Fine needle aspiration (FNA) was initially performed in the 1930s when Martin and Ellis first described the procedure and the potential implications in the medical field. Initially, interest in the procedure remained low due in part to concerns with regards to cancer seeding. Walfish later described the use of combined ultrasound guidance and needle aspiration cytology in increasing the diagnostic accuracy of specimen acquisition. The role of ultrasound-guided FNA has evolved over time as an important tool in the assessment of thyroid nodules.
The important aspects of the FNA procedure include:
The thyroid gland is a midline structure located in the anterior neck, specifically anterior to the trachea. The thyroid gland is divided into two lobes, which are connected in the midline by the isthmus. The isthmus crosses the midline of the upper trachea at the second and third tracheal rings. On ultrasound, the thyroid gland is easily identified due to its general hyperechogenicity with respect to the adjacent sternocleidomastoid muscle at the lateral margin and the sternohyoid muscle at the anterior margin. Nodularity within the thyroid gland can be considered a normal part of aging, with the incidence of nodules increasing with age. Thyroid nodules can be identified due to their difference in echogenicity when compared to normal surrounding thyroid parenchyma. Various studies have investigated the role of ultrasound features in understanding the malignant potential of nodules. Sonographic features that may relate to and/or raise concern for thyroid malignancy include hypoechogenicity, microcalcifications, lobulated appearance, taller-than-wide shape, irregular margins, and increased internal vascularity.
Ultrasound-guided FNA is often performed based on suspicious sonographic features on an initial thyroid ultrasound or imaging characteristic changes on follow-up examinations. The American Thyroid Association (ATA) and the British Thyroid Association (BTA) have published guidelines providing risk stratification based on ultrasound features. The American College of Radiology (ACR) has also provided Thyroid Imaging, Reporting, and Data System (TI-RADS) classification with regards to FNA or imaging follow-up recommendations based on sonographic features. One recent study demonstrated the ATA and TI-RADS systems demonstrate similar sensitivity and specificity in adequately characterizing malignant nodules. Regardless of classification criteria used by a provider, the primary indication for a thyroid nodule FNA is to further investigate sonographic features suspicious for malignancy.
Contraindications to thyroid nodule FNA include:
Evaluating the patient for a past medical history of bleeding diathesis is especially important. Local post-procedural hematomas in patients with bleeding disorders can exert a mass effect on the trachea and cause significant respiratory distress.
Different studies have suggested that FNA procedures can be performed safely in patients taking antithrombotic and anticoagulant agents without the need for discontinuation.. The Society of Interventional Radiology Guidelines classifies thyroid biopsies within the low bleeding risk category without the routine use of screening coagulation blood tests. These guidelines also do not recommend withholding anticoagulants in low-risk bleeding procedures.
A thorough discussion with a patient prior to the procedure includes the following:
The patient is placed in a supine position for the procedure. Prior to beginning the procedure, a brief preliminary ultrasound of the thyroid is performed to asses the nodule's location. The operator can take this time to better position the patient, probe, and monitor. This also allows the operator to visualize the proposed needle trajectory and surrounding vasculature via doppler mapping. The skin is then marked at the proposed site of entry.
Whether sterile technique and local anesthesia are employed for the procedure often remains dependent on provider and institution preference. The incidence of local infection from FNA is exceptionally rare. Studies have also suggested that anesthesia use can be limited to a small subset of patients. Due to the varying levels of operator proficiency, it is recommended to employ the sterile technique and local anesthesia.
The skin site is cleaned, and a sterile field is established with a fenestrated drape or with towel drapes. A high-resolution (7.5 MHz to 15 MHz) linear-array transducer is used for optimal visualization of the target nodule. A sterile cover is placed over the head of the probe. Sterile ultrasound gel can be avoided if the skin site is sterilized with an iodine-based solution, which can serve as an adequate ultrasound coupling medium. Local anesthesia is administered with approximately 1 mL to 3 mL of 1% lidocaine hydrochloride, either subcutaneously or through the proposed soft tissue trajectory of the FNA needle. This ultimately depends on nodule depth and location. A 22 gauge to 27 gauge needle with an attached syringe is used for specimen acquisition.
The ultrasound monitor should be located across the patient from the operator. A general linear alignment of the needle, the long axis of the transducer, and the monitor is important for optimal hand-eye coordination. This allows the operator to easily assess needle positioning within the soft tissues on the monitor while comfortably manipulating the needle.
The basic steps involved in an ultrasound-guided thyroid nodule FNA include the following:
Nonaspiration technique involves performing this maneuver without suction, with capillary action allowing the collection of the specimen within the needle hub. Aspiration technique allows for specimen acquisition via operator application of gentle suction during the to and fro maneuver. Although nonaspiration technique has occasionally been shown to be superior to the aspiration technique, two meta-analysis studies revealed no significant difference between the two. The needle-syringes may be delivered to a cytopathologist present during the procedure for further processing to ensure adequate acquisition. Multiple aspiration attempts are performed until a satisfactory tissue sample is confirmed by the cytopathologist.
The vast majority of thyroid FNAs are performed with ease and without complication. Development of a neck hematoma remains a concerning, yet rare, complication of thyroid FNA procedures. Other complications are also rare and include pain, transient swelling, infection, and vasovagal reaction.
The primary goal of performing an FNA of a thyroid nodule is to diagnose and/or exclude a malignant thyroid nodule. Ultrasound-guided FNA is imperative in directing the management of thyroid nodules, allowing for prompt diagnosis and treatment of thyroid malignancies. Additionally, the identification of benign thyroid nodules helps avoid unnecessary surgery and helps guide treatment and reduce the cost of care.
Papillary thyroid cancer rates have been gradually increasing in incidence. Papillary and follicular cancers are considered differentiated malignancies and are treated with total or partial thyroidectomy. In particular, identifying papillary thyroid cancer is especially important due to its relatively good prognosis, with mortality rate, which have been reported as low as 2%. Treatment includes surgical resection or administration of radioactive iodine. Anaplastic thyroid carcinoma, however, is extremely aggressive and occurs during the 6th and 7th decades. Clinically, anaplastic thyroid carcinoma commonly results in aggressive tumor expansion and growth, leading to compressive symptoms from mass effect on adjacent structures such as the cervical airway. The 5-year survival rate is around 5%.
The importance of prompt identification and diagnosis of thyroid cancer has also been demonstrated in the Surveillance, Epidemiology, and End Results database, where the mortality rate of localized thyroid cancer has been shown to be significantly better when compared to regional and metastatic disease. Ultrasound-guided FNA is an inexpensive and safe modality in the evaluation of thyroid nodules and will continue to guide clinical management for years to come.
The role of an ultrasound technologist is exceptionally important in helping the operator during the procedure. Patient comfort is critical in properly performing the procedure as significant patient movement can affect specimen acquisition and potentially injure adjacent anatomy such as blood vessels or the trachea. The assistant's role is essential in ensuring patient comfort, safety, and maintaining the sterile field. Quickly obtaining specimens is also important, considering blood infiltration within the thyroid parenchyma limits the quality of successive needle passes. If possible, an assistant ultrasound technologist can deliver the specimens to the cytopathologist while the operator continues acquiring additional specimens. Thus potentially improving the quality and adequacy of all aspirated specimens acquired during the procedure.
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