Tune up your cytology skills

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Veterinary medicine is all about interpreting information and decision-making. Use these tips as a foundation to tune up your cytology skills. (Bonus points if your coworker calmly holds your patient nearby as you deftly interpret slides.)

Cytology is a valuable tool in veterinary practice for several reasons. It's relatively inexpensive, can usually be performed on an outpatient basis and requires limited equipment that's readily available (e.g. needles/syringes, slides, stain). Cytology gets bonus points because it can help clients decide on the next diagnostic or therapeutic steps for their pet, and it's usually associated with very low risk to the patient.

Although there are lots of reasons to perform cytology, getting optimal results and correctly interpreting your findings can take some practice. During a recent lecture at Fetch dvm360 conference in San Diego, Kim Johnson, DVM, DACVIM (Oncology), president/CEO at Pet Cancer Consulting in Nashville, Tennessee, described some tips for improving your in-clinic use of cytology.

Streamline your next cytology exam

Dr. Johnson presented these five tips for obtaining viable cytology samples:

1. Aspirating a skin mass does not require shaving or surgical prep.
2. A 22-gauge needle with a 6-mL syringe is normally adequate for obtaining a sample, although a larger-gauge needle may be used.
3. When preparing slides, try to create a thin cell layer without deforming the cells.
4. Diff-Quik stain is adequate in most cases. More specialized stains (e.g. Giemsa) are rarely necessary.
5. Submit one slide to your laboratory pathologist, but keep one as backup in case you need an answer before laboratory results are available or if the first sample you submit contains insufficient material for analysis.

Subject to interpretation

Cytology results are always subject to interpretation, Dr. Johnson noted. If a dog presents with lymphadenopathy, the most likely cause is inflammation, infection or neoplasia. “Cytology can often identify the underlying process, or at least differentiate inflammation from neoplasia, but it also has some shortcomings that are hard to ignore,” Dr. Johnson said.

The biggest shortcoming is that the results are sometimes inconclusive. For example, when cancer causes localized inflammation and tissue necrosis, it can be difficult to rule cancer in or out because these other changes are happening concurrently. Dr. Johnson warned, “Reactive epithelial cells can be mistaken for carcinoma, and reactive mesenchymal cells can be mistaken for sarcoma cells.”

What's more, even when cytology has helped you identify cancer, it's hard to narrow the diagnosis as much as you might like. “Sometimes, we may be able to tell that a mass is a sarcoma, but we can't tell specifically which kind of sarcoma it is. We can only narrow it down to a certain family. In some cases, that can be prognostic, depending on what that family is,” she said. Also, even if a cytologic exam identifies cancer, th test can't tell us how aggressive or invasive a cancer process may be. Additional diagnostics are required for that.

Once a mass is determined to be neoplastic, cytologic exam can frequently differentiate a benign tumor from a malignancy, Dr. Johnson noted. Cellular characteristics that suggest malignancy include anisocytosis (variation in cell size), the presence of cytoplasmic vacuoles or granules, variably shaped nuclei, multiple nucleoli and/or nucleoli of varying sizes, differences in nuclear-to-cytoplasmic ratio and the presence of mitotic figures. “Although these features are not absolute, if you identify three or more of these things, it supports the likelihood that you're looking at a malignancy,” Dr. Johnson added.

Obtaining and analyzing samples

Obtaining a sample for cytology is usually straightforward. Some masses exfoliate (shed cells) readily, so bodily fluids (e.g. urine, cerebrospinal fluid), effusion from a body cavity (ascites or pleural effusion) or fluid samples from washes can provide acceptable specimens for cytology.

Most skin masses lend themselves well to fine-needle aspiration. Masses within body cavities may as well, but imaging guidance (e.g. ultrasound) may be necessary to isolate the target properly. “Fine-needle aspirates are my preferred sampling method for most masses,” Dr. Johnson said.

However, some tumor types are more likely than others to provide samples with adequate cellularity to make a cytologic diagnosis. Round cell tumors tend to exfoliate easier than others, and therefore lend themselves to cytologic analysis more readily. “In general, if you aspirate round cell tumors, like mast cell tumors, lymphomas, plasmacytomas or histiocytomas, you're likely to get a diagnosis because those tumor types exfoliate very well,” Dr. Johnson said. Round cell tumors tend to appear as discrete, individual cells with a generally round shape.

Epithelial cell tumors include adenomas, carcinomas and adenocarcinomas. These cells tend to cluster or form sheets, exhibiting cell-to-cell adhesion but, like round cell tumors, they also usually exfoliate well.

In contrast, sarcomas are less likely to exfoliate readily, so these cytology samples tend to have lower cellularity. Sarcoma cells may appear as single cells or clusters and may be spindle-shaped, round or oval. Examples include fibromas, fibrosarcomas, chondrosarcomas and osteosarcomas.

When examining a sample, Dr. Johnson recommends starting at low magnification and working your way up to high magnification. “Sometimes, a cluster of cells can look like a carcinoma cluster but turn out to be mast cells,” she said. Examining at progressive magnifications supports diagnostic accuracy. As with most other veterinary skills, clinicians can improve their efficiency and diagnostic capabilities by performing cytologies more regularly and becoming familiar with the subtleties of microscopic interpretation. In skilled hands, cytology can benefit your practice and your patients.

Dr. Todd-Jenkins received her VMD degree from the University of Pennsylvania School of Veterinary Medicine. She is a medical writer and has remained in clinical practice for over 20 years. She is a member of the American Medical Writers Association and One Health Initiative.