Tailgut cyst
Overview
- Rare congenital lesion arising from vestigial remnants of the embryonic tailgut (postanal gut)
- Typically benign multicystic mass in the presacral/retrorectal space
- Lined by various epithelia (squamous, transitional, columnar) reflecting hindgut origin
- No communication with rectal lumen (distinguishes from duplication cyst)
- Potential for malignant transformation (rare: adenocarcinoma, neuroendocrine tumour)
- Usually asymptomatic/incidental; symptoms from mass effect (pain, constipation, infection)
Epidemiology
- Predominantly middle-aged females (female:male 3–5:1)
- Age at presentation 30–60 years (can present from infancy to elderly)
- Extremely rare overall
Key radiological manifestations
CT
- Well-circumscribed, uni- or multilocular cystic mass in presacral space
- Posterior to rectum, anterior to sacrum/coccyx
- Thin walls, no solid components in benign cases
- Variable attenuation: mucoid content → higher density (20–40 HU)
- Occasional fat, calcification, or soft-tissue elements
- No enhancement of cyst contents; minimal wall enhancement if infected
MRI (modality of choice)
- T1WI: variable signal (low if serous, high if mucoid/proteinaceous)
- T2WI: high signal cystic locules (multilocular "honeycomb" appearance classic)
- Thin septa, no thick/nodular enhancement
- May show haemorrhage or debris levels
- Malignant change: solid enhancing nodules or invasive margins
Ultrasound (transrectal or endovaginal)
- Multilocular anechoic/hypoechoic mass posterior to rectum
- Limited role due to deep location
Classic appearance
Multicystic presacral mass with thin septa, no rectal communication, and variable T1 hyperintensity due to mucin → highly suggestive of tailgut cyst.
Differential diagnosis
| Diagnosis | Key differentiators |
|---|---|
| Rectal duplication cyst | Communicates with rectal lumen, single locule common, muscle wall layers |
| Anterior sacral meningocele | CSF signal intensity, contiguous with thecal sac, sacral defect |
| Epidermoid/dermoid cyst | Fat (dermoid) or restricted diffusion (epidermoid), may have calcification |
| Chordoma | Solid enhancing mass, sacral destruction, T2 hyperintense but not purely cystic |
| Neurogenic tumour (schwannoma) | Solid or solid-cystic, nerve origin, enhancement |
| Abscess | Thick/irregular walls, rim enhancement, clinical infection |
Common pitfall
Unilocular or mucoid tailgut cysts mistaken for meningocele or duplication → always confirm no sacral defect or rectal communication on sagittal MRI.
Imaging approach
- Initial detection: CT pelvis (often incidental)
- Characterisation: MRI pelvis with multiplanar T2 and T1 pre/post-contrast
- Sagittal and axial essential for anatomic relations
- Assess for solid components (malignancy)
- Preoperative: full sacral evaluation for bony involvement
- Biopsy/resection often required due to malignancy risk and differentials
Pearls
- Location strictly presacral (between rectum and sacrococcyx) is key
- Multilocularity and mucoid content more common than in other retrorectal cysts
- Infection or fistula rare but can complicate imaging (mimics abscess)
- Complete surgical excision (posterior or trans-sacral approach) is curative for benign lesions
- Follow-up if incomplete resection due to recurrence/malignancy risk
Reporting tip
Describe "well-defined multicystic presacral mass without sacral erosion or rectal communication, favoured to represent tailgut cyst; recommend surgical referral for excision and histological confirmation."
