False Negative Thyroid Biopsy: A Case Report Highlighting the Importance of Ultrasound Evaluation.

[INTRODUCTION] The increasing use of advanced imaging modalities has led to a growing number of Incidental thyroid nodules (ITNs). Although most ITNs are benign, dedicated evaluation with a thyroid ultrasound (US) is strongly recommended to assess malignancy risk with a dedicated.

[CASE PRESENTATION] We report a case of a 66-year-old male with large fluorodeoxyglucose (FDG)-avid right thyroid mass identified incidentally on a positron emission tomography (PET) scan. Initial fine needle aspiration (FNA) suggested benign pathology. However, subsequent thyroid US demonstrated highly suspicious features concerning for lymphoma or sarcoma, prompting a core biopsy with flow cytometry and immunohistochemistry, which confirmed a diagnosis of small lymphocytic B-cell infiltrating the thyroid gland.

[CONCLUSION] This case underscores the diagnostic limitation of FNA in detecting thyroid lymphoma and highlights the critical role of thyroid US in identifying concerning features. When US findings are highly suspicious for lymphoma, additional diagnostic approaches - including core biopsy with flow cytometry and immunohistochemistry - are essential to ensure accurate diagnosis and appropriate management.