Fine Needle Aspiration Cytology
Know the reporting systems
Thyroid FNA uses the Bethesda System: I nondiagnostic, II benign, III AUS/FLUS, IV follicular neoplasm, V suspicious for malignancy, VI malignant - each with an implied risk of malignancy and management pathway. Salivary gland uses the Milan System (nondiagnostic, non-neoplastic, AUS, neoplasm, suspicious, malignant). Breast aspirates are often reported C1–C5. Lymph node and pancreatic FNA have their own structured schemes. Questions often ask which category drives surgery: AUS/FLUS and follicular neoplasm lead to diagnostic lobectomy or molecular testing, not to a cancer diagnosis.
Morphology worth memorizing
Papillary thyroid carcinoma: nuclear grooves, pseudoinclusions, powdery chromatin, psammoma bodies. Medullary carcinoma: plasmacytoid cells, amyloid, calcitonin. Follicular neoplasm: crowded microfollicles with scant colloid - but cytology cannot separate adenoma from carcinoma. Capsular or vascular invasion on histology is required. This is the single most tested concept in thyroid FNA.
Technique and adequacy
Rapid on-site evaluation reduces nondiagnostic rates by confirming cellularity and triaging material for flow cytometry, cell block, or microbiology. Necrotic or cystic aspirates may be nondiagnostic, and a “negative” call on an acellular smear is a trap. Always correlate with imaging and clinical history.
Worked example
A thyroid aspirate shows a monotonous population of microfollicles with little colloid and no grooves or inclusions. Report follicular neoplasm (Bethesda IV) - not “follicular adenoma,” and not “benign” - because malignancy requires histologic evaluation of the capsule.