Metastatic Tumors Involving the Skin

Tumors metastatic to skin are clinically important because they may represent the first manifestation of an unrecognized internal malignancy or the first evidence of recurrence of a previously treated primary tumor. It is critical to recognize that the tumor is in fact a secondary deposit and not to mistake it for an unusual primary tumor. Determining the site of the primary tumor, if unknown, is often very difficult and sometimes impossible. However, certain primary sites may be suspected from the histopathologic features or the immunoprofile of the cutaneous metastatic deposit.

Secondary tumors may involve the skin by direct spread from adjacent noncutaneous structures; by lymphatic or hematogenous spread; or, rarely, as a consequence of implantation following a surgical or other diagnostic procedure. Although tumors involving the skin as a consequence of direct spread from adjacent noncutaneous structures are not true metastases, they may be misdiagnosed as unusual primary cutaneous tumors unless the possibility of a noncutaneous origin of the tumor is considered. The presence of skin metastases usually occurs in the clinical setting of a known primary tumor, often with accompanying widespread visceral metastatic disease. In such cases, a pathologist, provided with a thorough and accurate clinical history, will usually have little difficulty in correctly categorizing secondary tumors. In other situations, there is a real risk that a secondary tumor may be incorrectly diagnosed—for example, when there is inadequate clinical information on the pathology request form; or when a metastasis is solitary, isolated, and presents a long time after the primary tumor was initially diagnosed; or when a cutaneous metastasis represents the first indication of an underlying visceral cancer or other tumor. Conversely, uncommon variants of primary cutaneous malignancies, including basal cell carcinoma (BCC), squamous cell carcinoma (SCC), and melanoma, may occasionally be mistaken for metastases. Misdiagnosis of a metastasis as a primary tumor or vice versa may result in inaccurate prognostic assessment, inappropriate management, and a potentially poorer clinical outcome. A high index of suspicion and careful clinicopathologic correlation are important to prevent misdiagnosis.