- 1. Mammary tumors are extremely common in intact bitches, and account for at least 50% of all reported neoplasms. At least 70% of intact bitches will develop a clinically detected mammary tumor if they live to 15 years, and virtually 100% of them will have microscopic tumor foci.
- 2. Ovariohysterectomy prior to the first heat cycle virtually eliminates the risk of developing mammary neoplasia later in life. The benefits of spaying are dramatically reduced the longer one waits: the risk is reduced by about 90% if spaying is between the first and second heat cycles, and by about 70% if it is between the second and third. There is no statistical benefit (in terms of reducing mammary neoplasia) if the spaying is delayed until after the fourth heat cycle.
- 3. Ninety-to-ninety five percent of all canine mammary neoplasia is behaviorally benign.
- 4. By far the most reliable predictor of true behavioral malignancy in canine mammary tumors is local invasion. This is easily detected by microscopic examination of excised tumors, but it can also be predicted with 80% accuracy by your own clinical examination. Tumors that seem to be fixed to underlying tissue, that cross the midline, or that are otherwise obviously infiltrative are very likely to be true malignancies. Only a very small percentage of mammary tumors will be a "surprise" malignancy since most of them give very unmistakable clinical evidence of malignancy before you even subject them to the scalpel.
- 5. Remember that benign mammary tumors develop from hyperplastic foci in mammary glands that have been rendered "fertile soil" by repeated cycles of hyperplasia and atrophy during normal estrus cycles. We know that a bitch with one detected mammary tumor is virtually certain to have dozens of microscopic foci in the same or other glands. It is probably appropriate for you to alert the owners to that fact - without worrying them unnecessarily - by explaining that most of these foci will never amount to anything significant during the life of their dog. With increasing life expectancy, however, we can certainly expect to see more bitches developing multiple benign mammary tumors.
Cats:
Mammary tumors in cats have not been as thoroughly studied as they have in dogs, probably because the overall prevalence is substantially lower. In contrast to the estimate of a 50% prevalence of macroscopic tumors in intact bitches, the prevalence in cats is estimated at approximately 25 tumors per 100,000 cats. Their relative infrequency is counterbalanced, however, by a much greater malignancy.
- 1. Ninety percent of all mammary tumors in cats are invasive tubular adenocarcinomas, and all should be considered behaviorally malignant.
- 2. The most important prognostic variable is the size of the tumor. Cats with mammary malignancies less than 1 cm in diameter have a 4.5 year median post-operative survival, while those with tumors greater than 3 cm in diameter at the time of excision have only a 6 month median survival. I see no histologic difference between the large and small tumors, so I conclude that the key to success is early detection and wide excision.
- 3. In contrast to the situation in bitches, there is no evidence that ovariohysterectomy exerts any influence over the prevalence of neoplasia in queens.
- 4. Finally, just a reminder: beware of rapid enlargement of one or more mammary glands in young queens in the springtime. This is hormone-dependent feline mammary hypertrophy, and it is not an indication for radical mastectomy! Some people use the arrival of birds or the sprouting of bulbs as signals that spring is coming. I, of course, use the dramatic increase in biopsies of feline mammary hypertrophy!
Brian Wilcock, D.V.M., PhD.
wilcock@histovet.com
21 Vardon Drive, Guelph, Ont. N1G 1W8
Toll Free Phone/Fax: 1-800-853-PATH
Outside Canada: 519-822-4486
- Mammary Tumors in Dogs and Cats: What Do I Tell the Owner?
I consider mammary neoplasia to be one of the great embarrassments of veterinary pathology, for we have historically been excessively pessimistic about the behavior of these tumors in both dogs and cats. Most of the attention has been paid to mammary tumors in dogs. Based on a review of the worldwide literature, 50% of canine mammary tumors are described as histologically malignant. That piece of information has, unfortunately, been translated into predictions of 50% behavioral malignancy. This is a massive overestimate of the true malignancy of canine mammary tumors. The mistake is in assuming that the histologic criteria of malignancy can be applied equally to all kinds of tumors. What we have discovered, in the last 10 years or so, is that these criteria of malignancy need to be interpreted differently for each kind of tumor, so that things like hyperchromasia and high mitotic index may signal the kiss of death when found in one kind of tumor, yet be quite irrelevant to prognosis for another.
In order to assist you in giving the appropriate prognostic information to owners concerned about mammary neoplasia in their pets, I have listed for you some of the more recent information about the behavior of mammary tumors in both dogs and cats.
Dogs:
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