Background: The association of a seborrheic keratosis with other common cutaneous neoplasms such as basal cell carcinoma and Bowen disease has been reported, but the association between a seborrheic keratotis and a malignant neoplasm with sebaceous differentiation is very unusual.
Objective: We present a case of two contiguous neoplasms, a seborrheic keratosis and a sebaceous carcinoma, and discuss the possibility of malignant change in a seborrheic keratosis as an explanation for the findings.
Methods and results: A 57-year-old man presented with an asymptomatic tumor on the skin of his abdomen that was composed of two separate but contiguous lesions. The central lesion, about 0.9 cm in diameter, was nodular, irregular, and reddish and was surrounded by a blackish lesion about 3 cm in greatest dimension. Histopathologic examination revealed that the plaque was composed of two different adjacent tumors, including a central portion showing findings consistent with a sebaceous carcinoma and a peripheral part showing a seborrheic keratosis.
Conclusion: Although the association is likely to be a coincidence and probably represents a collision tumor, the possibility that the sebaceous carcinoma represents malignant degeneration of the seborrheic keratosis cannot be entirely excluded.