Malignancy arising in seborrheic keratosis: a report of two cases.

We describe the pathological features of two cases of malignant transformation occurring in seborrheic keratosis. The lesions affected elderly individuals: a 79-year-old Chinese lady and a 79-year-old Chinese gentleman, both of whom had long-standing seborrheic keratoses with recent increase in size. Histologically, the malignancies identified were invasive well-differentiated squamous cell carcinoma and Bowen's disease (squamous carcinoma in situ), respectively. Pre-existing seborrheic keratoses demonstrating a zone of transition into the malignant foci were present. Aetiologic factors that have been proposed to explain these exceedingly rare occurrences have included prolonged sun damage and chronic low dose radiation exposure.