Gastrointestinal stromal tumor (GIST) is a distinct group of mesenchymal neoplasms recently shown to exhibit differentiation toward interstitial cells of Cajal. Although previous studies have shown that the clinical outcome of patients with GIST is associated with mitotic activity, the proliferation index determined by the Ki-67 labeling index, immunophenotype (CD34 and/or p53) and mutation in exon 11 of the c-kit, a definitive discrimination between benign and malignant GIST has not yet been established. We report a patient in whom malignant GIST in the abdomen recurred five times. In this case, the primary GIST and the five recurrent GIST were associated with c-kit immunoreactivity, but the mitotic index of the GIST tended to be increasingly higher with subsequent recurrences. Mutational analysis of the c-kit revealed that the primary and recurrent GIST were mutant-negative. These data indicated that 'morphologically appearing benign' tumors with lower proliferative parameters may also have the capacity of metastasis and recurrence.
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