442 Cellular inflammatory myofibroblastic tumor with rhabdomyoblastic differentiation: Report of a rare tumor with previously unreported immunohistochemical features

Introduction/Objective Inflammatory myofibroblastic tumor (IMT) is a neoplasm of uncertain etiology that predominantly occurs as an abdominal or pulmonary lesion in children and young adults. Histologic patterns include myxoid, hypercellular, and hypocellular fibrous. While hypercellular IMT is well-established, immunohistochemical evidence of rhabdomyoblastic differentiation within these tumors has not been reported previously. We report a cellular IMT with rhabdomyoblastic differentiation in left cheek of a 24-year-old male. Methods/Case Report The patient presented to oral surgery for a 3.5 cm cheek lesion with ulceration. The wide excision specimen revealed a variably cellular proliferation of spindle cells with eosinophilic cytoplasm arranged in short fascicles, with a partially myxoid matrix, chronic inflammatory infiltrate, minimal atypia, and low mitotic activity. This morphology is interrupted by a sharp transition to areas of increased cellularity and mitotic rate. Immunohistochemically, the tumor showed reactivity for SMA, desmin, and ALK1. Transitional areas of increased cellularity showed evidence of rhabdomyoblastic differentiation in their additional expression of myogenin and Myo D1. Gene fusion panel identified CLTC-ALK fusion, supporting the diagnosis. The patient is alive without evidence of recurrence two months after reresection. Results NA Conclusion Our case presents previously unreported immunohistochemical findings in an IMT, which may have implications for therapeutic and diagnostic approaches to myofibroblastic tumors.