Fine Needle Aspiration Thyroid Biopsy

Ultrasound imaging of the thyroid gland is one of the first steps in assessing a thyroid nodule or an enlarged thyroid.  Ultrasound is the preferred imaging modality for the thyroid gland, and is performed in the office here in Atlanta by your thyroid surgeon.  A thyroid biopsy can also be performed in the office when necessary.  Ultrasound does not use radiation, and is safe for everyone (including babies).

Thyroid nodules are concerning as they can be malignant (cancer).  However, approximately 90% of thyroid nodules are completely benign, and require no treatment.  The purpose of a fine needle aspiration is to determine the risk of a nodule being malignant, because malignant nodules need to be removed surgically.

If a thyroid nodule is found that has certain size or imaging characteristics, a fine needle aspiration biopsy (FNA) will be recommended.  This procedure involves using an ultrasound to place a fine needle (the diameter of a pin) inside the nodule to obtain a sample of cells.  The cells are then examined under a microscope by a pathologist (physician trained in the microscopic appearance of cells), and if necessary, the cells can be processed to determine the presence of genetic mutations for an even more accurate risk analysis.  Local (injected) anesthetic is generally all that is necessary, and the procedure takes just a few minutes.

The sampling of the cells from the thyroid is the easiest portion of having the prodcedure.  The difficult part is the interpretation of the cells from the sample.  This is best done by an experienced pathologist who spends much of their time (if not all) looking at thyroid pathology and cytopathology.

In 2009, a group of pathologists and thyroid specialists got together in Bethesda, MD and decided to standardize the reading of these thyroid samples so that accurate risk stratification can be calculated.  They created a total of 6 categories which can be used for accurate risk stratification which is called the Bethesda System for reporting thyroid cytopathology.

The Bethesda System for reporting thyroid cytopathology
I      non-diagnostic or unsatisfactory

Interpretation:  not enough cells to give an accurate assessment

Usual management:  repeat ultrasound guided FNA  in 3 months (a biopsy performed sooner than this introduces the risk of biopsy changes being confused for malignancy)

II     benign

Interpretation:  normal, benign appearing thyroid nodule

Risk of malignancy:  0-3%

Usual management:  follow-up ultrasound in 1 year to assess for growth or changes in appearance

III   follicular lesion of undetermined significance (atypia of undetermined significance)

Interpretation:  cells appear somewhat abnormal, but not enough to be considered cancer

Risk of malignancy:  5-15%

Usual management:  risk stratification with molecular markers, repeat biopsy in 3 months, or surgical lobectomy (for high risk lesions)

IV   follicular neoplasm (or suspicious for follicular neoplasm)

Interpretation:  these types of nodules cannot be determined to be benign or cancer based on the appearance of the cells alone

Risk of malignancy:  15-30%

Usual management:  risk stratification with molecular markers, followed by diagnostic thyroid lobectomy or total thyroidectomy

V    suspicious for malignancy

Interpretation:  pretty sure this is thyroid cancer

Risk of malignancy:  60-75%

Usual management:  thyroid lobectomy or total thyroidectomy

VI  malignant

Interpretation:  thyroid cancer

Risk of malignancy:  99-100%

Usual management:  total thyroidectomy has historically been performed, but a thyroid lobectomy is now considered adequate surgical treatment for low risk malignancies

This classification is now the standard of care for reading thyroid cytopathology.  Even years later, not all pathologists are using this system.  If the pathologist reading your biopsy slides isn’t an experienced thyroid cytopathologist who uses the Bethesda system, it means that you are more likely to ultimately have your thyroid removed through surgery.  An inexperienced pathologist tends to overcall the appearance of the cells they see (which is reasonable, better to error on the side of caution), but in reality, the great majority of these nodules are benign and do not need to be removed.

While fine needle aspiration is very useful, the accuracy decreases as the size of the nodule increases.  Nodules that are very large (>4 cm) require surgical excision because the risk of a false negative fine needle aspiration biopsy (benign biopsy, when in fact cancer is present) is about 15%.

Indeterminate results (Bethesda III and IV) can be further assessed with molecular markers, which can allow greater risk stratification for thyroid cancer.  This allows low risk nodules to be observed, where high risk nodules need to be fully evaluated after being removed surgically.