Inflammatory Rhabdomyoblastic Tumor: A Clinicopathological and Immunohistochemical Study of 22 Cases.

Inflammatory rhabdomyoblastic tumor (IRMT) is a newly defined subtype of skeletal muscle neoplasm with uncertain malignant potential. Herein, we present a series of 22 IRMT cases to further expand its clinical and pathologic spectrum. This cohort enrolled 20 males and 2 females, with a median age of 42 years (range: 18 to 60 y). The tumors were predominantly located in the deep soft tissues of the lower extremity (n=11, 50%), followed by the trunk (n=4, 18.2%), head and neck region (n=3, 13.6%), upper extremity (n=2, 9%), and abdomen/retroperitoneum (n=2, 9%). Most patients presented with a slowly growing, deeply seated soft tissue mass, with a median diameter of 6 cm (range: 3 to 23 cm). Histologically, 12 cases (54.5%) showed a spindled to pleomorphic morphology, 7 cases (32%) exhibited a monomorphic spindled morphology, and 3 cases (13.6%) displayed a spindled to epithelioid phenotype. All lesions were characterized by dense inflammatory and histiocytic infiltrates. Focal calcification, cholesterol clefts, and histiocytic necrosis were noted in a subset of cases. Three cases demonstrated progression to rhabdomyosarcoma. Immunohistochemically, all tumors exhibited diffuse and strong desmin expression, with variable MyoD1 immunoreactivity and limited myogenin staining. Molecular analysis identified somatic mutations in NF1 (n=4), TP53 (n=2), and KRAS (n=1) genes among the tested cases. With a median follow-up of 17.5 months (range: 3 to 77 mo) in 22 patients, 2 cases with rhabdomyosarcomatous transformation developed lung metastases, whereas the remaining patients remained disease-free following complete surgical resection. Our study further validates IRMT as a distinct myogenic neoplasm with an indolent behavior. Correlation of clinical manifestations and morphologic features with ancillary studies is essential to avoid overdiagnosing IRMT as a high-grade rhabdomyosarcoma.