Practical Principles of Fine-Needle Aspiration by Sang Wu, M.D.
Aspiration cytology is a diagnostic technique involving the collection and evaluation of cells obtained from a thin-gauge needle (typically a 22 to 25 gauge needle). Superficial and palpable nodules from the thyroid, breast, salivary glands, lymph nodes, and subcutaneous tissue, can be adequately sampled for diagnostic evaluation. Lesions in the deep organs (lung, mediastinum, liver, pancreas, kidney and retroperitoneum) can also be aspirated with radiographic guidance. Fine-needle aspiration cytology can be utilized to confirm malignancy, to confirm a benign lesion, or to provide material for ancillary studies. The advantages of fine-needle aspiration include the following: minimally invasive procedure, greater cost effectiveness, low risk of complication, and comparable diagnostic accuracy in comparison to histology.
The superficial fine-needle aspiration (FNA) procedure involves using a fine-gauge needle to extract cells from a palpable mass or nodule. The most common anatomic sites for superficial FNAs include thyroid, breast, lymph node, salivary glands, and subcutaneous soft tissue. Superficial FNAs are often performed by the pathologist or a clinician with technical expertise in performing the procedure. Deep FNAs on non-palpable masses of the lung, liver, pancreas, kidney, adrenal gland, or lymph nodes are performed by radiologists or endoscopists.
Many pathologists will evaluate for specimen adequacy using the microscope during the procedure, which may represent a valueadded service to the patient. The cytology obtained is subsequently examined microscopically and triaged for further studies if deemed necessary. It is important to recognize that fine-needle aspiration represents a minimally invasive diagnostic technique to evaluate neoplasms, infections, inflammation, or infiltrations.
Indications for FNA include the following: diagnosis in lieu of surgical tissue biopsy, therapeutic cyst drainage, or collection of specimen for special studies such as flow cytometry or culture. The advantages of a fine-needle aspiration can be summarized by the acronym "SAFE."
- Simple - Outpatient, minimal pain, no anesthesia, no pre- or post-procedure restrictions, can be repeated easily, no scars, does not interfere with further study.
- Accurate - High sensitivity, specificity and accuracy (>95%).
- Fast - 20-30 minute procedure with rapid results (usually a 12-24 hour turnaround time).
- Economical - Most cost-effective method of obtaining a diagnosis: hospital admission is unnecessary and special kits are not needed.
There are no dietary or other restrictions necessary prior to a fineneedle aspiration procedure. All regular medications can still be administered. The patient should be informed that several passes (averaging 4-6 passes) may be necessary to obtain ample cells for diagnosis, and that for certain lesions, a definitive diagnosis may not be made based on this procedure alone. After thoroughly discussing the risks and benefits of the procedure and answering any questions the patient may have, an informed consent form should be signed and witnessed.
Prior to performing the aspiration procedure, positioning the patient in an optimal and stable stance is important. Sitting up with the head tilted backwards is preferred for some thyroid and neck lesions, while other superficial lesions may become more accessible with the patient lying either supine or prone. The skin surface should be cleaned with either alcohol or iodine. Local anesthesia is not necessary, and may cause interpretive difficulties on cytology. However, a topical spray may be used for intraoral lesions if deemed necessary. After locating and evaluating the suspected mass, it should then be immobilized in between the operator's thumb and index finger. The fine needle with an attached 10 or 20 mL syringe is subsequently inserted into the lesion, and suction is applied either manually or with the assistance of a "pistol-grip" device. The needle should be manipulated in a cutting motion radially throughout the lesion to ensure adequate sampling. The suction is then released, and the fine needle is withdrawn from the site. Firm pressure is applied to the aspiration site with sterile gauze.
In preparation of the cytology smears, one drop of aspirate material is placed onto the center of a glass slide. The needle is removed from the syringe, and any additional contents is expelled onto additional glass slides. The first slide is opposed with another glass slide (preferably a glazed slide) and the aspirate is smeared by gently pulling the two slides apart with no pressure applied. After smearing, immediately spray-fix or alcohol-fix the glazed glass slide and allow the mirror-image slide to air dry. Any remaining aspirate material from the needle is rinsed in a tissue culture medium where ancillary studies such as a cell block (histologic section), culture, or flow cytometry can be performed following review of the smears.
Air-dried slides are immediately stained with Diff-Quik (similar to hematology stain) to assess the adequacy of the specimen. Staining characteristics of the Diff-Quik stain are good for hematologic or lymphoid disorders, mucin, colloid, extracellular matrix. The sprayfixed or alcohol-fixed slides are stained by the Papanicolaou method (PAP stain), which tends to provide good nuclear and cytoplasmic cytomorphology without air-drying artifact or cellular distortion.
At the end of the procedure, the patient's skin surface is cleaned with alcohol and minimal bleeding may be observed. A bandage is applied locally, and the patient is asked to follow-up with his referring physician for discussion of the findings from the fine-needle aspiration.
Contraindications for a fine needle aspiration procedure are few and limited to certain clinical situations. Some authors have advocated the discontinuation of aspirin or other agents that affect coagulation for several days before the procedure. Lesions known to be highly vascular, such as hemangiomas, can be a contraindication due to the risk of severe bleeding. For thyroid FNAs, patients who cannot suppress a cough may pose difficulty to the pathologist in localizing the mass due to unexpected movements during the procedure. In liver FNAs, a suspected hydatid cyst is a contraindication due to the possibility of an anaphylactic reaction.
The primary limitation of a fine-needle aspiration is the lack of tissue architecture, especially in instances in which documentation of capsular or vascular invasion may be necessary to confirm malignancy. A thyroid follicular carcinoma, for example, cannot be diagnosed by cytology alone. Furthermore, FNAs requires considerable training and experience for accurate diagnosis. FNAs are also difficult to perform on children less than 13 years old without the assistance of the referring physician, and sedation may be required in some instances.
Serious complications are exceedingly rare, and the incidence increases with increasing needle diameter. For superficial lesions, excessive bleeding may rarely occur, and application of local pressure adequately prompts clot formation. Local infection may also occur, although thorough cleaning of the skin surface with alcohol or iodine minimizes any such risk.
The sight of needles may prompt some individuals to experience a vasovagal reaction. The procedure should be immediately terminated, and the patient should be placed in a supine position for adequate re-circulation. A rare but potentially serious complication in transthoracic FNAs is a pneumothorax, which will require close clinical follow-up and possible hospitalization.
FINAL CYTOLOGY REPORT
The final cytology report should contain the following information:
3) specific diagnosis, differential diagnosis, or descriptive diagnosis
Cytologic findings should not be interpreted within a vacuum; it is important to correlate cytologic findings with clinical and radiologic findings. Further work-up, including possible tissue biopsy, is indicated if there is a lack of correlation between the cytologic findings and clinical suspicion.
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