Carcinoma Cuniculatum of the Back: Dermoscopic and Ultrasonographic Findings.

Carcinoma cuniculatum (CC) is a rare, well-differentiated variant of cutaneous squamous cell carcinoma that is frequently misdiagnosed because of its deceptively benign clinical appearance. We report an unusual case of CC arising on the back that initially mimicked an epidermal cyst. A 55-year-old Japanese woman presented with a slowly enlarging subcutaneous nodule on her upper back that had been present for three years. Physical examination suggested an epidermal cyst, whereas ultrasonography demonstrated atypical findings that prompted excisional biopsy. However, the lesion rapidly enlarged within one month, ulcerated, and became a pedunculated tumor. Histopathological examination of an incisional biopsy revealed well-differentiated squamous cell carcinoma. Wide local excision was subsequently performed. Examination of the resected specimen demonstrated characteristic deeply penetrating keratin-filled branching sinus tracts with minimal cytological atypia, establishing the diagnosis of CC. This case emphasizes that rapidly progressive changes in lesions initially diagnosed as epidermal cysts should prompt reconsideration of the diagnosis and timely histopathological evaluation.