Introduction

Tailgut cysts, also known as retrorectal cystic hamartomas and mucus-secreting cysts, are rare congenital lesions that are thought to be derived from remnants of the embryonic postanal gut, which incompletely regresses during development [1–3].

They are almost invariably located in the retrorectal space [2], but have also been reported in the perirenal space [4–6], the perianal skin [3, 7–9] and, very rarely, prerectally [2, 10, 11]. We present a retrorectal tailgut cyst and also a much rarer prerectal tailgut cyst, which has only been reported four times before.

Case reports

Case 1

History and examination

A 24-year-old female presented to the Accident and Emergency Department with acute urinary retention requiring urethral catheterisation. She gave a 2-year history of low backache, but had no abdominal or anal symptoms. One year previously, she had a thyroidectomy for a follicular adenoma. Clinical examination of the abdomen revealed a pelvic mass and digital rectal examination revealed a smooth, large, immobile retrorectal mass displacing the rectum to the right-hand side. It was not possible to get above the mass on rectal examination.

Investigations

Initial investigation with transabdominal ultrasonography confirmed a pelvic mass. Computed tomography (CT) with contrast demonstrated a low attenuation lesion in the retrorectal space. Magnetic resonance imaging (MRI) with contrast confirmed a 9 × 11 × 11-cm cystic mass in the retrorectal space (Fig. 1), displacing the rectum, anal canal and distal sigmoid to the right and the vagina anteriorly. The cyst was well-defined, thin-walled, unilocular and extended into the right ischioanal fossa. The cyst was adherent to the left posterolateral wall of the rectum, but did not invade the sacrum. On T1-weighted MRI, the lesion was low signal and, on T2-weighted MRI, the lesion was high signal. There were no areas of heterogeneity or irregularity that enhanced with contrast.

Fig. 1
figure 1

Sagittal T2-weighted MRI scan showing the retrorectal tailgut cyst (black arrow) and the rectum (white arrow)

Full size image

Treatment

The patient underwent a low anterior resection with a hand-sewn coloanal anastomosis and loop ileostomy. The retrorectal cyst was excised completely (Fig. 2). Postoperative recovery was complicated by a left ischioanal fossa collection secondary to a contained leak from the coloanal anastomosis. This required two CT-guided percutaneous drainages. The patient then made a good recovery. A gastrograffin enema, at 3 months postoperatively, showed no leak from the coloanal anastomosis and the patient had uncomplicated ileostomy closure.

Fig. 2
figure 2

Opened tailgut cyst and specimen of rectum showing the pedicle and the inner lining of the tailgut cyst

Full size image

Histopathology

On histopathological examination, the macroscopic specimen consisted of a unilocular 11 × 6.5 × 4-cm collapsed cyst attached to the posterior mesorectal surface by a 3 × 2-cm stalk of connective tissue. The inner aspect of the cyst had an irregular, yellow surface with some mucoid fluid content (Fig. 2).

Microscopically, the cyst was partially lined by non-keratinising squamous epithelium (Fig. 3). In other areas, it was lined by granulation tissue with numerous foamy macrophages, scattered foreign body type multinucleated giant cells and cholesterol crystals. The underlying stroma showed chronic inflammation and fibrosis.

Fig. 3
figure 3

Tailgut cyst wall partially lined by non-keratinising stratified squamous epithelium (magnification ×50)

Full size image

Case 2

History and examination and investigations

A 60-year-old gentleman, with a past medical history of idiopathic epilepsy, had an abdominal ultrasound scan for the investigation of very mildly deranged liver function tests. The ultrasound scan coincidentally detected a pelvic cystic mass and left-sided hydronephrosis. Abdominal and digital rectal examinations were unremarkable. A subsequent CT scan with contrast revealed a partially calcified, lobulated cystic mass situated posterior to the bladder and anterior to the rectum. The cyst displaced the bladder to the right and was extrinsically compressing the distal left ureter causing left hydronephrosis (Fig. 4). The differential diagnosis based on the CT scan included a large bladder diverticulum with tumour within it or a tumour of the seminal vesicles or prostate. However, a transrectal ultrasound scan showed the lesion was not arising from the seminal vesicles or prostate and cystoscopy demonstrated that the bladder was normal. Percutaneous needle biopsy of the cyst was inconclusive, showing only mucinous and inflammatory material with no malignant cytology.

