Stephen J. Birchard DVM, MS, Diplomate ACVS

Tuesday, September 19, 2017

Should We Spay Dogs When Removing Mammary Tumors? This study answers that question.

V.M. Kristiansen, A. Nødtvedt, A.M. Breen, et. al. Effect of ovariohysterectomy at the time of tumor removal in dogs with benign mammary tumors and hyperplastic lesions: A randomized controlled clinical trial 
J Vet Intern Med 2013;27:935–942 


The authors of this study sought to answer the following question: is there a clinical benefit to performing ovariohysterectomy (OHE) at the time of benign mammary tumor removal in dogs.  Their hypothesis was that removal of ovarian hormones by OHE would decrease the development of new benign mammary tumors and thus would have important clinical ramifications. In a randomized, controlled clinical trial, dogs having surgery to remove benign mammary tumors were placed in one of 2 groups: those also having OHE performed at the time of tumor removal (n=42), and those not having OHE performed (n=42). All tumors were analyzed by histopathology. The dogs were followed for at least 80 months and long term follow-up information obtained either by phone calls to owners or by recheck examinations by veterinarians. 

New benign mammary tumors developed in 64% of in tact dogs compared to 36% of dogs having OHE. This was statistically significant and showed the clinical benefit of concomitant OHE with benign mammary tumor removal. Nine of the intact dogs also went on to develop disorders of the ovaries or uterus, such as pyometra. There was no difference in survival data between the 2 groups. 


For years surgeons have recommended OHE in combination with removal of mammary tumors, not because of any proven benefit related to the mammary neoplasia, but to prevent other disorders such as pyometra. This study provides evidence of the benefit of performing OHE in combination with mammary tumor removal. The authors make the case that since dogs that develop benign mammary tumors are also at increased risk of developing malignant mammary tumors, prevention of future malignant tumors may also be a positive factor of OHE.

The lack of difference in survival between the two groups may be misleading since some of the dogs were euthanized. The many and varied issues surrounding the owner’s decision to euthanize make the survival data, in the author’s words, a “soft and biased endpoint in the study.” In contrast, the different rates of tumor recurrence between the 2 groups is objective data in which statistical significance was found.


This prospective randomized study provides objective evidence of the benefit of OHE in dogs being surgically treated for mammary tumors. Clinicians can recommend OHE at the time of mammary tumor removal as evidence based medicine approach to treatment, not just in the prevention of reproductive disorders such as pyometra and ovarian tumors, but in prevention of benign mammary tumors.

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