fine needle aspiration

**Technique:** A thin needle (22–27G) is passed into a lesion, often under ultrasound or CT guidance, and cells are aspirated with or without suction. Smears are made or the sample is placed in liquid-based fixative for cytologic evaluation. Cell blocks, flow cytometry, and immunohistochemistry or molecular testing can be done on the sample. Complications are minor (local hematoma, pain, rare infection). Tumor seeding along the tract is rare.

**Key advantages:** It is quick, cheap, and outpatient, needs no general anesthesia, and has a low complication rate. It can often avoid unnecessary surgery.

**Key limitations:** - It gives cytology only, with no tissue architecture. It cannot assess capsular or vascular invasion. For this reason, **follicular adenoma vs follicular carcinoma** cannot be distinguished on FNA, and a follicular neoplasm requires diagnostic lobectomy or surgical excision. - Lymphoma diagnosis and subtyping generally needs **excisional lymph node biopsy**, because nodal architecture matters. Core biopsy is an alternative. - Sampling error can produce non-diagnostic or false-negative results.

**Classic clinical uses (high-yield):** - **Thyroid nodule:** Check TSH first. If TSH is low, get a radioiodine scan (a hyperfunctioning "hot" nodule is rarely malignant). If TSH is normal or high, do ultrasound, then FNA of suspicious nodules (size and sonographic features). Results are reported by the **Bethesda system** (I nondiagnostic, II benign, III AUS/FLUS, IV follicular neoplasm, V suspicious for malignancy, VI malignant). Papillary carcinoma shows Orphan Annie eye nuclei, nuclear grooves, and intranuclear pseudoinclusions. Medullary carcinoma shows amyloid, and calcitonin staining is positive. - **Breast mass:** In a palpable mass, FNA can distinguish a cyst from a solid lesion. A simple cyst yields clear or green fluid and the mass resolves. Bloody fluid or a residual mass needs further workup. Core needle biopsy is preferred for solid masses because it gives histology and receptor status (ER/PR/HER2). - **Lymphadenopathy:** Used for suspected metastatic carcinoma (e.g., Virchow node), and for infections such as TB. - **Others:** Salivary gland masses (parotid, where open biopsy is avoided because of the risk of seeding and facial nerve injury), pancreatic masses (endoscopic ultrasound-guided FNA), lung nodules (CT-guided), and liver lesions.

**Comparison with other biopsies:** - **FNA:** cells only (cytology). - **Core needle biopsy:** a tissue core that preserves architecture. - **Excisional biopsy:** removes the whole lesion. It is the gold standard for suspected lymphoma and melanoma. - **Incisional biopsy:** removes part of a lesion.

**Boards pearl:** A solitary cold thyroid nodule with normal TSH leads to FNA. A "follicular neoplasm" result leads to surgery, because invasion cannot be assessed on cytology.