Cytology for 3 common dermatological tumors

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Abnormal cell growth results in the formation of tumors and, for dogs in particular, skin tumors are the most prevalent, with this abnormal cell growth happening in the tissue just beneath the skin.¹ Discovering skin tumors in pets can sometimes be easier than internal tumors because of the visibility on the outside. Skin tumors can also be more common for pets because of exposure to various environmental factors, such as chemicals, solar radiation, and viruses. Additionally, the development of skin tumors may be affected by hormonal imbalances and genetic factors.¹

In a session at the Directions in Veterinary Medicine symposium in Nashville, Tennessee, Kate Baker, DVM, DACVP (Clinical Pathology), CEO of Pocket Pathologist and VetHive, presented on common skin tumors in pets and how veterinary professionals can use cytology to identify and diagnose them.²

3 common skin tumors and their cytologic features

Lipoma

Lipomas are benign lumps that form because of an overgrowth of fat cells. These dermatological tumors can vary from very small to very large in size and can feel soft or firm. Lipomas are also usually well circumscribed and freely movable when touched, according to Baker.²

Greasy or oily material may come out when this tumor is aspirated; however, Baker warned veterinarians to be cautious about automatically diagnosing a tumor as lipoma just because it shows greasy material. “It’s tempting to just look at a slide, see greasy material, and call it a lipoma. Most of the time they are, but I really want you to take a quick peek under the microscope just to make sure there isn’t anything hiding, because any kind of other pathology can hide in fat. Because two tumor types that I see most commonly hiding in fat are mast cell tumors and soft tissue sarcomas,” she said.²

For those worried that a benign lipoma could be a liposarcoma during physical examination, Baker stated that these 2 tumors look different cytologically. Liposarcomas under the microscope typically have spindled borders and large, prominent nuclei. Baker also shared that in her experience, liposarcomas tend to be less common than lipomas. Overall, she emphasized that this is one of many reasons cytology can be crucial in tumor diagnoses.

Mast cell tumor

Mast cell tumors are often involved in allergic reactions and are the most common malignant tumor seen in dogs. They release histamine, which causes irritation and itching, and other chemicals that may cause shock.¹ These mast cell tumors can sometimes look like other types of tumors during initial examination and can vary from a small discrete mass or plaque to generalized swelling of a large area of the body. Cytologically, they have a moderate to large amount of cytoplasm that contains uniform purple granules and round, centrally located nuclei that are often obscured by granules. This unique granulation makes mast cells identifiable with cytology.² “They can be poorly granulated, so be aware of that. And if you’re moving too fast through your cytology interpretations, you can sometimes miss [something]...so spend a little bit of time with your cytology,” Baker said.²

Soft tissue sarcoma

Sarcomas consist of skin’s connective tissue (fat, muscle, nerves, blood vessels, etc). Sarcomas on the surface of the skin tend to be benign, whereas deep-seated sarcomas lean toward malignancy. The larger the tumor, the more likely it is to be malignant.¹ As previously mentioned, soft tissue sarcomas can often mimic lipomas physically but look different cytologically under the microscope.

“Very common, soft tissue sarcomas. Soft tissue sarcoma is a big category; [there] are lots of different types of soft tissue sarcomas,” Baker said. However, most soft tissue sarcomas have long tails or tentacles branching off and are often called spindle cell sarcomas.

Baker also stated that these tails can get tangled and stuck together. “Anytime you’re looking at dense cellular samples, be careful not to just hop right in the middle of that superdense area. I don’t even look in that area; I can’t tell what those cells are, because I can’t see their borders [or] I can’t see how they’re interacting with each other. So, I’m looking around that periphery and saying, ‘Oh, these are long, wispy tails; this is a spindle cell tumor [with] features of a soft tissue sarcoma—specifically those 2 types because of how long those tails are.’”²

Takeaways

These were just 3 examples of dermatological tumors but, overall, the benefits of cytology used in diagnosing skin tumors in pets lies in its ability to provide quick and relatively noninvasive insights into the nature of the tumor. In some instances, a histopathological examination of a biopsy may be recommended for a more comprehensive evaluation. The combination of cytology and other diagnostic methods allows veterinarians to gather a comprehensive understanding of the nature of a skin tumor in pets, facilitating more informed treatment decisions.

References

1. Villalobes AE. Tumors of the skin in dogs. Merck Veterinary Manual. Updated October 2022. Accessed November 15, 2023. https://www.merckvetmanual.com/dog-owners/skin-disorders-of-dogs/tumors-of-the-skin-in-dogs#v3207674
2. Baker K. Cytology of 12 common skin problems. Presented at: Directions in Veterinary Medicine; September 15-16, 2023; Nashville, TN.