How is Ultrasound useful to determine which thyroid nodules warrant FNA?
US has relatively high specificity for identifying papillary thyroid cancers based on imaging features. If extension of a lesion beyond the thyroid capsule or if metastatic lymph nodes are present, a thyroid malignancy can be diagnosed. Current imaging guidelines require evaluation of the lateral cervical nodes in all patients undergoing neck US because the detection of metastatic thyroid cancer to the lateral cervical lymph nodes may aid in the definitive preoperative diagnosis of malignancy in nodules with indeterminate cytologic results and alter the surgical approach to patients with known nodal disease. Specific imaging features that have a high correlation with thyroid malignancy include presence of microcalcifications, coarse and dystrophic calcifications, marked hypoechogenicity, infiltrative and/or microlobulated margins, and taller-than-wide shape. However, not all malignancies demonstrate these features, such that their absence cannot reliably exclude a malignancy. Additionally, there is overlap in the appearance of benign follicular adenomas and follicular cancers. Both may appear as predominantly solid noncalcified vascular lesions, often with an identifiable capsule or halo.
Imaging Features Associated with Thyroid Malignancy
- • Invasion of adjacent structures
- • Lymphadenopathy
- • Microcalcifications
- • Large, coarse calcifications
- • Marked hypoechogenicity
- • Infiltrative or microlobulated margins
- • Taller-than-wide shape
There are some sonographic features that correlate with a very high likelihood that a thyroid nodule is benign, due to a hyperplastic focus rather than a neoplasm. Nodules that are nearly entirely cystic without an identifiable solid component or have a spongiform appearance, defined as interspersed cystic spaces without marked vascularity, calcifications, or irregular margins, are two features of thyroid nodules that indicate a very low (less than 1%) risk of malignancy. Small spongiform nodules are not typically biopsied when under 2 cm in size but when larger in size may be followed by serial US to assess for the development of suspicious features or a rapid growth pattern.
Sonographic features therefore play an important role in determining which thyroid nodules warrant FNA. Other factors that also influence the need for FNA include the patient’s overall health, risk factors for thyroid malignancy such as a history of head and neck irradiation as a child or a known predisposing genetic risk, and the method of discovery of the nodule including rapid growth or detection by 18 F-fluorodeoxyglucose (FDG) positron emission tomography (PET). When multiple nodules are present in the thyroid gland, the decision about which nodules would need FNA is based on the sonographic features of each nodule rather than nodule size alone.