After an FNA is performed, the pathologist will examine the cells that were sampled from the thyroid nodule. There is a uniform method of reporting FNA results that nearly all pathologists use. It is called the Bethesda grading system and breaks down results into six groups. Here are possible outcomes from an FNA:
Non-diagnostic FNA – Bethesda 1
A nondiagnostic FNA sample is not really a result. This occurs when there are not enough cells for the pathologist to make a firm recommendation based upon the findings. This happens less than 10% of the time and if this occurs, then the FNA will need to be repeated to gather more samples. These findings would be considered Bethesda category 1.
Benign FNA Results -Bethesda 2
Of course, the most desired result from an FNA is that the pathologist would look at the cells and determine that the findings are benign. Normal thyroid cells have a distinct characteristic that allow for the pathologist to comfortably state that the nodule is benign and results from benign biopsies tend to be highly accurate. These findings would be Bethesda category 2.
Indeterminate FNA Results – Bethesda 3
The indeterminant category means that the pathologist looked at the samples taken from the FNA and could not firmly conclude that they were benign normal cells, nor did it have all the distinguishing features of thyroid cancer. There are several names used to describe this category of lesions. AUS is a typical designation which stands for atypical cytology of undetermined significance. This means that the thyroid cells that were seen were not normal but whether they were cancer could not be determined by the biopsy. Also seen in this indeterminant category are follicular lesions of undetermined significance (FLUS). Follicular lesions could represent benign follicular adenoma which is not a cancer and does not have risk of spread into other parts of the body. A minority of times, a follicular lesion can turn out to be a cancer which does have the potential for spread beyond the thyroid. Both AUS and FLUS are Bethesda category 3.
Indeterminate FNA Results – Bethesda 4
Sometimes the pathologist sees clear evidence that there is a tumor, also called a neoplasm. In the thyroid, it is not possible to determine whether a follicular neoplasm is benign or cancerous with a FNA. This type is Bethesda category 4 and described as either a follicular neoplasm or suspicious for a follicular neoplasm. The indeterminant category is by far the most difficult category to determine the next best steps for. Advice on next steps should be highly individualized based upon a number of risk factors from the patient history, as well as the ultrasound characteristics and the level of suspicion for each individual lesion. Molecular testing has come to play a prominent role in helping patients with indeterminant results. The use of molecular testing should be selective and weighed against the benefits of the surgical biopsy vs. the risk involved.
Malignant FNA Results – Bethesda 5 or 6
Thyroid cancer frequently has some very distinct characteristics which make it recognizable on even a small sampling from the fine needle aspiration. Cancer characteristics lead the pathologist to categorize the findings as either highly suspicious for cancer or cancer. Both of these would require surgical intervention for optimal treatment. Bethesda category 5 is suspicious for cancer while Bethesda category 6 indicates cancer. Whether a pathologist uses category 5 or 6 is often reflective of the confidence in the findings. Category 6 implies that there is no room for doubt and the findings lead to a firm conclusion of cancer. Frequently, pathologist will hedge – calling a result that is convincing for cancer a category 5, instead of 6. This leaves room for error, and probably more importantly, protects the pathologist from litigation risks should the final pathology appear different from the FNA sampling. This is an unfortunate reality of our current healthcare environment – where the threat of legal action alters the practice of medicine. As a result, the Bethesda category 5 is used for most cancer cases and the rate of cancer in category 5 is far higher than the 75% stated in the original Bethesda classification system. In actual practice, Bethesda category 5 represents cancer in more than 90% of the cases we see.