Breast Cancer in Dogs: What You Need to Know

Though you're likely aware of how common breast cancer is in humans, you may not know how common breast cancer in dogs is. One in four intact (not spayed) female dogs will be diagnosed with malignant mammary tumors, according to Animal Hospital Maple Orchard, and, as with humans, early detection is key.

How Common Is Breast Cancer in Dogs?

Mammary tumors are one of the most common tumor types seen in female dogs and are the most common tumor in intact female dogs. Around 50% of dog mammary tumors are benign and 50% are malignant or cancerous, noted the American College of Veterinary Surgeons (ACVS).

Middle-aged and older female dogs are most commonly affected, and an increased incidence of mammary tumors in certain breeds suggests a genetic component, much like in humans. A few of the breeds at increased risk of malignant mammary tumors include German shepherds, cocker spaniels, Dobermans and poodles.

How Can You Catch Signs of Mammary Tumors Early?

To catch tumors early, you can start regularly monitoring your dog for any mammary growths when they're around 5 to 6 years old. If a female dog is intact or was spayed later in adulthood, she's considered at higher risk for mammary cancer; however, this regular monitoring should be done in all female dogs as a precaution.

Normal mammary tissue should be soft and pliable without any firm lumps. If you feel any lumps during an at-home assessment, take your dog to your veterinarian to discuss further testing and possible removal. In many cases, your dog may otherwise appear healthy and happy despite a palpable mammary tumor, but it's still important to have them evaluated by a vet because catching cancer early is essential.

How Is Breast Cancer in Dogs Diagnosed?

The three most important factors to determine a prognosis are mammary tumor size, lymph node involvement and metastasis (whether the cancer has spread to other sites).

In general, large tumors and cancer that's spread to lymph nodes or other organs lead to a poorer prognosis. Additional factors that can negatively affect prognosis include the presence of skin ulceration, cancer that's invasive/attached to adjacent tissue or rapidly growing, and a higher clinical stage at the time of diagnosis. According to NC State Veterinary Hospital, carcinoma tumors are one of the most common mammary cancers seen in dogs, which includes many subsets that all behave slightly differently.

A cytology exam (the evaluation of a sample of cells) of the tumor can help your vet to identify non-mammary tumors, as it allows them to see other tumor types in this location. Unfortunately, cytology alone is generally not helpful in determining whether a mammary tumor is benign or malignant, so a biopsy is often required to obtain a diagnosis.

In many cases, the surgeon will attempt an excisional biopsy (complete removal of the tumor), which can result in a diagnosis and potentially a cure in one surgery. Sometimes, additional surgeries are needed for complete removal.