A small, multi-center cohort study of SBRT in treatment of low-risk prostate cancer

Published reports on the safety and efficacy of extreme hypofactionated radiation therapy (stereotactic body radiation therapy or SBRT, a.k.a. CyberKnife radiation therapy) have, to date, come from just a couple of key centers.

In a new report, four different centers report as a group on their initial experience of SBRT using the CyberKnife technology with a median follow-up of just under 4 years. This is an important paper because it provides multi-institutional, multi-author support for the general principle that SBRT shows strong indications of early efficacy and reasonable safety in the treatment of low-risk prostate cancer, although The “New” Prostate Cancer InfoLink still believes we need 10-year outcome data to give a real blessing to this type of radiation therapy.

McBride et al. enrolled men with low-risk adenocarcinoma of the prostate into a Phase I, multi-institutional trial of SBRT. wnere evaluated for biochemical progression-free survival (bPFS), PSA “bounce,” radiation toxicities, and health-related quality of life factors (using the Sexual Health Inventory for Men [SHIM], American Urological Association [AUA], and Expanded Prostate Cancer Index Composite [EPIC] questionnaires)

The key findings from theis study (to date) are given below:

  • 34/45 patients (75.5 percent) received 7.5 Gy delivered in five fractions (37.5 Gy in total).
  • 9/45 patients (20.0 percent) received 7.25 Gy delivered in five fractions (36.25 Gy in total).
  • 2/45 patients (4.5 percent) received other regimens.
  • Average (median) follow-up for surviving patients was 44.5 months (range, 0 to 62 months).
  • The rate of bPFS was 97.7 percent at 3 years.
  • The patients’ median PSA level declined from 4.9 ng/ml at diagnosis to 0.2 ng/ml at last follow-up.
  • The median decrease in PSA level at 12 months post-treatment was 80 percent.
  • With respect to PSA “bounces”
    • Nine men had at least one PSA “bounce” ≥0.4 ng/ml.
    • Four men had two PSA bounces.
    • The median time to first PSA bounce was 11.6 months (range, 7.2 to 18.2 months).
    • The size of the average (mean) PSA bounce was 1.07 ng/ml.
  • With respect to side effects and complications of treatment
    • One patient (2 percent) had a late grade 3 urinary obstruction.
    • Two patients (4 percent) had late grade 3 proctitis.
    • There was a significant late decline in SHIM and EPIC sexual scores.
    • There was a small, late decline in the EPIC bowel domain score.

The authors conclude that, “In a select population, extreme hypofractionation with stereotactic radiosurgery was safe and effective for the treatment of low-risk prostate adenocarcinoma.”

The abstract of this paper does not specify this, but we can probably assume with safety that no patient actually died of prostate cancer during this study.

This appears to be a carefully conducted, mult-center study in which the authors set out to monitor all patients for consequences of treatment. There are clearly side effects associated with the use of SBRT by physicians with limited experience of this form of radiation therapy. That hardly comes as a surprise. It would have been helpful to be able to know the frequency of grade 2 toxicities, and this information may be available within the full text of the report.

Perhaps the one worrying thing about this report is the “significant late decline in … sexual scores.” In other words, at 12+ months after treatment there appears to have been a meaningful loss of sexual functionality. Could this be associated with early physician experience with this type of radiation treatment? Is it possible that there are other reasons for this finding? Or is there a degree of inevitability to some loss of sexual function associated with CyberKnife radiation therapy? We are going to need more data before the answer to this question becomes evident.

2 Responses

  1. These are very, very, very disappointing results.

    For 50% of men who are young (let’s say < 65 years) and have no other health problems, current radical therapies equate to spending somewhere between half and two-thirds of their adult lives with measurably diminished or no capacity for sexual function. And these outcomes of treatment are far, far, far from definitively linked to the seriousness of their prostate cancer itself. The problem here is treatment methods.

    As this long-needed conversation on risk vs. benefit for indolent and low-risk disease management gears up, it's imperative that the conversation be framed in blatantly explicit terms about the comparable reality of risks to their longevity associated with stage of disease and risks to their quality of life associated with modes of treatment.

    It appears to me that not only do patients/consumers need to be educated on these issues, but many, many clinicians and advocates do too.

    "First do no harm." "First do no harm." "First do no harm." … Hhhhmmm, anybody not clear on this concept?

  2. So let’s think about this. We have an unreliable indicator of cancer cure (PSA) in a group of men who don’t need to be treated at this time, with a follow-up that is essentially unimportant (4 years) with a treatment that has no long-term survival data. Wow, that seems wonderful, let’s treat everyone with this. Oh, by the way, I wonder if there is any chance that the risk of pelvic cancer will be higher with this fractionation than it is with 70 Gy?

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