Low-grade fibromyxoid sarcoma in abdominopelvic and retroperitoneal locations: a clinicopathologic analysis of 26 cases and a review of the literature.

Low-grade fibromyxoid sarcoma (LGFMS) is a rare, slow-growing deceptively bland-looking fibroblastic sarcoma that usually develops in the deep soft tissues of the extremities. Development of the tumor in the abdomen, pelvis, and retroperitoneum is extremely rare. We analyzed 26 cases of LGFMS originating at these sites. The patients age ranged from 5 to 75 years (median, 39) with a slight female predominance (54%). Most tumors were located in the abdominal wall/abdomen or peritoneal tissues (n = 11; 42%), followed by the intestinal wall/mesentery (n = 7; 27%), pelvis (n = 4; 15%), retroperitoneum (n = 3; 11%) and the perirectal perineum (n = 1; 4%). Median tumor size was 10 cm (range, 1.9-68). Histologically, tumors were characterized by spindle cells with mild-to-moderate nuclear atypia arranged into short fascicles and whorling pattern within alternating variably myxoid to collagenous stroma rich in arcuate vessels with perivascular hyalinization. Eight tumors (31%) showed areas of sclerosing epithelioid fibrosarcoma (SEF), hence qualifying as hybrid LGFMS/SEF. Large collagen rosettes were observed in 8 tumors (31%). All 26 tumors (100%) were immunohistochemically diffusely positive for MUC4. FUS or EWSR1 gene rearrangements were identified in 9 of 9 (100%) tumors tested. One of 17 tumors showed variable nuclear Betacatenin reactivity (this case harbored EWSR1::CREB3L1 fusion). Of 15 patients with follow-up data (range, 6-84 months; median, 28.5), 2 (13%) died of disease, 3 (20%) developed metastases, and 2 (13%) had local tumor recurrence. In total, 7 (47%) patients experienced disease progression. Eight (53%) patients were disease-free at 5-84 months follow-up (median, 45). LGFMSs at these rare sites are similar to their extremity counterparts but likely peruse a more aggressive clinical course with adverse events being noted in half of patients at a median follow-up of 45 months, suggesting a possibly more unfavorable outcome at long-term.