Abstract
A 64-year-old man was hospitalized with right lower abdominal pain and a protruding abdominal mass. Abdominal CT scan and MRI showed a large tumor occupying the entire abdomen. The tumor adhered to the appendix and part of the ileum. An operation was performed to extract the tumor and deal with the intestinal tract adhesions. The tumor was 33 cm in diameter. On histology, a mesenchymal tumor which partially infiltrated the subserous layer from the serous layer of the enteric tract was diagnosed. On immunohistology, the tumor was thought to be a gastrointestinal stromal tumor (GIST) ; it was KIT protein-positive and CD-34 protein-positive. However, extragastrointestinal stromal tumor (EGIST) was the final diagnosis because on histopathology it was thought that the tumor originated from the peritoneum. The tumor was considered to be malignant, because the tumor was large and contained many mitotic figures. This EGIST case was considered to be clinically malignant based on GIST clinical practice guidelines.
