Mammary Gland (Breast) Cancer in Dogs
What are mammary gland (breast) tumors?
Mammary glands are female organs that produce milk for feeding the young. Dogs have 5 glands on each side which are
drained by lymph nodes in the armpit and the groin. The development of cancer in the mammary glands has been shown
to be dependent on hormones and mammary cancer is one of the most common cancers in the female dog. Based on
available data, approximately 50% masses in the mammary glands are malignant.


It is important to know that mammary cancer does not refer to a single disease but there are many different subtypes, each
responding differently to treatment and each with a different prognosis. The subclassification of mammary cancer is as

Benign tumors                                   Malignant tumors
Adenoma                                           Non-infiltrating carcinoma
Simple adenoma                                Complex carcinoma                                               
Complex adenoma                             Simple carcinoma
Basaloid adenoma                             Tubulopapillary carcinoma  
Fibroadenoma                                   Solid carcinoma
Low-cellularity fibroadenoma             Anaplastic carcinoma
High-cellularity fibroadenoma            Special types of carcinomas
Benign mixed tumor                           Spindle cell carcinoma
Duct papilloma                                   Squamous cell carcinoma
                                                   Mucinous carcinoma
                                                    Lipid-rich carcinoma

How common are breast tumors in dogs?
Spaying dogs prior to their first heat cycle significantly reduces the cancer risk later in life. Spaying after the second heat
cycle does not provide any advantage.

What are the symptoms of breast cancer in dogs?
Mammary tumors can develop either as a single mass or as multiple masses within the mammary gland. Dogs and cats
have multiple mammary glands, all of which can develop one or more benign or malignant tumors. Benign tumors are
typically well defined and firm upon examination whereas the clinical symptoms of malignant tumors include rapid growth, ill-
defined boundaries, ulceration and/or inflammation. Inflammatory carcinoma subclass of mammary tumors is typically
rapidly growing, affects multiple mammary glands and is characterized by firmness, warmth, edema, erythema, thickening
and signs of pain. Extensive swelling (lymphedema) of a limb adjacent to the affected glands can be also observed.

How is the diagnosis made?
The diagnostic procedures include physical examination, blood and serum chemistry profiles, X-rays, and abdominal
ultrasound. Collecting cells with
fine-needle aspiration for cytology evaluation is not a sensitive method to distinguish
malignant from benign cells but can help rule out other lesions such as inflammatory lesions or mast cell tumors. The most
definitive way to get a diagnosis on the pet's suspected mass is to obtain a
tissue biopsy.

Does cancer cause pain in dogs?
Pain is common in pets with cancer, with some tumors causing more pain than others. In addition to pain caused by the
actual tumors, pets will also experience pain associated with cancer treatments such as surgery, radiation therapy or
chemotherapy. Untreated pain decreases the pet's quality of life, and prolongs recovery from the illness, treatment or
injury. It is, therefore, essential that veterinary teams that are taking care of pets with cancer should also play a vital role in
educating pet owners about recognizing and managing pain in their pets. The best way to manage cancer pain in pets is to
prevent it, a term referred to as preemptive pain management. This strategy anticipates pain ahead of time and
administers pain medication before the pet actually experiences pain, thus ensuring the pet's maximum comfort.

To learn more about which tumors are likely to cause a lot of pain, how to recognize pain in pets with cancer and what
cancer pain management options are available for your pet, please visit the
Cancer Pain Management section.

Is nutritional support important for dogs with cancer?
Cancer cachexia (a term referring to progressive severe weight loss) is frequently observed in pets with cancer. Pets with
cancer lose weight partly because of lack of appetite and partly because of cancer-induced altered metabolism. Some of
the causes for decreased appetite are related to the cancer itself (for example, tumors may physically interfere with food
chewing, swallowing, and digestion process) and some may be related to the side effects of cancer treatment (for example,
some chemotherapy drugs cause nausea and vomiting, and radiation therapy can cause mouth inflammation).

