Epidermotropic metastatic malignant melanoma simulating melanoma in situ. A report of 10 examples from two patients.

Epidermotropic metastatic malignant melanoma (EMMM) is a well-recognized entity that can simulate primary malignant melanoma, and in the past reports of numerous (> or = 100) such lesions were misconstrued as multiple primary lesions. We present the cases of two patients who had not only numerous epidermotropic metastases that simulated primary melanomas but also 10 lesions that mimicked melanoma in situ. After removal of a primary malignant melanoma, the two patients developed 35 and 22 epidermotropic metastases over a 4-year period before dying with brain and pulmonary metastases, respectively. In these patients, 29 and 22 of the skin lesions were excised, respectively. All had epidermotropic metastases, of which seven and three showed pagetoid melanocytes in a pagetoid pattern exclusively within the epidermis and epithelial structures of adnexa, an "epidermal only" (in situ) pattern. Other lesions showed a continuum of dermal involvement, from the more conventional description of EMMM with the extent of dermal involvement > or = epidermal (n = 2) to epidermal involvement > dermal disease (n = 36). This histologic spectrum of dermal versus epidermal involvement in conjunction with the extremely small size (2.8 mm average histologic diameter), symmetry, large number, and time course of development argues strongly that these lesions represent metastases rather than multiple primary melanomas. The lesions illustrate the diagnostic dilemma posed by EMMM that simulates primary melanoma and further exhibits an "epidermal only" (melanoma in situ) pattern.