The success or failure of radiation therapy for prostate cancer can be predicted as early as 3 months after completion of therapy, according to a presentation at the 45th Annual Meeting of the American Society for Therapeutic Radiology and Oncology (ASTRO).

The study was designed to evaluate the prognostic value of early changes in prostate specific antigen level in order to establish a model for analyzing improvements in radiation therapy, including intensity-modulated radiation therapy, a rapidly developing technique that pinpoints tiny radiation beams to destroy tumors but spare surrounding healthy tissue.

The American investigators studied blood samples from 855 men treated at a major center with external beam radiotherapy. They compared the early response in level of prostate specific antigen to the clinical outcome of the patients. The average patient in the study was followed for more than 6 years. None of the men underwent hormone therapy, and no patients with cancer outside the prostate gland were included in the study.

The researchers found that men whose blood levels of prostate specific antigen stayed above 3.0 ng/mL 3 months after radiotherapy were 30 percent less likely to have relapse-free survival at end of follow-up than men who had a greater reduction in blood levels (level lower than 3.0 ng/mL, 87.8 percent relapse-free compared with level above 3.0 ng/mL, 57.2 percent relapse-free).

“Patients generally have their prostate specific antigen tested soon after therapy, but unless there is a dramatic increase, many physicians wait a year or 2 and analyze the trend before interpreting the information. My colleagues and I disagree with that approach,” said Sean X. Cavanaugh, M.D., lead author of the study. “We found that prostate specific antigen levels at 3 or 6 months after radiotherapy were significantly prognostic for long-term outcome. Analysis of early antigen response enables us to accurately identify those patients who have an 80 percent or better chance of being cured of prostate cancer. Once confirmed by follow-up studies, this method may help us to identify men who will most benefit from the addition of hormone therapy, which has some serious side effects and should not be started in all men.”

“A number of scientific papers have been written on prostate specific antigen, and we used some of the excellent work by other authors as a starting point for our research,” Cavanaugh said. “Our approach was thorough and novel and I think we have produced some of the most important work to date on the prognostic value of early prostate specific antigen changes after radiotherapy.”