Seborrheic inclusion cyst of the skin positive for cytoplasmic inclusion bodies and HPV antigen.

Seborrheic inclusion cyst (SIC) is a very rare variant of epidermal cyst of the skin. SIC shows seborrheic keratosis (SK)-like lesion in epidermal cyst. SIC is extremely rare; only 6 case reports have been published in the English literature. However, no immunohistochemical study of SIC has been reported. A 41-year-old Japanese man noticed a subcutaneous tumor in the neck. Physical examination showed slightly mobile tumor in the subcutaneous tissue, and total excision was performed. Grossly, the tumor (1 x 1 x 0.8 cm) was cyst containing atheromatous keratin. Microscopically, the lesion is a cyst containing keratins. About one half of the cyst showed features of epidermal cyst consisting of mature squamous epithelium with granular layers. The other one half showed SK-like epidermal proliferation. The SK-like area showed basaloid cell proliferation with pseudohorn cysts. No significant atypia was noted. Many eosinophilic cytoplasmic inclusion bodies were noted in the SK-like area. Immunohistochemically, the SK-like area was positive for pancytokeratin AE1/3, pancytokeratin CAM5.2, p63, and Ki-67 (labeling=8%) and HPV, but negative for p53. The pathological diagnosis was SIC.