Background : Desmoid-type fibromatosis is a very rare disease that has no characteristic image findings, so it is often difficult to differentiate from gastrointestinal stromal tumor (GIST). A case of desmoid-type fibromatosis that was difficult to differentiate from GIST is reported. The decisive factor in the diagnosis was positive nuclear immunostaining for β-catenin nucleus. Case presentation : A man is his 30s had no significant past medical history, including no abdominal surgery. A medical check-up found a large tumor in the right lateral abdomen. After some examinations, a preoperative diagnosis of GIST was made, and open ileocecal resection was performed. However, the final diagnosis based on the pathological findings was desmoid-type fibromatosis. Conclusions : We should consider desmoid-type fibromatosis when we find a large abdominal mass, but it may be difficult to diagnose based only on imaging findings. Immunohistochemical examination of the specimen may make the diagnosis
