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A 64-year-old man was referred to our institution after he was found to have a retrorectal mass on a computed tomographic scan he underwent for persistent rectal pain and discomfort. Family and medical history were unremarkable. No laboratory abnormalities or specific findings were noted on physical examination. At digital rectal examination, the mass was not palpable and the rectal mucosa was intact. The computed tomographic scan showed a 40-mm mass in the ischiorectal fossa adherent to the levator ani muscle and rectum, and while it was suspicious for invasive rectal carcinoma, the exophytic appearance was suggestive of a soft tissue neoplasm. The patient also underwent magnetic resonance imaging, which revealed an enhancing, 35-mm soft tissue mass in the right ischiorectal fossa adherent to the wall of the rectum with questionable involvement, extending through the right levator ani muscle, suspicious for a rectal carcinoma vs a soft tissue mass (Figure 1). Transrectal ultrasonography was performed and a 30-mm solid hypoechoic mass with a 4-mm calcification in the left lateral perirectal space 9 to 11 cm from the anal verge was noted without signs of rectal wall invasion. No lesions suspicious for locoregional invasion were identified. Fine-needle (22-gauge) aspiration was performed and the cytological analysis revealed the presence of neoplastic elements positive for AE1/AE3 and synaptophysin but not for chromogranin and p63.

Based on the radiological and cytological findings, which did not rule out a potential malignancy, the patient underwent surgical excision of the mass. An arcuate incision was performed, extending from the posterior midline of the anal margin, with preservation of the sphincter complex to the right of the coccyx. The coccyx was excised to achieve adequate exposure, the anococcygeal ligament was divided, and the retrorectal space was entered. A 5-cm calcified mass involving the posterior aspect of the retrorectal space adjacent to the tip of the coccyx extending toward the right was identified and excised to normal healthy tissue, confirmed by multiple frozen sections. At gross examination, the specimen consisted of a well-circumscribed mass of rubbery consistency with a cystic component and a 1-cm calcified area (Figure 2).

The postoperative course was uneventful and the patient was discharged on postoperative day 1.

A.  Metastatic mesorectal lymph node

B.  Neuroendocrine carcinoma of a tailgut cyst

C.  Dermoid cyst

D.  Chordoma

Answer

The final pathologic diagnosis was low-grade neuroendocrine carcinoma arising from a tailgut cyst. At the initial 3-month follow-up, the patient was doing well, with no signs of recurrence on physical examination and transrectal ultrasonography. His follow-up will consist of physical examination and transrectal ultrasonography every 3 months for the first year, every 6 months for the second, and yearly after that for a total of 5 years. Computed tomographic scan of the abdomen and pelvis will be obtained every year for 5 years. Regular colonoscopy protocol will be followed as per average-risk patients.

Tailgut cysts, or cystic hamartomas, are rare developmental cysts derived from tailgut remnants, an embryological precursor of the gastrointestinal system, thus explaining the common finding of glandular columnar epithelium within.¹ The differential diagnoses include other congenital developmental (epidermoid cysts, dermoid cysts, and teratomas) and miscellaneous (chordomas, anterior meningocele, rectal duplications, and adrenal rest tumors) cystic lesions, inflammatory lesions (granulomas and abscesses), neurogenic neoformations (neurofibromas, neurolemmomas, ependymomas, ganglioneuromas, and neurofibrosarcomas), osseous tumors (osteomas, osteogenic sarcomas, sacral bone cysts, and Ewing tumor), and other neoplastic lesions (metastases, lymphangiomas, desmoid tumors, leiomyomas, and fibrosarcomas).² Congenital lesions account for 55% to 70% of retrorectal lesions, and 60% of them are developmental cysts.³ Tailgut cysts can occur at any age but are more frequent in middle-aged women and most of the time are asymptomatic and incidentally diagnosed by routine physical examination or imaging studies obtained for nonspecific and often unrelated symptoms.⁴ Malignant transformation is unusual but the development of adenocarcinomas, carcinoids, squamous cell carcinomas, and sarcomas has been described.⁵ Digital rectal examination can provide essential information on the location, consistency, and mobility of the lesion, while endoscopic ultrasonography, computed tomographic scan, and magnetic resonance imaging are valuable tools in defining texture, extent, and relationships with surrounding structures of the lesion, the latter being the diagnostic imaging tool of choice.¹ The role of transrectal or presacral needle biopsy is controversial because of the low likelihood of obtaining a significant sample for an adequate diagnosis, along with the potential risk of tumor seeding.⁶ A primary carcinoid tumor arising in a retrorectal tailgut cyst is a rare finding, with only 12 cases previously reported in the literature.^5,7 Neuroendocrine carcinomas arising in tailgut cysts have a real malignant potential, as the occurrence of distant metastases has been reported in the literature.⁸ Besides the oncological concerns, the need for surgical excision of tailgut cysts resides in the potential for infection and fistulization.⁹ Surgical resection is the keystone in the management of all retrorectal tumors, aiming to excise the mass intact and surrounded by macroscopically intact tissue. For low lesions (below the fourth sacral element), a posterior approach is preferred to the transabdominal route to better assess local invasion without violating the rectal wall, unless it is involved.¹⁰ As confirmed in this case, a parasacrococcygeal approach with resection of the coccyx provides optimal exposure, thus facilitating radical en bloc resection of the tumor, sparing the uninvolved surrounding structures, with minimal surgical trauma and morbidity and fast recovery.