Conversations With Prostate Cancer Experts

Radiation Therapy: IMRT + SBRT

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Dr. Michael Zelefsky, a radiation oncologist, is Professor of Radiation Oncology, Chief of the Brachytherapy Service, and co-leader of the Genitourinary Disease Management Team at Memorial Sloan Kettering Cancer Center in New York City.

Prostatepedia spoke with him about the pros and cons of radiation therapy versus surgery in terms of cancer control and side effects. (Download Prostatepedia’s August issue to read the interview.)

What are some of the forms of radiation therapy available to today for prostate cancer?

Dr. Zelefsky: One of the accurate ways of delivering standard external photon radiotherapy is IMRT, which stands for intensity-modulated radiation therapy. With the help of sophisticated computer programs, we can modulate and change the intensity of the radiation beam so it is much more targeted and more intense in the areas of the tumor. However, in the regions of the normal surrounding tissues, such as the bladder and the rectum, there is less intensity of the radiation given. Intensity-modulated radiation has become a standard approach in the delivery of radiation.

A newer form of IMRT is IGRT, or image-guided radiation therapy. In IGRT we use either markers or beacons placed within the prostate to pinpoint the prostate. The position can move from one day to the next and even during the actual radiation treatment itself.

By using imaging techniques to target the radiation beams, we can treat with less of a margin. That means we don’t need to include as much normal tissue in our focusing of the radiation beam. Ultimately, we observe fewer side effects with this approach.

We can also make real-time adjustments during the radiation beam to more accurately target the location of the prostate. We can then even further diminish those margins we use around our radiation beam and even more accurately pinpoint it. That is what has been done with SBRT, or stereotactic body radiation therapy.

SBRT delivers treatments at a higher dose at each session, condensing the typical radiation schedule from nine or 10 weeks to about seven to 11 days. We give a high dose of radiation at each session, but because the margins we utilize are even tighter and we are excluding more normal tissue, so far patients tolerate this higher dose. We nevertheless tell people that SBRT is a newer type of radiation treatment. It’s the same radiation beam that we used before, but the delivery is done in a shorter period of time with higher doses at each session.

SBRT has been used in a number of centers around the country for the last five to eight years. The results to date have been very good, indicating that tumor control rates look as good as those after prolonged sessions. People are tolerating these kinds of procedures well.

We don’t know yet whether it’ll actually be better than the nine to 10 weeks of treatment. We don’t have randomized trials to tell us definitively whether one is better than the other, but we are monitoring these patients very carefully to try to understand the success rate of these newer approaches.

For now, in my mind, SBRT represents an exciting development in the delivery of radiation for prostate cancer. It’s a convenient way of delivering treatment, and so far, very well tolerated.

Is the attraction of the abbreviated course of radiation offered by SBRT the convenience—i.e. it’s more efficient both for the medical center and for the patient? Or is there also an associated decrease in side effects because patients aren’t having as many sessions?

Dr. Zelefsky: Although there are fewer sessions, there is a higher dose at each session. Nevertheless, while there clearly is a convenience aspect that is a real advantage to many patients, there may be a biologic advantage associated with these higher doses condensed in shorter periods of time. It’s well recognized that, at least from a biologic perspective, those kinds of fractionation regimens, or delivery of treatment in condensed ways, may have a greater biologic potential for cancer cell kill. We haven’t seen long-term results yet demonstrating clear-cut differences, but I wouldn’t be surprised if those differences show up in the future.

We also have to be very cognizant about the fact that there are risks of long-term side effects that could be associated with SBRT. Patients need to be monitored very carefully not only for the short term, but also for the long term as well.

Author: Prostatepedia

Conversations about prostate cancer.

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