[Histopathological and cytological findings in a case of gastrointestinal stromal tumor presenting in the pelvic cavity].

Gastrointestinal stromal tumor (GIST) usually occurs in the gastrointestinal tract, mainly in the stomach. Recently, GIST arising in extragastrointestinal organs was been reported. This case was a 49-year-old female who was diagnosed as having uterine leiomyoma and pelvic tumor in the rectovaginal septum. The pelvic tumor showed no connection with the uterus and rectum. Grossly, the pelvic tumor was light brown and solid. Cytologically, the tumor cells of the pelvic tumor showed vague interlacing fascicles with spindle/fibrous cytoplasm and elongated nuclei. No necrosis or mitosis was present. Histological findings of the pelvic tumor revealed an interlacing bundle fashioned with one to two mitotic figures per 50 HPE No necrosis was found in the tumor. The tumor cells were positive for CD117 (c-kit protein), CD34, and vimentin by immunohistochemistry, but negative for actin, desmin, and S-100. The tumor was diagnosed as GIST in the pelvic cavity. GIST rarely presents in extragastrointestinal regions such as this case. It is difficult to make a differential diagnosis between GIST and other mesenchymal spindle cell tumors only from a cytologic specimen. A positive reaction for CD117 is useful for the diagnosis of GIST.