As retrorectal space, also known as presacral space, contains remnants derived from embryonic neuroectoderm, notochord, and hindgut, several kinds of congenital lesions may arise from this anatomic location. This kind of tumors are uncommon and mostly congenital and cystic hamartomas represent a small proportion of them.  During embryogenesis, the tailgut is the most caudal part of the hindgut distal to the future anus. It normally involutes by the 8^th week of embryonic development. If a tailgut remnant persists, it may become a tailgut cyst.

It can be found at any age but is more common in women between 40 and 60. Its clinical presentation is variable, depending on size and presence of complications. Half of developmental cysts are asymptomatic. Some patients have symptoms including constipation (usually the most common debut symptom), painful defecation, lower abdominal, sciatic, rectal or perianal pain and dysuria conditioned by local compression on the lower urinary tract. Their first symptoms are commonly caused by complications such as a chronic fistula, infection, bleeding or malignant degeneration.

In MRI studies the tailgut cysts present as retrorectal multilobulated thin-walled masses, usually well circumscribed, with typical low-intensity signal in T1 and high-intensity signal in T2, but in some cases show heterogeneous mixed signal. High intensity on T1-weighted images is secondary to mucoid, protein or haemorrhagic content.  Low intensity signal in T2 is also possible and related to haemorrhagic content. Rare cases of associated thin calcifications in the cyst wall are described (none of aur 5 patients).  Three lesions were located above the pelvic floor, two of them extended above pelvic diaphragm, through the levator ani muscle. Chronic fistulas may communicate with the cyst, involving levator ani muscle, posterior perianal skin, or anal canal’s in the midline.

The main  problem is whether a retrorectal cystic lesion should be considered malignant. Malignant transformation of the epithelial component of a tailgut cyst has only been reported on rare occasions. Malignant tumors that can develop from a tailgut cyst include adenocarcinomas, carcinoid tumors, neuroendocrine carcinomas, endometrioid carcinoma, adenosquamous carcinoma, squamous cell carcinoma, and sarcoma.

The most important finding that indicates malignancy is associated solid tissue component. Focal irregular wall thickening with intermediate signal intensity on both T1- and T2 weighted images, enhancement after contrast material injection, loss of discrete margins and involvement of contiguous structures, also suggests malignant degeneration. However fibrous tissue within the cyst caused by concurrent infection or chronic inflammation may show similar findings. In order to decrease risk of malignant degeneration and complications, complete surgical excision is recommended. 

Tailgut cysts present recurrence as common complication, therefore, MRI controls are recommended. Two of our patients had a local recurrence on first year post-surgery follow-up MRI and required a second surgical intervention.

Differential diagnosis of cystic hamartoma is important because of the malingnant potential of a tailgut cyst , unilocular or multilocular characteristic may help to do it and includes:

- Epidermoid cysts, they can be observed throughout the body, but are rare in the perirectal region. They are usually uniloculated and may contain calcifications and show heterogeneous signal intensity on T1- and T2-weighted MR image.  

- Dermoid cysts, they are usually uniloculated and contain variable amounts of fatty component.

- Duplication cysts, which are usually uniloculated.

- Cystic sacrococcygeal teratomas present mixed solid and cystic component,  50% content fat or calcifications. Teratomas are the most frequently encountered presacral lesion in the pediatric age group. - Anal gland cyst

- Cystic lymphangioma, multilocular

-Anterior meningocele or other neurogenic lesions, ussually unilocular