QUIZ: No. 35, 17/11/2025

QUIZ No. 35

ANS – Tailgut cyst (Retrorectal cystic hamartoma)

Findings: USG shows a well-defined retrorectal cystic lesion that is anechoic to hypoechoic with internal low-level echoes and no internal vascularity. CT reveals a well-circumscribed, thin-walled, low-attenuation cyst located in the presacral/retrorectal space with smooth margins, anterior displacement of the rectum, and no calcification, fat, air, solid components, or sacral erosion. MRI demonstrates a retrorectal cystic mass that is markedly hyperintense on T2 and predominantly hypointense on T1, with thin peripheral rim enhancement without diffusion restriction. No features of invasion, fistula formation, or malignant mural nodules are seen, and the surrounding pelvic structures are preserved.

Tailgut cysts arise from remnants of the embryonic post-anal gut.
Common in presacral space of middle-aged adults.

Though usually benign, they can undergo malignant transformation (adenocarcinoma, neuroendocrine tumour, squamous carcinoma) in up to 10–13% of cases.

Lack of fat, air, or significant enhancement helps differentiate it from dermoid, epidermoid, abscess, or rectal duplication cyst.

Complete surgical excision is the definitive treatment due to malignant risk and potential for infection.

Take home points

A retrorectal, well-defined, multilocular cystic lesion in an adult should strongly suggest a tailgut cyst.

MRI is the best modality: it displays multilocularity, T2 hyperintensity, mucinous contents, and absence of solid enhancement.