Fine Needle Aspirate Technique In Veterinary Practice
Fine needle aspirate (FNA) is a procedure akin to biopsy. It is a form of cytology. It is a relatively non-invasive, nearly painless way to have a look into a mass in an effort to determine the character of the mass.
Notice that we said “in an effort.” FNA is not foolproof. There are several techniques, which vary according to the cell type suspected to be inside the mass.
In all cases the process begins with clipping hair and sterilizing the skin so as not to introduce infection into a mass.
In most cases the so-called “woodpecker” technique is used. Woodpecker is especially helpful when cells inside a mass are suspected to be fragile, such as inside a lymph node. The technique is simple. A needle is removed from its syringe and inserted deeply into the mass. Then it is pulled back, but not completely out, and inserted into an adjacent area of the mass. At each insertion we expect fluid and cells to “exfoliate,” or be given up, into the needle. After 1-5 reinsertions, the needle is placed back onto the syringe and its contents are squirted onto a microscope slide. A special technique is used to spread the material into a thin layer and the slide(s) is allowed to dry before being sent to a pathologist for microscopic analysis. Some general practitioners and most oncologists are also adept at reading their own slides.
The “aspirate” part of the name comes into play when we expect the internal structure of a mass to contain fibrous material and/or cells that can withstand rough treatment. In this case a larger-bore needle might be used and the procedure begins with the needle still on the syringe. The needle is inserted into a mass, at which time the plunger is pulled back on the syringe, creating a strong vacuum. Again, cells and fluid enter then needle and/or the syringe. Gently, the syringe’s plunger is allowed to seat again, at which time the needle is removed. Air is drawn into the syringe, the needle is reattached, and fluid and cells are delicately blown onto a microscope slide(s), then spread.
If a veterinarian is totally unsure about the character of the material inside a mass, he may begin with the gentler woodpecker technique and a few slides, then follow up with a more aggressive aspiration technique.
The potential downfall of these cytology processes is that we evaluate an extremely small percentage of the interior of a mass. Suppose you used the technique on an orange. If you examined the results and saw tiny cells of pulp, you would never know whether seeds were present inside. Likewise, if a mass’ interior is heterogeneous, we could get a report from the pathologist that everything he saw was normal, while having missed a seed-sized area of aggressive cancer with our needle.
We always take that limitation into account with every fine needle aspirate.
Another way FNA results can fool a doctor is in a mixed tumor. Canine mammary tumors, for example, are notorious for being “mixed,” having both non-cancerous areas adjacent to areas that may have more than one cancerous cell type. FNA cytology can be very misleading if the report comes back “rosy” while dangerous cancer lurks inside a mammary gland.
Tucker, pictured above, is an excellent example of the value of the fine needle aspirate technique. Recently, we removed a mass from his throat area, submitted it for histopathology, and it proved to be a grade 2 mast cell tumor.
Recovery from the surgery was uneventful, but he came in this week with a swollen lymph node. Mast cell tumor tends to spread in lymph nodes, so we obtained an FNA and are awaiting the pathologist’s report.
See you tomorrow, Dr. Randolph.