Teaching Case

A 32-year-old male patient presented with a right-sided gluteal mass. His past medical history was unremarkable. On rectal examination there was right-sided fullness.

Ultrasound of the right gluteal region showed a unilocular, thin-walled lesion with intralesional reflections (Fig. 1). MRI of the pelvis confirmed a well-circumscribed mass located posteriorly to the bladder causing left-sided deviation of the rectum. The lesion was slightly hyperintense compared to muscle on T1-weighted images (WI) (Fig. 2, 3) and of intermediate to high signal on T2-WI (Fig. 4). There were multiple intralesional areas of low signal on T1-WI and of high signal on T2-WI. On fat-suppressed T2-WI, the lesion was heterogeneously hyperintense (Fig. 5). The lesion did not enhance after intravenous administration of gadolinium (Fig. 6). Based on imaging findings a tentative diagnosis of dermoid cyst was made and subsequently surgical excision was performed. Diagnosis of dermoid cyst was confirmed on histopathological examination.

Developmental cysts are rare congenital cysts arising in the retrorectal space including epidermoid, dermoid, enteric and neuroenteric cysts. They have an incidence of 1 per 40000 and most commonly affect middle-aged women (with a female-to-male ratio of 3:1) [1-3].

Approximately 50% are asymptomatic and present as an incidental finding during clinical examination or imaging [1, 2]. Physical examination may show a funnel-shaped dimple in the postanal midline or a chronic perianal fistula. This fistula may communicate with the cyst. A smooth, firm mass may be felt in the retrorectal space during digital rectal examination [1, 2].

Large lesions may cause compression on adjacent organs, such as the rectum and urethra. Rectal compression may cause constipation, rectal fullness, painful defecation and tenesmus, whereas local compression on the urethra can cause dysuria and urinary frequency [1-4]. Complications of developmental cysts consist of infection and bleeding. Malignant degeneration may occur in enteric cysts [2].

Dermoid cysts are lined with squamous epithelium and contain, unlike epidermoid cysts, skin appendages such as hair follicles, sweat glands, sebaceous glands and/or tooth buds [2, 3]. In our case, a histopathological diagnosis of dermoid cyst was made based on the presence of sebaceous glands in the surgical specimens.

On ultrasound, dermoid cysts are seen as unilocular or multilocular retrorectal lesions, often containing internal echoes due to mucoid material or inflammatory debris [1, 2].

CT shows well-circumscribed, uni-or multilocular hypoattenuating lesions anterior to the sacrum and posterior to the rectum. In rare cases, thin calcifications may be seen. Secondary infection may result in thickening of the cystic wall with adjacent inflammatory changes [1-3]. Dermoid cysts may be associated with sacroccocygeal deformities, which were not present in our case [1, 2].

Whereas retrorectal developmental cysts are of low signal intensity on T1-WI and of high signal intensity on T2-WI, dermoid cysts may show high signal intensity on T1-WI due to the presence of sebaceous contents and intermediate (to high) signal intensity on T2-WI [3, 4]. Intralesional foci of low signal may be due to keratin debris [5]. Use of fat-suppressed T2-WI and gadolinium-enhanced MR images may be useful to demonstrate the cystic nature of dermoid cysts, as there will be no fat suppression and no central enhancement of gadolinium [5]. The differential diagnosis with epidermoid cyst is not always straightforward on imaging alone and histopathological examination is often needed for final diagnosis.

Surgical excision of dermoid cysts is only recommended in symptomatic cases [2-3].