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3. Fine needle biopsy - AAES GUIDELINES

Fine needle biopsy is the principal diagnostic technique used to assess the potential for thyroid cancer in a suspicious nodule. The decision as to which nodules should be biopsied depends upon their ultrasound and clinical characteristics (see Imaging). Fine needs biopsy is performed with a very small needle making multiple passes through the nodule aspirating cells which are examined by a cytopathologist. In most circumstances the yield of cells will be optimised when performed with ultrasound guidance. The biopsy results are then classified using the Bethesda System which provides standardised reporting linked to specific management recommendations. There are five categories with Bethedsda 2 being a benign thyroid nodule with 99% confidence  through to Bethesda 6 being a thyroid cancer with a greater than 95% confidence.

Recommendation 7: FNAB is a standard component  of thyroid nodule evaluation, and its indications should follow established guidelines based on US characteristics, size and clinical findings.


Fine needle biopsy is a very safe and straightforward procedure and I believe is best performed under ultrasound guidance by a radiologist experienced in thyroid biopsies. This allows immediate assessment by a cytopathologist of the adequacy of the sample.  Whilst most of the categories are self-evident, there is often confusion about Bethesda 3 and Bethesda 4 reports. Category 3 (Atypia of Uncertain Significance) simply describes abnormal cells which may be due to inflammation such as Hashimoto's but may represent an early tumour with the recommendation for treatment depending on the clinical situation. Category 4 (Follicular Neoplasm) represents a true tumour which may be either benign (Follicular Adenoma) or malignant (Follicular Cancer) and always requires removal for diagnosis.

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