Stereotactic body radiotherapy (CK) for prostate cancer

from the IV MB, by yyy60:

>>Stereotactic body radiotherapy (CK) for prostate cancer

UroToday
Friday, 02 April 2010

Beyond the Abstract - Stereotactic body radiotherapy for organ-confined prostate cancer, by Alan J. Katz, MD, JD

BERKELEY, CA (UroToday.com) - Our paper reports on the largest series of patients treated with CyberKnife® for prostate cancer at one institution.

In the year since we compiled the data, we have continued to be encouraged by the efficacy and toxicity profile. We still have not seen a local failure in the low or intermediate risk patients, with our early group treated with 35gy having a median followup of 42 months. For two reasons, we have decreased our dose back from 36.25 Gy to 35 Gy. First, 35 Gy has been very effective, with a 36 month median PSA of 0.15, which is highly predictive of excellent long-term control according to the literature. Secondly, our 36.25 group did experience a small increase in the rate of Grade2-3 urethral toxicity.This is predictable based on the radiobiology. If the alpha-beta ratio for prostate cancer is 1.5 (that is, prostate cancer cells are highly sensitive to dose per fraction), then 35Gy in 5 fractions is the equivalent of 92 Gy in 1.8Gy fractions. This dose is on the flat part of the dose response curve, and therefore dose escalation is not necessary to maximize control. The alpha beta ratio for late complications is probably around 3, and therefore the 35Gy dose is in the 70-75 Gy range for late complications. Therefore, I believe we are on the steep part of the dose response curve for complications and the 3% increase in the dose to 36.25 predictably increased toxicity in the urethra by bringing us to almost 80 GY dose equivalent. Adjusting the dose down should and does reduce late toxicity.

We did not see a concomitant increase in rectal toxicity, probably because we spared the rectal tissues with the conformality achieved with CyberKnife® technology, and we used the radioprotector Amifostine intrarectally prior to each fraction. Our potency preservation remains at 80%.

I am especially encouraged by our excellent results with intermediate and high risk disease with CyberKnife® alone. I will be publishing a series of patients who received a CyberKnife® boost after 45Gy to the pelvis for this category. There appears to be no benefit to adding the external beam in terms of efficacy and there is slightly greater rectal toxicity. If CyberKnife® alone is optimal treatment, this will afford a wide range of prostate cancer patients the chance to reduce their treatments from 45 to 5. This would be a huge benefit to older patients, especially those that have to travel long distances for radiation therapy. In addition, there is a great cost benefit to society. Medicare reimbursement for a five day CyberKnife® treament is less than IMRT and a small fraction of the cost of Proton beam therapy.

I believe that my series and the other two series, from Naples and Stanford, provides substantial early evidence that this dose fractionation scheme is highly effective and safe when delivered with CyberKnife® technology. Since my PSA nadirs are so low, I am very optimistic that our data will hold up with more follow up over time. It should be time for those who claim this treatment to be experimental to soften their position. To date, no form of radiotherapy has been compared to another with a prospective randomized trial. Therefore, I do not believe this to be necessary before offering patients the option of CyberKnife®. However,I would welcome such a trial against IMRT,protons or brachytherapy.