Benefits and Risks of Neutering, an Evidence Update: Neutering and Mammary Cancer in Female Dogs

This is another in my series of evidence updates on the risks and benefits of neutering in dogs and cats. I will be updating the evidence and conclusions of my original 2010 review based on a series of systematic reviews being produced by a research group in the UK. The first such update concerned the effect of neutering and age at neutering on urinary incontinence in female dogs. This update concerns one of the most important reasons to considering neutering female dogs: mammary cancer.

Beauvais W, Cardwell JM, Brodbelt DC. The effect of neutering on the risk of mammary tumours in dogs–a systematic review.J Small Anim Pract. 2012 Jun;53(6):314-22.

In my 2010 review, I found evidence suggesting that mammary cancer is very common in intact female dogs, that it is malignant about half the time, and that neutering dramatically reduces the risk of this disease. I also found evidence suggesting that neutering is most effective in preventing mammary cancer if performed before the first heat, and that the benefit disappears after the third estrus cycle. This is a major reason why I still recommend neutering young female dogs.

The UK group found only 4 research reports that addressed the question of the impact of neutering on mammary cancer risk and that met the minimum quality criteria for review. One of these also addressed the issue of whether age at neutering was associated with mammary cancer risk. They reported the following results: 

  1. All studies had significant methodological flaws, and none provided information that could be confidently generalized to the general dog population.
  2. One study found a strong protective effect of neutering. Another found some protective effects but these were inconsistent. The remaining two studies found no association between neutering and mammary cancer risk.
  3. The one study that evaluate age at neutering found a significant decrease in the protective benefits of neutering with each of the first three heats, and no benefit after the third heat.

Once again, the general conclusions are consistent with the current consensus, that neutering young female dogs likely has a meaningful beneficial effect on reducing the risk of mammary cancer later in life. However, the existing evidence is extremely weak, and this conclusion has to be viewed as tentative. While current evidence does not justify a change in the common practice of recommending neutering to prevent mammary cancer, we cannot view our current conclusion with any great confidence, and we desperately need more and better quality research to determine if the current evidence is correct or if a significant change in our recommendations is called for.

As always, the decision to neuter must be based on the entirety of the known risks and benefits, with a clear understanding of the uncertainty involved, as well as on the unique circumstances of each individual. The value of this review is not so much in answering our questions about the subject, which it cannot do, but in making it clear what the strengths and weaknesses are in the evidence behind our current recommendations and in guiding us in the development of better evidence that will permit more confident conclusions.



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11 Responses to Benefits and Risks of Neutering, an Evidence Update: Neutering and Mammary Cancer in Female Dogs

  1. Art says:

    In my 2010 review, I found evidence suggesting that mammary cancer is very common in intact female dogs, that it is malignant about half the time >>>>
    I have read that in the dog the mammary tumors usually start out benign and if left alone turn malignant. If they are removed early when benign would that take them out of the tumor data in the studies?
    Art Malernee dvm

  2. skeptvet says:

    That’s different from my understanding. I believe they are either malignant or benign from inception (about 50:50), so early removal might reduce the chances of mets but not the character of the tumor. Perhaps you have a paper that desxribes what you’re talking about?

  3. Art says:

    Maybe this is what I remember and got confused:)

  4. Art says:

    Here is the quote from the link above from a boarded guy in Canada.
    “3. Ninety percent of all canine mammary neoplasms are behaviorally benign.”

  5. Art says:

    My bad. Maybe he is not boarded just a Dvm Phd

  6. skeptvet says:

    The author doesn’t say where that claim comes from, so it’s hard to evaluate. He also cites the statistics about OVH before first estrus, but as the systematic review shows that is based on one study of not very good quality. It seems like to make a claim like this you would have to identify a large number of mammary tumors early and follow them without treatment for some time, which doesn’t sound like research that I’ve hard reported anywhere.

