Mammary Cancer in Dogs: Stages, Causes and Treatment

Photo of author
Updated On

I Love Veterinary blog is reader-supported, and we may earn a commission from products purchased through links on this page, at no additional cost to you. Learn more About Us and our Product Review Process >

What Is A Mammary Tumor in Dogs?

Can dogs get breast cancer? Sadly, they can!

In veterinary medicine, mammary cancer in dogs is something that we see way too often lately. Neoplastic growths originating from the tissue of mammary glands in dogs describe these tumors. Tumors of the mammary tissue are evident in all animals, but most often in dogs and cats. 

These mammary tumors can be benign (non-cancerous) or malignant (cancerous), with the potential to metastasize to other organs or nearby lymph nodes. 

Types of Mammary Tumors in Dogs

Veterinary oncology faces quite a challenge when it comes to breast cancer in dogs because a significant percentage of all mammary tumors show malignant characteristics. These specific neoplasias stem from the mammary glands’ epithelial cells, and their metastatic potential and biological behavior can vary.

Below are some of the different types of canine tumors and their properties.

Benign Mammary Tumors

Benign mammary gland tumors in dogs have the characteristics of cells that cannot invade the surrounding tissues or metastasize to distant organs. 

The most common benign types of mammary tumors are fibroadenomas (well-defined, wobbly, and moving under the skin with glandular tissue composition) and adenomas (smaller, less complex, originating from the glandular epithelium). 

Malignant Mammary Tumors 

All malignant tumors have one thing in common – aggressive growth and rapidly spreading throughout the body. 

Predominant among the malignant tumors are carcinomas, which can be solid, papillary, tubular, or anaplastic. 

Sarcomas, on the other hand, are less common but more aggressive. They originate from the connective tissue of the mammary gland. 

Benign mammary tumor on a dog removed (Adenomyoepthelioma mammae)
Malignant mammary tumor removed from a female dog (Adenomyoepthelioma mammae). Image courtesy of Natasha Boycheva

Mixed Mammary Tumors 

These types of tumors can contain multiple components, such as connective and epithelial tissue, at the same time. They can be benign or malignant, and their behavior will depend on the biological characteristics of the malignant part. 

Inflammatory Mammary Carcinomas

This type of canine breast cancer is very aggressive, and it grows rapidly, causing severe inflammation along the way. Clinical signs very often look like mastitis and can be misdiagnosed. The prognosis is very poor. 

Dog Breast Cancer: Signs and Symptoms

Signs and symptoms of dog breast cancer can vary based on the nature or malignancy of the tumor, its location, and its size. It is imperative to recognize the signs of breast cancer as early as possible to offer timely intervention and treatment and potentially a better prognosis and outcome for the affected patient. 

What should we look for when looking out for mammary tumors in dogs? 

Palpable Masses

The lumps are the most notable and common in presenting mammary tumors. These lumps can appear anywhere along the mammary tissue and are noticeable under the fingers with palpation. 

These lumps can vary from very small (like a rice grain) to huge (like a tennis ball). The lumps can be singular (only one lump) or multiple (like a bunch of hard grapes bunched together).  

Appearance Change 

Besides the lumps, the tissue surrounding them can change its appearance. It can become reddened, swollen, ashy, or ulcerative. The continuity of the skin around the lumps will appear distorted.


Some mammary tumors can cause fluid buildup in the mammary ducts, which will appear clear, bloody, or pus-like. Mammary gland discharge most often indicates additional underlying pathology of that gland. 

Discomfort and Pain 

Some mammary tumors can go unnoticed by the dog, especially in the early stages, but others can be pretty painful to the touch. The dog might lick obsessively, bite the tumor, or become aggressive when touched where the cancer is. 

Systemic Signs 

In cases where the mammary tumor is malignant, particularly if there is metastasis to distant organs, the dog might show signs of 

  • Lethargy
  • Difficulty breathing
  • Inappetence
  • Weight loss
  • Coughing
  • Lameness
  • Weakness
  • Enlarged lymph nodes

Causes of Mammary Tumors in Dogs

Mammary cancer in dogs is affected firstly by genetics, then hormones, and lastly environmental factors. It is vital to understand their etiology to ensure early detection and prevention of canine mammary tumors.


