Canine Health Menu

Mammary Tumor

Issue Description
A mammary tumor is a tumor originating in the mammary gland. It is a common finding in older female dogs and cats that are not spayed, but they are found in other animals as well. The mammary glands in dogs and cats are associated with their nipples and extend from the underside of the chest to the groin on both sides of the midline. There are many differences between mammary tumors in animals and breast cancer in humans, including tumor type, malignancy, and treatment options. The prevalence in dogs is about three times that of women. In dogs, mammary tumors are the second most common tumor (after skin tumors) over all and the most common tumor in female dogs. The lifetime risk of intact (not spayed) female dogs to develop mammary tumors has been estimated to be about 25 percent, with a much lower risk (about 1 percent) in male dogs and a risk in cats about half that of dogs.

Female dogs who are not spayed or who are spayed later than the first heat cycle are more likely to develop mammary tumors. Specifically, dogs spayed before their first heat have 0.5 percent the risk of intact female dogs, and dogs spayed after just one heat cycle have 8 percent the risk. The tumors are often multiple. The average age of dogs with mammary tumors is ten to eleven years old. Obesity at one year of age and eating red meat have also been associated with an increased risk for these tumors, as has the feeding of high fat homemade diets.

Breeds At Increased Risk
  • Poodle
  • Brittany Spaniel
  • English Setter
  • Pointer
  • Fox Terrier
  • Boston Terrier
  • Cocker Spaniel
  • Lhasa Apso

  • Symptoms
    There can be a single or several tumors, and they can occur in one or more glands. The last two sets of glands (the 4th and 5th glands) are most commonly affected. The tumors can be firm or soft, well-defined lumps or diffuse swellings. Tumors can be attached to underlying tissues or moveable, skin-covered or ulcerated. They can be different sizes, and they may grow slowly or quite fast. Most dogs are seen by the veterinarian for signs associated with the primary tumor and are otherwise feeling well. A few dogs are diagnosed with advanced metastasis (tumors that have spread to elsewhere in the body, such as the lungs and lymph nodes) and might be feeling ill from their tumors when they come for treatment.

    Types Of Tumors
    A benign glandular tumor for which no treatment is necessary.

    "Mixed" Mammary Tumor
    What is mixed is the type of cell that makes up the tumor: the epithelial cells that line the glandular tissue and the mesenchymal cells that make up the non-glandular portion. ("Mixed" does not refer to a mix of benign and malignant cells.) The mixed tumor can be either benign or malignant and the biopsy will indicate this.

    Adenocarcinomas can be "tubular" or "papillary" depending on that gland cells the tumor arises from. Adenocarcinomas behave malignantly but how aggressively malignant they are depends not on whether they are tubular or papillary but on other cellular characteristics described by the pathologist (such as how quickly the cells appear to be dividing and how closely they resemble normal gland cells). When the oncologist reads the description he or she will be able to determine how aggressively to combat the tumor.

    Inflammatory Carcinoma
    A highly malignant tumor that generates tremendous inflammation locally with ulceration, pus, and discomfort. This type of tumor tends to spread early in its course and is difficult to treat. Fortunately, this especially tragic tumor type accounts for less than 5% of mammary tumors.

    Appearance and location of the tumor is enough to identify it as a mammary tumor. Biopsy will give type and invasiveness of the tumor.

    Surgical removal is the treatment of choice, but chest x-rays should be taken first to rule out metastasis. Removal should be with wide margins to prevent recurrence, taking the whole mammary gland if necessary. Because 40 to 50 percent of dog mammary tumors have estrogen receptors, spaying is recommended by many veterinarians. A recent study showed a better prognosis in dogs that are spayed at the time of surgery or that had been recently spayed. Chemotherapy is rarely used.

    Size of the tumor is thought to be a good prognostic indicator. Dogs with smaller (less than 5 cm in diameter) malignant tumors have a better prognosis for long-term survival. Metastasis is observed in about half of malignant tumor cases. Histologic evaluation is critical not only to determine the origin of the neoplastic cells, but also to find microscopic evidence of possible metastases. Early surgical intervention and the method of surgery play important roles in prolonging survival. Ovariohysterectomy at the time of tumor excision has been recognized to increase survival time.

    Horse Herd