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Case Reports

Abstract

Seborrheic keratoses (SKs) are among the most common benign epidermal tumors and are frequently treated without histologic evaluation; however, they can occasionally coexist with or harbor malignant lesions, including melanoma. Recognizing this rare association is critical, as failure to diagnose melanoma within an SK can delay treatment and compromise staging, particularly when the biopsy is transected. We present the case of a 71-year-old woman with a pigmented lesion on the left cheek. Histopathologic evaluation revealed melanoma in situ colonizing a seborrheic keratosis with adnexal extension. The specimen was broadly transected at the base, precluding the exclusion of invasive melanoma. Microscopic findings demonstrated a nodular histologic pattern, a minimum tumor thickness of 1.5 mm, a Clark level of at least III, a radial growth phase, and no ulceration, mitotic figures, regression, lymphovascular or perineural invasion, or an associated nevus. Both the deep and peripheral margins were involved, resulting in a provisional pathologic stage of at least pT2a. SOX10 immunostaining confirmed melanocytic proliferation and delineated the tumor’s extent. Complete excision was recommended to determine the true Breslow depth and finalize staging. This case underscores the importance of submitting SKs for histologic evaluation, especially when they display atypical features or clinical change. Melanoma arising within a benign lesion can easily be overlooked if treated empirically. While immunohistochemistry assists in tumor mapping, full-thickness excision remains essential to confirm invasion, achieve accurate staging, and guide optimal patient management.