Plexiform Fibromyxoma: A Distinctive Benign Gastric Antral Neoplasm Not to be Confused With a Myxoid GIST

A great majority of gastric mesenchymal tumors are gastrointestinal stromal tumor (GIST). A rare group of non-GISTs include myxoid mesenchymal neoplasms. In this report, we describe 12 cases of a distinctive gastric tumor, named here as plexiform fibromyxoma. These tumors occurred in 5 men and 7 women of ages 7 to 75 years (median, 41 y). All tumors were located in the gastric antrum and 6 of them also extended into extragastric soft tissues or into the duodenal bulb. The tumors measured from 3 to 15 cm (median, 5.5 cm). Histologically typical was a plexiform intramural growth with multiple micronodules containing paucicellular to moderately cellular myxoid to collagenous and fibromyxoid neoplastic elements. A prominent, sometimes plexiform capillary pattern was typically present. Extramural components included subserosal nodules, and sometimes more cellular, solid nonplexiform spindle cell proliferation. The tumor cells varied from oval to spindled and had limited atypia and mitotic activity <5/50 high-power fields. Frequent ulceration, mucosal invasion, and vascular invasion (4 cases) had no adverse significance in these tumors. Immunohistochemically, the tumor cells were positive for α smooth muscle actin, and variably for CD10, and were consistently negative for KIT, DOG1, CD34, desmin, and S100 protein. No KIT or platelet-derived growth factor receptor alpha mutations were present in the 3 examined cases. None of the 4 patients who were followed from 9 to 20 years (median, 19 y) developed recurrences or metastases. Additional 3 patients survived 14 to 25 years with unknown tumor status. Review of large numbers of mesenchymal tumors in the esophagus and intestines did not reveal similar tumors. Plexiform fibromyxoma is a distinctive benign gastric antral neoplasm that should be separated from GIST, nerve sheath tumors, and other fibromyxoid neoplasms.