What is that bump? - Cutaneous mast cell tumors in cats

Just like any cat that comes through our front doors, Mr. A and Mr. B get wellness exams periodically. On February 7, one of our technician discovered a small, raised bump on Mr. B's hock.

Since new bumps on a cat should be looked at to ensure that they are not cancerous, we had Dr. Demos give Mr. B a thorough exam and take a sample from the mass with a needle. This is called a Fine Needle Aspirate (FNA), and can often give us a good preliminary idea of the kind of lump we are dealing with.

Once we had a sample of the mass, the images we saw under the microscope showed this:

 Very dark-staining cells filled with small granules and lots of small granules in the background. These are mast cells.

Mast cells

 Mast cells can easily be compared to other abnormal cells, like these white blood cells, called lymphocytes, that are abnormal due to the presence of a cancer called lymphoma.

Lymphoma

 Since we were suspicious of a mast cell tumor, we scheduled Mr. B for surgery to remove the mass. He fasted prior to surgery, and then was given preliminary pain medications. We anesthetized him and shaved and scrubbed the surgical site. Even though the lump was quite small, the surgery was very delicate, because the hock is an area with prominent blood vessels and tendons, and has very little fat.

Once the mass was removed, we placed it in formalin to send to a pathologist for analysis. Even though we have a good idea what the mass is before we remove it, we like to confirm it with the experts, and also have the mass checked to ensure that we removed all of it. This is done by measuring the "margins" around the edge of the abnormal tissue.

"Clean margins" (a ring of completely normal tissue around the mass) mean that the mass is less likely to recur. If the margin of normal tissue around the mass is too small, there increased risk of the mass re-occurring. The need to have clean margins around the mass is also the reason that you may notice the area where a mass was removed is often quite a bit larger than the mass was, originally. In some areas, such as the leg, face and tail or the top of the head, where there is not a lot of extra skin, it may be very difficult to create good margins for a mass without risking the integrity of other parts of the body. This is why it is a good

After surgery, we placed Mr. B in a cage, to ensure that he was fully recovered before we let him run around. Usually recovery from a short surgical procedure like a skin mass removal takes just a few minutes from the time that the anesthetic gas is turned off. In Mr. B's case, he was sitting up and asking for post-surgical treats within about 10 minutes.

First, Mr. B fasted for his surgery. He was not very excited about that.

Next, we set up for surgery.

We placed Mr. B under anesthesia, intubated him and shaved and prepped his surgical site.

Post-surgery, we cleaned the sutured incision site, and let Mr. B recover on his warm water blanket.

Mr. B completed his recovery in a cage, and then enjoyed a post-surgical meal. His eyes are very dilated from the pain medication we used. It made him VERY happy.

What is a Mast Cell? Mast cells are a normal cell type found in the body that is involved in various functions including the immune system, wound healing, allergies and inflammation. The granules that they contain are really little sacs that carry important chemicals such as heparin, histamines and antimicrobial chemicals.They are developed in the bone marrow and disperse throughout the body to the skin, connective tissues and the lining of the abdominal organs such as the stomach and intestines.

Despite their beneficial activities in the body, sometimes, mast cells go bad. Mast cells play a big role in anaphylactic reactions (the kind of serious allergic reaction many people experience due to bee stings). Also, in addition to forming mast cell tumors, they can also be involved in autoimmune diseases. In humans, mast cells have been implicated in rheumatoid arthritis.


About 20% of all skin tumors in cats are mast cell tumors, and these types of tumors occur less frequently in cats than in dogs. About 90% of cutaneous mast cell tumors in cats are benign, and excision (surgical removal) is the cure. Mast cell tumors in the internal organs are a different matter.

Once we had sent in Mr. B's mass for biopsy, we received the following report from the pathologist a few days later:

Microscopic Description	Small left hock skin mass contains a dermal solid proliferation of minimally pleomorphic small neoplastic mast cells. Mast cells efface adnexal structures. One mitotic figure per 10 high power fields is seen. Tumor free margins measure approximately 1 mm.
Microscopic Interpretation (Biopsy)	Cutaneous mast cell tumor
Comments:	Histologic features of this skin tumor are consistent with a benign dermal mast cell tumor. Cutaneous mast cell disease accounts for approximately 21% of all feline skin tumors. Grading scheme similar to canine mast cell tumors has not been developed for feline neoplasms. Mitotic count (over 5/10 HPF) is the only criteria that has prognostic significance in determining benign from malignant (Vet. Pathol 2010; 47(4); 643-653). Neoplasms with high mitotic rate may have concomitant increase in mast cell pleomorphism.

Now we can be confident that the tumor was removed entirely, and that it was benign.

Mr. B is now strutting his stuff as usual, with lots of commentary on his healing progress. In 10-14 days, he can have his sutures out and in 6-8 weeks, his hair will have regrown, and no one will even see his scar!

We wonder whether Mr. B is asking for a matching shaved spot on his other legs.