Recent studies show evidence that certain breeds of female dogs are more prone to develop canine tumors than others. These breeds are Poodles, Dachshunds, and Yorkshire Terriers. Research in veterinary oncology is ongoing and trying to spot the genetic markers responsible for tumorigenesis [1]. 


Reproductive hormones have a strong link with the development and growth of canine mammary tumors, especially progesterone and estrogen. Estrogens are primarily secreted in the ovaries (small amounts in the adrenal glands and the adipose tissue) and play a significant role in the reproductive health of the bitch. 

Progesterone is a hormone produced mainly by the adrenal cortex and some by the ovaries. It plays an essential role in ovulation and during pregnancy because while the bitch is pregnant is also excreted by the corpus luteum to protect and keep the fetuses alive till term. 

Exposing mammary tissue to these hormones over time is considered to increase the risk of developing breast cancer. Studies have shown that spaying female dogs before their first reason reduces the risk of developing mammary tumors to 0.5% [2].

Environment and Lifestyle

Although not researched enough, the experts believe that certain risk factors, such as diet, obesity, and exposure to environmental carcinogens, can contribute to the development and growth of mammary cancer in dogs. 

Reproductive History

The risk of mammary tumors in bitches increases with age, and the percentage of documented cases is higher in dogs bred after the second oestrus. Bitches that have never been bred and remain unspayed also risk developing mammary tissue tumors. 

How is Breast Cancer in Dogs Diagnosed?

Diagnosing canine breast cancer requires multiple diagnostic tools, starting with a physical examination. Some mammary tumors may appear connected to the abdominal wall if the bitch is in good physical condition; some may droop freely in the skin tissue. 

Before opting for surgery, some veterinarians may perform FNA (fine needle aspiration). The procedure requires using a thin needle with which the veterinarian will puncture the tumor to collect cells for cytology. These allow us to determine the nature of the cancer, whether it’s benign or malignant. 

When surgery is involved, the tumors are almost always sent to biopsy, either a whole tumor or a sample, to determine the definitive character of the cancer. Veterinarians advise, when removing the mammary tumor, to spay the bitch to control the recurrence of the tumors. 

X-rays and MRIs can help to determine the degree of the tumor and the possibility of metastasis. The primary metastatic sites looked for on the x-ray are the lungs and the regional lymph nodes, but mets in the liver and spleen have also been evident. 

Dog Mammary Cancer Stages Explained

The staging process involves categorizing tumors according to the Tumor, Node, Metastasis (TNM) system, which comprises five classifications:

Mammary Cancer in Dogs 1080 x 1080 I Love Veterinary - Blog for Veterinarians, Vet Techs, Students
  • Stage 1 (T1N0M0): Tumor size is less than 3 cm, with no regional or distant metastasis.
  • Stage 2 (T2N0M0): Tumor size ranges from 3 to 5 cm, with no regional or distant metastasis.
  • Stage 3 (T3N0M0): Tumor size exceeds 5 cm, with no regional or distant metastasis.
  • Stage 4 (T1-3N1M0): Any tumor size with evidence of regional metastasis but no distant metastasis.
  • Stage 5 (T1-3N0-1M0): Any tumor size with or without regional node metastasis and evidence of distant metastasis.

Treatment Options for Mammarian Cancer in Dogs

Veterinarians should tailor the treatment of breast cancer depending on the tumor type (whether the tumor is benign or malignant), the stage of the cancer, and of course, the bitch’s overall health.


The most common treatment path is surgical excision of the tumor with a clear margin of healthy tissue. Sometimes, this might mean that a partial or total mastectomy is necessary. 

Removal of canine mammary tumor. Video courtesy of Natasha Boycheva.


When the diagnosed tumor is malignant, chemotherapy or radiation therapy might be a possible option, especially if the risk for metastasis is high. The choice of which chemotherapy drug will be an option will depend on the type of tumor. Some of the most common chemotherapy drugs used in treating mammary tumors in dogs are Doxorubicin, Carboplatin, Mitoxantrone, and Paclitaxel. 


We can use radiation therapy to locally control the tumor, especially in cases where the cancer is inoperable, or the surgical margins are very narrow. 

Drugs and Hormone Therapy

In some cases, doctors consider canine hormonal therapy and give anti-estrogen drugs to the patient. With hormonal therapy, we are targeting the hormonal influence on the growth of the tumor, but this is limited and specific to certain types of mammary tumors. 

