A Large Gastrointestinal Tumor With Cystic Degeneration Initially Mistaken for Gastric Splenosis: A Case Report.

Gastrointestinal stromal tumors (GISTs) are the most common mesenchymal neoplasms of the gastrointestinal tract and usually present as submucosal gastric lesions. Atypical morphologies, such as exophytic or cystic growth, are uncommon and can lead to lesions being mistaken for hepatic, pancreatic, or splenic origin. We report a case of a 63-year-old woman referred for evaluation of unexplained weight loss. Imaging revealed a 4 cm lobulated lesion adjacent to the spleen with peripheral cystic changes. The mass enhanced similarly to splenic tissue and was initially interpreted as splenosis. Final pathology revealed a GIST with cystic degeneration, negative margins, and no evidence of metastasis. This case highlights the diagnostic challenges posed by atypical GISTs and emphasizes the importance of endoscopic ultrasound (EUS) with tissue acquisition for accurate diagnosis when cross-sectional imaging is inconclusive.