Case Presentation: A 50-year-old man with hypothyroidism and dyslipidemia presented with one week of bilateral upper abdominal pain; subjective fevers; chills; sweats; and anorexia. His abdominal pain was constant, aching, relieved with lying supine, and unaffected by eating. He denied nausea, vomiting, early satiety, weight change, or change in bowel habits or stool characteristics. He further denied chest pain, dyspnea, cough, dysuria, hematuria, or change in urine output.
He had low-grade elevated temperature. Vital signs were otherwise normal. Tenderness and firmness of the epigastrium and left upper abdominal quadrant were noted. There was no palpable mass.

Leukocytosis was noted. Basic metabolic profile, hepatic chemistries, lipase, lactate, and urinalysis were all normal.

Computed tomographic scan of the abdomen and pelvis showed an 8.0 x 9.0 x 9.6 cm, heterogeneous, left-sided abdominal mass, inseparable from adjacent loops of small bowel, with irregular peripheral enhancement and evidence of central necrosis. There was no bowel obstruction. Based on the imaging findings, the interpreting radiologist raised concern for malignancy.

Percutaneous biopsy of the abdominal mass was planned. However, fevers persisted, leukocytosis worsened, and repeat imaging showed further enlargement of the mass, with findings suspicious for bowel perforation and abscess. Percutaneous drainage of 650 milliliters of purulent fluid was performed. Culture of this fluid ultimately grew intestinal flora.

The patient subsequently underwent exploratory laparotomy, with resection of the mesenteric mass and adjacent jejunum, and regional lymph node sampling. Pathology showed gross and immunohistochemical evidence of mesenteric fibromatosis and abscess, but no evidence of malignancy.

Discussion: Mesenteric fibromatosis is a sub-category of desmoid tumors, which are benign, slowly-growing, fibroplastic neoplasms with a propensity for local recurrence, even after complete surgical resection, particularly in patients with familial adenomatous polyposis (FAP). Desmoid tumors can develop at virtually any body site, but three main anatomic sites are described: trunk/extremity, abdominal wall, and intra-abdominal (bowel and mesentery). Most arise sporadically, although there are associations with FAP (5-15% of cases) and pregnancy.

Desmoid tumors are rare, accounting for only 0.03% of all neoplasms and < 3% of all soft tissue tumors. Although lacking malignant potential, they are locally infiltrative and cause morbidity through destruction of adjacent vital structures and organs. Treatment is with aggressive surgical resection, as able.

Conclusions: Intra-abdominal malignancy is often discovered by hospitalists in the course of investigating suggestive clinical and imaging findings. However, in this case, a benign mesenteric neoplasm and associated abscess masqueraded as malignancy. The purpose of reporting this case is to describe mesenteric fibromatosis, an unusual etiology of intra-abdominal mass, and to remind hospitalists to be vigilant of the heuristic of premature closure.