Clinical outcomes of CyberKnife stereotactic radiotherapy for localized prostate cancer: comparison of 35 Gy in 5 fractions and 36 Gy in 4 fractions

This retrospective study evaluated the clinical outcomes of two CyberKnife (CK) stereotactic body radiation therapy (SBRT) regimens in patients with localized prostate cancer (LPC). A total of 249 patients were treated, with 123 receiving 35 Gy in 5 fractions and 126 receiving 36 Gy in 4 fractions, with a median follow-up of 50 months. The primary endpoint was the incidence of grade ≥2 genitourinary (GU) and gastrointestinal (GI) toxicities, assessed using CTCAE Version 5.0. Secondary endpoints included 3-year biochemical recurrence-free survival (BCRFS), locoregional recurrence-free survival (LRRFS) and distant metastasis-free survival (DMFS). To address baseline imbalances between groups, inverse probability of treatment weighting (IPTW) was applied. Acute grade 2 GU toxicities occurred in 19 patients (7.6%) and chronic grade 2 in 15 patients (6.0%). Acute and chronic grade 2 GI toxicities were rare, occurring in 3 (1.2%) and 2 (0.8%) patients, respectively. There were no significant differences in cumulative GU or GI toxicity rates between fractionation regimens either before or after IPTW adjustment. After IPTW, the 3-year BCRFS was 96.5% in the 5-fraction group and 96.8% in the 4-fraction groups (P = 0.571). The 3-year LRRFS rates were similarly favorable (99.2% vs. 100%). The 3-year DMFS was 100% in the 5-fraction group and 97.5% in the 4-fraction group (P = 0.058). Overall, CK-SBRT using 36 Gy in 4 fractions was well tolerated, with low rates of treatment-related toxicity and no statistically significant differences in oncologic outcomes between regimens. Longer follow-up is required to determine the long-term efficacy of this dose-escalation strategy.