Canine Mammary Carcinoma

Benefits of Spaying in Regards to Canine Mammary Carcinoma

By Boyd Hardy

                Mammary tumors are a commonly diagnosed tumor in female dogs, and canine spaying is a preventative measure to decrease the incidence of mammary tumor development in female dogs. Different forms of mammary tumors can occur, but they most commonly occur in middle to older aged dogs. Currently, the treatment of choice is surgical removal of the tumor, and the type of surgical procedure selected is determined by the size and number of the tumors as well as local lymph node involvement. Canine mammary carcinomas are malignant tumors that develop from epithelial cells and can spread via lymphatics to metastasize in other areas of the body. Approximately 50% of mammary tumors are malignant or mixed malignant in nature.


                Mammary carcinoma is the most common form of malignant neoplasia in dogs, and a higher incidence has been noticed in intact dogs versus spayed dogs.2 Estrogen and progesterone are hormones that are necessary for mammary development, and receptors for these hormones have been found in many of these tumor types. One study showed that approximately 70% of benign tumors and 50% of malignant tumors have estrogen and progesterone receptors inside of the tumor4.This leads to the theory  that estrogens may play a direct and an indirect role in tumor development.5 Since estrogen and progesterone are produced by the follicle and corpus luteum on the ovaries, respectively, early spaying of female dogs has benefits in decreasing the risk of mammary tumor development, which is often noted between 8.4 and 11 years of age depending on the study.1, 2


                An ovariohysterectomy, or spay, removes the ovaries, uterine horns, and uterine body from the animal. Spaying is often done to prevent pregnancy from occurring in dogs, but spaying also has a place in preventing the development of mammary tumors. It has been shown that there is a significant increase in risk of mammary tumor development for intact female dogs compared to spayed female dogs. Dogs that are spayed before their first estrus cycle are considered to have their risk of mammary tumor development reduced to 0.5%, and those spayed after one estrus have their risk reduced to 8% for development of mammary tumors.3 Spaying after the second estrus has occurred reduces the risk to 26%.5 Therefore, spaying female dogs before their first heat cycle will give them the most benefit. The presence of estrogen receptors in epithelial tumors is thought to be the reason that early spaying of female dogs appears to be protective against mammary tumor development. Consequently, spaying is considered to have a place in the treatment plan for developed mammary tumors.5


                After a mammary tumor has been identified, immediate surgical removal of the tumor is considered the treatment of choice. The surgery being performed will be more aggressive or conservative depending on the size and number of tumors present.1 Regional lymph node involvement or distant metastasis has shown a decreased 2-year survival rate according to one study, so early detection helps provide the optimal treatment outcome.2 Studies have looked at the two-year survival rates of dogs in relation to performing a spay at the time of the surgical mass removal versus within two years of the surgical mass removal. Certain studies have shown that there was no difference between survival rates of spayed and intact dogs that underwent treatment,4 but this study did not differentiate when the animal was spayed in relation to mammary development and treatment. Another study showed a benefit to spaying within a certain time from of mammary development and surgical removal. This study found that intact dogs that were spayed two years before tumor development or during the tumor removal surgery lived 45% longer than dogs left intact or spayed more than two years before tumor development.5 Another paper has shown that dogs with complex carcinoma had a better prognosis when spayed rather than left intact.2 Therefore, evidence in favor of spaying is available.


                In conclusion, mammary tumors are one of the most common tumors in female dogs, and approximately 50% of them are malignant in nature. The treatment of choice for mammary carcinoma is surgery, and the type of surgical procedure for removal depends on the size and number of mammary tumors present as well as lymph node involvement.1 Spaying has been shown to have positive effects on dogs’ lives before mammary tumor development as well as during tumor removal. A spay early in a female dog’s life can reduce her risk for development of mammary tumors to as low as 0.5%, and the literature shows that a spay within two years of mammary development or along with surgical removal of the mammary tumor can allow dogs to live 45% longer than if they were left intact.3, 4, 5


  1. "Canine Mammary Tumors." The Ohio State University Veterinary Medical Center, n.d. Web. 2 Oct. 2016.
  2. Chang, Shih-Chieh, DVM, MVS, Chao-Chin Chang, DVM, PhD, Tien-Jye Chang, DVM, PhD, and Min-Liang Wong, PhD. "Prognostic Factor Associated with Survival Two Years After Surgery in Dogs with Malignant Mammary Tumors: 79 Cases (1998-2002)." Journal of Animal Veterinary Medical Association 227 (2005): 1625-629. Web.
  3. "Overview of Mammary Tumors: Mammary Tumors.” Merck Veterinary Manual. N.p., Oct. 2014. Web. 2 Oct. 2016.
  4. Philibert, Jeffrey C., Paul W. Snyder, Nita Glickman, Larry T. Glickman, Deborah W. Knapp, and David J. Waters. "Influence of Host Factors on Survival in Dogs with Malignant Mammary Gland Tumors." Journal of Veterinary Internal Medicine (2003): 102-06. Web. 8 Oct. 2016.
  5. Sorenmo, Karin U., Frances S. Shofer, and Michael H. Goldschmidt. "Effect of Spaying and Timing of Spaying on Survival of Dogs with Mammary Carcinoma." Journal of Veterinary Internal Medicine (2000): 266-70. Web. 8 Oct. 2016.