GASTROINTESTINAL STROMAL TUMORS WITH PROMINENT MYXOID MATRIX - CLINICOPATHOLOGICAL, IMMUNOHISTOCHEMICAL, AND ULTRASTRUCTURAL-STUDY OF 9 CASES OF A DISTINCTIVE MORPHOLOGIC VARIANT OF MYOGENIC STROMAL TUMOR

Nine cases are presented of a distinctive morphologic variant of myoge nic gastrointestinal stromal tumor characterized by an unusually promi nent myxoid stromal background reminiscent of a neural neoplasm but la cking the immunohistochemical or ultrastructural features of periphera l nerve sheath or ganglionic differentiation. The patients included si x women and three men aged 42 to 86 years (mean, 70). The lesions occu rred 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 cy stic cut surface. Histologically, the lesions were composed of a proli feration 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 staini ng reaction was abolished by treatment with hyaluronidase, indicating an abundance of connective tissue mucosubstances rich in hyaluronic ac id. Immunohistochemical stains showed strong positivity of the tumor c ells with vimentin antibodies in all cases and focal weak to moderate positive staining with muscle actin (HHF35) in eight cases and with de smin in two. Stains for keratin, S-100; epithelial membrane antigen, a nd collagen type IV were uniformly negative. Ultrastructural examinati on carried out in all cases showed features consistent with those prev iously described for myogenic gastrointestinal stromal tumors, namely, scattered mitochondria and prominent Golgi apparati, strands of rough endoplasmic reticulum, focal accumulation of intracyloplasmic microfi laments with occasional focal condensations, subplasmalemmal attachmen t plaques and immature cell junctions, focal extracellular basal lamin a material, and surface-oriented micropinocytotic activity. The myxoid changes observed in these tumors may represent a secondary, nonspecif ic reaction pattern of the tumor cells to some noxious stimulus, or th ey may be a form of degenerative phenomenon such as that commonly obse rved in smooth-muscle tumors of the uterus and other sites. Myogenic g astrointestinal stromal tumors with prominent myxoid stroma should be distinguished from benign schwannoma of the stomach and gastrointestin al autonomic nerve tumors. Because of the differences in prognosis for these entities, immunohistochemical and ultrastructural examinations are recommended for the evaluation of gastrointestinal stromal neoplas ms with prominent myxoid features.