JOSEPHINE KANG, MD, PhD
Advances in radiation therapy have enabled physicians to deliver high doses of radiation treatment with extreme precision, shortening treatment duration and reducing exposure to normal organs. For men with localized prostate cancer that hasn’t spread outside the prostate gland, this has led to radiation treatments that can be completed within just five treatments.
Initially this approach was developed on the Cyberknife Radiosurgery Platform. Cyberknife radiosurgery doesn’t actually refer to a knife or traditional “surgery,” but rather a specific type of machine that delivers radiation. Now, this five-treatment radiation for prostate cancer can be delivered using many different types of radiation machines, and as a result has undergone a bit of a name change. We now refer to this treatment as “stereotactic body radiation” or SBRT for short.
There are multiple reasons to select SBRT as treatment for prostate cancer. First, it only requires a total of five treatments over the span of one to two weeks, in contrast to standard external beam radiotherapy, which requires nine weeks of daily treatments. Additionally, prostate cancer success rates from SBRT appear comparable to other treatment modalities based on monitoring for up to nine years post-treatment. These outcomes were recently reported at the 2016 American Society of Clinical Oncology (ASCO) Genitourinary meeting.
This retrospective study reflects the longest follow up monitoring to date and demonstrates local control of the cancer, with nine-year freedom from PSA failure (rise of 0.2 ng/ml above nadir) of 95% for low-risk patients, 89% for intermediate-risk patients, and 66% for high-risk patients (determined based on National Comprehensive Cancer Network risk criteria). In the study, toxicity from radiation was low, and the dose utilized was 7.0 – 7.25 Gy per fraction. In prostate SBRT, appropriate treatment dosing is critical as higher doses have been linked with unacceptably high rates of toxicity. At the level used in the study, patients reported some bowel and urinary side effects that lasted less than one year. Overall, this study suggests that increased radiation doses and additional hormonal therapy did not improve outcomes; however, prospective studies are ongoing.
There is now enough data regarding using SBRT for prostate cancer treatment that it is an accepted treatment regimen by the American Society of Radiation Oncology (ASTRO), and the National Comprehensive Cancer Network (NCCN) guidelines. However, these guidelines explicitly state that prostate SBRT should, when elected, be performed at a center with high-volume and expertise. The physicians at Weill Cornell Medicine and NewYork-Presbyterian are very experienced in delivering SBRT for localized prostate cancer and have published many articles on this approach and when it should be used.
We will soon be opening a randomized study looking at prostate SBRT in conjunction with rectal spacer versus endorectal balloon in an ongoing effort to refine our treatments. The rectal spacer is a gel, placed between the prostate and rectum. By displacing the rectum from the prostate, it reduces exposure of the rectal wall to radiation. The spacer degrades over a period of three months and has been shown to reduce toxicity in patients undergoing standard external beam radiation therapy. We hypothesize that there will be similar toxicity reduction after SBRT. An endorectal balloon is another way to reduce overall rectal wall dose, by pushing the posterior rectal wall away from the prostate during radiation treatment.
As data for prostate SBRT continues to mature, more individuals with low- and intermediate-risk prostate cancer will likely opt for this convenient and efficacious form of radiation over more protracted courses.