Microinvasive squamous cell carcinoma arising within seborrheic keratosis.

Squamous cell carcinoma (SCC) arising within seborrheic keratosis (SK) is rare. We report an 84-year-old woman who presented with a rapidly growing black tumor on her left palpebral eyelid of several years' duration. Clinical examination revealed an elevated hemorrhagic black tumor that measured 0.9 x 0.9 x 0.6 cm. A clinical diagnosis of SK was made, but basal cell carcinoma could not be ruled out; therefore, excision with wide margins was performed. Histologically, the tumor was symmetrical and composed of benign basaloid cells with pseudohorn cysts in a reticulated pattern. The tumor showed heavy melanin deposition. The features were indicative of SK. An atypical cell cluster was seen in the central low area. These cells showed keratin pearls, individual keratinization, mitotic and apoptotic figures, nuclear atypia, and microinvasion, indicating microinvasive SCC. Immunohistochemistry revealed the microinvasive SCC area was true SCC. This case suggests that microinvasive SCC can arise within pigmented reticulated SK.