5-year SBRT trial: high cancer control, low toxicity

(9/10/2018)
Meier et al reported the results of a 5-year multi-institutional trial, (also reported at the 2017 ASTRO meeting), finding that SBRT had high rates cancer control and low toxicity.

This was a prospective clinical trial in which all 21 institutions treated 309 patients according to the same protocol. The institutions were community, regional and academic hospitals across the US. All patients were low (56%) or intermediate risk (44%). Of the 137 intermediate risk patients,  61% were favorable and 39% were unfavorable intermediate risk. The treatment was:

• 40 Gy in 5 treatments to the prostate
• 36.25 Gy to the seminal vesicles in intermediate risk patients
• No concurrent or adjuvant androgen deprivation therapy was allowed.

At five years after SBRT treatment, the following oncological outcomes were reported:

• 97.1% had no biochemical progression; that is, no increases in PSA to over 2 ng/ml from the lowest value achieved 
  o 97.3% for low risk patients, compared to 92.3% for IMRT historically
  o 97.1% for intermediate risk patients, compared to 91.3% for IMRT historically
       - 100% among favorable intermediate risk
       - 93.1% among unfavorable intermediate risk

By five years after SBRT treatment, the late toxicity outcomes were reported:

• No grade 3 (serious) rectal side effects
• Grade 2 rectal side effects in 2%

• Grade 3 (serious) urinary side effects in 4 of the 309 patients (1.3%)
• Grade 2 urinary side effects in 12%

These are certainly excellent outcomes, and are in-line with or better than retrospective SBRT studies that have previously been reported. So far, the longest running SBRT single institution study has been reported by Alan Katz (see this link). I’ve heard that a ten-year update is in the works. That will be as long and larger than the longest running IMRT trial.

SBRT is about half the cost of IMRT, and at only 5 treatments, is certainly a lot less bother for the patients. It has excellent outcomes even without adjuvant ADT in unfavorable intermediate risk patients. With large long-term studies now available, it is difficult to understand why some insurance companies still don’t cover it.