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We have all heard of breast cancer in women. With approximately one woman in eight or nine falling victim to this form of cancer, there are awareness campaigns from numerous health care agencies and research continues. In the cat, mammary cancer is the third most common cancer, with t he most common victim being a senior female cat around age 10-12 years.
Dogs are lucky, as only about 50% of mammary tumors are malignant for them. For the cat, approximately 90% are malignant, with rapid spread to adjacent glands and the nearest set of lymph nodes. Cats generally have 8 mammary glands (thoracic, cranial abdominal, caudal abdominal, and inguinal – see illustration). The most commonly affected glands are the thoracic and inguinal glands. An owner should be accustomed to feeling for even small lumps in these areas. Because veins connect both the right and left sets of glands, it is easy for tumor cells to cross from one side to the other though usually the glands on the same side as the original tumor are seeded first.
At first the tumor is small and may feel like a pebble or dried pea. The tumor should be removed as soon as possible in hope of removing it completely. If left alone, mammary tumors get larger and harder and ultimately burst through the skin creating a smelly, infected ulcer.
Tumors removed when they are less than
0.8 inches (2cm) in diameter have a
median survival time of 4 and a half years.
Tumors removed that are greater than
1.2 inches (3cm) in diameter have a
6 month median survival time.
Tumors spread from the mammary glands to local lymph nodes and then on to the chest, brain, bone, and even spleen. Expect chest radiographs to assess tumor spread to the lung to be needed before surgery can proceed. Basic blood tests will also be needed.
- Not surprisingly, being female is the most significant factor.
- Siamese cats are overrepresented. They also tend to develop mammary tumors at a younger age than other breeds (9 years is average for Siamese and 14 for non-Siamese).
- Exposure to progestin hormones (not used much any more but previously used for behavioral issues such as urine marking) is a risk factor.
Early spay is the single most significant protective factor. Spaying before age 6 months results in a 91% reduction in risk. Spaying before age 1 year results in an 86% reduction in risk. Spaying before age 2 years leads to an 11% reduction in risk. Having given birth to kittens has no effect on mammary cancer risk for cats. Spaying after age 2 years does not reduce the risk of mammary cancer development at all.
Feline mammary tumors are generally either carcinomas or adenocarcinomas. Papillary or tubular carcinomas tend to be the least aggressive. Ductular carcinomas seem to kill cats about four times faster. Anaplastic carcinomas are the worst kind, usually dispatching the patient three times faster than that.
The pathologist examining the biopsy tissue uses different cellular criteria of malignancy to grade the tumor as Grade I, Grade II, or Grade III. A grade I tumor generally has a good chance of a one year survival time depending on its size and tumor type. Grade II tumors have a 57% chance at survival after a year unless chemotherapy accompanies surgery. Grade III tumors treated with surgery alone had no survivors at one year from the time of diagnosis.
The first step in treatment is surgical removal of as much of the tumor as possible. This means removing not just the affected mammary glands but the enter chain of mammary glands on that side. If both sides are involved, then both sides will have to be removed though there is generally not enough skin to remove both mammary chains at one time.
Aggressive surgery yields a disease free interval of
1 year in about 50% of cats and 2 years in 32% of cats.
If lymph node involvement is already present at
the time of surgery or if the tumor recurs after the first surgery,
then 51-66% of cats will have a 5 and a half month disease-free interval.
If the tumor is in an obviously advanced state at the time of the first surgery, then a less aggressive surgery to palliate the infection may be more appropriate. This will at least make the cat more comfortable by losing the smelly, infected external tumor.
 To maximize disease-free interval, surgery is followed by chemotherapy. The most common drugs used are Adriamycin and Cyclophosphamide. This combination was tested in a group of cats who had either incomplete surgical removal of their tumor or obvious tumor spread to lymph nodes or beyond at the time of surgery. About 50% of cats showed at least some response though 21% actually achieved remission (i.e. no detectable tumor at least temporarily). In the 50% of cats that showed at least some response to therapy, median survival time increased to 150-180 days as opposed to 75-86 days in the 50% that did not respond. Carboplatin has been another promising medication either added in to the above combination or used alone.
For the most up to date information on chemotherapy,
we recommend an appointment with a veterinary oncologist.
In our area, we recommend the Veterinary Cancer Group
at the City of Angels Veterinary Complex
(www.cityofangelsvets.com)
In other areas see www.acvim.org to locate a veterinary cancer specialist
There are two problems which tend to lead to the demise of the cat. One is simply the infected ulcerated external tumor. The primary tumor smells rotten and bleeds. Secondary tumors arise and also break through the skin. Cats are fastidious creatures and do not like having growths of this type on their bodies plus owners tend not to like rotten, bleeding flesh in their homes. Cats are commonly euthanized due to the progress of the external tumor.
The other problem is tumor spread to the chest. The lungs are invaded by tumor cells and ultimately the cat will not be able to breathe.
Cats with advanced lung involvement at the time the tumor is diagnosed have a median survival time of only one month.
Criteria for a euthanasia decision are largely personal but some guidelines have been established to assist in making this difficult decision. Please visit our Euthanasia page for more guidance on what to watch for.
Information in this article was largely drawn from Wypij, J., Fan, T.M., de Lorimer L. Malignant mammary tumors: Biological behavior, prognostic factors, and therapeutic approach in cats. Veterinary Medicine June 2006, p352-366.
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