Dermoid cyst in the subcutaneous tissues of the back: A rare case with multimodal imaging and pathologic correlation.

Dermoid cysts are lined by keratinizing squamous epithelium and consist of skin appendages such as hair follicles, apocrine glands, and sebaceous glands. They are usually diagnosed during infancy or early childhood, commonly reported in the head and neck region. A dermoid cyst on the back is extremely rare, with only three cases in the pediatric and adult populations. We report a rare case of a dermoid cyst in the subcutaneous layer of the back in an adult. A 75-year-old man presented with a soft, painless mass on his left upper back. Computed tomography revealed a low-density mass nearly identical to the subcutaneous fat in the subcutaneous layer with nodular soft-tissue density components. It was a heterogeneously hyperechoic mass without internal vascularity on ultrasonography. On magnetic resonance imaging, the lesion showed nearly identical signal intensity (SI) to subcutaneous fat on T1 and T2-weighted images. The soft tissue component was intermediately hyperintense on T1- and T2- weighted images with enhancement. This lesion was pre-operatively suspected as a lipoma variant or a well-differentiated liposarcoma/atypical lipomatous tumor because of the fat density or SI and enhancing portion. We demonstrated and reviewed the multimodality imaging features of dermoid cysts at an unusual location and suggested imaging features that could help readers differentiate dermoid cysts from lipomatous tumors. When a mass shows fat density or SI with or without enhancing soft tissue components at the trunk or extremity, dermoid cysts as well as lipomatous tumors could be considered in the differential diagnosis.