Mammary Tumors in Cats

“Lump on chest” was the brief description in the appointment book for my 4:00 appointment.  Todd Pechter had noticed a small, firm lump on his cat, Kina, a 13 year-old spayed female domestic shorthair. He couldn’t be sure how long it had been there; he had only noticed it 4 days ago.

On physical examination, Kina looked fine.  She weighed a robust 12 and a half pounds, and appeared to be in excellent health, except for a small, firm mass involving mammary gland #2 on the left side of her chest.  The seven other mammary glands felt normal.  I informed Mr. Pechter that I believed this to be a mammary tumor and that we should schedule surgery as soon as possible.

It is estimated that one out of 4000 cats develop breast cancer.  This may seem like a small incidence, but in fact, breast cancer is the third most common tumor in cats and accounts for 10 to 12 percent of all diagnosed feline tumors.

The average age at onset is 10 to 12 years.  Siamese females are at twice the risk of developing this type of cancer compared to other cat breeds.  Siamese also tend to develop these tumors at a younger age.  Males rarely develop breast cancer. In fact, less than 1% of cats that develop mammary cancer are males.

Intact cats are at an increased risk of breast cancer.  Spaying a cat, especially before their first heat, greatly reduces this risk. A hormonal influence is known to exist in dogs, and it is clear that estrogen and progesterone play a similar role in tumor development in cats, although this role is not as well understood.  It is hypothesized that, under the influence of hormones, the epithelial cells of the mammary gland enlarge, progressing to a pre-cancerous state, where it continues to grow until it has transformed into a carcinoma.

The benefit of spaying in regard to reducing the risk of mammary cancer in dogs is well documented. Dogs spayed before their first heat have only 0.5% the risk of mammary tumor development compared to an intact dog.  After their first heat, the risk jumps to 8%, and after two or three heat cycles, 26%!  Spaying a dog after 4 or more heat cycles or after 2.5 years of age has no protective effect.  Similar effects are seen in cats.  Spaying a cat prior to 6 months of age leads to a 91% reduction in the risk of mammary cancer development.  In other words, a cat spayed prior to their first heat (around 6 months of age) has only 9% of the risk of mammary tumor development compared to an unspayed cat. If spayed after 6 months, but before 1 year, the risk is 14% compared to an unspayed female. When Mr. Pechter adopted Kina, she was already somewhere between 2 and 3 years of age, and he did not know for sure at what age she had been spayed.

Typically, a middle-aged or older cat presents to the veterinarian after an owner notices a lump associated with the mammary gland(s).   Cats have four pairs of mammary glands: the four on the left side form the left “chain”, and the four on the right comprise the right chain.  The glands are numbered one to four, with gland #1 being closest to the head, and gland #4 being closest to the tail. On physical exam, cats may have a single nodule like Kina, or it may have multiple nodules associated with the mammary gland.  In dogs, the glands closest to the groin are most often affected.  In cats, the tumors occur with nearly equal frequency in all glands, with perhaps a slightly higher incidence in the most cranial (#1) and caudal (#4) glands.

Mammary tumors should be removed surgically.  “There is no way to determine whether a mammary tumor is benign or malignant from the visual appearance of the tumor”, says Dr. Avenelle Turner, a board certified veterinary oncologist at the Fifth Avenue Veterinary Specialists in New York City.  “The vast majority of mammary masses in cats are malignant, therefore, every mammary-associated lump or mass should be considered malignant until proven otherwise”, says Dr. Turner.  Because most of the affected cats are elderly, a full pre-surgical evaluation of the patient is important.  A complete blood count, serum biochemistry panel, thyroid evaluation, and urinalysis should be performed.  Radiographs should be taken as well, to determine if the cancer has already spread to the lungs at the time of surgery.  If the cancer has spread, surgery won’t be curative, and it may be best to cancel the surgery direct all efforts on supportive care to keep the cat comfortable.  If the tumor is ulcerated and bleeding or infected, it may be necessary to proceed with surgery even if the tumor has already spread to the lungs or elsewhere.  “Changes in the nearby lymph nodes, such as a change in size or texture, may be the first indicator of metastasis”, warns Dr. Turner.  “Any enlarged lymph nodes should also be removed at the time of surgery. Some surgeons feel that all nearby lymph nodes should be removed even if they appear normal, because they may contain microscopic disease.”

The goal of surgery is to remove the entire tumor by the simplest procedure possible.  Small tumors may be able to be removed via a “lumpectomy”, i.e. removal of just the lump.  Larger tumors may need a “mammectomy” – removal of the entire mammary gland.  If tumors are present in multiple glands, they maybe removed individually, or via a “chain mastectomy” in which the entire chain of mammary glands is removed, making one long incision.  Again, the choice of procedure depends on ease of removal of all affected tissue. Because each gland within a mammary chain is connected to each other by lymphatic vessels, some surgeons feel that a radical chain mastectomy is the procedure of choice, since mammary tumors initially spread via the lymphatic vessels. Kina’s tumor, at 2 centimeters in diameter, required a mammectomy.

In dogs, 50% of these tumors are benign.  Of the 50% that are malignant, half of them can be removed completely, resulting in a cure.  The other half will either recur or metastasize (spread to other parts of the body) following surgery.  In cats, the picture is more grim: approximately 86% are malignant.  Not surprisingly, Kina’s mammary tumor fell into this category; it was a malignant mammary carcinoma, however, no invasion of the lymphatic vessels by cancer cells was seen by the pathologist, and he reported that the mass appeared to be completely removed.

There are differing opinions regarding the efficacy of chemotherapy for feline mammary gland tumors. Chemotherapy is often recommended as adjunct therapy in cats whose tumors show evidence of invasion into the blood vessels or lymphatic vessels.  Others recommend chemotherapy in all cases, given the high metastatic potential of feline mammary tumors. A consultation with a veterinary oncologist would be prudent to assess whether a particular cat is an appropriate candidate for post-surgical chemotherapy.

Prognosis depends on several factors, the most important being the size of the tumor at the time of diagnosis.  If the tumor is less than 2 centimeters in diameter, the prognosis is better; cats often survive over 3 years.  Tumors larger than 3 centimeters are associated with a survival time of only 4 to 6 months.  These statistics clearly illustrate what has essentially become common knowledge regarding cancer in people and animals: early detection is of paramount importance.  This fact cannot be overstated.  Kina’s tumor, at 2 centimeters, was relatively small, and her pathology report looked favorable, however, almost exactly a year after her tumor was removed, she presented to our office with difficulty breathing, and x-rays revealed several small masses in the lungs as well as some fluid in the chest cavity.  Analysis of the fluid confirmed the presence of carcinoma cells.  Pulmonary metastasis (spread of the cancer to the lungs) is the most common cause of mammary-cancer related death, and sadly, Kina succumbed to her illness soon afterward.