Monitoring of prostate-specific antigen during external beam radiotherapy for carcinoma of the prostate.

In 35 patients with histologically confirmed carcinoma of the prostate confined to the pelvis, the value of prostate-specific antigen (PSA) was evaluated during external beam radiotherapy to the prostate and draining pelvic lymph nodes. In eleven patients initial prostate-specific antigen levels were more than 10 ng/ml and in twelve patients between 4 and 10 ng/ml. In the remaining twelve, initial prostate-specific antigen levels were less than 4 ng/ml. In the course of radiotherapy we could see a significant decrease of the prostate-specific antigen, even in those with levels between 4 and 10 ng/ml. This decrease seems to follow a logarithmic course but, because only three measurements during radiotherapy were made, this needs further study. With higher levels (more than 20 ng/ml), we rarely saw a value of less than 10 ng/ml at the end of radiotherapy but had to wait for several months for lower values to be reached. In several cases prostate-specific antigen decrease took up to three months after the end of the radiation course. Our results indicate that prostate-specific antigen values actually start decreasing during the radiation course itself and may, therefore, be useful for monitoring response to radiotherapy.