Fig. 4
figure 4

Axial plane CT scan showing the low attenuation tailgut cyst (black arrow) in front of the rectum (white arrow) and behind the bladder (dashed arrow)

Full size image

Management

The patient was managed with the insertion of a JJ stent at cystoscopy to relieve the left-sided hydronephrosis. Laparotomy revealed an anterior-rectal cystic lesion, adherent to both ureters and to the small bowel and rectum. Frozen section of the cyst wall was inconclusive showing only inflammatory cells. An anterior resection and defunctioning ileostomy was performed and the patient made an uneventful recovery.

Histopathology

On histopathological examination, the macroscopic specimen consisted of a unilocular collapsed cyst attached to the posterior mesorectal surface. The inner aspect of the cyst had an irregular, yellow surface with mucoid fluid content.

Microscopic examination of the cyst wall showed mucinous columnar epithelium with denuded areas lined with foamy macrophages and scattered regions of fibrotic connective tissue associated with multinucleated foreign body giant cells with cholesterol clefts. There was no bowel mucosa, organised smooth muscle layers or neural plexus (Fig. 5).

Fig. 5
figure 5

Tailgut cyst wall lining showing columnar epithelium (magnification ×100)

Full size image

Discussion

Origin

Tailgut cysts are believed to arise from the remnants of the embryonic hindgut. During the 3.5- to 8-mm gestational stage (28–35 days) of human development, the embryo possesses a true tail, which is caudal to the site of the subsequent formation of the anus. The primitive hindgut extends into this tail, which usually completely regresses by the eighth week of gestation [12]. The persistence of this postanal gut in the retrorectal space is hypothesised to be the origin of the tailgut cyst [1, 2].

Clinical features

Tailgut cysts predominantly present in middle-aged women with an average age of 35 and a male to female ratio of approximately 1:3 [2, 13, 14]. Tailgut cyst has been described in infants and in the foetus, strengthening the theory that it is a congenital condition [3, 15].

Symptoms of mass effect or pain are present in 50% and include rectal fullness, bleeding and pain on defaecation, constipation, lower abdominal and back pain and symptoms associated with genitourinary obstruction [2, 13, 16].

Misdiagnosis and infection

A large proportion of cases are initially misdiagnosed. The patient may present with a history of multiple drainage procedures for recurrent pilonidal abscess, perianal abscess or fistula in ano [2].

Examination and locations

On digital rectal examination, a mass is usually palpable in the retrorectal space [2]. However, few cases have been reported showing that tailgut cysts can occupy other positions in the perirenal space [4–6], the perianal skin [3, 7–9] and prerectally [2, 10, 11].

Malignancy

Cases of malignancies that have been reported to be associated with a tailgut cyst include adenocarcinomas, carcinoid tumours, neuroendocrine carcinomas, endometrioid carcinoma, adenosquamous carcinoma, squamous cell carcinoma and sarcoma. The majority are adenocarcinomas and carcinoid tumours [2, 9, 13, 14, 17–24].

Dysplasia

Adenocarcinomas arising from tailgut cysts may have p53 gene mutations similar to colorectal adenocarcinoma. These cysts also demonstrated dysplasia, suggesting a dysplasia–carcinoma sequence [19].

Tumour markers

Adenocarcinoma arising in a tailgut cyst has occasionally been shown to be positive for CEA and serum CEA and CA19.9. Tumour markers that have been shown to indicate tailgut cyst tumour recurrence [18, 20, 21, 25].

Radiological features

Localisation

Localisation of the cyst in the retrorectal space may be difficult with transabdominal ultrasound and can be mistaken for an adnexal mass [26]; however, it may reveal whether the cyst is unilocular or multilocular.

Transrectal ultrasound may be useful in demonstrating the integrity of the layers of the rectum, showing absence of invasion in a benign lesion [27]. Cross-sectional imaging techniques are more reliable at defining the location of the cyst in the retrorectal space [26].

Differential diagnosis of retrorectal cyst

When the location of a retrorectal cyst is known there is still a large radiological differential diagnosis; epidermoid cyst, dermoid cyst, rectal duplication, neuroenteric cysts, cystic sacrococcygeal teratoma, anterior sacral meningocele, necrotic rectal leiomyosarcoma, extraperitoneal adenomucinosis, cystic lymphangioma, pyogenic abscess, neurogenic cyst, necrotic sacral chordoma and tailgut cyst [14].

A prerectal cyst differential diagnosis includes utricle cyst of the prostate, simple cyst of the prostate, rectal duplication, simple cyst of the seminal vesicle and bladder diverticulum [11, 28, 29].

CT

CT usually demonstrates a thin-walled, well-circumscribed retrorectal mass with septations. When the mass is large, the rectum may be displaced. The low attenuation of the contents varies according to the consistency of mucin and inflammatory debris [26, 30].

