M 86, mid back, poorly circumscribed gray lesion

    An-86-year-old male with a history of multiple actinic keratoses and seborrheic keratoses of the head and trunk presented with a mid-back skin lesion. The lesion was poorly circumscribed, flat, and gray, with a pink-tan, well-circumscribed scaly nodule within it. The biopsied lesion was composed of the usual features of hyperkeratotic seborrheic keratosis, but with focal atypical melanocytic proliferation with nesting along the dermal-epidermal junction (Figure 1). These neoplastic melanocytes displayed pleomorphic nuclei with prominent nucleoli. There was neither any significant upward migration of the neoplastic cells, nor was there any dermal invasion. Numerous melanophages were seen within the papillary dermis [Figure 2]. The atypical melanocytes were immunoreactive with S-100, HMB-45, MART-1, and MITF (Figure 3). We interpreted this lesion as a melanoma in-situ arising within a seborrheic keratosis.

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