Fine-Needle Aspiration
Fine needle aspiration (FNA) draws cells and fluid from a mass, node or collection through a thin needle, usually 22 to 27 gauge. The cells are spread on glass slides, or rinsed into a liquid medium, and examined by a cytopathologist.
It is quick, cheap and safe, needs little anaesthesia, and gives a result within hours to days. It reaches superficial lesions by palpation and deep lesions under ultrasound, CT or endoscopic ultrasound. It samples cells, not tissue, so it does not show architecture, and a core biopsy is added when more detail is needed.
Common targets are thyroid nodules, lymph nodes, breast lumps, salivary glands, soft tissue masses, and deep masses in the pancreas, lung, mediastinum and liver. It also drains cysts and collections for analysis.
The procedure takes 5 to 15 minutes. Rapid on site evaluation, in which a cytologist checks smears while the patient is still present, cuts the number of non-diagnostic results.
Diagnostic accuracy depends on the site, the sampling skill and the cytologist. For thyroid, 70 to 80% of aspirates are benign, 5 to 10% are malignant or suspicious, and 5 to 15% are non-diagnostic. For breast lumps, sensitivity for cancer is about 90 to 95%, and a core biopsy is used when the result is inconclusive.
Indications.
- Thyroid nodule with suspicious features on ultrasound.
- Enlarged lymph node of unknown cause, or suspected nodal metastasis (including staging of cancer).
- Breast lump or axillary node.
- Salivary gland, soft tissue and subcutaneous masses.
- Cysts and collections: for drainage, cell content and chemistry.
- Deep masses by image control: pancreas, lung, mediastinum, liver, kidney, adrenal and retroperitoneum.
- Suspected infection (tuberculosis, fungal infection, parasites), with material for culture, PCR and stains.
- Metastasis of unknown primary.
- Obtaining material for molecular testing in advanced cancer.
Not indicated when a core biopsy or excision is clearly needed for diagnosis, for example suspected lymphoma needing architecture, or a suspected sarcoma needing grading.
Contraindications and precautions.
- Uncorrected coagulopathy, especially for deep sites.
- Vascular lesions, and lesions next to major vessels.
- Suspected hydatid cyst: specialist planning is needed because of the risk of spillage and anaphylaxis.
- Suspected phaeochromocytoma: prior adrenergic blockade is needed.
- Uncooperative patient.
- Infection at the site.
- Pregnancy: safe, and ultrasound control is used.
- Lesions near the lung: risk of pneumothorax.
Preparation and technique.
The lesion is located by palpation or by ultrasound. The skin is cleaned. Local anaesthetic is optional for superficial lesions and is used for deep targets.
The needle is introduced into the lesion, and moved back and forth in several directions within it for a few seconds, with suction by a syringe or without it (the capillary or non-aspiration technique, preferred in vascular tissues such as the thyroid because it gives less blood). Several passes are made from different parts of the lesion.
Material is expelled onto slides. Smears are either air dried for Romanowsky staining, or fixed at once in alcohol for Papanicolaou staining. Alternatively the needle is rinsed in liquid based medium. Leftover material is saved for cell blocks, immunocytochemistry, flow cytometry, molecular tests and culture.
Rapid on site evaluation checks adequacy and triages extra material. Pressure is applied to the site for several minutes, and the patient is observed briefly. For cysts, the fluid is drained completely, and the residual mass is rechecked.
