The key is a substance known as prostate-specific antigen (PSA), which is often found at high levels in the blood of men who have prostate cancer. If the patient's PSA drops to a low level after treatment with external beam radiation therapy, stays normal for four or five years, and there are no other signs the disease is returning, it is highly likely the cancer has been cured, said Frank Vicini, MD, of William Beaumont Hospital in Royal Oak, Mich. Dr. Vicini was one of the authors of a study on prostate cancer radiation therapy published in the journal Cancer (Vol. 86, No. 8).

Dr. Vicini and colleagues were interested in setting standard criteria for determining if prostate cancer has been eliminated. In the past, it has been easy to know when treatment failed, but not whether it has been completely successful, Dr. Vicini explained.

The researchers developed criteria after analyzing information on 871 patients with prostate cancer who had been treated with external beam radiation between 1987 and 1994. Because long-term follow-up data was available, the researchers could estimate a time-frame for evaluating the treatment's success, Dr. Vicini said. PSA has been used in diagnosing prostate cancer only since the mid-1980s. Until recently, there was not enough long-term data on PSA to reach conclusions about treatment outcomes, he said.

The analysis showed patients with a normal PSA level five years after treatment have about a 95 percent certainty of being cured. The scientists also learned the outcome was likely to be better if the drop in PSA levels took place gradually over two or more years. "The longer it takes the PSA to go completely down, the better," Dr. Vicini said.

Patients whose PSA level was low before treatment and who have a lower Gleason score (a measure based on examination of the cancer under a microscope) may need longer follow-up and more evaluation before treatment can be declared completely successful. However, it is not possible to give patients a 100 percent guarantee that they have been cured under any circumstances, Dr. Vicini said.

"Overall, this is a good review of a lot of patients and supports the conclusions many of us have already reached," said H. Joseph Barthold, MD, director of the Louis B. Hager Cancer Center at Bassett Healthcare in Cooperstown, NY, and a member of the American Cancer Society (ACS) prostate cancer advisory board. However, he pointed out most patients in the study were treated with low-dose radiation therapy, while today most prostate patients receive a higher dose of radiation as well as temporary hormone therapy.

Prostate cancer is the most common type of cancer among men and the second most deadly. The ACS estimates that 179,300 men will be diagnosed with prostate cancer and 37,000 will die from the disease in 1999.