Fine Needle Aspiration

Indications:

This technique is most commonly used to collect cells from cutaneous and subcutaneous masses and lymph nodes. Examination of the cells from cutaneous masses and cysts may allow differentiation of inflammatory and neoplastic processes. This procedure is of particular use as a screening process and is very helpful in any subsequent surgical planning (e.g.' in the case of a mast cell tumour where the advised surgical margins are greater than would normally be considered).

Lymph node aspirates are most commonly used in the investigation of lymphadenopathy. They may be particularly useful in the confirmation of lymphoproliferative disease. However, if metastatic disease of the node is suspected, then a biopsy and histological evaluation would be advisable. Histological examination of the node allows a study of the architecture and gives a better chance of identifying small neoplastic foci.

Fine needle aspiration of internal organs may also be a very useful technique, but it presents additional considerations. Aspiration of the liver or spleen may yield cells that reflect the pathological process if the changes are diffuse. The aspiration of focal lesions require ultrasound guidance. Significant complications associated with aspiration of internal organs are not common (although their incidence is a controversial issue), but include internal haemorrhage and the potential seeding of neoplastic cells throughout the peritoneum.

Equipment and Procedure:

Needle size may vary from 21 to 25 gauge, with the finer needles being more appropriate for internal organs or deep masses. The skin should be clipped and prepared and the mass fixed by digital pressure (if possible). The needle is inserted into the tissue and 2-5 ml of suction applied. The needle should continuously move during the application of suction (either in a straight line or along a number of different tangents within the mass).

The amount of material collected may be very small (especially when sampling from solid tissue) and may be contained within the hub of the needle. The syringe is then disconnected, filled with air and the contents of the needle hub gently ejected onto a slide. Smears should be made immediately. Wedge preparations are suitable where a moderate amount of material is present. For small amounts, a squash preparation may be more helpful. When slides are submitted to the laboratory, a detailed description of the size and the appearance of the mass gives the best chance of a meaningful cytological interpretation.

Problems:

Where a lesion has multiple areas of varying appearance, (e.g., a solid area and a cystic space), is important that cells are aspirated from each region. Often the examination of cyst fluid alone is unrewarding since the cells within the fluid may not be representative of the underlying cause.

There are inherent difficulties associated with some tumour types. Vascular neoplasia generally results in aspiration of a large amount of blood. In these cases, haemorrhage cannot be avoided. In other cases, excessive haemorrhage may be caused by use of a large bore needle and failure to keep the needle moving while suction is applied. Mesenchymal neoplasms may yield very small numbers of cells. Again, this may be an unavoidable problem, but, occasionally, use of a larger bore needle is helpful.

Smear-making

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The aim of smear-making is to display the cells in a monolayer while causing minimal disruption. Rapid drying of the smears is important in all preparations to maintain the cellular morphology, but this is particularly important in viscous fluids (mucus-containing samples, synovial fluid). The use of a hairdryer (cool/warm, but not hot) will facilitate rapid drying. The air should be directed onto the back of the glass slide from a distance of at least 6 inches.

Wedge Preparations:

This technique is used to prepare smears of peripheral blood, high-cellularity fluids with the viscosity of blood, and smears of the sediment of body cavity fluids. It should be ensured that the feathered edge of the smear is on a part of the slide that can be easily stained and examined under the microscope (i.e., away from the edges).

Squash Preparations:

This method is ideal for synovial fluid, tracheal and nasal washes and bone marrow aspirates. A small amount of the sample is placed on one slide and a second slide placed on top. The second slide may be perpendicular or parallel to the first. The material will spread between the two slides (Figure 1a). If necessary, very gentle pressure may be used to facilitate spreading (Figure 1b) and the top slide is gently pulled across the bottom until the two slides are separated (Figures 1c & 1d). The slides should slide apart, and should not be lifted away from each other.

Figure 1.