Thyroid nodules are abnormal growths of cells within the thyroid gland, found in up to 60% of adults, often discovered incidentally during imaging scans. These lumps often do not cause any symptoms. The main concern is determining if the nodule is benign or malignant, as approximately 5 to 15% of all thyroid nodules harbor cancer. A biopsy is the established method used to gather cells from the nodule to assess this risk.
The Role of Fine Needle Aspiration (FNA)
The standard first step in evaluating a thyroid nodule is a Fine Needle Aspiration (FNA) biopsy. This minimally invasive procedure uses a very thin, hollow needle, typically under ultrasound guidance, to collect a small sample of cells. The primary goal is to obtain enough cellular material for a pathologist to perform a cytological examination.
The pathologist examines the physical structure of the cells under a microscope to determine their appearance. This examination is often highly accurate, providing a definitive diagnosis of either a benign or a malignant tumor. However, the FNA procedure has limitations because the pathologist must rely only on the physical appearance of the cells, which can sometimes be ambiguous.
When Cytology Results Are Indeterminate
Genetic testing becomes necessary when the initial cytological examination of the FNA sample yields an indeterminate result. Pathologists use a standardized reporting system, such as the Bethesda System for Reporting Thyroid Cytopathology, to classify these findings.
Specifically, results falling into Bethesda Category III, termed “Atypia of Undetermined Significance,” or Category IV, “Follicular Neoplasm,” are considered ambiguous. These categories indicate that the cell structure shows some irregular features but not enough to definitively diagnose cancer. The estimated risk of malignancy in these nodules typically falls within a range of about 13% to 34%.
Because the pathologist cannot definitively rule out cancer based on cell shape alone, the next step is to use molecular testing to clarify the underlying biological risk. This genetic analysis is performed on the same cell sample collected during the initial FNA procedure. The test helps prevent unnecessary diagnostic surgery for a large percentage of patients whose ambiguous nodules are ultimately benign.
What Molecular Tests Identify
Molecular tests work by searching for specific genetic alterations, such as mutations and gene fusions, that are known to drive the development of thyroid cancer. These tests analyze the DNA and RNA within the cells, focusing on genes involved in key cellular growth pathways like the MAPK and PI3K pathways. The presence or absence of these changes provides objective biological information that complements the subjective cell examination.
Common genetic markers sought include the BRAF V600E mutation, which is highly associated with papillary thyroid cancer and can indicate a more aggressive tumor. Tests also look for RAS gene mutations and structural changes like RET/PTC or PAX8/PPARĪ³ fusions, which are found in other forms of thyroid cancer.
The results are compiled into a statistically calculated molecular risk score. This score indicates the likelihood of the nodule being cancerous based on its unique genetic profile. The molecular analysis can effectively rule out cancer with a high degree of certainty if no high-risk markers are found. Conversely, the detection of a specific cancer-associated mutation, such as the BRAF V600E mutation, significantly increases the probability of malignancy, sometimes to nearly 100%.
How Genetic Results Guide Treatment Decisions
The clinical utility of the genetic test results lies in how they transform an ambiguous diagnosis into a clear, actionable treatment plan. By providing a quantified risk score, the molecular profile directs the next step for patient management. The test results help determine whether the patient can safely avoid surgery or if an operation is warranted.
For nodules with an indeterminate cytology result, a molecular test that comes back as low-risk (indicating a probability of malignancy of less than 4-5%) allows the patient to proceed to active surveillance. This involves regular ultrasound monitoring of the nodule, avoiding the risks and costs associated with immediate surgery. This is a significant benefit, as historically, most of these indeterminate cases would have proceeded to a diagnostic surgical removal of at least half the thyroid gland.
Conversely, if the molecular test identifies a high-risk mutation, such as BRAF, confirming a high probability of cancer, the patient is recommended for definitive surgical intervention. Knowing the genetic profile beforehand allows the surgeon to plan the appropriate extent of the operation, such as a lobectomy (removing half the thyroid) or a total thyroidectomy (removing the entire gland), based on the tumor’s predicted aggressiveness and size.