• Liang-Ji Lu, Xiao-Pei Xu, Fei Dong, and Jie Min.
• Department of Radiology, the Second Affiliated Hospital, Zhejiang University School of Medicine.
• Medicine (Baltimore). 2020 Oct 2; 99 (40): e22493.

RationaleGastrointestinal stromal tumors that present outside the gastrointestinal tract are known for extra-gastrointestinal stromal tumors (EGISTs) and they share the same morphological and immunohistochemical characteristics with gastrointestinal stromal tumors. Here we report a rare case of diffuse primary EGIST arising at peritoneum.Patient ConcernsA 57-year-old male presented to the hospital with abdominal pain and right lower abdominal tenderness.DiagnosisThe core needle puncture biopsy showed epithelial-like cells and the nuclei were ovoid and focally elongated. Immunohistochemical examination was consistent with a primary EGIST of the peritoneum.InterventionsThe patient was treated with Imatinib mesylate.OutcomesFive months later, there is no complication resulting from treatment. The follow-up abdominal contrast-enhanced CT showed the lesion was significantly decreased in size, and was evaluated as partial response. The patient continued the treatment with Imatinib as prescribed by the oncologist.LessonsEGISTs are rare and should be considered in the differential diagnosis of the peritoneal tumors and immunohistochemistry helps to confirm the diagnosis. Further study with longer follow-up is desired to characterize these uncommon tumors.

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