Canine Mammary Tumors

Breast Cancer Awareness: Canine Version

Maritza Rodriguez, VMD

Case:

Untitled1“Molly” is a beautiful 6 year old female Catahoula Leopard dog that presented to Rt 516 Animal Hospital for a mass noticed near one of her mammary glands.  At the time, Molly was not spayed as she was previously intended to be bred. On examination, we found an approximately 2 x 3 cm mass associated with her right 3rd mammary gland. The remainder of her physical examination was normal.

Mammary Masses in Dogs:

Untitled2Mammary masses are actually the most common type of tumor in intact (not spayed) female dogs. We tend to see mammary tumors develop in dogs that are older (8-10 years old), not spayed or those that were spayed later in life. The risk of mammary cancer is associated with the duration of exposure to ovarian hormones early in life. If the pet is spayed before her first heat cycle, the risk of developing mammary tumors is only 0.5%. This number increases to 8% if she is spayed after her 2nd heat and to 26% if she is spayed after the third heat. There has also been a link to decreased risk of mammary tumors in dogs that were in lean body condition at the age of 9-12 months old.

Some of the breeds that tend to have an increased risk of mammary tumors are: Yorkshire terriers, Dachshunds, English Setters, Pointers, German Shepards, Malteses, Toy and Miniature poodles, English Springer Spaniels, Cocker Spaniels and Brittany Spaniels. The mammary glands that are most commonly affected are the 4th and 5th mammary glands.

If your dog is suspected to have a mammary tumor, we perform a full physical exam to localize and measure all masses and assess the local lymph nodes. We also recommend full blood work and imaging (chest radiographs and an abdominal ultrasound) to evaluate for potential spread or metastasis. Surgical excision and biopsy is then advised.

Surgery is the mainstay of treatment for canine mammary tumors. The goal of surgery is to remove all tumors with clean (tumor-free) margins and obtain the tissue for histopathology via biopsy. It is very important to biopsy the mass so that we know the tissue of origin and outlook or prognosis for your pet. We recommend biopsying all tumors since one pet can have different types of mammary tumors.

About 50% of canine mammary masses are malignant. Malignancy depends on the size, type and the grade (changes present on the biopsy report) of the tumor. Some signs of malignancy from the history or physical examination include: rapid increase in size, fixation to the underlying structures, ulceration, pain and inflammation. If these signs are not present, the mass can still be malignant.

Surgery can be curative for benign tumors and dogs with small, malignant tumors with favorable characteristics on their biopsy report. Dogs with larger, high grade masses that are at risk for metastasis or recurrence will require additional treatment such as chemotherapy.  If the pet is still intact, we will recommend spaying her to remove the hormonal influence. The overall prognosis for a pet is dependent on the tumor size, type, grade, lymph node involvement and presence of spread.

Update on Molly:

Untitled3Molly went on to have to surgery to spay her and surgically excise her mammary mass. The biopsy report of the mass indicated it was a low grade complex mammary gland carcinoma. Though the mass is malignant, the characteristics seen on the biopsy were not associated with aggressive or metastatic behavior. The margins were free of tumor cells but were narrow in size. Surgery is likely curative for Molly and it was determined that she is not at a high risk for metastasis. Molly is being examined every 3 months to evaluate for local recurrence of the mass, lymph node involvement and chest radiographs to screen her for any potential spread. Molly recovered well from surgery and has been doing great at home!!

Key Points:

  • Spaying your pet early can help prevent mammary tumors!
  • Risk of Developing Mammary Tumors Timing of Spay
    0.5% Spayed before 1st heat
    8% Spayed before 2nd heat
    26% Spayed before 3rd heat
  • 50% of canine mammary tumors are malignant and 50% are benign.
  • A biopsy is very important to help determine the prognosis for your pet.