Unusual presentation of a dermoid cyst in the ischiorectal fossa. Magnetic resonance imaging and ultrasound appearances.

The ischiorectal fossa may give rise to a wide variety of pathological entities, although it is composed of relatively few structures. Developmental cysts are included among the list. Large epidermoid cysts in the ischiorectal fossa have been previously described (Fujimoto et al., Clin Imaging 17:146-148, 1993; Ng et al., Can J Surg 49:435-436, 2006). However, to the best of our knowledge, there is no published case in the English literature of a dermoid cyst within the ischiorectal fossa. Using magnetic resonance (MR) imaging and a subsequent ultrasound-guided biopsy, we were able to offer a focused differential that included a dermoid cyst within the ischiorectal fossa in a 55-year-old man presenting with a painful mass on the buttocks. Hair and fatty components were obtained by targeted ultrasound-guided biopsy. On MR imaging, the mass was seen to be well circumscribed and registered a heterogeneous T1-weighted signal that corresponded to layers of fat and debris on short-tau inversion recovery (STIR) imaging. A well-defined ball of fat was noted centrally within the lesion, with a speckled low T1 and low T2 signal within it. Hair admixed with fat was obtained from it by targeted ultrasound-guided biopsy. There was no enhancement of the lesion after administration of gadolinium. On ultrasound, the lesion was well circumscribed and heterogeneous; the echogenic area corresponded to the fat signal seen on magnetic resonance imaging (MRI). The lower level echoes within the lesion corresponded to the debris seen on MRI. The central rounded area of speckling, registering fine posterior shadowing corresponded to the hairy contents obtained by the targeted ultrasound-guided biopsy. A differential diagnosis of all lipomatous lesions was included in the pre-biopsy report: fat necrosis within a lipoma; well-differentiated liposarcoma; myxoid liposarcoma and dermoid cyst. Histopathological diagnosis following complete surgical resection was that of a dermoid cyst.