Understanding Follicular Neoplasms
Follicular neoplasms are abnormal growths of thyroid cells, forming a distinct nodule within the thyroid gland. These growths result from an uncontrolled proliferation of follicular cells, the primary cell type in the thyroid responsible for producing hormones that regulate the body’s energy use, temperature, and organ function.
These growths are typically surrounded by a clear boundary, or capsule. The challenge with follicular neoplasms is that their microscopic appearance, particularly via fine needle aspiration (FNA) biopsy, can look very similar whether benign (non-cancerous) or malignant (cancerous). While most thyroid nodules are benign, a significant portion of those classified as follicular neoplasms require additional evaluation due to this diagnostic ambiguity.
Initial Detection and Clinical Assessment
Thyroid nodules are often first identified incidentally during imaging tests or routine physical examinations. Most do not cause symptoms, but larger ones might create a painless lump that can be seen or felt.
An initial clinical evaluation involves a physical examination of the neck. The doctor assesses the nodule’s characteristics and checks for enlarged lymph nodes. A patient’s medical history, including family history of thyroid cancer or past radiation exposure, is an important part of this preliminary assessment, as these factors can influence the overall suspicion level.
Diagnostic Indicators of Suspicion
When a thyroid nodule is found, specific features identified through diagnostic tests can raise suspicion for a follicular neoplasm. Ultrasound imaging is a primary tool. Suspicious ultrasound features suggesting a higher risk of malignancy include solid composition, hypoechogenicity (appearing darker than surrounding tissue), irregular margins, microcalcifications, a taller-than-wide shape, or increased blood flow within the nodule.
Fine Needle Aspiration (FNA) biopsy collects cells from the nodule for microscopic examination. For follicular neoplasms, FNA results are often “indeterminate” within systems like the Bethesda System for Reporting Thyroid Cytopathology, classified as “follicular lesion of undetermined significance” (FLUS) or “follicular neoplasm/suspicious for follicular neoplasm” (FN/SFN). This indeterminate classification arises because a definitive diagnosis of benign versus malignant requires evaluating the entire nodule’s architectural integrity, specifically for capsular or vascular invasion, which cannot be reliably determined from a small FNA sample.
Reaching a Definitive Diagnosis
Once a follicular neoplasm is suspected based on FNA and ultrasound findings, further steps are necessary to determine if it is a benign follicular adenoma or a malignant follicular thyroid carcinoma. This distinction is important because follicular carcinomas can spread to other parts of the body, unlike adenomas. The definitive diagnosis typically requires surgical removal of the nodule.
During surgery, such as a lobectomy (removal of half the thyroid gland), the entire nodule can be examined by a pathologist. The presence of capsular invasion (cells growing through the nodule’s boundary) or vascular invasion (cells entering blood vessels) are the hallmarks that differentiate a malignant follicular carcinoma from a benign adenoma. Molecular testing of FNA samples can help predict malignancy risk, potentially guiding treatment or avoiding surgery for benign cases. However, surgical pathology remains the primary method for definitive diagnosis. Most follicular neoplasms are ultimately found to be benign, and even malignant ones generally have a favorable prognosis when identified and treated early.