Calcifications may be seen in the cyst wall although they are more commonly a feature seen in dermoid cysts, teratoma and other retrorectal space lesions (e.g. neuroblastoma, chordoma and mucinous adenocarcinoma) [24, 26, 31].

Infection may cause wall thickening along with other signs of local inflammation. Invasion of adjacent structures and loss of discrete margins are important signs of malignancy [26, 30].

MRI

Tailgut cysts usually have a low signal intensity on T1-weighted images and high signal intensity on T2-weighted images [32, 33].

MRI is more sensitive than CT at differentiating unilocular and multilocular masses and especially for the detection of small peripheral cysts [33]. The cyst wall and septa usually enhance with gadolinium, as do areas of malignant transformation and infection. Malignancy is suspected if there is a heterogeneous thickening with an irregular margin, invasion into the pelvic sidewall or adjacent pelvic viscera and intermediate signal before contrast on both T1- and T2-weighted images with enhancement after contrast [20, 23, 31, 32]. Recurrent benign tumours may be heterogeneous and have irregular margins and be indistinguishable from malignancy [34].

Histopathological features

Tailgut cyst

The histopathological features that help differentiate tailgut cysts from the other types of developmental cysts are the cell types in the lining of the cyst.

Gross examination of a tailgut cyst usually reveals a multilocular cyst ranging in size from 1 to 15 cm with a clear, thick mucoid fluid content. The cyst wall can be lined by a variety of epithelia: stratified squamous, columnar, cuboidal, ciliated columnar, mucinous, gastric and transitional. Scattered bundles of smooth muscle fibres are usually present in the cyst wall separated from the epithelium by a thin layer of fibrous tissue [2, 13, 14].

Other types of retrorectal/developmental cyst

Epidermoid cysts are unilocular and lined with stratified squamous epithelium and contain clear fluid. Dermoid cysts are similar to epidermoid cysts, but also contain skin appendages (e.g. hair follicles, sweat glands). Rectal duplication is defined by three histological criteria: continuity with the rectum, a smooth muscle coat with myenteric plexus and a mucosal lining reflective of the gastrointestinal tract with muscularis mucosae, villi and crypts. Teratomas contain derivatives from all three embryonic germ cell layers: ectoderm (squamous epithelium and skin appendages), endoderm (respiratory or gastrointestinal epithelium and pancreas) and mesoderm (connective tissue, smooth muscle, cartilage and bone). Neuroenteric cysts differ from tailgut cyst in that they contain a well-defined lamina propria and a more mature mucosa of endodermal origin (e.g. intestinal, bladder) [2, 13, 14, 31].

Teratoma

It has been suggested that tailgut cysts may be classified as teratomas because they contain evidence of ectoderm (squamous epithelium), endoderm (respiratory ciliated epithelium) and mesoderm (smooth muscle) [13, 25]. However, the ciliated columnar epithelium may have persisted from the embryological gastrointestinal tract and squamous epithelium may be due to metaplasia [2].The term teratoma is reserved for cysts demonstrating dermal appendages, neural elements, cartilage or bone [14].

Differential diagnosis of a prerectal cyst

The differential diagnosis of prerectal tailgut cyst includes: rectal duplication, utricle cyst of the prostate, which is lined by columnar or cuboidal epithelium and continuous with the utricular wall [35], and simple cyst of the seminal vesicle, which is lined by cuboidal or squamous epithelium with a fibrous wall and sperm within [11].

Management

All retrorectal cysts should be assessed for malignancy with radiological imaging and biopsy for histology. Unfortunately, definitive diagnosis and particularly differentiation of tailgut cysts from the other developmental cysts is usually not possible on diagnostic biopsies, including needle biopsy and frozen sections, as was the case in the second of our patients. Although diagnosis has been reported with ultrasound-guided needle biopsy [36], a diagnostic biopsy is generally not advised because it may miss foci of malignancy. Even if the biopsy does sample a malignancy, it carries the risk of spillage and seeding of tumour cells [14].

If there is a low index of suspicion for malignancy and patients are asymptomatic, routine surveillance may be appropriate. However, surgical resection provides a specimen that is sufficient for histopathological examination to give a definitive diagnosis, relieves symptoms caused by mass effect, avoids subsequent infection and rules out malignant transformation.

Conclusion

The preoperative diagnosis depends primarily on correct localisation of the cyst, which is usually in the retrorectal space, this is most accurately provided by MRI. The recommended treatment is complete surgical excision to provide a definitive diagnosis, relieve symptoms and exclude malignancy.