Immunohistochemical labelling for prostate specific antigen in non-prostatic tissues

Immunohistochemical detection of prostate specific antigen (PSA) in metastases of adenocarcinomas is widely used as an aid to identify the prostatic origin of metastatic cells. However, on the one hand PSA may not be expressed in some poorly differ entiated prostatic carcinomas, while on the other, PSA immunoreactivity has been found in small amounts in non-prostatic tissues. The aim of the current study was to evaluate the prevalence of PSA immunoreactivity in normal non-prostatic tissues and in breast carcinoma. PSA was localized by immunohistochemistry with four commercial antibodies in 34 different normal human tissues, and in 15 ductal and seven apocrine breast carcinomas. Concentrations of PSA in tissue homogenates of prostate and nine non-prostatic tissues from autopsied subjects were measured by a two-site immunoradiometric assay. Weak PSA immunoreactivity was found by immunohistochemistry in kidney, parotid gland and pancreatic tissues. Variable PSA immunoreactivity was seen in three cases of ductal (20%) and two cases of apocrine breast carcinoma (28%). No consistent PSA immunoreactivity was found in homogenates of non-prostatic tissues by the immunoradiometric assay. We conclude that PSA is a quite specific marker of prostatic tissue. However, there are some non-prostatic neoplastic and normal tissues that express PSA. Therefore, a definite diagnosis of metastasis of prostatic origin cannot be made on the basis of immunolabelling for PSA alone.