Heterotopic mesenteric ossification: a distinctive pseudosarcoma commonly associated with intestinal obstruction.

Heterotopic mesenteric ossification (HMO) is a rare intraabdominal bone-producing pseudosarcoma with fewer than 14 reported cases in the literature. We report our experience with 6 additional cases, all of which were referred to us with a diagnostic consideration of extraskeletal osteosarcoma (EO) or "sarcoma" and emphasize features which distinguish HMO from EO. Six intraabdominal lesions coded as "heterotopic mesenteric ossification," "ossifying pseudotumor," or "reactive myofibroblastic proliferation with ossification" were retrieved from our consultation files. Clinical follow-up information was obtained. Lesions occurred exclusively in males, with a mean patient age of 49 years (range, 22-72 years). The tumors occurred in the mesentery (N = 4), omentum (N = 1), or both (N = 1) and were preceded by significant abdominal surgery (4 cases) or trauma (1 case) in all but 1 case. Five patients presented with bowel obstruction and 1 with abdominal sepsis. Tumors were difficult to precisely measure; the mean size of the resection specimens was 11.8 cm (range, 3.5-20 cm). Grossly, the tumors resembled fat necrosis and often cut with a gritty sensation. Microscopically, all lesions demonstrated an exuberant, reactive (myo)fibroblastic proliferation resembling nodular fasciitis, with extensive hemorrhage and fat necrosis. All tumors produced abundant bone and osteoid, often "lace-like," and 2 contained cartilage. The proliferating (myo)fibroblasts, osteoblasts, and chondroblasts were mitotically active but cytologically bland. Follow-up (4 cases; mean, 47.3 months; range, 5-120 months) showed 3 patients alive without disease and 1 dead of unrelated causes. One case was recent. HMO is a distinct intraabdominal ossifying pseudotumor that typically occurs in males, almost always after surgery or abdominal trauma, and frequently presents with symptoms of intestinal obstruction. This clinical history, presence of clearly reactive zones resembling nodular fasciitis, thick osteoid, and absence of nuclear atypia, necrosis, and atypical mitotic figures allow the distinction of HMO from its most important morphologic mimic, EO.