneous control cohort treated with prostate-only implantation (SV-), used to quantify incidental SV-prox dose.

Results:

All SV-targeted implants were successfully delivered and evaluable on day-30 post-implant imaging, with prostate volumes, target coverage, and urethral and rectal dose constraints remaining within accepted post-implant limits and comparable between cohorts.

Intentional SV-prox targeting resulted in higher SV-prox coverage compared with prostate-only implants (median SV-prox D90: 87.6 Gy [IQR 64.8–121.4], 61% of the prescription dose, vs 35.2 Gy [IQR 29.2–44.9], 24% of the prescription dose; p < 0.001). At 6 mm, median SV-prox D90 in SV+ was approximately two-fold higher than in SV-. In the prostate-only cohort, 57% of patients received less than 50% of the prescription dose to at least half of the SV-prox volume.

SV-prox targeting required a median of 16 additional seeds (IQR 13–21), representing a ~12% increase relative to the median prostate seed count.

Conclusion:

Prostate-only LDR brachytherapy provides limited incidental dose to the proximal seminal vesicles, resulting in suboptimal SV-prox coverage. Intentional SV-prox targeting produces a substantial and consistent increase in SV-prox coverage while maintaining acceptable prostate dosimetry and a modest technical burden, supporting its feasibility as a refinement of LDR monotherapy in appropriately selected patients.