  7. Art says:

    I emailed the author of the 90% quote. His website says 90-95 % which goes along with my ,you never can trust, clinical impression. So far I have had no reply. Can anyone show me online data for the 50-50 claim? The phd taught vet school in Canada . There must be a student of his that had to listen to him promote the 90-95 figure who can help.
    Art Malernee Dvm

  8. skeptvet says:

    There is some variation in the numbers, but something close to 50:50 seems to be most common in studies evaluating tumors submitted for histopath. As far as I know, there hasn’t bee a prospective study identifying mammary tumors and not treating them to see how many are behaviorally malignant or benign, and I imagine such a study is unlikely to be done.

    Schlafer, D. H.; A review of mammary gland neoplasia in the bitch and queen. Clinical Theriogenology, 2012, 4, 3, pp 206-212
    “About half of all tumors of the bitch arise from mammary glands and from one third to one half of these are malignant…”

    Pedraza-Ordóñez, F. J.; Ferreira-de-la-Cuesta, G.; Murillo-Menjura, S. M.. Clinical and pathological description of 124 cases of canine mammary gland neoplasm in Manizales, Colombia. Veterinaria y Zootecnia, 2008, 2, 2, pp 21-28,

    “This research analyzed 124 patients with canine mammary gland neoplasm which were attended during a one-year period in several veterinary clinics in Manizales, Colombia, including the Veterinary Hospital of the Caldas University. Forty-one cases (33.1%) did not have a report from the pathology laboratory, while 83 cases (66.9%) were diagnosed by a defining histological study. The results indicate a high appearance frequency of the Mixed Malignant Tumor in 21 cases (25.3%), followed by Carcinoma with 20 cases (24.1%), Benign Mixed Tumor 17 cases (20.5%), Adenomas 16 cases (19.3%) and other diagnosis 9 cases (10.8%). The largest filing of cases occurred in animals older than 7 years; and with regards to breeds, 53 patients were French Poodles, 15 mixed breeds, 9 Boxers, 6 Pinschers, 5 Cockers, 5 Labradors, and in less number, other breeds. However, it was proportionally established that the most affected dog breeds were French Poodle, Cocker, Boxer and Pinscher, while mixed breeds and Labradors showed a lower prevalence of the disease. Due to differences with other global reports, it can be concluded that it is important to continue with research regarding risk factors, biology of tumor growth and epidemiology of these neoplasm in Manizales.”

    Priya, S.; George, V. T.; Balachandran, C.; Manohar, B. M.; Incidence of canine mammary tumours in Chennai, Tamil Nadu.
    Indian Veterinary Journal, 2006, 83, 10, pp 1054-1056,

    “Of the 115 neoplastic tumours, 38% were benign and 32% were malignant.”

    Torres Vidales, G.; Botero Espinosa, L.; A retrospective histopathological study of mammary gland neoplasia in canines (1975-2000).
    Revista Orinoquia, 2008, 12, 1, pp 80-88,

    “41% (70/173) of the neoplasia so evaluated were classified as being benign; the most frequently occurring were benign mixed mammary tumour (93%; 65/70), papillary cystic adenoma (5.7%; 4/70) and simple adenoma (1.3%; 1/70). 59% (103/173) were classified as being malignant; the most frequently occurring were simple tubular carcinoma (41%; 42/103), mixed malignant mammary tumour (37%; 38/103), simple papillary carcinoma (6.9%; 7/103), compound tubular carcinoma (5.9%; 6/103) and simple papillary cystic carcinoma (4.9%; 5/103).”

    This study, however, found a much lower frequency of malignant tumors.

    Kim YeongHun; Ahn NaKyoung; Roh InSoon; Yoon ByungIl; Han JeongHee; Retrospective investigation of canine skin and mammary tumors in Korea.
    Journal of Veterinary Clinics, 2009, 26, 6, pp 556-562,

    “In case of mammary gland tumors, 201 (83.8%) were benign and 39 (16.3%) were malignant with a benign to malignant ratio of 5.15. The most frequent mammary gland tumor was benign mixed tumor (35.0%) followed by mammary adenoma-complex type (31.7%).”

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