Pigmented squamous cell carcinoma is a rare neoplasm arising mostly in mucous membranes or in sun-exposed skin. We describe a case of this tumor in a 70 year-old man who presented with a pigmented ulcerated nodule on his lower back. Microscopic examination revealed a squamous cell carcinoma (SCC) with atypical proliferation of cells with individual cell keratisation and keratin pearls' formation. Apart from the squamous cell carcinoma a population of cytologically bland dendritic melanocytes was also present, a process that has been described as tumor cell colonization. Differential diagnosis of pigmented SCC includes melanoma with pseudoepitheliomatous hyperplasia, pigmented metatypical basal cell carcinoma and squamomelanocytic tumor, a recently described combined tumor. A small number of cases of pigmented SCC have been referred in the literature mainly located in mucosal surfaces such as the oral and nasal cavity and the conjunctiva but also in sun-exposed cutaneous sites especially at the skin of the head. In the present study, a case of pigmented SCC arising in sun protected skin is described. The authors review the published literature and discuss the histologic differential diagnosis of pigmented SCC as well as the possible mechanisms of melanocytes' colonization in pigmented non-melanocytic malignant neoplasms.

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