| Discussion: |
Gastrointestinal carcinoid tumor
Gastrointestinal carcinoid tumors arise from neuroendocrine cells in the bowel mucosa or submucosa and are the most common malignant neoplasms of the small intestine. Approximately 40 to 80% spread to the mesentery, either by direct extension or via the local lymphatics . The ileum is the most frequent location of the primary lesion. The mesenteric involvement is usually discovered first, when patients present with non specific abdominal pain. Alternatively, patients with hepatic metastases may present with the carcinoid syndrome caused by the release of vasoactive substances into the systemic circulation.
At CT, the most common manifestation of mesenteric carcinoid is that of an enhancing soft tissue mass with linear bands radiating in the mesenteric fat. Radiologic-pathologic correlation has shown that these radiating strands of soft tissue result from the intense fibrotic proliferation and desmoplastic reaction in the mesenteric fat and the adjacent mesenteric vessels caused by the release of serotonin and other hormones from the primary tumor. Calcifications are visible in up to 70% lesions at CT . Thickening of adjacent small bowel loops caused by tumor infiltration or ischemia as well as angulation and or obstruction secondary to fibrosis are common associated findings . The primary tumor is often small and not always diagnosed at CT.
Desmoid tumors are rare locally aggressive non encapsulated masses resulting from a benign proliferation of fibrous tissue. Although they can occur sporadically and develop anywhere in the abdomen including in the musculature of the abdominal wall, the retroperitoneum and the pelvis, abdominal desmoids developing in the mesentery are especially common in patients with Gardner syndrome, particularly if the patient has undergone abdominal surgery   At CT, they present as soft tissue masses, often with poorly defined borders and strands radiating into the adjacent mesenteric fat . Large size (over 10cm), multiple desmoids as well as extensive infiltration of the small bowel and entrapment of the ureters are poor prognostic signs .
Several intra-abdominal malignancies, including gastric, pancreatic and colon cancer may extend directly into the leaves of the mesentery or spread along the mesenteric vessels. About 40% of patients with newly diagnosed adenocarcinoma of the pancreas have unresectable, locally advanced disease with tumor extension along the root of the mesentery and encasement of the major mesenteric vessels .
Sclerosing mesenteritis is a rare inflammatory condition of unknown etiology affecting the root of the mesentery. The mesenteric fat is involved with variable amount of inflammation, fatty necrosis and fibrosis. When the inflammation predominates, the so- called mesenteric panniculitis, patients generally present with acute pain. On CT, this entity presents as a focal area of increased attenuation within the mesenteric fat surrounded by a pseudocapsule. Areas of fibrosis within the inflamed fat appear as linear bands of soft tissue attenuation . In retractile mesenteritis, the fibrosis predominates and the disease manifest itself as large masses of soft tissue attenuation which may contain calcifications. Some masses are poorly defined with whiskers of soft tissue thickening extending into the adjacent fat . The infiltrative nature of the fibrosis may lead to result in serious complications including thrombosis the mesenteric vessels with secondary variceal bleeding. Scarring with retraction of the mesentery and encasement of small bowel loops can lead to ischemia or obstruction.