Basal Cell Carcinoma Arising within Seborrheic Keratosis

Seborrhoeic keratoses are benign epidermal neoplasms that are rarely associated with other skin lesions especially malignancies. In this study, the aim was to assess the incidence of associated lesions occurring either adjacent to or contiguous with a seborrhoeic keratosis. A retrospective case series over a 12-month period was carried out. In total, 639 consecutive histologically diagnosed seborrhoeic keratoses were identified, of which 85 (9%) were found to be associated with other lesions. Of these associated lesions, 44 (7%) were malignant, with four of these found to be arising within the seborrhoeic keratosis. These associated lesions included premalignant lesions, malignancies, melanocytic lesions and miscellaneous lesions. Men (59%) were affected more commonly than women. The average age was 74 years (range 33-98 years). Seborrhoeic keratoses associated with other lesions were found most commonly on the head and neck. The incidence of associated lesions, in particular malignancy arising within seborrhoeic keratoses may be higher than previously thought. This may be an incidental phenomenon, or seborrhoeic keratosis could represent a precursor lesion. Therefore, seborrhoeic keratoses that have undergone recent clinical change should be considered for biopsy and histological examination.

The association of malignant neoplasms arising contiguous with or adjacent to seborrheic keratoses has been previously documented. In this study a retrospective analysis was performed to further characterize these combined neoplasms. A total of 54 examples of malignant neoplasms in conjunction with seborrheic keratoses were found, of which 43 were basal cell carcinomas, six were Bowen's disease, three were keratoacanthomas, and two were malignant melanomas. The average age of the patients was over 65 years. Men were affected more frequently then women. The posterior thorax was the most common site for malignant neoplasms associated with seborrheic keratoses. The superficial type of basal cell carcinoma was the most common type of basal cell carcinoma found in this association. All subtypes of seborrheic keratosis may be found together with malignant neoplasms. We believe the appellation collision tumor is not valid because it is unknown whether the association of seborrheic keratoses with malignant neoplasms is a random event or whether there is, in fact, a pathogenic relationship in the development of two distinctive neoplasms together. Therefore, the term compound tumor is proposed to designate the finding of two distinctive neoplasms either directly contiguous with each other or immediately adjacent to each other.

Seborrheic keratoses (SKs) are common epidermal tumors composed of benign keratinocytes. Malignant skin tumors including basal cell carcinoma (BCC) rarely arise within SKs. We report a rare case of an 82-year-old man with nodulocystic BCC that appeared at the center of a scaly hyperpigmented SK that had been presented for more than 10 years. It was histologically confirmed that CK19-positive BCC arose directly from the wall of the pseudohorn cyst, a part of the SK. Nodular and/or cystic BCC also rarely arise within SKs while the most common histologic type of BCC within SKs is the superficial type. Careful observation of SKs is important even though it is rarely a background condition for malignant transformation.