Lichen planus appearing subsequent to generalized lichen nitidus in a child.

in children as young as our patient (7 years) have been reported (3). Histologically, MEC is characterized by three cell types: mucinous, squamous, and clear (4). Lesions are typically well-circumscribed and demonstrate peritumoral fibrosis, variability in the epithelial lining, mucinous, and papillary features (2,5). Immunohistochemical analysis reveals positive staining for cytokeratin and carcinoembryonic antigen. Histologic grade correlates well with clinical behavior, independent of primary tumor location, and low-gradeMEC rarely metastasizes (2,6). Local recurrences may occur after incomplete excision (6,7). Clinical diagnosis of MEC of the skin is challenging (2). First,MECof the skinmaybe primary ormetastatic. Whereas primaryMECof the skin tends tobe low-grade, metastatic skin MEC tends to be high-grade and is considered to indicate a poor prognosis. A recent review (2) identified only eight well-documented occurrences of primaryMECof the skin (two deaths) and three ofMEC metastatic to the skin. Secondly, MEC of the skin is commonly confused with adenosquamous carcinoma (ASC) of the skin (5), which may present at various primary sites, including the cervix, lungs, intestines, pancreas, throat, and skin. This entity is characterized histologically as a more high-grade, aggressive malignancy with areas of both squamous cell carcinoma and adenocarcinoma. It is less circumscribed, and does not demonstrate peritumoral fibrosis, variability in the epithelial lining, mucinous, or papillary features (2,5). Additionally, MEC is dermal-based whereas ASC may be intraepidermal (2,5). In summary, to our knowledge, this is the first report of primary MEC of the skin occurring in childhood. Primary MEC of the skin should be distinguished from both MEC metastatic to the skin and ASC, given the important morphologic and prognostic differences.