Inflammatory Rhabdomyoblastic Tumor with Malignant Progression of the Neck: Unique Copy Number Profile Identified by Nanopore Sequencing.

Inflammatory rhabdomyoblastic tumors (IRMT) are newly described entities characterized by a near-haploid karyotype. Most IRMTs exhibit indolent behavior; however, rare malignant transformation has been reported. We report a case of IRMT with malignant progression. A 67-year-old man presented with a painless mass in the left neck. Radiology revealed a 40-mm tumor in the left sternocleidomastoid muscle, with no evidence of distant metastasis. Core needle biopsy showed densely proliferating polygonal cells with pleomorphic nuclei, necrosis, and atypical mitoses. Based on histology, a preliminary diagnosis of pleomorphic rhabdomyosarcomas was made, although the clinical behavior seemed atypical. Nanopore DNA sequencing was subsequently performed and, within days, revealed characteristic molecular features, including relative gains of chromosomes 5, 7, and 22, a TP53 mutation, and DNA methylation profiling consistent with IRMT with malignant progression. The tumor was surgically resected, and pathological examination of the resected specimen confirmed a minor conventional IRMT component. Furthermore, infiltration of CD163-positive histiocytes was prominent, and fluorescence in situ hybridization analysis revealed two red signals (COL1A1 on chromosome 17) and four green signals (PDGFB on chromosome 22) in most tumor cells. Collectively, these histopathological, immunohistochemical, cytogenetic, and epigenetic findings established a final diagnosis of IRMT with malignant progression. Given the small tumor size, chemotherapy and radiotherapy were not administered. Nevertheless, the long-term behavior of IRMT with malignant progression remains unclear; thus, careful follow-up is warranted.