A Multicenter Phase 2 Study of Ultrahypofractionated Stereotactic Boost After External Beam Radiotherapy in Intermediate-risk Prostate Carcinoma: A Very Long-term Analysis of the CKNO-PRO Trial

BACKGROUND: Genitourinary (GU) or gastrointestinal (GI) complications and tumor relapse can occur in the long term after radiotherapy for prostate cancer. OBJECTIVE: To assess the late tolerance and relapse-free survival (RFS) in patients undergoing hypofractionated stereotactic boost therapy after external beam radiotherapy (EBRT) for intermediate-risk prostate cancer. DESIGN, SETTING, AND PARTICIPANTS: Seventy-six patients with intermediate-risk prostate carcinoma between August 2010 and April 2013 were included. The first course delivered a dose of 46 Gy by conventional fractionation; the second course was a boost of 18 Gy (3 × 6 Gy) within 10 d. OUTCOME MEASUREMENTS AND STATISTICAL ANALYSIS: GU and GI toxicities were evaluated as the primary outcomes. The secondary outcomes were overall survival and RFS. The cumulative incidence of toxicity was calculated using a competing-risk approach. Overall survival and RFS were estimated using the Kaplan-Meier method. RESULTS AND LIMITATIONS: The median follow-up period was 88 mo (range, 81–99 mo). Sixty (79%) patients were treated with the CyberKnife and 16 (21%) using a linear accelerator. The cumulative incidences of GU and GI grade ≥2 toxicities at 120 mo were 1.4% (95% confidence interval [CI]: 0.1–6.6%) and 11.0% (95% CI: 5.1–19.4%), respectively. The overall survival and RFS rates at 8 yr were 89.1% (95% CI: 77–95%) and 76.9% (95% CI: 63.1–86.1), respectively. CONCLUSIONS: A very long follow-up showed low GU and GI toxicities after a hypofractionated stereotactic boost after EBRT for intermediate-risk prostate cancer. Dose escalation of the boost delivered by hypofractionated radiation therapy appears safe for use in future trials. PATIENT SUMMARY: We found low toxicity and good survival rates after a short and high-precision boost after external beam radiotherapy for intermediate-risk prostate cancer, with a long-term follow-up of 88 mo. This long-term treatment is safe and should be considered in future trials.