Prostatic specific antigen as an indicator of response to radiotherapy in prostate cancer

The effectiveness of prostatic specific antigen (PSA) as an indicator of response to radiotherapy and as a predictor of tumor recurrence was studied. Serum PSA values of 71 patients with regionally confined prostate cancer, who were irradiated with aim at cure, were measured before irradiation, at the end of radiotherapy, 6 weeks later, and in 6-month intervals thereafter. The patients were followed for 12-30 months with a median follow-up time of 18 months. PSA was determined using a polyclonal radioimmunoassay, a level of 12.2 ng/ml was considered the normal limit. Sixty-five per cent (46/71) of the patients had elevated PSA levels before starting irradiation. Six weeks after completion of therapy, 60% (28/46) of the primarily elevated PSA levels fell below 12.2 ng/ml. Six months later, 74% (34/46) of the PSA levels returned to normal. Twelve months after completion of radiotherapy, only two additional patients showed a normalization of PSA levels raising the rate to 78% (36/46). Out of the remaining ten patients with persisting elevated PSA values at 12 months, five were screened for metastases and found to be positive. We conclude that PSA values which fail to normalize 6 months after the resumption of treatment carry a high risk of recurrence. PSA values were found to correlate well with estimated clinical tumor stage. Comparison of PSA and prostatic acid phosphatase (PAP) levels indicate PSA to be the more sensitive tumor marker, allowing earlier detection of tumor at diagnosis and during follow-up.