Eye of the beholder.

Top Line: How does late toxicity from prostate SBRT compare to fractionated IMRT?

The Study: PACE-B is one of the large randomized trials comparing ultra-hypofractionated prostate radiation (SBRT) and conventionally or moderately hypofractionated radiation (IMRT). 874 men with low and intermediate risk (3+4 only) prostate cancer were randomized to receive SBRT (36.25Gy in 5 fractions) or IMRT (78Gy in 39 fractions or 62Gy in 20 fractions). It’s important to read the planning details for PACE-B, though, because the prescription wasn’t “just” 36.25Gy in 5–the CTV received a simultaneous integrated boost of 40Gy. ADT was not allowed. A few years back, we learned that the rate of RTOG grade 2+ toxicity at 12 weeks was similar with SBRT (23%) and IMRT (27%). Here we have 2 year toxicity outcomes. The short version is that there were no differences in the primary outcomes of RTOG grade 2+ GU (3% SBRT v 2% IMRT) and GI (2% SBRT v 3% IMRT) toxicity at 2 years. The long version is more complicated. There was a wide variety of toxicity outcomes measured on the trial–too many to unpack in a single QuadShot. The overall takeaway is that several of the clinician-reported GU toxicity outcomes, although mild, were worse with SBRT. For example, CTCAE grade 2+ GU toxicity was higher with SBRT (12% v 7%) as was cumulative RTOG grade 2+ GU (18.3% v 10.6%) and cumulative CTCAE grade 2+ GU (32.3% v 19.8%). Another interesting finding was that some measures of GU toxicity were significantly lower with CyberKnife-based SBRT compared to LINAC-based SBRT, which may have been driven by the fact that most CK centers were high-volume centers with considerable SBRT expertise. Were these differences clinically relevant? Overall, there were no major differences in patient-reported toxicity outcomes. Bladder QOL was slightly worse after SBRT while bowel QOL was slightly better. Still to come are the disease control outcomes.

TBL: Differences in grade 2+ toxicity 2 years after SBRT and IMRT for prostate cancer are likely small. GU toxicity is slightly higher with SBRT, depending on the scale, but there is no detectable increase in GI toxicity. | Tree, Lancet Oncol 2022