The hormonal, or anti-estrogen, therapy that is in play in these cases is selective estrogen receptor modulators (SERM) and diethylstilbestrol drugs. Examples of SERM are Raloxifene, Tamoxifene, and Torimefene. The vets and pet owners administer these drugs in tablet form.

What to Expect After Dog Mammary Tumor Treatment

After treating a mammary tumor in a dog surgically, the prognosis and the expected outcomes will vary drastically, whether the cancer is benign or malignant, the presence (or absence) of metastasis when diagnosed, and the general health of the patient. 

The incision, or surgery site, will recover within two to three weeks, depending on the size of the excised tumor. Maintaining proper wound management is crucial, as cleaning the wound daily and preventing the dog from licking and biting the incision site with an Elizabethan collar. The stitches, if present, are removed after 10 to 14 days post-surgery. 

Some tumors, usually aggressive malignant types, can regrow on the same site or adjacent in this healing period, but this is rare. 

Observing the incision site for signs of infection, swellings, or wound opening is crucial. 

After the healing period, veterinarians advise conducting regular veterinary checkups to search for signs of tumor recurrence or the growth of new tumors, using physical examination and, if necessary, x-ray imaging. If metastasis occurs, the most common organs affected are the lungs, liver, and others. 

If surgeons completely remove tumors with benign characteristics, the dog has an excellent prognosis and can expect to lead a normal and healthy life. When the cancer is malignant, the prognosis will vary on the type of tumor and aggressiveness.  

Mammary Tumors in Dogs: Life Expectancy and Outcome

The prognosis and outcomes for canine mammary tumors depend on the tumor’s type, size, and stage and the dog’s overall health. These variables are crucial for the veterinarian and the pet owners to establish realistic expectations and make decisions for the treatment and care of the dog. 

Dogs usually have a good or excellent prognosis with benign tumors if surgeons remove the tumor surgically. These benign mammary tumors typically grow slowly and do not invade the surrounding tissues, and the risk for mets is minimal. The likelihood of recurrence or complications after surgery is minimal. 

When it comes to malignant mammary tumors, the prognosis will depend on several factors. High-grade tumors and the ones diagnosed at a later stage, especially ones with metastasis and lymph node involvement, have a very poor prognosis. Confined, early-stage tumors will have a better prognosis. 

Overall survival time after surgery will depend on the type and grade of the tumor, where benign and small, wholly excised tumors have a better survival rate of a couple of years. Patients with high-grade tumors and/or metastasis tend to live a few months to a year.  

The Bottom Line

Mammary cancer in dogs is something that we see very often at veterinary clinics. Studies show that spaying can lower the chance of a mammary tumor to 0.5%, but not all pet owners know this. 

Mammary tumors can be of benign or malignant character, and this determines the outcomes and prognosis for our oncological patients. Surgical removal is always the golden rule for breast cancer in dogs, and sometimes, chemo and radiation therapy are an option after surgery. 

Life expectancy will significantly depend on the size, type, and grade of the tumor, with benign tumors having excellent outcomes, whereas patients with malignant ones are not so lucky. 


What Does Breast Cancer Look Like in a Dog?

Mammary cancer in dogs has a lumpy appearance, making the mammary glands look uneven when examined. These lumps vary in size.

How Fast Do Mammary Tumors in Dogs Grow?

The progress rate of canine mammary tumors depends on the type of tumor. Benign tumors grow slower and malignant faster. Also, hormones play a significant role in breast cancer growth, and more rapid growth is evident during and after the bitch’s oestrus cycle. 

What is the First Sign of Mammary Cancer in Dogs?

The first sign is palpating a lump in the mammary glands.

Should I Remove My Dog’s Mammary Tumor?

You should discuss the removal of a mammary tumor with a veterinarian. The vet should thoroughly examine and determine the best options for every patient. 

Is Breast Cancer in Dogs Painful?

Yes, breast cancer in dogs can be painful.

Sharing is caring!

Photo of author


Natasha brings six years of hands-on experience working in small animal veterinary clinics to her writing. Her time in these clinics has provided her with invaluable real-world knowledge of veterinary practices, making her a trusted source for veterinary-related content. She combines her clinical experience with her writing skills to deliver accurate and informative articles to her readers.