Pigmented Bowen disease arising in pigmented reticulated seborrheic keratosis

A 76-year-old man consulted to our hospital complaining of a black tumor of the temporal head. The tumor was nodular mass measuring 1 × 1 × 0.6 cm and black. A clinical diagnosis of basal cell carcinoma was made, and an excision with wide margins was performed. Histologically, the tumor was symmetrical, with horny keratinization in the center. An aggregate of atypical cells regarded as malignant cells was present in the center. No invasion was recognized. The cells were heavily pigmented. Mitotic figures, individual keratinization, and clumping cells were also recognized. Numerous dendritic melanocytes were recognized in the malignant lesion. Immunohistochemically, strong p53 expression was present in all malignant cells, and the malignant cells showed a high Ki-67 labeling of 60%. They were negative for melanosome and S100 protein and positive for cytokeratins. The periphery of the tumor showed typical features of pigmented reticulated seborrheic keratosis. There were gradual merges between the two lesions. This case suggests that pigmented Bowen disease can arise in pigmented reticulated seborrheic keratosis.