Data from large, retrospective study shows 5-year efficacy for SBRT in localized prostate cancer

Data from a large, retrospective, multi-center study of stereotactic body radiation therapy (SBRT) with CyberKnife technology in the treatment of men with localized prostate cancer were presented on Wednesday afternoon at the annual meeting of the American Society for Radiation Oncology (ASTRO).

According to a media release issued by ASTRO, this study (presented by Katz et al.) was designed to evaluate “the biochemical relapse-free survival (bRFS) rates over a five-year period for patients with organ-confined prostate cancer.”

Here are the core data from the study:

  • The study included 1,101 men, all treated between 2003 and 2011.
    • 59 percent of men had low-risk disease.
    • 30 percent of men had intermediate-risk disease.
    • 11 percent of men had high-risk disease.
  • Treatment was carried out at one of eight different institutions.
  • The average (median) dose of SBRT was 36.25 Gy (range, 35-40 Gy) delivered in either four or five fractions.
  • Biochemical relapse was defined as a rise > 2 ng/ml above nadir PSA level (the so-called Phoenix criteria).
  • Average (median) follow-up for all cases was 36 months (range, 1 to 66 months).
  • 49/1,101 men (4.5 percent) had a biochemical relapse post-treatment.
    • Nine of these patients had resolution of this relapse and showed no clinical signs of relapse.
  • 149/1,101 men (13.5 percent) received neoadjuvant or adjuvant androgen deprivation therapy (ADT).
    • The authors state that such ADT “appeared to make no difference in the outcome of biochemical relapse.”
  • Estimated (actuarial) rates of biochemical disease-free survival at 5 years were
    • 95 percent for low-risk patients
    • 90 percent for intermediate-risk patients
    • 80 percent for  high-risk patients (but this was based on a relatively small number of assessable patients)

There seems to be little doubt that SBRT is a viable therapeutic option for the treatment of men with low- and intermediate-risk prostate cancer, and that it comes with the distinct benefit of relatively rapid treatment (over the course of just one week) by comparison with other forms of external beam radiation therapy (including proton beam radiation therapy). What is not yet clear is whether SBRT is either necessary or curative (over a 10 year period) for many of the men whose data have been collected to present the above findings. In addition, there is no mention of the risk for or rates of adverse reactions to SBRT among these 1,101 men (although prior data suggest that the side effects of SBRT are relatively low, and no higher than those seen from other forms of external beam radiation therapy).

What we are missing here, as usual, is any good basis for comparison of the outcomes for the men in this study to the outcomes for men treated with other forms of radiation therapy (let alone surgery). Frankly, the lack of such data should, by now, be embarrassing to the radiation oncology community, and we shall have more to say on this topic. Most of these institutional data series are pointless today unless they can be reliably compared to data from other institutions.

One Response

  1. I cannot agree more with your conclusion. Several weeks ago I was given a percentage for my prostate cancer-free survival. Although it’s quite acceptable, I later asked myself what the reference class was for that value. I shall ask that and more in several weeks.

    I see two problems with such statistics. First, how can an ethical test be designed and carried out to its pre-set end-point? One trial I know of was ended quite early, when it became clear that one arm was giving consistently good results whilst the other arm was giving consistently poorer results. Maybe that’s considered a fine outcome, I don’t know. The other problem (and I might have missed the relevant articles) is that all articles I’ve seen that compare treatment modalities are retrospective. Some conditions are then uncertain or some data cannot be obtained. This seems serious to me, but if that’s the best that can be ethically done today, experiential comparisons seem acceptable if and only if the data used are sifted as carefully as possible. But still, different sets of retrospective data will probably give different statistics, so how can one choose among them?

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