Treatment strategies and outcomes of inflammatory myofibroblastic sarcoma: Insights from a retrospective single-center study.

e23540 Background: Epithelioid inflammatory myofibroblastic sarcoma (EIMS) is a rare, aggressive variant of inflammatory myofibroblastic tumor (IMT) that occurs in children and young adults. The most recent retrospective review documented just over 55 cases in the literature. In the last year, our hospital system has had two EIMS patients present with novel characteristics. Due to the rarity of EIMS, this retrospective analysis was conducted to understand prognostic factors and treatment patterns of EIMS compared with its more benign variant, IMT. Methods: In this single-institution retrospective analysis, patients with IMT and EIMS were identified between 2002-2024 from both our pediatric and adult sarcoma programs. Patient data was extracted from the electronic medical record (EMR). Relevant clinical factors including demographics, molecular testing, treatment, and outcomes were obtained. Results: Seventeen patients were identified at Indiana University Simon Comprehensive Cancer Center and Riley Children’s Hospital with IMT or EIMS. Median age at diagnosis was 8.5 years with a range of 1-24 years. 76% of the patients were White, 11% Black, and 11% Asian. Biological sex ratio was 8M:9F. The most common primary tumor site was bladder (N = 4) and abdominal/peritoneum (N = 4) with others arising in the lung/pleura, liver, mediastinum, trachea, and brain. Most tumors were locally advanced, with three cases of metastasis at the time of diagnosis, both of the EIMS cases and one IMT of the lung. Both patients over the age of 18, one EIMS and one IMT, were metastatic at diagnosis. Molecular analysis was performed on 6 of the cases, all since 2019. One of the EIMS patients had a CLTC-ALK fusion which is more commonly found in IMTs. This patient with the CLTC-ALK fusion progressed on crizotinib therapy but has had a partial response to 2 nd generation ALK inhibitor, alectinib. The other patient with EIMS had recurrence after stopping crizotinib post tumor resection, but has had a sustained complete response with reintroduction of crizotinib. The median OS for the living cohort (N = 16) is 68 mos (range 6-174 mos), and 1 patient with IMT passed away in 2002 with limited documentation prior to EMR being available. Conclusions: EIMS is an aggressive variant of IMT with a paucity of cases reported in the literature. This single institution retrospective review displayed that EIMS was more likely to recur and/or present with metastatic disease compared to IMT, with the first known report of a CLTC-ALK fusion in EIMS which usually harbors RANBP2-ALK fusions. A multi-institutional study will be helpful to understand the full slate of prognostic factors and biology to tailor the best treatment regimen for this biologically distinct sarcoma.