Tailgut cysts are uncommon congenital anomalies that originate from remnants of the embryonic hindgut.¹ During embryonic development between 28 and 35 days of gestation, the embryo forms an appendage extending from the embryonic hindgut, known as the tailgut. Failed regression of the embryonic hindgut results in the development of a tailgut cyst.² In rare cases, however, tailgut cysts have been associated with meningothelial proliferation and benign thyroid tissue exhibiting oncocytic changes. In addition, some reports have identified the presence of pancreatic acini, islets of Langerhans, and even glomus bodies within these cysts. These findings contribute to the ongoing debate regarding the precise etiology of tailgut cysts.^2,3 Pathophysiologically, these lesions manifest as cystic structures lined with various epithelial types in the retrorectal space, which may lead to symptoms or complications through infection, compression of the adjacent structures, or malignant transformations.⁴

These lesions are usually located in the retrorectal space, lying anterior to the sacrum and posterior to the rectum,² but they are rarely found at the perirenal,⁴ perianal,⁵ subcutaneous⁶ and intradural sites.⁷ They usually present with non-specific symptoms, such as abdominal pain, discomfort, tenesmus, and dysuria, but they can also be asymptomatic.^1,2 Most tailgut cysts are benign, but a malignant transformation can occur, albeit rarely.⁸ Adenocarcinoma and neuroendocrine tumors are the most common malignant transformation patterns when the tailgut cysts are present.² A recent article has reported malignant transformation rates as high as 27%, suggesting that the true incidence of malignancies may be higher than previously assumed.⁹ No specific clinical variables, such as age or tumor size, have been found to predict a malignant transformation reliably.¹⁰

Surgery is the treatment of choice for a tailgut cyst.^2,11,12 If tailgut cysts are not completely resected, complications such as the development of cancer and recurrent draining sinuses with intractable infection can occur in the residual tissues.^2,12 Benign and malignant recurrences can occur, and recurrent lesions of either type require a re-resection. More concerningly, a case of distant metastasis has also been reported.¹² A complete excision of the cyst wall significantly reduces the risk of recurrence, which in turn lowers the overall morbidity in this patient population.¹¹ Therefore, the primary goal of surgery is a complete resection to avoid complications.

The use of an endoscopic resection has been steadily increasing in recent years. Endoscopic resection techniques such as EMR and endoscopic submucosal dissection (ESD) can remove lesions that invade the submucosal layer, offering a curative option for select early-stage tumors. An endoscopic full-thickness resection (EFTR) can facilitate the successful resection of tumors invading the muscularis propria layer.¹³ Most tailgut cysts are located external to the rectal wall, rather than within the submucosal layers.⁹ Nevertheless, certain lesions are located within the muscularis propria, a layer that now falls within the expanding indications for endoscopic resection. Thus far, only a single case report has documented successful endoscopic resection.¹⁴

Here, this paper reports a case of a 60-year-old woman with a tailgut cyst located in the rectum, which was treated successfully with EMR, and reviews the literature on this condition.

A 60-year-old woman visited the authors’ clinic for a health screening examination. She denied any previous medical history or other symptoms such as fever, weight loss, and abdominal pain. She was afebrile, and her blood pressure and pulse were normal. Her abdomen was soft and not distended, and there was no tenderness. The liver and spleen were not palpable, and there were no palpable superficial lymph nodes. A digital rectal examination revealed a small, hard, and firm yet movable mass. The laboratory findings, including complete blood cell count, liver function tests, and a renal function study, were within the normal limits. The level of serum carcinoembryonic antigen was 1.23 ng/mL (normal range: 0–5 ng/mL).

Colonoscopy revealed a 5-mm solitary subepithelial lesion with a yellowish surface, located in the lower rectum near the anal sphincter. The lesion appeared as a smooth, sessile subepithelial mass with an intact overlying mucosa and no surface ulceration or erythema. Superficial thin vessels were observed on the surface, without evidence of vascular distortion (Fig. 1). Therefore, narrow-band imaging was not performed. The lesion was hard and firm on palpation and exhibited positive rolling signs, indicating that it moved together with the forceps upon manipulation. Based on these findings, the lesion was initially suspected to be a neuroendocrine tumor. Endoscopic ultrasonography (EUS) revealed a 6 mm hypoechoic lesion in the lower rectum, characterized by an ill-defined margin and a thin hyperechoic wall, without internal septations (Fig. 2). The layer of origin could not be clearly identified owing to its proximity to the anal sphincter. No evidence of invasion into the adjacent rectal wall or surrounding tissues was observed. Contrast-enhanced chest, abdomen, and pelvis computed tomography (CT) showed no significant abnormalities.

