Thyroblastoma Dominated by a Primitive Small Round Cell Component: A Diagnostic Pitfall

present (Fig. 1C), with partial embedding in fibrous stroma and focal continuity with thyroid follicles (Fig. 1D).Three lateral cervical lymph nodes showed metastases composed of small round cells with multifocal rosettes (Fig. 1E), abundant necrosis, and high mitotic activity, without stromal or follicular/tubular differentiation.Immunohistochemically, tumor cells were largely cytokeratin-negative with focal CK18 positivity (Fig. 2A), and negative for TTF1, PAX8, and thyroglobulin (positive in adjacent follicles; Fig. 2B).SALL4 was strongly expressed (Fig. 2C), synaptophysin was weak and heterogeneous (Fig. 2D), and chromogranin A and INSM1 were negative.No stromal expression of desmin (Fig. 2E) or myogenin was detected.Tumor cells were also negative for calcitonin, S100, CD99, SOX10, CD45, CEA, BRAFV600E, NUT, and OCT4.Nuclear SMARCA4 expression was retained, and p53 and retinoblastoma protein (RB1) showed a normal expression pattern.The proliferative index was very high (> 90%, Ki-67; Fig. 2F).Targeted next-generation sequencing identified two pathogenic DICER1 variants: a splice-site mutation (c.5603 + 2T > C, variant allele frequency 40%) and an RNase IIIb hotspot mutation (c.5127T > A; p.D1709E, 37%) consistent with a typical two-hit mechanism. DiagnosisThyroblastoma with a prominent blastemal component. CommentsThyroblastoma is a rare, high-grade thyroid neoplasm with a typically triphasic architecture: primitive small cells, a sarcomatous component (often rhabdomyosarcoma-like), Case HistoryA 46-year-old woman presented with a long-standing thyroid nodule that had rapidly enlarged in recent months.Evaluation showed a 2.5 cm nodule in the left thyroid lobe and three enlarged ipsilateral cervical lymph nodes up to 3 cm.Intraoperative frozen sections of the thyroid nodule and one lymph node revealed a malignant small round blue cell tumor, prompting left hemithyroidectomy with central and lateral lymphadenectomy.