Thyroid Myofibroma in a Child With Turner Syndrome: Integrated Cytologic, Histopathologic, and Molecular Characterization.

Most thyroid nodules encountered in clinical endocrinology are epithelial in origin. In contrast, primary thyroid mesenchymal tumors are exceedingly rare but increasingly recognized with advances in immunohistochemistry and molecular diagnostics. Thyroid myofibroma is an exceptionally uncommon benign fibroblastic-myofibroblastic neoplasm, with only a limited number of cases reported to date. We report a 9-year-old girl with Turner syndrome (TS) and a prior myofibroblastic proliferation of the parotid gland 5 years earlier, who presented with a solitary thyroid nodule. Fine-needle aspiration (FNA) demonstrated a spindle-cell proliferation suspicious for an extrafollicular process. Lobectomy revealed a well-circumscribed, unencapsulated spindle-cell tumor composed of bland myofibroblastic cells arranged in fascicles within a collagenous stroma. The immunophenotype supported myofibroma. Targeted molecular testing identified a pathogenic PDGFRB variant and a novel variant of uncertain significance, confirming the diagnosis. No prior reports have described thyroid myofibroma associated with TS or a preceding salivary gland myofibromatous lesion, making this case unique and diagnostically and pathogenetically significant. Awareness of this entity is critical to avoid misclassification as other spindle-cell thyroid lesions on FNA. Integrated cytomorphology, immunophenotyping, and PDGFRB-focused molecular analysis provide a robust diagnostic approach and may offer insight into shared pathogenetic mechanisms linking TS and myofibroblastic proliferations.