Fluid-fluid level in a giant epidermal cyst of the buttock.

We describe a case of a giant epidermal cyst of the buttock that demonstrated a fluid-fluid level on imaging. There have been no previously reported cases of epidermal cysts containing a fluid-fluid level, so our case was considered to be very exceptional. A 39-year-old man had had a slowly enlarging giant subcutaneous mass in the left buttock for more than 10 years. It was elastic-soft, well-circumscribed and smooth-surfaced. Ultrasonography showed a well-circumscribed hypoechoic mass with a fluid-fluid level in the subcutis of the left buttock. Magnetic resonance imaging (MRI) showed a well-circumscribed cystic mass measuring 8 cm x 5.5 cm x 5.5 cm in the same place that the ultrasonography indicated. It was hypointense on the T1-weighted image, hyperintense on the T2-weighted image, and contained a fluid-fluid level on both the T1- and T2-weighted images. It bordered on the anus, but neither adhered nor formed a fistula with it. A surgical excision was performed. The mass was diagnosed histopathologically as an epidermal cyst. The occurrence of fluid-fluid levels in epidermal cysts should be kept in mind in the differential diagnosis of subcutaneous soft tissue tumors.