Cap-assisted EMR was planned to ensure accurate histological diagnosis and effective treatment. A cap-fitted single- channel colonoscope (CF-HQ290I; Olympus, Tokyo, Japan) equipped with a transparent C-cap featuring a 7 o’clock snare slot, and a 25×2,300 mm EMR-C lower snare (Olympus), was used. A mixture of normal saline, indigo carmine, and epinephrine (1:100,000 dilution) was injected into the submucosal layer to achieve adequate lesion lifting. The lesion was resected en bloc using a snare after sufficient submucosal elevation (Fig. 3). The lesion was successfully suctioned; the resected specimen contained a yellowish component. Hemostasis was achieved by coagulating the blood vessels with a Coagrasper. No immediate bleeding or perforation was observed during the procedure. The total procedure time was approximately 10 minutes. Routine histology with hematoxylin–eosin staining revealed a benign cyst lined by a ciliated columnar epithelium (Fig. 4A, 4B). The biopsy specimens were interpreted as suggestive of a rectal tailgut cyst.

The patient remained asymptomatic on the night of the procedure and was discharged the following day. At the two-week outpatient follow-up, the patient remained asymptomatic, with no signs indicative of delayed complications. The patient has been on a regular follow-up schedule at the outpatient clinic and showed no evidence of recurrence at 12 months after EMR.

Tailgut cysts are most commonly identified in middle-aged women, as in the present case. Previous studies indicated a female-to-male ratio ranging from 3:1 to 9:1, showing a female predominance.^2,15 The diagnosis of tailgut cysts can be made by history taking, physical examination, CT, magnetic resonance imaging (MRI), or incidental detection during surgery. Preoperative tailgut cysts are challenging to diagnose because of their rarity.^1,2 Although patients have undergone CT or MRI prior to the diagnosis, it is often difficult to establish a definitive diagnosis. Sometimes, they are initially misdiagnosed as having a perianal fistula or abscess, which is then corrected after surgery.^1,8

Histopathologically, tailgut cysts are typically multicystic or multiloculated, with adherent surrounding fibroadipose tissue. They are usually well demarcated and soft in consistency, containing clear serous or translucent mucoid fluid. Microscopically, the cysts are lined with a variety of epithelial types, including stratified squamous, transitional, stratified columnar, mucinous or ciliated pseudostratified columnar, and gastric-type epithelium. In most cases, the cyst wall contains focal, well-formed smooth muscle fibers. Occasionally, the foci of glomus bodies, pancreatic acini and ductules, or islets of Langerhans can also be identified.^4,16

Various surgical procedures, including exploratory laparotomy, laparoscopic, posterior trans-sacral, trans-sphincteric, inter-sphincteric, perineal, and parasacrococcygeal approaches, and trans-anal endoscopic microsurgery, have been applied depending on the location, size, and malignant potential of the tailgut cyst.^15,17-19 The surgical outcome for tailgut cysts is generally favorable. The reported complete resection rates exceed 90%, with no procedure-related mortality within 30 days of the procedure. Documented complications include wound infection and small bowel obstruction.¹⁹ The reported recurrence rates have ranged from 0% to 16%, with a median follow-up duration of approximately one year.⁸

Nevertheless, surgical resection is inherently invasive and carries a high risk of postoperative complications. As with other colonic adenomas managed endoscopically, there is a growing need for less invasive treatment options. A recently published case report on the endoscopic resection of a tailgut cyst suggested a potential expansion of therapeutic strategies.¹⁴ In that case report, the tumor measured 2 cm in diameter and was located in the mid-rectum, 11 cm from the anal verge. EUS confirmed that the tumor originated from the muscular layer. A retrorectal extraluminal endoscopic resection (REER) was successfully performed.

The present case involved a 60-year-old woman with an asymptomatic rectal tailgut cyst, which was incidentally detected on screening colonoscopy and removed en bloc via EMR. To the best of the authors’ knowledge, this is the second reported case of EMR for a tailgut cyst located in the rectum, following a previous report by Kiosov et al.¹⁴, and the first such case reported in Korea. This represents an innovative use of EMR in the management of a tailgut cyst. Theoretically, an endoscopic resection may be considered inappropriate for most tailgut cysts because they are typically located external to the muscularis layer. As observed in the present case, however, some lesions are small, superficially located, and clearly visualized on EUS with well-defined margins. In such cases, EMR, ESD, or EFTR may represent reasonable therapeutic options when the lesions are technically amenable to endoscopic resection. These clinical scenarios may serve as potential selection criteria for an endoscopic resection.

The follow-up schedule has no established guidelines. Consistent with other reports,^8,19 a 12-month interval follow-up colonoscopy with CT was planned to detect recurrence and potential distant metastasis. Serum laboratory monitoring may also be warranted, as with other malignancies. Lifelong follow- up may not be necessary because most tailgut cysts follow an indolent course, and the majority of recurrences occur within one year.²⁰ In conclusion, despite their rarity, tailgut cysts should be considered in the differential diagnosis of rectal subepithelial tumors. Complete endoscopic resection is essential for managing rectal tailgut cysts because residual lesions may carry a risk of malignant transformation. Nevertheless, further follow- up is necessary to determine the long-term clinical outcome of this treatment approach.