Morphologic studies of gastric stromal tumors (GSTs) indicate that mitotic counts (MCs) and tumor size are major discriminants predictive of biologic behavior. The purpose of this study is to improve the understanding of GST behavior, including the prognostic factors and surgical treatment of GSTs. A retrospective analysis (1990-1997) of the clinical course for 116 patients with GSTs was completed, with a median follow-up of 43 months. Tumors were categorized as malignant GSTs (n = 17) when the MC was > 5/50 high-power fields (HPF) and the size > 5 cm or as benign GSTs (n = 99) when the MC was less than or equal to 5/50 HPF and the size less than or equal to 5 cm, MC less than or equal to 5/50 HPF and size > 5 cm, or MC > 5/50 HPF and size less than or equal to 5 cm. None of 99 benign tumors recurred or metastasized, whereas 7 of 17 malignant tumors recurred. MCs had a close correlation with tumor size. Immunohistochemical studies using CD34, smooth muscle actin, S-100 protein, and synaptophysin have shown positive rates of 61%, 33%, 14%, and 3% respectively. Smooth muscle actin reactivity was more common in the benign tumors (p = 0.046) and synaptophysin reactivity in the malignant tumors (p = 0.010). Univariate analysis shelved that the following clinicopathologic factors were potentially related to poor survival of patients: (1) MC > 5/50 HPF (p = 0.0001); (2) severe pleomorphism (p = 0.0062); (3) necrosis (p = 0.0173); (4) marked cellularity (p = 0.0112); (5) presence of ulceration of overlying gastric mucosa (p = 0.0091); (6) tumor size > 5 cm (p = 0.0195); and (7) exogastric growth pattern (p = 0.0344). Tumors with MC > 5/50 HPF and size > 5 cm were found to be strong indicators of an unfavorable prognosis. The type of surgery and tumor site did not affect the prognosis of the patients.