Proper nutrition while undergoing cancer treatment is essential to maintain your pet's strength, improve survival times,
quality of life and maximize response to therapy. Adequate nutritional support was shown to decrease the duration of
hospitalization, reduce post-surgery complications and enhance the healing process. Additionally, pets with cancer need to
be fed diets specifically designed to provide maximum benefit and nutritional support for the patient. To learn more, please
visit the
Cancer Nutrition section.

What are the treatment options for breast cancer in dogs?
The primary goal of surgery is to remove the tumor and it remains as the treatment of choice with a few exceptions. There
are a variety of different procedures that can be used to remove the breast tumors and the choice of procedure is
determined by various factors of your pet's condition such as the size of the tumor, the number of lesions, etc. Lumpectomy
(also called nodulectomy) is typically performed on small benign nodules. Mammectomy refers to the procedure that
removes the affected mammary gland(s). Mastectomy refers to the procedure that removes all of the mammary glands at
once. The mammary tumors can spread to the adjacent lymph nodes, however, it is not very common in dogs. The lymph
nodes should be removed only if they are enlarged and cytologically positive for cancer or whenever gland 5 is removed
due to the close association of the lymph node with this node.

While chemotherapy is the standard of care for humans, its benefit in canine mammary tumors has not been fully
evaluated. Small studies support the benefit of chemotherapy but additional studies with larger number of patients need to
be done for confirmation.

Radiation therapy
As is the case for chemotherapy, veterinary oncologists don't have enough information to confirm or rule out the benefit of
radiation therapy in the case of mammary tumors.

What are the treatment associated risks?
The risks associated with the surgical removal of the mammary tumors include rare complications such as anesthetic death
and infection.

How do I find a qualified veterinary oncologist?
To locate a qualified veterinary oncologist worldwide who can discuss with you appropriate cancer treatment plan for your
pet's cancer condition, please visit the "
Locate a veterinary oncologist" section.  

Are there any clinical trials investigating new treatments for breast cancer in dogs?
There is one ongoing clinical trial in canine mammary cancer at Auburn University in Alabama (click here for more details).
In addition, there are several clinical trials available for dogs with any tumor type for which your pet may qualify. To learn
more these trials (which are partially or fully funded by the institutions), please visit the
Dog Clinical Trials (any tumor type)
Cat Clinical Trials (any tumor type) section.  

To learn more about veterinary clinical trials in general, please visit the
Pet Clinical Trials section.

What is the prognosis for dogs with breast cancer?
There are several factors that will influence the pet's prognosis including the type of the tumor, its size, lymph node
involvement, presence of metastasis, etc. The following table summarizes the prognostic factors which should not be taken
as absolute but rather relative:

Source: Withrow Stephen J, and David M. Vail. Small Animal Clinical Oncology. St Louis: Saunders Elsevier, 2007.

Additional Resources:

  • Withrow Stephen J, and David M. Vail. Small Animal Clinical Oncology. St Louis: Saunders Elsevier, 2007.
  • Morrison Wallace B. Cancer in Dogs and Cats: Medical and Surgical Management. Baltimore: Williams&Wilkins, 1998
Good prognostic factors
Poor prognostic factors
Indifferent prognostic factors
Tumor <3cm in diameter
Tumor >3cm in diameter
Well defined tumor boundaries
Ill defined tumor boundaries,
Negative lymph nodes
Positive lymph nodes
Carcinoma-well differentiated,
complex, tubular/papillary
Carcinoma-poorly differentiated,
simple, solid, anaplastic; inflammatory
carcinoma; sarcoma
Type of surgery (simple or radical)
Tumor grade I
Tumor grade III
Number of tumors
Positive estrogen and progesterone
Negative estrogen receptors
Glands involved
Index of proliferation
Low Ki-67
Index of proliferation
High AgNOR
High Ki-67
p53 gene mutation
Comprehensive guide to cancer diagnosis and treatment in cats and dogs
© 2007 Pet Cancer Center. ALL RIGHTS RESERVED.
Last updated 02/19/2017