KEY POINTS
- This retrospective dosimetric substudy included 286 consecutive patients with localized prostate cancer treated with HDR brachytherapy monotherapy between 2005 and 2013. The cohort was evenly divided between intermediate-risk (50.6%) and high-risk (49.4%) disease, with a median PTV of 74.8 cc.
- Patients received 19-20 Gy × 1, 13 Gy × 2, or 10.5 Gy × 3 using a single implant. The respective groups contained 49, 135, and 102 patients. Median EQD2 D90 was 122.7 Gy, median V100 95.1%, V150 30.5%, and DHI 69.0%.
- At a median follow-up of 108 months, 10-year biochemical relapse-free interval was 72.8%. Patients below versus above the median EQD2 D90 had 10-year bRFI rates of 73.5% versus 71.2%, respectively, with no significant difference (p=0.52).
- Higher EQD2 D90 was not independently associated with biochemical control. Per 10-Gy increase, the adjusted hazard ratio for biochemical relapse was 1.12 (95% CI 0.84-1.48, p=0.44). The adjusted hazard ratio for overall survival was 0.87 (95% CI 0.67-1.12, p=0.27).
- No significant relationship between EQD2 D90 and biochemical control was found within the 1-, 2-, or 3-fraction schedules individually. Higher D90 was associated with better overall survival only in the 2-fraction subgroup, an exploratory finding not reproduced in the other schedules.
- V100, V150, and dose homogeneity index were also unrelated to biochemical control or overall survival. Greater applicator use was associated with higher D90 and V100, while larger prostate volumes were associated with lower V150 and greater dose homogeneity.
- The absence of a D90 dose-response should not be interpreted as evidence that fractionation does not matter. The study used sequential, nonrandomized treatment cohorts, had a relatively narrow range of already-high delivered doses, relied on simplified EQD2 modeling, lacked dominant-lesion dosimetry, and did not assess treatment toxicity.
CLINICAL TAKEAWAY
Once prostate HDR monotherapy achieved consistently high target coverage, further incremental increases in D90 were not associated with better long-term biochemical control. The result supports prioritizing robust implant quality rather than pursuing D90 escalation for its own sake, but it does not establish equivalence between fractionation schedules or support single-fraction HDR monotherapy.