Ask the Vet

by Dr. Cheryl Burke, DVM, CCRP

My 6-year-old Lab, Cooper, has a lump on his shoulder that was at least 1.5 inches when I found it, but when I got to my vet’s office two days later, it was gone. Now it’s back! My vet thinks it might be a mast cell tumor. Before I go to my recheck appointment next week, I want to learn as much as possible about what that could mean. Can you help?
Worried in Woodlawn

Dear Worried:
I’ll help you as much as I can. First, I want you to take a permanent marker or nail polish and mark the hair right where that lump is. Draw a circle around it if you like. It’s important that your vet can examine the exact spot of the “on and off ” lump. Without taking a cell sample from the lump, we cannot know for certain if Cooper’s lump is a mast cell tumor: Nevertheless, it’s worth a discussion of the basics of this tumor type. Here are the things that I tell my clients when a mast cell tumor is suspected or diagnosed.

  1. Mast cells are normally found in the body. They are chemically active cells that are found in parts of the body that are environmentally exposed. These are the respiratory system, skin, and gastrointestinal systems. They participate in immune response and contribute to allergic and inflammatory reactions. When you see redness, itching, and swelling from a bee sting, you’re seeing mast cell activity.
  2. Mast cell tumors are abnormal proliferations of the cell type. Although the tumor cells retain the chemical “abilities,” they lack normal constraints and can divide, spread, and invade without normal restrictions. This is also known as cancer. The inflammatory properties of this cell type explain why a lump might change size or even seem to go away.
  3. Mast cell tumors are the most common skin cancers in dogs accounting for 20% of all skin cancers. They are extremely variable in appearance, as the pictures indicate. Several breeds of dogs are at higher-than-average risk to get this cancer, with Boxers at No.1 on the list. Labradors, beagles, dachshunds, and bulldogs are also all an increased risk for this cancer.
  4. We can often diagnose this tumor by putting a fine needle into the lump and extracting a few cells, which is called a fine needle aspirate. These cells can be prepared and stained on a slide and reviewed in the vet’s office under the microscope. The cells have characteristic granules that are usually easy to identify.
    If a growth is confirmed to be a mast cell tumor and is surgically approachable, then surgery is the treatment of choice. Your veterinarian will recommend some staging tests that help to establish whether the tumor cells have escaped to other areas of the body.
    These tests will include X-rays of the chest and/or abdomen, lymph node examination and/or aspiration, blood work, and sometimes more involved testing if the tumor appears aggressive. There are pathologists that are grading the mast cell tumors from cytology but that is not routinely available. In general, we would not suggest surgery for the original tumor if there is obvious spread (metastasis).
    After a mast cell tumor is removed or biopsied a pathologist can grade the tumor. Grading is a scale of how malignant (dangerous) a tumor is. Mast cell cancer is graded I to III, with III being the most aggressive. Only a pathologist looking under the microscope can grade a mast cell tumor. However, certain tumor locations such as the trunk, groin, and face are known to be more aggressive. Similarly, ulcerated, angry tumors are presumed even presurgically to be more aggressive until pathology proves otherwise.

At surgery, your veterinarian or surgeon will plan to remove tissue 2 cm on all sides of the lump, and one full layer beneath the lump. This type of excision is designed to create clean margins of no less than 1 cm wide and a full tissue plane deep. For 95% of grade I and Grade II tumors, these clean, wide margins will be a surgical cure. If the surgery is not successful or if the tumor has already spread, there are several chemotherapy options, including the first
FDA-approved animal chemotherapy drug, Palladia. Radiation therapy is known to be effective when clean surgical margins cannot be obtained or if the tumor is inoperable. There are exciting developments and new treatments that are not yet FDA approved that may provide additional choices in the future. If faced with complex or recurrent mast cell disease, your veterinarian may suggest you consult with a cancer specialist.

Worried, you have done a great job so far in finding that lump so quickly. Hopefully, your doctor will give you and Cooper good news next week.

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