The histological diagnosis of metastases to the breast from extramammary malignancies.

This study aims to review histological and immunohistochemical features that are useful in the diagnosis of metastases to the breast. Histological features were compared between non-haematological metastases to the breast and 100 consecutive core biopsy specimens of primary invasive carcinomas of the breast. 18 non-haematological metastases to the breast were diagnosed over a 10-year period (0.3% of malignant mammary tumours). Elastosis and carcinoma in situ were seen only in primary mammary cancers. Two-thirds of tumours had features raising the possibility of metastasis, such as clear cell carcinoma suggestive of renal origin and small cell carcinoma suggestive of pulmonary origin. The features observed in haematological metastases are also described. Immunohistochemical panels to distinguish mammary carcinoma (oestrogen receptor, gross cystic fluid protein-15) from common metastases to the breast, including carcinoma of the lung (thyroid transcription factor-1), malignant melanoma (S100, HMB45, melan-A) and ovarian serous papillary carcinoma (Wilms' tumour 1), are discussed. The pathologist has a key role in considering the diagnosis of metastasis to the breast if the histological features are unusual for a primary mammary tumour. The clinical history is vital in some cases. Immunohistochemistry plays a useful supplementary role.