MAMMARY TUMORS IN DOGS
PROTECTION VIA SPAYING
A female puppy spayed before her first heat cycle can expect never to develop a mammary tumor of any kind. The incidence of tumor development in this group is nearly zero.
If she is allowed to experience one heat cycle before spaying, the incidence rises to 7% (still quite low).
If she is allowed to experience more than one heat cycle, the risk is driven up to one in four.
If your dog is unspayed, was known to have had puppies, or was spayed in adulthood, she fits into the high risk group for mammary cancer development. It is important to be somewhat familiar with the normal mammary anatomy of the female dog. There are five sets of mammary glands as shown though the average female dog has only nine. (It is not unusual for asymmetry of mammary glands to be found). The normal glands should be soft and pliant, especially towards the rear legs. There should be no firm lumps. If a lump is detected, see your veterinarian at once regarding possible removal. Most tumors occur in the glands nearest the rear legs.
BENIGN VS. MALIGNANT
Approximately 50% of malignant mammary tumors in the dog have receptors for either estrogen or progesterone. This means that the presence of these female hormones promotes the growth of these tumors. Benign tumors also have female hormone receptors and can also be stimulated by hormonal cycling of the female dog. This means that=2 0spaying is important even if a tumor has already developed; in one study, female dogs spayed at the time of mammary tumor removal or two years prior lived 45% longer than those who remained unspayed.
TYPES OF TUMORS
The following are common classes of mammary tumors that might be found on a biopsy:
A benign glandular tumor for which no treatment is necessary.
“MIXED” MAMMARY TUMOR:
What is mixed is the type of cell that makes up the tumor: the epithelial cells that line the glandular tissue and the mesenchymal cells that make up the non-glandular portion. (“Mixed” does not refer to a mix of benign and malignant cells.) The mixed tumor can be either benign or malignant and the biopsy will indicate this.
Adenocarcinomas can be “tubular” or “papillary” depending on that gland cells the tumor arises from. Adenocarcinomas behave malignantly but how aggressively malignant they are depends not on whether they are tubular or papillary but on other cellular characteristics described by the pathologist (such as how quickly the cells appear to be dividing and how closely they resemble normal gland cells). When the oncologist reads the description he or she will be able to determine how aggressively to combat the tumor.
A highly malignant tumor that generates tremendous inflammation locally with ulceration, pus, and discomfort. This type of tumor tends to spread early in its course and is difficult to treat. Fortunately, this especially tragic tumor type accounts for less than 5% of mammary tumors.
In general: approximately 50% of malignant mammary tumors will have already spread by the time of surgery.
This, of course, means that the other 50% are locally confined and surgery is curative.
WHAT ELSE DETERMINES PROGNOSIS?
The type of tumor is obviously very important in determining the prognosis; further, spaying at the time for tumor removal or prior is also an important factor in determining prognosis. Other factors include:
The biopsy sample will not only identify the tumor type, it will also indicated whether or not the tumor was completely removed (so called “clean” or “dirty” margins).
If the tumor was not completely removed, one may wish to consider a second surgery to remove more tissue.
Both radiation therapy, chemotherapy and anti-estrogen therapy have been used for incompletely removed tumors. Sometimes it is most appropriate to monitor for recurrence with periodic chest radiographs.
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Page last updated: 6/5/2012