Elevated serum prostate-specific antigen levels in a woman with metastatic breast cancer.

To the Editor: Prostate-specific antigen (PSA) is a protein produced almost exclusively by the prostate. Although various tissues in females have been found to produce very small amounts of PSA, serum PSA levels in females are quite low and are measurable only with ultrasensitive PSA immunoassays. In a study of 1064 women, the highest serum PSA levels were less than 1 μg per liter, and only 1.5 percent of the women had levels that were higher than 0.1 μg per liter; the median value was 0.002 μg per liter. Significantly higher values have been reported in women with benign breast diseases. PSA is a hormonally regulated protein, and administration of the synthetic progestin megestrol acetate in women with metastatic breast cancer may induce the production of PSA. Elevated serum PSA levels appear to have prognostic value in such patients.