Sclerosing mesenteritis Chronic mesenteric inflammation, usually involving the mesentery of the small bowel (rarely peripancreatic, omentum, and retroperitoneum) Cause Unknown cause, but often associated with other idiopathic inflammatory disorders like retroperitoneal fibrosis, sclerosing cholangitis, Riedel thyroiditis, orbital pseudotumor May be related to prior abdominal surgery Coexist with malignancy (69%) (lymphoma, breast, lung, colon cancer, melanoma) 3 subtypes, based on predominant tissue type within mass Mesenteric panniculitis: acute inflammation with/without fat necrosis Mesenteric lipodystrophy: fat necrosis > inflammation or fibrosis Retractile mesenteritis: fibrosis/retraction > inflammation or fat necrosis

Imaging Findings of Sclerosing Mesenteritis Acute mesenteritis “Misty mesentery”: non-specific sign, increased attenuation of jejunal mesentery, without discrete soft tissue mass Thin pseudocapsule encasing inflamed mesentery Cluster of mesenteric lymphadenopathy Mesenteric vessel and node have “halo” of uninflamed fat (“fat ring” sign) Chronic phase Solid soft tissue mass in mesentery root (fibrous tissue) Often has stellate appearance Encasement of mesenteric and collateral vessels With or without calcification Fatty necrotic cystic mass may be seen Fat ring sign

Differential Diagnosis Lymphoma Usually multiple large discrete or confluent mesenteric nodes No calcification unless treated (treated mesenteric lymphoma can look identical to mesenteritis) Preservation of fat-ring sign favors sclerosing mesenteritis Carcinoid tumor Usually involves ileal mesentery Primary hypervascular tumor with/without liver metastasis Mesenteric edema Seen in conditions like cirrhosis, heart failure, vasculitis, etc. Desmoid tumor (fibromatosis) Usually discrete solid mass Others: carcinomatosis (metastasis), amyloidosis, inflammatory pseudotumor, IgG4-related disease