Tailgut cysts, also known as retrorectal cystic hamartomas, are rare congenital lesions arising from remnants of the embryonic hindgut. Embryologically, the cloacal membrane covers the distal aspect of the future hindgut at the fourth week of gestation and normally regresses by the eighth week; persistence of this structure leads to a tailgut cyst (1). Histologically, they are composed of various epithelial linings but are distinguishable from teratomas due to the absence of skin or mesenchymal tissue (2). They exist in the presacral space, just posterior to the rectum, and are thus often asymptomatic, but may present with nonspecific symptoms such as perineal pain, defecatory dysfunction, or may be identified during imaging for unrelated concerns (3).

While typically benign, tailgut cysts carry a small risk of malignant transformation, most commonly into neuroendocrine tumor or adenocarcinoma (3, 4). Malignant transformation is often an incidental finding, complicating diagnosis and treatment. In rare circumstances, malignancy may be detected due to a secondary infection, which may manifest as a perianal or pilonidal abscess (5). We report a case of a 68-year-old woman who presented with a spontaneously ruptured sacral mass and presumed abscess, ultimately diagnosed as mucinous adenocarcinoma arising from a tailgut cyst.

A 68-year-old Caucasian female with a past medical history of remote pulmonary embolism, type 2 diabetes, iron-deficiency anemia, and hypertension presented to our institution’s Emergency Department (ED) after experiencing spontaneous rupture of a presumed sacral abscess. The lesion had been gradually increasing in size for 7 months prior to the ED presentation (Figure 1). The patient had previously undergone a biopsy at an outside hospital, which revealed acellular mucinous tissue. She was treated with a course of oral amoxicillin/clavulanate but did not experience significant improvement. She denied systemic symptoms such as fever, chills, loss of appetite, or fatigue.

On admission, laboratory results revealed lactic acidosis (lactate 4.2 mmol/L) and elevated inflammatory markers—ESR 62 mm/hr and CRP 11.7 mg/L—consistent with an acute inflammatory or infectious process. The patient had mild anemia (hemoglobin 10.4 g/dL, hematocrit 34.9%) with normal platelet and white blood cell counts. Tumor markers were within normal limits (CA 19-9: 14 U/mL; CA 125: 8 U/mL; CEA: 3.4 ng/mL, slightly above non-smoker reference). CRP levels peaked at 71.5 mg/L on day 4, then declined steadily to 30.5 mg/L (day 7), 21.1 mg/L (day 9), and 23.8 mg/L (day 10), indicating a robust inflammatory response followed by gradual clinical improvement. Blood cultures eventually grew methicillin-sensitive Staphylococcus aureus (MSSA), confirming bacteremia. Overall, findings were initially consistent with systemic inflammation due to MSSA bacteremia, mild anemia, and an appropriate biochemical response to treatment without obvious evidence of malignancy.

A computed tomography (CT) scan of the abdomen and pelvis revealed a multiloculated complex lesion centered at the sacral coccyx with possible bony erosion (Figures 2A–C). She was started on empiric intravenous (IV) cefepime and vancomycin. Subsequently, a superficial fluid swab also grew MSSA, prompting a switch to IV cefazolin (Ancef).

Further evaluation with magnetic resonance imaging (MRI) revealed a large presacral and post-sacral mass with features concerning for a myxoid neoplasm or chordoma (Figures 2D–F). Additionally, a 1 cm enhancing lymph node on the left pelvic wall raised concern for metastasis, though the malignancy did not appear to originate from the rectum. To obtain a definitive diagnosis, the interventional radiology team performed a CT-guided fine needle biopsy, which identified mucinous adenocarcinoma arising from a tailgut cyst on histology.

The histopathological and immunohistochemical findings are shown in Figure 3. Histologic sections showed a portion of a cyst wall and multiple fragments of atypical mucinous epithelium with mucin-rich glands (Figures 3A–C) supporting the diagnostic features of an adenocarcinoma. In immunohistochemical studies, the neoplastic cells were positive for CK20 and CDX2 and negative for CK7, supporting an intestinal immunophenotype (Figures 3D–F). Taken together, the clinical, radiological, and histopathological findings were consistent with mucinous adenocarcinoma arising in a tailgut cyst.

Given the initial MSSA-positive culture from the mucinous tissue, the infectious disease team recommended a total 10-day course of IV cefazolin. Repeat biopsy cultures showed no microbial growth. During hospitalization, the patient also received IV iron (Venofer) for iron-deficiency anemia and fluconazole (Diflucan) for vulvovaginal candidiasis.

The patient completed concurrent chemoradiation 3 months after the initial ED visit, receiving external-beam radiotherapy to a total dose of 50 Gy in 25 fractions, along with capecitabine 1,000 mg twice daily by mouth during radiotherapy. The colorectal surgery team advised that complete tumor resection would require a proctectomy with permanent colostomy. The patient declined surgical intervention (due to a history of childhood psychological and sexual trauma), acknowledging that this choice would limit the likelihood of achieving the optimal oncologic outcome. Follow-up CT at an outside facility 3 months after chemoradiation demonstrated suboptimal response with ongoing mass measuring 9.2 x 4.0 x 7.2 cm.

