Collision Tumor of Seborrheic Keratosis and Malignant Melanoma

Abstract

An 89-year-old man noticed a tumor on the right lower abdomen approximately 20 years ago and it gradually increased in size. Physical examination revealed a 35×26×8mm nodule composed of red to reddish brown areas and dark brown areas, and its surface was focally erosive and papillated. It was pedunculated without surrounding macules. On dermoscopy, most of the tumor was consistent with seborrheic keratosis, but milky red areas of malignant melanoma were observed. Biopsy of the milky red areas led to the diagnosis of malignant melanoma and wide local excision was performed. On histological assessment of the surgical specimen, seborrheic keratosis and malignant melanoma were adjacent and mixed at the boundary. We diagnosed the patient with a collision tumor of seborrheic keratosis and malignant melanoma. It is important to perform biopsy when a tumor exhibits worrisome dermoscopic features even focally. Skin Research, 20 : 203-207, 2021