Squamous Cell Carcinoma Arising in an Epidermal Cyst in the Setting of Dowling–Degos Disease and Hidradenitis Suppurativa: A Rare Tetrad

Dowling–Degos disease (DDD)/reticulate pigmented anomaly of the flexures is a rare autosomal dominant genodermatosis characterized by abnormalities of the follicular epithelium and reticulate hyperpigmentation of flexural areas. Mutations impacting keratinization pathways, such as k eratin 5 , p rotein O-fucosyltransferase 1 , and protein O-glucosyltransferase 1 have been linked to the disorder. There is growing evidence linking DDD to follicular occlusion conditions like hidradenitis suppurativa (HS). However, it is extremely uncommon for follicular lesions in DDD to undergo malignant transformation. We describe a rare case of squamous cell carcinoma developing in an epidermal cyst in a patient who also had HS and DDD. The coexistence of these entities points to a common pathophysiologic mechanism involving aberrant epithelial proliferation and follicular occlusion. Persistent chronic inflammation within follicular structures may predispose to dysplasia and malignant transformation. Recognition of such associations is important for early diagnosis and management of suspicious lesions in patients with follicular disorders.