KEY POINTS
- The analysis used prospectively collected data from 294 patients treated on an institutional protocol with prostate SABR at 45 Gy in 5 consecutive fractions without ADT. Treatment incorporated MRI-based target definition, intrafraction tracking, urethral-sparing dose painting, and a 2-mm PTV margin.
- After a median follow-up of 83.9 months, 39 biochemical recurrences occurred. Seven-year biochemical recurrence-free survival was 100.0% for low-risk, 96.2% for favorable intermediate-risk, 83.4% for unfavorable intermediate-risk, and 73.7% for high-risk disease.
- The earliest model combined ISUP grade group ≥3 with ΔPSA-D from baseline to 3 months >0.138 and separated seven-year bRFS at 96.7% versus 62.7% (p<0.001). Both ISUP ≥3 (HR 2.17, 95% CI 1.12–4.20) and the adverse PSA-D term (HR 4.09, 95% CI 2.12–7.89) remained independently associated with recurrence.
- The 3–6-month model combined ISUP ≥3 with ΔPSA-D >0.033 and separated seven-year bRFS at 93.5% versus 53.4% (p<0.001). The PSA-D kinetic term remained independently prognostic (HR 3.41, 95% CI 1.69–6.85; p<0.001) alongside ISUP ≥3 (HR 2.26, 95% CI 1.17–4.40; p=0.016).
- A three-feature score combining ISUP ≥3 with both early PSA-D intervals produced biochemical relapse rates of 5.8%, 21.8%, 47.8%, and 66.7% for scores 0–3 and had the strongest fixed-time discrimination (AUC 0.772).
- Static PSA-D measurements were also prognostic: at 3 months, median PSA-D was 0.0667 versus 0.0427 ng/mL/cm³ in patients who ultimately relapsed versus remained controlled (p=0.00039). However, the authors found dynamic kinetic models more informative because they incorporate treatment-response velocity.
CLINICAL TAKEAWAY
Early PSA-density kinetics may identify unfavorable response phenotypes within the first 3–6 months after prostate SABR, long before conventional PSA nadir metrics mature. The concept is clinically interesting, but the thresholds were derived within the same cohort, no formal calibration or optimism correction was performed, and external validation is required before using them to intensify surveillance or treatment.