Session Item

Prostate stereotactic body radiotherapy (SBRT) is associated with low levels of toxicity but randomised data comparing SBRT with standard radiotherapy schedules is lacking. Here we present late toxicity, to 2 years after treatment, for men treated in the PACE-B trial.

Men with NCCN low/lower intermediate risk prostate cancer (Gleason 4+3 excluded) were randomised (1:1) to 62Gy in 20 fractions/78Gy in 39 fractions over 4-8 weeks (collectively called SOC) or 36.25Gy in 5 fractions (concomitant boost to CTV to 40Gy) over 1-2 weeks (SBRT). Androgen deprivation therapy was prohibited. Patients were recruited from 37 centres across the UK, Ireland and Canada. After 12 weeks post-treatment, men were assessed for side effects 3-monthly for the first 2 years according to CTCAE and RTOG scales. Follow up is ongoing prior to analysis of the primary endpoint of freedom from biochemical/clinical failure. Proportion of participants with grade 2 or higher (G2+) gastrointestinal (GI) and genitourinary (GU) RTOG toxicity and CTCAEs at 2 years are compared between groups using chi-squared test. Kaplan-Meier estimates of cumulative late G2+ toxicity rate at 2 years are presented.

874 men were randomised (441 SOC; 433 SBRT); follow-up is complete to two years post treatment. 9% of participants had low risk and 91% had intermediate risk prostate cancer. Median PSA at trial entry was 8.0 ng/ml. Baseline characteristics were similar in both groups. Worst grade CTCAE GI and GU toxicities at 2 years are shown in Table 1.

The incidence of G2+ GI toxicities was low with no differences between groups at 2 years: CTCAE: SOC 3.8% v SBRT 4.0%; p=0.87; RTOG: SOC 3.1% vs SBRT 2.0%; p=0.35. Two-year cumulative incidence rates of G2+ CTCAE GI toxicity were 10.0% (95% CI: 7.5% to 13.3%, 42 events) for SOC and 12.2% (9.4% to 15.8%, 51 events) for SBRT. Two-year cumulative incidence rates of G2+ RTOG GI toxicity were 7.6% (95% CI: 5.5% to 10.6%, 32 events) for SOC and 8.4% (95% CI: 6.1% to 11.5%, 35 events) for SBRT.

At 2 years, the incidence of CTCAE G2+ GU toxicity was higher with SBRT (6.4% vs 11.1%, p=0.02); the increase in RTOG G2+ GU toxicity was not statistically significant: SOC 2.3% vs SBRT 4.6%; p=0.08.  Two-year cumulative incidence rates of G2+ CTCAE GU toxicity were 18.8% (95% CI: 15.4% to 22.8%, 79 events) for SOC and 29.1% (95% CI: 25.0% to 33.7%, 122 events) for SBRT. Two year cumulative incidence rates of G2+RTOG GU toxicity were 12.9% (95% CI: 10.0% to 16.5%, 54 events) for SOC and 20.8% (95% CI: 17.2% to 25.0%, 87 events). The most frequently reported CTCAE GU G2+ toxicity was urinary frequency which peaked at 4.5% at 9 months for SOC and at 8.3% at 15 months for SBRT.

Side effects from radiotherapy were low in both groups. With 2 years follow-up, SBRT was associated with a higher rate of late G2+ GU toxicities. G3+ toxicity was rare.