Extragastrointestinal (Soft tissue) stromal tumors: An analysis of 48 cases with emphasis on histologic predictors of outcome

The clinicopathologic features of 48 tumors that were histologically simila r to gastrointestinal stromal tumors but occurred in the soft tissues of th e abdomen were analyzed to determine their overall similarity to their gast rointestinal counterpart, their biologic behavior, and the parameters that predict risk for adverse outcome. Classic leiomyomas and leiomyosarcomas we re specifically excluded. The tumors occurred in 32 women and 16 men, who r anged in age from 31 to 82 years (mean, 58 years). Forty tumors arose from the soft tissue of the abdominal cavity, and the remainder arose from the r etroperitoneum. They ranged in size from 2.1 to 32.0 cm and varied from tum ors composed purely of rounded epithelioid cells to those composed of short fusiform cells set in a fine fibrillary collagenous background with some c ases showing a mixed pattern. Tumors displayed variable amounts of stromal hyalinization, myxoid change, and cyst formation. The tumors expressed CD11 7 (c-kit receptor) (100%), CD34 (50%), neuron-specific enolase (44%), smoot h muscle actin (26%), desmin (4%), and S-100 protein (4%). Tumors were eval uated with respect to several parameters: size (<10 cm or >10 cm), cellular ity (low or high), mitoses (0 to 2 per 50 high-power fields, >2 per 50 high -power fields), nuclear atypia (1 to 3+), cell type (epithelioid, spindled, or mixed), and necrosis (absent or present). These parameters were then ev aluated in univariate and multivariate analysis with respect to adverse or nonadverse outcome, the former defined as metastasis or death from tumor. F ollow-up information was obtained for 31 patients (range, 4 to 84 months; m edian, 24 months). One patient presented with an adverse event and, therefo re, was excluded from subsequent analysis. Twelve patients (39%) developed metastases or died of tumor. In univariate analyses, cellularity, mitotic a ctivity (>2 per 50 high-power fields), and necrosis were associated with st atistically significant increases in the risk for adverse outcome. Despite the relatively small sample size, in a multivariable analysis mitotic activ ity (relative risk, 7.46; P = .09) and necrosis (relative risk, 3.75; P = . 07) displayed trends toward independent predictive value. No association wa s noted between histologic pattern and outcome. Although only 39% of tumors behaved in a malignant fashion, this figure probably represents a conserva tive estimate because long-term follow-up (>5 years) was available for only a limited number of patients. Stratification of patients who have extragas trointestinal stromal tumor into those with 0 to 1 adverse histologic facto rs versus those with 2 to 3 offers the advantage of separating patients int o two groups that have a markedly different risk for adverse outcome in the short term (0.02 events versus 0.54 events per person-year; P < .001, resp ectively). Extragastrointestinal (soft tissue) stromal tumors are histologi cally and immunophenotypically similar to their gastrointestinal counterpar t but have an aggressive course more akin to small intestinal than gastric stromal tumors.