Primary thyroid rhabdomyosarcoma in an adult: a challenging case with histomolecular diagnosis and literature review

We report a case of primary thyroid rhabdomyosarcoma in a 61-year-old woman. Histologically, the neoplasm was composed of sheets of pleomorphic or spindle-shaped cells with eosinophilic cytoplasm and few large and very pleomorphic cells admixed with the spindle cell proliferation, without any thyroid epithelial component. Immunohistochemically, the tumour cells were positive for muscular markers and negative for epithelial and thyroid differentiation markers (AE1-AE3, EMA, CK5/6, TTF1, PAX8, thyroglobulin). Molecular tests revealed the presence of NF1, PTEN and TERT pathogenic mutations. Major morphological differential diagnoses include anaplastic thyroid carcinoma with rhabdoid phenotype, leiomyosarcoma, as well as other rare sarcomas. In this study, we describe the fifth case in the literature of primary thyroid rhabdomyosarcoma and the third in adults with, for the first time, an extensive molecular analysis.