Breast metastasis from prostate cancer and interpretation of immunoreactivity to prostate-specific antigen

A case of prostate cancer metastasized to the breast is presented, the latter being prostate-specific antigen (PSA) positive. This is the first of such cases reported in Hong Kong and China in the English literature. As PSA expression also can be found in primary breast cancer, prostatic acid phosphatase staining was employed to confirm the diagnosis. The relationship of PSA and non-prostatic tissues is reviewed. The differential diagnosis of breast enlargement in patients known to have prostate malignancy also is discussed.