[A case of squamous cell carcinoma that developed on chronic tar dermatosis].

A 79-year-old Japanese man visited our clinic for evaluation of a large tumor on the left wrist and multiple keratotic tumors. He had handled creosote oil, which is a purified product of coal tar production, for 50 years. Physical examinations revealed poikiloderma with multiple hyperkeratotic tumors on the back of his hand and forearm bilaterally, and a cauliflower-like tumor, 80 x 60 x 15 mm in size, on the left wrist. Histopathologically, the large tumor showed a proliferation of atypical squamoid cells with many keratinization foci, indicating well-differentiated squamous cell carcinoma.