FNAB – The fine art of diagnosing thyroid nodules

Thyroid nodules are common. It is estimated that up to 60% of our population may harbour thyroid nodules. The majority of nodules are not symptomatic and may never come to light. For patients who have been diagnosed with nodules, this may become a source of anxiety. Many questions abound in the minds of patients. Is the nodule cancerous? Will it become cancerous eventually? Is the diagnosis reliable? Is surgery necessary?

Thyroid ultrasound

A high-resolution ultrasound is necessary for detecting a thyroid nodule unless it is easily visible and palpable in the neck. The ultrasound is also accurate in assessing for features of malignancy, otherwise known as suspicious features. These include a hypoechoic taller-than-wide nodule, irregular margins, increased intra-nodular vascularity and the presence of microcalcifications.

For some patients, serial ultrasound examinations over a period of months to years may be ordered. This is designed to detect subtle changes in the nodules which may be evident over time, and to pick them up early. Ultimately, an early cancer has the best chance of cure and survival, as with any other cancers in our body.

Occasionally, patients are referred after undertaking an MRI, CT or PET scan which detected an incidental thyroid nodule. An ultrasound is still the preferred imaging modality to enable a more accurate assessment of the thyroid gland.

Fine needle aspiration biopsy (FNAB)

The next step in the evaluation process is to obtain a sample from the nodule for examination under the microscope. In medical terms, this sample can be in the form of a histology or cytology. Histology involves removing a portion of the thyroid tissue while cytology relies on a microscopic sample obtained via suction into a needle (FNAB).

A histology sample generally involves some form of anaesthesia as the process can be quite painful. It is also done in a clean environment such as an operating theatre. The cytology sample on the other hand, involves sticking a fine needle into the nodule without the need for anaesthesia. Much like taking a blood sample from the arm, the process of FNAB is very quick and the discomfort is usually tolerable and short-lived.

Unlike ordinary biopsies, there is no special preparation needed for an FNAB so it can be performed as part of the consultation with a thyroid specialist. This is a huge advantage as there are no delays such as booking a separate appointment at a separate medical facility. The whole procedure is completed in a matter of minutes and the patient can go home. A repeat consultation is scheduled in two to three days to review the results of the FNAB.

Interpreting the FNAB

While the FNAB is generally very reliable and safe, sometimes a cancer may be missed. This is known as the false negative rate and can be as high as 20%. A higher rate is generally expected when the nodule is large, when the biopsy technique is poor or when the aspirate is mainly blood or cystic fluid.

The cytology is classified as one of six possible results on a scale (Bethesda System for Reporting Thyroid Cytology). Bethesda category 5 or 6 is associated with a high likelihood of papillary thyroid cancer and surgery is generally indicated. Bethesda category 3 or 4 are indeterminate and reflects the limitation of the cytology assessment. As such, a discussion with the thyroid surgeon is necessary to determine the next course of action. This may be a repeat FNAB at a later date or a diagnostic lobectomy to gain a definitive diagnosis.

Cost of an FNAB


An FNAB procedure has a surgical code. Thus, the cost can be covered by either your private health insurance or claimed through your Medisave account. Dr Ho has been teaching and performing ultrasound guided FNAB of the thyroid for over 10 years so you can be assured of the utmost expertise and professionalism. As the procedure is performed at the clinic during the consultation, there is almost no down-time to getting your thyroid nodule diagnosed. In addition, the costs are significantly reduced compared to the same procedure being performed in the radiology centre of a hospital.

Routine application of FNAB is a cost-effective evaluation of thyroid nodules. It has resulted in a decline in rates of unnecessary surgical treatment of thyroid nodules and increased the pick-up rates of thyroid cancer.

References

  • Haugen BR, Alexander EK, Bible KC, Doherty GM, Mandel SJ, Nikiforov YE, et al. 2015 American Thyroid Association Management Guidelines for Adult Patients with Thyroid Nodules and Differentiated Thyroid Cancer: The American Thyroid Association Guidelines Task Force on Thyroid Nodules and Differentiated Thyroid Cancer. Thyroid. 2016;26(1):1–133.
  • Ali S, Cibas E. The Bethesda System for Reporting Thyroid Cytopathology: Definitions, Critera and Explanatory Notes. New York, NY 2018.

Ultrasound guided fine needle biopsy being performed in the clinic

Ultrasound image of the needle and thyroid nodule

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