Analysis of acute toxicity from PACE-B: Comparing SBRT to CFMHRT for localised prostate cancer

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Published: 15 Feb 2019
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Dr Nicholas van As - The Royal Marsden NHS Foundation Trust, London, UK

Dr Nicholas van As speaks to ecancer at the 2019 ASCO Genitourinary Cancers Symposium about analysis of acute toxicity from the randomised phase III PACE-B trial comparing tereotactic body radiotherapy (SBRT) to conventionally fractionated or moderately hypofractionated external beam radiotherapy (CFMHRT) for localised prostate cancer.

He explains that the main difference between SBRT and CFMHRT is the dose as using modern techniques you can deliver larger doses more accurately and safely while increasing the effectiveness.

Dr van As reveals that SBRT, which was the lower dose of the two, had lower toxicity than expected and that importantly there was no statistical difference between toxicity of the two.

PACE is actually part of an umbrella of three studies: PACE-A, which is a randomised study comparing stereotactic radiotherapy and radical prostatectomy; PACE-B, which is the study we presented, is a randomised study comparing SBRT with conventional or moderately hyperfractionated radiotherapy; and PACE-C is a study still to be launched which is very similar to PACE-B but in a higher risk group population, the difference being that all the patients receive androgen deprivation in addition to radiotherapy which they didn’t in PACE-B. The main aims of PACE-B were to determine whether SBRT, or profoundly hyperfractionated radiotherapy, is non-inferior to conventional or moderately hyperfractionated radiotherapy.

We’re quite a way off knowing the end outcomes of the study but we had planned to do an analysis of acute toxicity when we had got all the patients past three months of follow-up and that’s the data that we presented yesterday.

What are the main differences between SBRT and CFMHRT?

Essentially it’s the dose. Historically in radiotherapy we’ve always used small doses per day over a protracted period of time but we’ve got good data in prostate cancer that using a moderately hyperfractionated, so a slightly larger dose going from 2Gy to 3Gy, was not inferior and that was the result of the CHHiP study.  So SBRT is essentially pushing that envelope and going to a much larger dose – instead of 3Gy per day we’re using 7.5Gy per day. The concept is that with modern techniques with new radiotherapy machines you can deliver these very large doses more accurately and therefore safely, the concept being that these large doses will be more effective than the smaller doses.

How was this study carried out?

What we did is we measured toxicity at baseline using both the RTOG scale and the CTCAE scale. The study was actually powered on RTOG and that’s what we presented yesterday.

All the toxicity was measured at baseline then for patients having the conventional fractionation it was measured two weekly whilst on treatment and for SBRT at the end of radiotherapy and then at two weeks, four weeks, eight weeks and twelve weeks following treatment for all patients. What we showed is that in fact the toxicity throughout the study was lower than we expected. So we expected toxicity rates within the region of about 30% for GI and about 45% for GU and that’s based on literature and also the previously conducted CHHiP study. But actually we found significantly lower results in both the control arm and the experimental arm. Very importantly we found no statistical difference between the conventionally fractionated radiotherapy and the stereotactic radiotherapy.

We think it’s important because we were concerned that with these large doses per day we may see acute toxicity being worse but that has not been borne out. If anything it looks slightly better but that wasn’t statistically significant, but it definitely wasn’t worse.

Has this study led to any recommendations?

It’s too early to make recommendations because we need to wait for the end outcomes of the study. I don’t feel that people should take our results and say we now should call this a standard of care treatment. It’s encouraging, it supports the data that’s out there already, but until we get the results in terms of the cancer outcomes and the late toxicity this should still be considered an option but not a standard of care treatment.

When will we see the results and these outcomes?

It’s not possible for me to say exactly because it’s event rate driven and I don’t get to see that data, the Independent Data Monitoring Committee do, but I would estimate in probably two and a half to three years.