Teaching Case

The patient, otherwise asymptomatic, presented with a painless right buttock mass. She referred a fullness sensation more prominent when sitting. The anal canal was displaced to the left side at digital rectal exam; an anal fistula was ruled out.

CE-CT showed an oval thin-walled cyst (8.5x6x12.5 cm in size) in the right ischiorectal fossa (Fig. 1).
The cyst was unilocular and homogenously hyperintense on MRI; the right levator ani, the vagina and the anus were displaced but not infiltrated; infraperitoneal pelvic fat was preserved (Fig. 2a-d). The cystic content was slightly hyperintense relative to muscles on T1-wheighted fat-sat image (Fig. 3a). Enhancement of the lower pole wall was appreciable on post-contrast MRI (Fig. 3b, c).
The resected specimen measured 14×13×6 cm in size and 650g in weight (Fig. 4). On histological examination the cyst was lined by keratinizing stratified squamous epithelium forming a well defined germinal, prickle cell, granular and corneal layer. Keratin lamellae covered the innermost corneal layer whereas an outer layer of collagenous tissue circumscribed the lesion; no glands or dermal appendages were evident (Fig. 5).

The ischiorectal fossae are triangular spaces lying on each side of the anal canal, communicating via the retrosphinteric space. The ischiorectal fossa is delimitated medially by levator ani and external anal sphincter, laterally by obturator internus muscle and obturator fascia, anteriorly by superficial and deep transverse perineal muscles, posteriorly by the lower aspect of the gluteus maximus muscle and the sacrotuberous ligaments; the perineal skin is the base of this wedge-shaped space [1–3]. It contains vessel, nerves and lymphatics, and it can be affected by developmental lesions, infection, hematomas and neoplasms. Besides infections, suggested by symptoms and clinical findings, other lesions of the ischiorectal space usually present as a mass or swelling of the buttock, the perineum and labia [1].
Epidermal cysts, lined by squamous epithelium and containing keratin, can occur in the ischiorectal space; according to pathogenic hypotheses they can be congenital (from ectodermal remnant) or acquired, if due to trauma, iatrogenic manoeuvre or sebaceous duct occlusion [4–6].
On sonography, epidermal cysts can present a pseudotestis pattern, frequently associated to bright reflectors and a threadlike anechoic zone, or alternatively a heterogeneous appearance; the concentric ring and target patterns are less frequent but suggestive of epidermal cysts [6, 7].
CT can be useful to localize a lesion within the ischiorectal space, characterization of which is virtually impossible since CT findings (hypoattenuating mass circumscribed by a thin wall) are wholly unspecific [4]. An epidermal cyst is unilocular on MRI, the content being hyperintense on T2w-images but low to high signal intensity on T1w-images, depending on the relative amount of sebaceous or protinaceous material [2, 8, 9]. Although subcutaneous epidermal cysts present enhancement of the thin wall on post-contrast images [5], there are no available data regarding epidermal cysts of the ischiorectal fossa.
Nevertheless, MRI can rule out a mature teratoma (containing fat), a tail gut cyst (usually multicystic with inhomogeneous fluid) or a solid mass [1–3, 10]; moreover, owing to the excellent soft tissue contrast and multiplanar imaging ability, MRI can more easily delineate the relationship between the cyst and adjacent structure as well as the extension out of the ischiorectal space.
Rupture and infection are possible complications of an epidermal cyst and share similar MRI appearances, i.e. presence of septa, thick enhancing rim, indistinct enhancement of adjacent soft-tissue on post-contrast images; in this setting differentiation from a malignancy is mandatory [5]. Excision of an uncomplicated and asymptomatic epidermal cyst is recommended for possible malignant transformation [5].