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What to Do About Lumps on Your Pet

Pet Column for the week of March 20, 2000

Office of Public Engagement
2001 S. Lincoln Ave.
Urbana, Illinois 61802
Phone: 217/333-2907
By Sarah Probst
Information Specialist

Last fall, I noticed two lumps on my beloved pal, my sweet Lab mix, Maxine. Fearing the
worst, I consulted clinicians at the University of Illinois Veterinary Medicine Teaching
Hospital in Urbana.

"Lumps are very common occurrences, especially in geriatric pets," says Dr. Timothy Fan,
a veterinarian who specializes in cancer care. "Approximately 30 percent of all tumors
found in dogs and cats occur on the skin. There are a couple of important questions owners
should ask themselves when they find suspect lumps on their pets."

Has the lump appeared suddenly, or has it been there awhile? Has the lump stayed the
same in consistency and appearance or has it changed recently? Does the lump seem to
separate from underlying tissue or is it attached? Is there only one lump or have you found
multiple similar lumps? And finally, are there changes in your pet�s behavior such as eating
less, losing weight, vomiting, diarrhea, or lethargy? Answering these questions may help
your veterinarian evaluate the seriousness of the lump's presence.

"Lumps are often benign accumulations of fat, known as lipomas. However, many lumps
are not lipomas, so all should be evaluated for the possibility of a more aggressive
malignancy," says Dr. Fan. In general, many benign lumps may grow slowly or not at all,
and remain unchanged for many months to even years. However, many malignant lumps
grow rapidly and may infiltrate into the underlying tissue.

"The easiest way for us to figure out if a lump is benign or malignant is to take a fine-needle
aspirate," says Dr. Fan. "We use the same needles that are used to give vaccines, and we
just extract a few cells and take a look at them under a microscope.

"Many skin tumors, benign or malignant, can be cured with surgery. However, skin tumors
could be just the tip of the iceberg," warns Dr. Fan. Diagnostic tests may be needed to
determine whether your pet's lump is a manifestation of a more widespread disease.

The most common malignant skin cancer in the dog is mast cell tumor. Mast cells are
normal cells in the body that serve a normal defense mechanism. "When you get bit by a
mosquito, it�s the mast cells that instigate the inflammatory response that causes the
production of a red, itchy, hive," explains Dr. Fan. Mast cells also mediate most allergic
diseases such as asthma and food allergies.

If the normal mast cell undergoes a malignant change, a mast cell tumor may be produced.
In the dog, mast cell tumors may be benign; however, many tumors possess varying
degrees of malignancy, ranging from local reoccurrence following surgical removal to
aggressive systemic disease, which may be fatal. The potential for mast cell tumor
aggressiveness may be determined by a biopsy. Other factors are also important in
predicting the biologic nature of mast cell tumors.

Although mast cell tumors primarily affect the skin, they have the potential to spread to
other areas of the body. Therefore, it is important not only to address the skin tumor, but
also to fully evaluate patients for systemic spread (metastasis) with blood work, thoracic
radiographs, and abdominal ultrasound. If there is no evidence for tumor metastasis,
surgical removal of the mast cell tumor may cure the problem. If complete surgical removal
of the mast cell tumor is not possible, radiation therapy or systemic chemotherapy should be

In the cat, mast cell tumors of the skin are generally benign, and surgery usually cures the
problem. Systemic spread from a primary skin mast cell tumor is rare in the cat, unlike the
dog. However, some cats may manifest with a distinctly different type of mast cell tumor
that primarily affects internal organs, such as the spleen, liver, and bone marrow. This type
of mast cell cancer is known as visceral mastocytosis and is much more aggressive than
mast cell tumors of the skin.

When you notice a lump or bump on your pet, it is best to have a veterinarian look at it
soon. Many pets have lumps all their lives that remain benign, but if a lump is malignant,
your pet has a better prognosis if treated early. "Skin cancers are quite common, and many
times are treatable and curable," says Dr. Fan.

Fortunately, Maxine's lumps were merely accumulations of fat, benign lipomas. Knowing
that the lumps are benign and that she is healthy gives me great peace of mind.

If you find a lump on your pet, contact your local veterinarian.