Keratoacanthoma occurring within the red dye of a tattoo.

Keratoacanthoma (KA) is a common keratinizing squamous cell neoplasm of unknown origin characterized by rapid growth and spontaneous involution. Trauma-induced forms have been observed with various types of skin injury. To our knowledge, reports of KA arising at tattoo sites are scarce in the literature. A 41-year-old woman with no medical history presented for a rapidly growing nodule confined to the red part of a tattoo located on the scapula. Histology showed a keratin-filled cuplike crater with an epithelial proliferation (hyperkeratosis, parakeratosis, no keratinocyte atypia). An inflammatory infiltrate in the dermis composed of lymphocytes and histiocytes intermixed with red ink-related exogenous pigments was noted. Lack of papillomatosis and viral inclusions ruled out the diagnosis of viral wart, absence of granulomatous reaction ruled out deep fungal or mycobacterial infection and lack of cytological atypia and frank architectural abnormalities did not favour a squamous cell carcinoma. KA should be included in the list of cutaneous complications related to tattooing. Diagnosis can be challenging as differential diagnoses include pseudoepitheliomatous hyperplasia and squamous cell carcinoma. Removal of the entire area, thorough histological examination and careful follow up are mandatory.