Tailgut cysts were first described in 1885 (5), as an entity presenting in the retro-rectal space (also referred to as the presacral space), being posterior to the rectum and anterior to the sacrum and coccyx. They are embryological remnants of the postnatal portion of the hindgut, and failure of regression of this structure leads to cyst formation (6). The incidence of tailgut cysts can be difficult to determine, though data from a review at the Mayo Clinic from 1960–1979 suggested an incidence of 1:40,000 with a strong female predominance (7). Other large reports, including a case series of 53 cases (8) and a systematic review of 196 cases (4), confirm between three- and four-to-one female-to-male predominance with the average presentation in midlife, and approximately half of these patients were asymptomatic (8). Due to a risk of malignant transformation, diagnosed tailgut cysts are generally surgically excised. The malignancy rate quoted has been variable, with a recent Mayo Clinic review of 73 patients suggesting an 8% rate of cancer in these patients (9). The case report reviews likely overestimate the true prevalence, as the authors themselves acknowledge, given the tendency for case reports to emphasize rare or striking presentations (4, 8). In contrast, the data from the Mayo Clinic indicate a much lower malignancy rate of approximately 8%, while other studies attempting to characterize the rate of malignant transformation have reported a broader range, from 1.9% to 26.6% (9).

The common neoplasms are adenocarcinoma, neuroendocrine tumors and carcinoid tumors, but other potential transformations include transitional cell carcinoma or squamous cell carcinoma (2–4, 10). Of note, Prasad reported 2 out of 5 tailgut cyst cases that transformed into malignancy, with one becoming mucinous adenocarcinoma and one become neuroendocrine tumor (2). Given the deep location and nonspecific presentation of mucinous adenocarcinoma, lesions may go undiagnosed until they reach an advanced stage (2).

We reviewed 23 additional mucinous adenocarcinoma cases in the literature, summarizing their initial clinical presentations, tumor marker profiles, immunohistochemistry findings, treatment approaches, and clinical outcomes (Table 1). Most patients presented with non-specific symptoms such as lower abdominal, pelvic, or rectal discomfort, changes in bowel habits (including subacute onset of constipation), or persistent purulent drainage that was initially suspected to represent an abscess and treated empirically with intravenous antibiotics. Treatment strategies varied, with patients receiving chemotherapy, radiotherapy, surgery, or a combination of multiple modalities. Reported follow-up durations ranged from 7 months to 4 years, with no documented disease recurrences, supporting the relatively indolent clinical course of mucinous adenocarcinoma in these cases.

In our case, the initial presentation mimicked an infected sacral abscess, delaying recognition of the underlying malignancy. The presence of acellular mucin in the initial biopsy was an early clue, but definitive diagnosis required targeted imaging and tissue sampling. A similar patient was reported in the literature, where the patient presented with perineal abscesses. In this case, the patient had a presumed abscess occurring 10 years prior to presentation resolving after incision and drainage, and another abscess occurring 3 years prior to presentation that did not respond to incision and drainage eventually requiring draining seton placement. This patient eventually presented with palpable left anterior perineal mass with biopsy confirming mucinous adenocarcinoma (11). They received adjuvant capecitabine and external beam radiation therapy (50.4 Gy) postoperatively, and remained free of disease as of 28 months (11).

The management of mucinous adenocarcinoma arising from a tailgut cyst typically involves a combination of surgery, chemotherapy, and radiation therapy. Complete surgical resection with clear margins is the preferred treatment when feasible. However, in cases with metastatic potential or extensive local invasion, neoadjuvant chemoradiation may be necessary before considering resection (12, 13). Among 20 previously reported cases, six were treated with surgery alone (2, 14–17), three with surgery and chemotherapy (18–20), four with surgery and radiotherapy (21–24), five with surgery and chemoradiotherapy (10, 11, 25–27), and one case did not specify the treatment approach (28). In our patient, the presence of an enhancing pelvic lymph node raised concern for metastatic spread, supporting the decision for neoadjuvant therapy before surgery.

The infectious component of this case also posed a diagnostic challenge. The initial MSSA-positive culture from the mucinous tissue raised the question of superimposed infection versus contamination. However, repeat cultures from a deeper biopsy site showed no bacterial growth, suggesting that the malignancy itself may have been the primary driver of inflammation rather than an active infection. This highlights the importance of correlating microbiologic findings with imaging and histopathologic results to avoid unnecessary prolonged antibiotic therapy.

Our patient’s case underscores the diagnostic complexity of presacral masses and the need for a high index of suspicion for malignancy in patients with atypical presentations. Early recognition and multidisciplinary management are essential to optimizing outcomes for these rare but aggressive tumors.

Furthermore, this case highlights the complex psychosocial challenges that can arise when surgical management involves a permanent colostomy. For some patients, the prospect of living with a colostomy bag can cause significant emotional distress, affect body image, and trigger feelings of loss of autonomy. In this case, the patient’s history of trauma added further complexity to treatment planning, influencing her decision-making process and tolerance for certain interventions. Our social work team, together with the oncology, surgical, and radiation teams, provided coordinated multidisciplinary support to address the patient’s psychological needs, ensure informed decision-making, and respect her autonomy while optimizing her overall care.