Advances in the Treatment of Mammary Neoplasia

Mammary tumours are the most common neoplasm in intact female dogs and account for approximately half of all neoplasms in the bitch. In contrast, mammary neoplasms are rare in male dogs, accounting for less than 1% of these types of tumours. The incidence of mammary tumours is higher in dogs than any other domesticated animal and is three times the incidence in humans. Approximately half of the tumours are malignant, and half of these have metastasized by the time they are initially diagnosed. Sex hormones certainly play a role in development of mammary tumours in the bitch. Intact females have a seven-fold increased risk of developing mammary cancer compared to neutered females. The age at which ovariohysterectomy takes place is directly proportional to the risk of developing mammary cancer. Data clearly indicate the preventive role of ovariectomy prior to the second estrus. Interestingly, when only the dogs that had 2 or more estrus cycles before spaying are considered, there is still a benefit to earlier spaying. If they were spayed before 2.5 years of age, then the risk was 6% of those dogs not spayed at all; (that approximates to the 8% quoted for 1 estrus cycle). If they were older than 2.5 years, the risk was 40% of unspeyed dogs, but that difference was not significant. Essentially the greatest reduction in risk comes if you spay before the first heat, but then as long as you spay before 2.5 years, the risk is very low. Counter to all current thought one group reviewed the statistics in all the literature regarding the risk of neutering and found the data weak. We continue to recommend speying at an early age.