Malignant tumor occurring within seborrheic keratosis (SK), which is one of the most common benign cutaneous tumors, is extremely rare. We report a case of basal cell carcinoma (BCC) arising within SK. Additionally, this is the first study to describe the immunohistochemical characteristics of this type of carcinoma. An 89-year-old Japanese woman presented with a persistent scaly plaque in the right auricle of her ear. Histopathological study revealed a superficial type of BCC arising within SK. Immunohistochemical studies showed that cytokeratin 17 (CK17), CK19, SOX9 and p53 protein were expressed in BCC, but not in SK. BCC is considered to originate from the follicular germinative cells, and the outer root sheath may be the possible origin. SK is also thought to be a benign skin appendage neoplasm showing follicular differentiation, especially follicular infundibula. Therefore, previous reports speculated that there was a pathogenic relationship between SK and BCC, with respect to a common follicular origin. However, the immunohistochemical characteristics of this study suggest that BCC does not arise directly from SK, but instead, that SK is the nidus of the carcinoma, resulting in the abutment of SK with BCC. Furthermore, the results of the present case suggest that immunohistochemical surveillance of the expression of CK17, CK19 and SOX9 and p53 protein is useful in differentiating minute BCC from the non-neoplastic hair buds.

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