Here, we present an uncommon case of inflammatory myofibroblastic
tumor (IMT) involving the mesentery. The
tumor was composed of loosely arranged round-to-spindle-shaped
tumor cells with amphophilic cytoplasm in an inflammatory and myxoid background. The mitotic activity was low (1 per 50 high-power fields) and the
tumor cells lacked cellular atypism. Immunohistochemically, the
tumor cells demonstrated strong nuclear membranous staining with
anaplastic lymphoma kinase (ALK). In situ hybridization for ALK gene rearrangement revealed a splitting apart of the two signals within the
tumor cells. Reverse transcription-polymerase chain reaction revealed that the
tumor harbored a
ran-binding protein 2 (RANBP2)-ALK rearrangement. IMTs are usually characterized by epithelioid-to-round cells featuring increased mitotic activity, occasionally demonstrating unusual
tumor cells and more aggressive clinical behavior. To date, 23 IMTs have been reported with
RANBP2 and ALK gene rearrangements. However, the present case demonstrated indolent cytological features, leading to a difficulty in differentiating it from
desmoid-type
fibromatosis.