Mammary Gland Tumors

We recently had a cat come in with a mammary gland tumor, which made me think that it would be good to discuss mammary tumors in dogs and cats.  Let's start with dogs.

Diagram of mammary glands in dogs

Of any of the domesticated animals, dogs have the highest incidence of mammary tumors.  Females who are spayed before their second heat cycle have a significant decrease in the incidence of mammary tumors compared to dogs who are left intact or spayed after their second cycle.  One of the good news for dogs with mammary tumors is that around half of the tumors act in a benign fashion.  This means that they typically grow locally in the mammary gland alone and do not spread.  Unfortunately, this means that the other half of tumors are more aggressive and have spread by the time they are diagnosed.  The smaller the tumor (<3 cm) the more likely that the tumor will be benign.  If the local lymph node is involved or the tumor has spread elsewhere (lungs), then these tumors tend to be more aggressive.

What will happen if a mass is felt on your dog.  Typically, complete staging should be performed prior to treatment.  This can involve performing labwork, taking chest x-rays (to check for lung nodules), aspirating the local lymph nodes if enlarged (to look for cancer cells), and possibly performing an abdominal ultrasound (to check for any nodules in the liver, spleen, etc).  If the tumor shows evidence of spreading, then additional therapy beyond surgery, such as chemotherapy, may be recommended.

Typically, the mainstay of treatment for the primary mass is removal of the affected mammary gland (mastectomy).  If multiple mammary glands are involved or a lymph node is involved, then all affected glands and the nodes should be removed.  After removal, the masses should be submitted to a lab where histopathology will be performed and the origin of the mass can be identified.  Although there are no definitive studies showing an added benefit of chemotherapy with mammary tumors, many veterinarians will recommend a course of chemotherapy in dogs where the tumors are thought to be more aggressive (metastasis, large size, etc).  Some vets may also recommend spaying (if not already done) the dog at the time of mass removal as some tumors may be responsive to hormones released by the ovaries.

Mammary mass form a dog

For dogs with small tumors, surgical removal may be completely curative and monitoring of the other mammary glands is the only long term recommendation.  However, tumors that are larger and more aggressive typically carry a worse prognosis of only a few months to a year.  It is important to bring your pet in for an annual exam as your veterinarian may be able to pick of a mammary mass before it becomes too large.

Cats can also develop mammary gland tumors.  Like dogs, female cats who are spayed earlier in life have a significant decrease in the incidence of mammary tumors.  Unlike dogs, most feline mammary tumors (greater than 80%) tend to be malignant.  Tumors that are smaller (<3 cm) tend to carry a better prognosis (possibly as long as 2 years), whereas prognosis for larger tumors tends to be poor (4-6 months).

Similar to dogs, staging with labwork, imaging, and evaluation of the local lymph nodes should be performed. With cats, most veterinarians recommend removal of all the glands on the affected side versus just a simple mastectomy.