Gastrointestinal stromal tumors with prominent myxoid matrix. Clinicopathologic, immunohistochemical, and ultrastructural study of nine cases of a distinctive morphologic variant of myogenic stromal tumor.

Nine cases are presented of a distinctive morphologic variant of myogenic gastrointestinal stromal tumor characterized by an unusually prominent myxoid stromal background reminiscent of a neural neoplasm but lacking the immunohistochemical or ultrastructural features of peripheral nerve sheath or ganglionic differentiation. The patients included six women and three men aged 42 to 86 years (mean, 70). The lesions occurred in the stomach (seven cases) and small intestine (two cases) and ranged in size from 2.5 to 9.5 cm. They were described grossly as well circumscribed, unencapsulated, with a prominently myxoid and often cystic cut surface. Histologically, the lesions were composed of a proliferation of round, spindle, or stellate cells embedded in an abundant myxoid stroma. Histochemical stains showed strong positive reaction of the myxoid stromal background with alcian blue at pH 2.5; this staining reaction was abolished by treatment with hyaluronidase, indicating an abundance of connective tissue mucosubstances rich in hyaluronic acid. Immunohistochemical stains showed strong positivity of the tumor cells with vimentin antibodies in all cases and focal weak to moderate positive staining with muscle actin (HHF35) in eight cases and with desmin in two. Stains for keratin, S-100; epithelial membrane antigen, and collagen type IV were uniformly negative. Ultrastructural examination carried out in all cases showed features consistent with those previously described for myogenic gastrointestinal stromal tumors, namely, scattered mitochondria and prominent Golgi apparati, strands of rough endoplasmic reticulum, focal accumulation of intracytoplasmic microfilaments with occasional focal condensations, subplasmalemmal attachment plaques and immature cell junctions, focal extracellular basal lamina material, and surface-oriented micropinocytotic activity. The myxoid changes observed in these tumors may represent a secondary, nonspecific reaction pattern of the tumor cells to some noxious stimulus, or they may be a form of degenerative phenomenon such as that commonly observed in smooth-muscle tumors of the uterus and other sites. Myogenic gastrointestinal stromal tumors with prominent myxoid stroma should be distinguished from benign schwannoma of the stomach and gastrointestinal autonomic nerve tumors. Because of the differences in prognosis for these entities, immunohistochemical and ultrastructural examinations are recommended for the evaluation of gastrointestinal stromal neoplasms with prominent myxoid features.