A Case of Malignant Gastrointestinal Stromal Tumor of the Transverse Colon: Evaluation of Proliferation Activity

Here, we report a colonic gastrointestinal stromal tumor (GIST) in a middle-aged man above 40. A histologically similar second tumor was detected 2 years after initial surgery. The primary GIST, measuring 5.5 × 6.0 cm, consisted of spindle tumor cells with a higher number of mitoses and Ki67 labeling index (about 20%) than those of the second tumor, implying a de novo GIST. These markers might be useful in evaluating malignant potential, as well as to differentiate between a de novo and a recurrent tumor in the colonic GIST.