Phase 2 study of simultaneous focal boost with stereotactic radiotherapy for localized intermediate to high-risk prostate cancer: 2-year outcomes

Stereotactic body radiotherapy (SBRT with focal boost to the dominant intraprostatic lesion (DIL) might enhance outcomes in high-risk prostate cancer while minimizing toxicity. The phase 2 SPARC trial, at the Royal Marsden NHS Foundation Trust (London, UK) evaluated this strategy in patients with newly diagnosed, unfavorable intermediate- to high-risk localized prostate cancer (Gleason ≥ 4+3, and/or MRI-defined T3a N0, and/or PSA ≥ 20) with up to two MRI-defined DILs. Participants received 36.25 Gy in 5 fractions with a simultaneous boost up to 47.5 Gy delivered using CyberKnife. All participants receive androgen deprivation therapy. Between 2013 and 2023, 20 participants were enrolled, with a median follow-up of 54 months. At 2 years, the cumulative incidence of RTOG-assessed grade 2 or greater GU and GI toxicity was 35% and 5%, respectively. There was no clinically significant worsening of EQ5D-5L assessed QOL, IPSS score, and urinary QOL scores at 2 years compared to baseline. There was a reduction in IIEF-5 scores for sexual function at 2 years from baseline. There is one case of biochemical relapse reported to date. The authors conclude that CyberKnife-based SBRT to the prostate with a simultaneous boost was well tolerated at 2 years.