Background: External beam radiotherapy (EBRT) is a curative treatment for LPCa. Large randomised controlled trials (RCTs) have shown moderately hypofractionated regimens (2.5–3 Gy/fraction(f)) as non-inferior to conventionally fractionated regimens (2 Gy/f). PACE-B aims to demonstrate non-inferiority of SBRT compared to CFMHRT for biochemical or clinical failure. Compared to CFMHRT, SBRT reduces patient (pt) attendances but compressed overall treatment time may influence acute toxicity severity. Methods: PACE is a phase III open-label multiple-cohort RCT. Men with LPCa, stage T1-T2, ≤ Gleason 3 + 4, PSA ≤ 20 ng/mL, unsuitable for surgery or preferring EBRT, were eligible for the PACE-B cohort. Between 08/12-01/18, 874 pts (38 centres) were randomised (1:1) to SBRT or CFMHRT. SBRT dose was 36.25 Gy/5f in 1-2 weeks (wks), CFMHRT as 78 Gy/39f over 7.5 wks, or 62 Gy/20f in 4 wks. Androgen deprivation therapy was not permitted. Clinician reported acute toxicity was assessed at baseline, 2-weekly during CFMHRT and at 2, 4, 8 & 12 wks post-treatment. Key toxicity outcomes were worst grade 2+ Radiation Therapy Oncology Group (RTOG) genitourinary (GU) and gastrointestinal (GI) acute toxicities, compared by Chi-square test with alpha 0.05 divided between the two measures. Results: By per protocol analysis n=430 received CFMHRT, n=414 received SBRT. Key characteristics seen in the CFMHRT and SBRT groups respectively were: mean age: 69.5 vs 69.3 years; T-stage ≥T2b: 51.8% vs 56.6%; Gleason Score 3+4: 80.2% vs 85.0%; PSA 10-20 ng/mL: 30.9% vs 31.6%. RTOG G2+ toxicity was not significantly different for GI events (CMFHRT 52/430 (12.1%) vs SBRT 42/414 (10.1%), p=0.368), nor GU events (CFMHRT 117/430 (27.2%) vs SBRT 96/414 (23.2%), p=0.179). Conclusions: Despite an accelerated treatment schedule, RTOG assessments show similar rates of acute GI and GU toxicity for SBRT and CFHFRT. Pt follow-up in PACE-B continues and results of late toxicity and biochemical/clinical failure are awaited. Clinical trial information: NCT01584258