Carcinoid Tumor Arising in a Tailgut Cyst of the Anorectal Junction With Distant Metastasis: A Case Report and Review of the Literature

Tailgut cysts are rare, congenital, hamartomatous lesions, and most occur in the retrorectal (presacral) space in all age groups. They are embryologically considered to arise from persistent remnants of the postanal gut (tailgut). Excluding the possibility of direct extension or metastases of carcinoid tumor from the rectum, a primary carcinoid tumor in the retrorectal space arising in tailgut cysts is rare and only 4 cases have been reported previously in the literature.1–3 Reports of other forms of malignancy arising in tailgut cysts are also rare and include approximately 10 cases of adenocarcinoma4–13 and 2 cases of neuroendocrine carcinoma.13,14 One of the 2 reported cases of neuroendocrine carcinoma can be categorized under carcinoid tumor based on the characteristic morphologic and immunohistochemical findings.13 We report the sixth case of carcinoid tumor arising in a tailgut cyst. This case is unique because it occurred at the anorectal junction, not in the retrorectal space, where all of the previously reported tumors have occurred, and it showed aggressive behavior with a distant metastasis.

A 41-year-old woman came to the hospital with a complaint of acute perianal pain. The patient was previously healthy. On digital rectal examination, a submucosal tumor was palpated at the anorectal junction. Values for tumor markers, including carcinoembryonic antigen and CA 19-9, were within normal ranges. By sigmoidoscopy, a submucosal ovoid polypoid mass measuring about 2 cm in greatest dimension was identified at 4 cm above the anal verge (Figure 1). The patient underwent an abdominoperineal resection. One year after surgery, the patient developed a liver metastasis and received 3 cycles of chemotherapy using 5′-fluorouracil. Three months after the chemotherapy, the patient complained of diplopia. By magnetic resonance imaging, an enhancing mass suggesting a brain metastasis was identified in the right cavernous sinus. The patient is currently on palliative radiation therapy.

Grossly, the excised mass was a multilocular cyst filled with yellow-tan serous fluid with partly gray-tan granular solid portions (Figure 2). Most parts of the multilocular cyst were located in the submucosa and the perianal sphincter muscle layers. The solid portions were present mostly between the multilocular cystic walls and focally in the submucosa of the anorectal junction. Microscopically, the various-sized multilocular cyst showed intervening solid portions containing oval or round cells (Figure 3). The cyst was lined by various epithelia, including ciliated columnar, squamous, and transitional cells, which were consistent with a tailgut cyst. Skin adnexal structures, neuronal elements, and heterologous mesenchymal tissues, such as bone and cartilage were not identified in the cystic areas. An area of transitional epithelial cells on the cystic wall focally abutted the tumor cells in the solid portions (Figure 4). The tumor cells in the solid portions predominantly demonstrated a trabecular growth pattern in the vascularized fibrous stroma (Figure 5). The tumor cells were oval or round with granular eosinophilic cytoplasm, uniform nuclei, fine chromatin, and inconspicuous nucleoli. There were areas of tumor necrosis and lymphovascular tumor emboli. Immunohistochemical staining of tumor cells in the solid portions showed positivity for cytokeratin AE1/AE3 (1:100; Zymed Laboratories, Inc, South San Francisco, Calif), synaptophysin (1:100; DiNonA, Seoul, Korea), and chromogranin (1:50; DiNonA), supporting the diagnosis of the carcinoid tumor (Figure 6). Lymph nodes in the perirectal soft tissue were free of tumor metastases.

Tailgut cysts, also commonly known as retrorectal cyst hamartomas, occur in all age groups and are more commonly reported in females.1–14 Most patients with tailgut cysts are symptomatic. Symptoms are related to mass effects, such as pain in the rectal area and discomfort when sitting. Some cases of tailgut cyst in asymptomatic patients are detected incidentally by routine physical examination or imaging studies conducted for other purposes.

The differential diagnoses for tailgut cysts include teratoma, dermoid cyst, duplication cyst (enterogenous cyst), anal gland cyst, and anterior sacral meningocele. In this case, the adjacent areas of the cyst contained no skin adnexal structures, neuronal elements, or heterologous mesenchymal tissues, such as bone and cartilage, excluding the possibility of teratoma or dermoid cyst. Absence of well-formed, 2-layer smooth muscle bundles with neural plexuses excluded the possibility of duplication cyst. The main location of the cyst in this case was the submucosa and the perianal sphincter muscle layer rather than the anal mucosa, ruling out the possibility of anal gland cyst. Cystic structures with a variety of epithelial linings, including squamous, ciliated columnar, and transitional epithelia, were diagnostic for tailgut cyst, excluding other diagnostic possibilities. In cases of infected tailgut cysts, there are additional differential diagnoses, such as pilonidal cyst, anorectal fistula, and recurrent rectal abscess.

Carcinoid tumors rarely arise in a tailgut cyst, and only 4 cases have been reported in the literature to date.1–3 Two other cases have been reported as neuroendocrine carcinomas.13,14 One of these 2 cases can be considered a carcinoid tumor,13 but the other did not show the histologic features of carcinoid tumor, despite positive immunoreactivity for neuroendocrine markers.14 Data for the 5 previously reported cases (including the 1 neuroendocrine carcinoma reported by Prasad et al13) and for the current case are summarized in the Table. All previously reported cases were located within the retrorectal (presacral) space, which is bounded anteriorly by the rectum, posteriorly by the sacrum, superiorly by the peritoneal reflection, inferiorly by the coccygeus and levator ani muscles, and laterally by the ureters and iliac vessels. No case was reported in the anorectal junction, as seen in our case. The differential diagnoses of a carcinoid tumor arising in a tailgut cyst include direct extension of colorectal carcinoid tumors into a preexisting tailgut cyst or metastases from other organs. However, thorough examination of the rectoanal mucosa in this case failed to demonstrate any evidence of carcinoid tumor in this area, and no other lesions were identified in other organs. Small areas where the lining epithelium of the cyst abutted the underlying carcinoid tumor cells suggest the possibility that neuroendocrine cells present in the cyst were the origin of the carcinoid tumor in this case. The prominent trabecular growth pattern of the carcinoid tumor in this case is very similar to that of a rectal carcinoid tumor, which is also a hindgut derivative.

The clinical significance of tailgut cysts includes the potential for infection, recurrent perianal fistulas, and rare malignant transformation. Because of a lack of familiarity with carcinoid tumor arising in tailgut cysts and its similar clinical presentation to other diseases, such as perianal fistula and abscess, the diagnosis is often delayed and prompt surgical intervention is not undertaken. Most cases of tailgut cyst are palpable on the digital rectal examination, and transrectal or presacral needle biopsies are indicated in patients with high risk for surgery. However, establishing a pathologic diagnosis based on the biopsy specimen alone is usually very difficult because biopsy specimens often contain only fibrous tissue without epithelia or only 1 type of epithelium, and the malignant foci in tailgut cysts can be very focal. Additionally, malignant cells can spill into the peritoneal cavity during the biopsy procedure. Thus, complete excision of tailgut cysts with a free margin of normal tissue and thorough gross examination with generous sampling should be performed.

Prognosis for tailgut cysts with malignant tumors depends on the status of complete surgical resection and tumor histology. Two cases with local recurrences and one case with a distant metastasis have been reported in adenocarcinomas arising in tailgut cysts in the literature.8,12 Previous literature reports of cases of carcinoid tumor have not cited metastases. However, it is important to appreciate the malignant potential of carcinoid tumors. We believe the current case represents the first case of carcinoid tumor arising in a tailgut cyst associated with distant metastases during follow-up (Table).