Presurgical evaluation and diagnostic imaging for canine mast cell tumors

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What should veterinarians know about the diagnostic investigation of a canine mast cell tumor (MCT)? At the 2024 New York Vet Show—happening November 7 to November 8—Ann Hohenhaus, DVM, DACVIM (Oncology, SAIM), director of Pet Health at the Schwarzman Animal Medical Center (AMC) in New York, talked about several components of a diagnostic investigation of canine MCTs, including the presurgical evaluation and diagnostic imaging. According to Hohenhaus, the second most common tumor seen on canine skin is the MCT, and it is the most common malignant skin tumor in dogs.¹

Presurgical evaluation

In her lecture “Mast Cell Tumors Part 1: The Diagnostic Investigation,” Hohenhaus delved into fine-needle aspiration and the preanesthetic evaluation—both essential components of a presurgical evaluation for patients with suspected MCTs.

Fine-needle aspiration

Regularly aspirating cutaneous tumors is crucial, as these tumors can often be MCTs, and performing a fine-needle aspirate of a skin mast is the first step in the presurgical evaluation. “Aspiration cytology is critical for identifying mast cell tumors because when you have a mast cell tumor, you need to be sure that you're going to have adequate surgical margins,” emphasized Hohenhaus during her lecture.¹ “So, to not know that a tumor is a mast cell tumor and go in and just cut a little bit around the mast is going to end up having that pet have to have a second surgery to get adequate margins. So that's the critical reason to aspirate skin masses.”

In dogs, skin masses generally look like pimples. However, on an aspirate, identifying a MCT is not a difficult feat. When aspirating, Hohenhaus recommends using a needle without a syringe on the hub, stabbing the mast several times, and then attaching a needle and squirting the material in the hub of the needle onto the slide. “That's sometimes a little bit more friendly to the cells in the mast cell tumor, because sometimes if you suck them into a syringe and squash them onto a slide, they just explode, and the pathologist can't really tell what it is,” explained Hohenhaus.

Aspirating the regional lymph nodes and looking at the patient’s complete blood count (CBC) and biochemical profile are also important components of the presurgical evaluation and should be routinely done.¹

Preanesthetic evaluation

Treatment for a canine MCT consists of surgery, most importantly. As such, a preanesthetic evaluation should be conducted as part of the diagnostic evaluation—this will aid in the planning of anesthesia.

A preanesthetic evaluation is especially important for older canines and those undergoing tumor removal or other nonelective procedures, as these groups have a higher risk of anesthetic death.¹ Additional risk factors for anesthetic death in patients include having an abnormal hematocrit (Hct) level and being underweight. Foregoing conducting or not recording a physical exam and pulse oximetry can also put a patient at risk for anesthetic death. Moreover, feline patients are at a higher risk of anesthetic death than their canine counterparts.

In canines, a buffy coat is not a discriminator for mast cell disease, said Hohenhaus. “Cats are a different story,” Hohenhaus continued. “If a cat has a positive buffy coat, then that cat highly likely has a mast cell tumor…somewhere.”

Diagnostic imaging

Thoracic radiographs

In discussing diagnostic imaging for MCTs, Hohenhaus emphasized that although chest X-rays can be useful for managing a patient with cardiac or respiratory disease, this method is generally not effective for detecting MCTs. “I've seen a lot of dogs with mast cell tumors over the years, and only 1 dog had a mast cell tumor in its thorax, and that dog, we found it on necropsy—it was not visible on a chest X ray, so a chest X ray in a dog with a mast cell tumor doesn't really help me manage that mast cell tumor better,” said Hohenhaus.

Abdominal ultrasound and cytology

An abdominal ultrasound should likely be done if the patient has enlarged popliteal or inguinal lymph nodes as this imaging will show lymphadenopathy associated with metastatic mast cell disease.

“If you have a dog with a grade 3 or a high-grade tumor, you probably want to look at that liver and spleen because those are the dogs that are at high risk for visceral metastasis,” said Hohenhaus, “And if you've got a dog with a whole bunch of mast cell tumors all over, you might want to think about imaging the abdomen and considering cytology for the liver and spleen”

For dogs with low-grade cutaneous mast cell tumors, imaging is often unnecessary, as studies show a low incidence of visceral metastasis in these cases.^1,2 Hohenhaus recommended waiting to do preoperative imaging until after biopsy results come back, suggeting that only dogs exhibiting signs of more aggressive tumors—such as edema or ulceration—should undergo further imaging.

Ultimately, understanding whether there is metastatic involvement is crucial for determining prognosis and appropriate treatment, including the potential need for chemotherapy and long-term histamine blockers to manage symptoms. “Benadryl and famotidine would be the 2 that we would typically use,” said Hohenhaus, of the long-term histamine blocking agents that would be used in those cases.

References

1. Hohenhaus A. Mast Cell Tumors Part 1: The Diagnostic Investigation. Presented at: 2024 New York Vet Show; November 7- 8, 2024; New York, NY.
2. Rinaldi V, Crisi PE, Vignoli M, et al. The role of fine needle aspiration of liver and spleen in the staging of low-grade canine cutaneous mast cell tumor. Veterinary Sciences. 2022;9(9):473. doi:10.3390/vetsci9090473