Symptoms and Treatment of Canine Mammary Tumors

ImmunityQ&AMeongsiljang Veterinary Advisory Board

Mammary gland tumors are the most common type of tumor in intact female dogs. We’ve compiled a comprehensive guide based on veterinary oncology textbooks, covering symptoms, how to distinguish between benign and malignant tumors, surgical procedures, and post-operative care.

What to Do When You Discover a Canine Mammary Tumor?

Illustration of a puppy undergoing a belly health checkup

Mammary tumors in dogs are neoplastic conditions caused by abnormal cell proliferation in mammary tissue, and they are common in intact female dogs. To get straight to the point, the prognosis for canine mammary tumors is good when detected early and surgically removed. Approximately 50% of mammary tumors are benign, and even if malignant, surgical removal while the tumor is small results in a low recurrence rate. According to veterinary oncology textbooks, tumors smaller than 3 cm have a relatively low recurrence rate when surgically removed, whereas tumors larger than 3 cm carry a higher risk of recurrence. It is most important to visit an animal hospital immediately upon feeling a lump.

Why Do Mammary Tumors Develop?

Canine mammary tumors are significantly influenced by female hormones. According to veterinary textbooks, the development of these tumors is known to be hormone-dependent. The timing of spaying is crucial: spaying before the first heat cycle reduces the risk of mammary tumors to a very low level, nearly eliminating it. If spaying is performed after the dog has experienced heat cycles, this preventive effect is greatly diminished. Age: These tumors are most common in senior dogs over 10 years old. Veterinary textbooks report the median age at diagnosis to be between 10 and 12 years, though they can rarely occur in young dogs under 2 years old. The risk increases with age. Breed: Poodles, Spaniels, Dachshunds, and German Shepherds are known to have higher incidence rates. Since mammary tumors are commonly found in intact female dogs, it is advisable to consider both spaying status and age when scheduling regular check-ups.

Check for These Symptoms

According to veterinary oncology textbooks, these are the hallmark signs of mammary tumors.

Lumps on the belly: You may feel firm or soft masses around the mammary glands (nipples). There may be a single lump or multiple ones.

Changes in size: Tumors often start small and gradually grow. Rapid enlargement should raise suspicion of malignancy.

Skin changes: The skin over the tumor may become red or develop ulcers (open sores with discharge).

Pain response: In the early stages, the area may not be painful to touch, but as the tumor grows, your pet may show discomfort when it’s handled.

Multiple simultaneous tumors: It’s common for a single animal to develop several tumors at once. Some may be benign, while others could be malignant.

An illustration of a puppy having its mammary area examined.

Mammary Tumor Staging — Based on Size and Metastasis

According to veterinary oncology textbooks, mammary tumors are staged based on tumor size, lymph node metastasis, and the presence of distant metastasis. Staging is crucial for determining treatment direction and prognosis.

Stage	Tumor Size	Lymph Node Metastasis	Distant Metastasis	Prognosis
Stage I	Less than 3 cm	None	None	Good
Stage II	3–5 cm	None or possible ipsilateral non-fixed lymph node metastasis	None	Fair
Stage III	5 cm or larger	Possible	None	Guarded
Stage IV	Any size	Possible	Present	Poor

Surgery Is the Key to Treatment

According to veterinary oncology textbooks, the first choice for treating mammary tumors is surgical removal. If there is no metastasis and it is not inflammatory cancer, surgery is the most effective option.

Lumpectomy: The tumor is removed with approximately 1 cm of surrounding tissue as a margin. This is suitable for small, superficial tumors.

Simple mastectomy: The entire affected mammary gland is removed.

Unilateral mastectomy: All five mammary glands on one side are removed. This is performed when there are multiple tumors on one side.

The veterinarian will determine the simplest yet most definitive surgical method based on the size, number, and location of the tumors.

Illustration of a puppy receiving treatment consultation at a veterinary clinic

Go to the Vet Immediately in These Cases

If a tumor is 3 cm or larger, growing rapidly, ulcerated, or firmly attached to surrounding tissues, it is highly likely to be malignant. According to veterinary oncology textbooks, about 50% of malignant mammary tumors have already begun to metastasize by the time of diagnosis. Therefore, it is most important to visit a veterinary clinic promptly while the lump is still small.

Post-Operative Care Instructions

Wound care: Keep the e-collar on to prevent your pet from licking or scratching the surgical site. Follow your veterinarian’s instructions for suture removal.

Biopsy results: Any tumor removed during surgery must undergo a histopathological examination. This is the most critical step to determine whether the tumor is benign or malignant and to confirm that the surgical margins are clear.

Regular check-ups: If the tumor was malignant, regular follow-up exams are essential to monitor for recurrence or metastasis. Since spread to the lymph nodes, lungs, or bones is possible, consult your veterinarian to determine the appropriate examination schedule and specific tests.

Additional treatment: Depending on the type and stage of the malignant tumor, your veterinarian may recommend chemotherapy.

Illustration of a puppy recovering while wearing a neck brace

Mammary Tumors Can Be Prevented

The most effective prevention method is early spaying.

Spaying before the first heat cycle: This significantly reduces the risk of mammary tumors to nearly zero. Veterinary textbooks report that spaying before the first heat cycle greatly lowers the risk of mammary tumors, making it the most effective timing.

If your dog is already an adult: While the risk-reduction benefits of early spaying are well-documented, it is known that the tumor-prevention effect of spaying diminishes with each subsequent heat cycle. For adult dogs, the preventive benefits of spaying may be limited, so please be sure to consult thoroughly with your veterinarian before deciding whether to proceed.

Regular abdominal palpation: Regardless of spaying status, it is crucial to develop the habit of checking for lumps while petting your dog’s belly. Early detection is the best treatment.

Reviewed by a veterinarian

Dr. Tony — Punnawat Phongkittirak

Veterinarian

A veterinarian who majored in veterinary medicine at Khon Kaen University, Thailand, and completed the IVSA program at North Carolina State University in the United States. Drawing on clinical experience at animal hospitals, he works in the pet healthcare field and is dedicated to building a digital care environment that connects pet parents with veterinarians.

Frequently Asked Questions

How can you tell if a mammary tumor is benign or malignant?

It’s impossible to tell for sure just by looking at it. A definitive diagnosis can only be made through a histopathological examination (biopsy) after surgically removing the tumor. While a fine-needle aspiration biopsy can provide a preliminary assessment before surgery, a tissue biopsy is still required for final confirmation.

If there are multiple mammary tumors, can they all be removed in one surgery?

Yes, if there are multiple tumors on one side, they can all be removed at once through a unilateral mastectomy. If tumors are present on both sides, surgery is often performed on one side at a time. Your veterinarian will develop a surgical plan based on the location and number of tumors.

Can male dogs get mammary tumors?

Although rare, male dogs can also develop mammary tumors. Since these tumors are more likely to be malignant in males, it's best to visit the vet immediately if you feel a lump on your male dog's belly.

Can mammary tumors recur after surgery?

If the tumor is benign, the chance of recurrence is low once it has been completely removed. If it’s malignant, there may be a risk of recurrence or metastasis depending on the stage and biopsy results. Regular follow-up check-ups after surgery are essential.

How much does surgery for a mammary gland tumor cost?

Costs vary significantly depending on the surgical procedure (such as mass excision, simple mastectomy, or unilateral mastectomy) and the clinic. You can receive an accurate cost estimate after consulting with a veterinarian. Rather than focusing solely on price, we recommend choosing a clinic based on the veterinarian’s experience in tumor surgery and the quality of their facilities.