Immunoreactivity of prostate-specific antigen in male breast carcinomas: two examples of a diagnostic pitfall in discriminating a primary breast cancer from metastatic prostate carcinoma.

Prostatic-specific antigen (PSA) is regarded as a specific marker secreted by normal and neoplastic acinar epithelial cells of the prostate gland; its detection by immunocytochemistry has been accepted as an indication of metastatic prostate cancer. This is ascribed to the commonly held belief that PSA is not found in extraprostatic tissues. However, this concept has recently been challenged, based on the observations that certain nonprostatic tissues and their neoplasms can also secrete PSA. Such a questionable belief could result in a diagnostic pitfall when using immunostaining for PSA on fine-needle aspiration (FNAC) cytology samples to differentiate metastatic prostate cancer from a primary carcinoma of an extraprostatic organ. In this communication, two cases of primary carcinomas of the male breast are reported in which PSA immunopositivity on FNAC led to the suggestion of a diagnosis of metastatic carcinoma of the prostate. Diagn. Cytopathol. 1999;21:167-169.