Cytologic Features of Inflammatory Rhabdomyoblastic Tumor Using Conventional and Liquid‐Based Cytology Preparations: A Case Report

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ABSTRACT Inflammatory rhabdomyoblastic tumor (IRMT) is a recently recognized skeletal muscle neoplasm of uncertain malignant potential, characterized by a prominent inflammatory infiltrate. Although its histologic features are increasingly being defined, reports describing its cytologic characteristics remain extremely limited. Herein, we describe the detailed cytologic findings of a case of IRMT across multiple cytologic preparations. A man in his 60s presented with pharyngeal discomfort and was diagnosed with tonsillar diffuse large B‐cell lymphoma (DLBCL). Subsequent staging positron emission tomography‐computed tomography revealed focal abnormal fluorodeoxyglucose uptake in the left tibialis anterior muscle. Although lymphomatous involvement was considered, ultrasound‐guided biopsy suggested IRMT. The patient underwent surgical resection of the soft tissue mass. Imprint, crush, and liquid‐based cytology (LBC) preparations were obtained from the resected tumor. Cytologic examination across all preparations revealed a characteristic biphasic pattern: numerous atypical cells with enlarged, variably shaped nuclei, finely granular chromatin, prominent nucleoli, and abundant cytoplasm, admixed with a rich background of reactive lymphocytes and histiocytes. No cross‐striations were identified. Mitotic figures and necrosis were absent. On immunocytochemistry, the atypical cells were positive for desmin and MyoD1, while the background histiocytes expressed CD163. The final histopathologic diagnosis was IRMT. Following resection, the patient received R‐CHOP chemotherapy for DLBCL. At the most recent follow‐up, less than 6 months after resection, there was no evidence of local recurrence or distant metastasis of IRMT. This case highlights the cytologic features of IRMT across conventional and LBC preparations, supported by confirmation of rhabdomyoblastic differentiation by immunocytochemistry. Recognition of atypical rhabdomyoblastic cells within a lymphocyte‐ and histiocyte‐rich background is an important diagnostic clue.