A case report of RRBP1-ALK fusion gene-positive epithelioid inflammatory myofibroblastic sarcoma with collagenous stroma and good prognosis

Background: Epithelioid inflammatory myofibroblastic sarcoma (EIMS) is a rare variant of inflammatory myofibroblastic tumors with distinct morphological features and malignant clinical behavior. ALK is positive in all EIMS cases, the RANBP2-ALK fusion gene has been detected in most EIMSs, but several RRBP1-ALK and EML4-ALK fusion genes have also recently been reported.Case presentation: Here, we report a case of RRBP1-ALK fusion-positive EIMS in the colon. The patient was a 26-year-old man. A pelvic mass was found in a routine physical examination and was completely resected. Microscopically, the tumor cells were rounded and epithelioid in shape. Diffuse collagenous stroma and focal inflammatory infiltration were present. ALK was positive, with a pattern of cytoplasmic staining with perinuclear accentuation, and the RRBP1-ALK fusion gene was detected by targeted next generation sequencing. The patient began to take crizotinib from October 2019 to July 2020, and the patient is presently in good condition. No synchronous or metachronous metastases was found.Conclusions: EIMSs with different ALK fusion partner genes have unique staining patterns, which may provide clues for diagnosing EIMS. EIMS cases with RRBP1-ALK fusion gene and with diffuse collagenous stroma may have a good prognosis.