Dosimetric Analysis of Single-Fraction HDR Brachytherapy Combined with Ultra-Hypofractionated Elective Pelvic RT for Unfavorable and High-Risk Prostate Cancer
Abstract
Purpose
To evaluate dosimetric characteristics and biological dose delivered via abbreviated regimen combining single-fraction HDR brachytherapy (15 Gy) with ultra-hypofractionated elective pelvic radiation therapy (ePUHRT, 25 Gy in 5 fractions) for unfavorable intermediate and high-risk prostate cancer patients.
Methods
Patients with unfavorable intermediate to very high-risk prostate cancer enrolled in an IRB-approved phase II trial underwent real-time transperineal ultrasound-guided HDR brachytherapy with hyaluronic acid rectal spacer placement, followed by fiducial marker implantation and highly conformal VMAT-based ePUHRT delivered within 2–6 weeks. Biological dose (EQD2) was calculated using α/β ratios of 1.5 Gy for prostate and 3 Gy for organs at risk (OAR). Dosimetric parameters were evaluated for protocol constraint compliance.
Results
Ten high-risk prostate cancer patients (mean prostate volume 41.5 ± 9.8 cc) completed treatment between March and November 2025. HDR brachytherapy achieved excellent target coverage and organ sparing, with mean prostate V100 of 95.9 ± 0.8%, rectum V75 of 0.44 ± 0.32 cc, and urethra D10 of 112.1 ± 1.9%. All patients met key prostate and rectal constraints, and 90% achieved all protocol criteria. ePUHRT plans demonstrated robust coverage (mean PTV 858.0 ± 120.2 cc; V100 98.2 ± 1.2%; D99 99.5 ± 0.7%) with substantial OAR sparing. Mean rectum, bladder, and bowel V25 values were 3.1%, 5.8%, and 2.6%, respectively, corresponding to 61–83% below protocol limits. Composite biological dose analysis confirmed a total prostate EQD2 of 117.1 Gy₂ (HDR 70.7 Gy₂ + ePUHRT 46.4 Gy₂), comparable to ASCENDE-RT, delivered over a median of 44 days with 100% treatment completion.
Conclusion
This abbreviated regimen with an HA rectal spacer and fiducial-based motion management achieves biological dose escalation comparable to conventional 6-week therapy, with excellent dosimetric feasibility and organ-at-risk sparing. The consistent achievement of stringent protocol constraints with substantial safety margins supports continued clinical investigation, with outcome and toxicity analyses ongoing.