A metastasis is a neoplasm that has spread from its original site such that it is no longer in continuity with, and often not in close proximity to, the primary lesion. Neoplasms can invade the skin by contiguous spread or may spread into the skin by direct extension into surgical scars and needle biopsy tracts, but the usual pathway of regional or distant metastases is believed to involve lymphatic or blood vessels.1-3 Such lymphatic and vascular spread may be not only intraluminal but also extravascular by means of tumor cell migration along the abluminal surfaces of vascular channels. The frequency of cutaneous metastases in patients with cancer ranges from 0.6% to 10.4% in various studies, accounting for approximately 2% of all skin tumors.4-10 Autopsy studies reporting lower incidence rates ranging from 1% to 5.3% might underestimate the frequency of cutaneous metastases both due to successful antecedent therapy and incomplete postmortem skin examination.11 In a study of 4,020 patients with metastatic carcinoma and melanoma, 420 patients (10.4%) had cutaneous metastases, and cutaneous metastases were the first sign of extranodal metastatic disease in 306 patients (7.6%).10 In another study of 7,316 cancer patients, 5% had skin involvement. Skin involvement was the first sign of cancer in less than 1% of patients.9 A little more than a third of these rare patients had direct extension of the primary cancer into the skin, a third had local (satellite) metastases, and only about a quarter represented distant metastases to the skin. Thus, although skin metastases are in general relatively uncommon, they can quite rarely be the initial presentation of an internal malignancy.
Cutaneous metastases reflect the biologic behavior and population-based incidence of their associated primary tumors. The greatest incidence of cutaneous metastases is therefore in the fifth, sixth, and seventh decades of life, and the incidence and distribution of cutaneous metastases are correlated with gender. The incidence of primary tumors in women and men with skin metastases from the classic studies of Brownstein and Helwig on 724 patients is listed in the left-hand columns of Table 35-1.4,5 These authors found that the most common primary tumors in women were breast at 69%, followed by large intestine, melanoma, lung, and other internal sites. In men, the lung and large intestine were the most common sources (Table 35-1). Skin metastases were the presenting sign of malignancy in men in 37% of cases and in women in 6% of cases, presumably because primary breast cancer in women is often diagnosed at an earlier stage than primary lung and colorectal cancer are diagnosed in men. In a later study by Lookingbill et al.10 of cutaneous metastases in a population of patients with metastatic carcinoma and melanoma (right-hand columns of Table 35-1), the most frequent tumors to metastasize to the skin in 300 women were the breast and melanoma. In 127 men, the most frequent cutaneous metastases ...