[A Case of Gastrointestinal Stromal Tumor(GIST)Originating in the Anal Canal].

We report a case of a gastrointestinal stromal tumor(GIST)that originated in the anal canal. A 70's woman with a subcutaneous tumor reaching from the anal canal was referred to our hospital. After a thorough examination, the tumor was resected percutaneously in the jackknife position. Histopathological examination showed proliferation of spindle-shaped tumor cells arranged in irregular bundles. Immunohistochemical staining showed that the tumor was positive for c-kit and CD34, and negative for a-SMA and S-100, so the tumor was diagnosed as GIST. As a-SMA-positive smooth muscle cells were seen around the tumor, we suspected that this tumor originated from the internal sphincter muscle.