Pet Cancer Awareness Month

In honor of National Pet Cancer Awareness Month, we want to highlight the most common, yet largely preventable, cancer in intact pets: mammary gland neoplasia.

Author: Emi Kooyman, DVM | Founder, Spay Neuter Vets

Published date: November 2, 2026

Mammary neoplasms arise from mammary glandular tissue or supporting structures. Tumors may be benign or malignant. Mammary gland tumors are the most common tumor in intact female dogs. In populations where most dogs are sexually intact, mammary tumors represent 50-70% of all tumors in females, and approximately 50-73% of them are malignant. For cats, mammary gland tumors are the second or third most common tumor type overall. Cats have a high rate of malignant mammary neoplasms (80-90%) and a high rate of metastasis (>80%).

Here are the stats:  

DOGS

• Spayed before the first heat cycle: 0.5% risk (1 in 200 dogs)

• Spayed between the first and second heat cycles: close to 8% risk

• Spayed after two or more heat cycles: 26% risk

• Spayed after 2.5  years of age: no risk reduction

CATS

• Spayed before the first heat cycle: 91% risk reduction

• Spayed before the second heat cycle: 86% risk reduction

• Spayed before the third heat cycle: 11% risk reduction

• Spayed after 2 years of age: no risk reduction

These figures are based on several papers, which have been interpreted in various ways. The first study to look at the incidence of mammary gland tumors was published in 1969 (Schneider et al.).

Hormone exposure plays a role in mammary tumor development. Therefore, dogs and cats spayed early have a much lower chance of developing this tumor than those spayed later or not spayed at all. Spaying after the age of 2.5 years in dogs, 2 years in cats, is not considered protective against mammary tumors, but spaying at any age will prevent other diseases of the reproductive system, such as pyometra, which is life-threatening.

Lastly, while much less likely, mammary neoplasia can occur in males.

Surgery is the treatment of choice for mammary gland neoplasia that has not metastasized. It can be invasive and extensive, with removal of the entire mammary chain recommended in cats. In dogs, recurrence is also common. For tumors that have metastasized, chemotherapy and radiation therapy often follow.

Spaying your pet before their first heat cycle significantly reduces the chances of mammary gland tumors. We encourage you to discuss spaying or neutering your pet early with your veterinarian, or to contact us for more information.