Skin metastases from unknown origin:: role of immunohistochemistry in the evaluation of cutaneous metastases of carcinoma of unknown origin

Determining the primary origin of skin metastases might be a challenging issue for pathologists, especially when there is no primary history or when this history is unavailable. The poor specificity of morphological appreciation is challenging, emphasizing the need for ancillary studies. We have retrieved 44 cases of skin metastases from our pathology files. Paraffin blocks were collected and homemade tissue arrays were made. We have tried to assess the primary origin based on morphological data alone, and then using 13 antibodies (cytokeratins (CK) 5/6, 7, 19, 20, thyroid transcription factor-1, carcinoembryonic antigen, PS100, tumor-associated glycoprotein 72, BerEP4, estrogen receptor (ER), progesterone receptor (PR), CD10, and E-cadherin). Most metastases in our series were from breast (13) and colorectal cancers (six) as they are the main clinical activity in our hospital. Only 44% of cases were correctly assessed based on the sole morphology, emphasizing the need for ancillary studies. CK 20, ER, and PR were the most helpful markers to determine the primary origin of skin metastases by highlighting colorectal origin and mammary origin, respectively. By far, clinical information and morphological evaluation are more reliable than the use of ancillary techniques, which have to be used in the absence of the former one and the poor differentiation of the latter ones.