Mammary Tumors - Female

If a dog is spayed before the first heat cycle there is about a 0.5% chance that this pet will develop mammary cancer

The larger the breast tumor the greater the risk that it has spread to the lymph nodes, lungs, or other parts of the body

When a breast tumor has been found, there is a 50% chance that it is malignant and a 50% chance that it is benign

Of the malignant breast tumors, 50% have already spread at the time of diagnosis

• Dogs have 5 mammary glands on each side (total of 10 breasts)
• Cats have 4 mammary glands on each side (total of 8 breasts)
• The lymph nodes in the groin and arm pits drain the mammary glands

Risk Factors for breast or mammary cancer in dogs

• If a dog is spayed before the first heat cycle there is about a 0% chance that this pet will develop mammary cancer
• The risk of developing mammary tumors increases with each successive heat that a dog
  • one heat cycle = 0.5%
  • two heat cycles = 8%
  • three heat cycles = 26%
• Spaying a dog after 2 years of age will not decrease the risk for developing mammary cancer
• The larger the breast tumor the greater the risk that it has spread to the lymph nodes, lungs, or other parts of the body
• When a breast tumor has been found, there is a 50% chance that it is malignant and a 50% chance that it is benign
• Of the malignant breast tumors found 50% have already spread at the time of diagnosis
• If your pet has multiple mammary tumors, some may be malignant and some may be benign
• Size matters: if the tumor is less than 3 cm in size the recurrence rate is relatively low, versus greater than 3 cm has a fairly high recurrence rate
• If the biopsy report indicates that the breast tumor has spread to the lymph nodes, lymphatic channels, or blood vessels the prognosis is poor
• If the biopsy report indicates that the tumor is surrounded by cells called lymphocytes, a better prognosis is expected
• If chest radiographs (x-rays) indicate that there is signs of metastasis (spread of cancer) to the lungs, long-term survival is not expected

Diagnostic tests prior to surgery

• Compete blood cell count
• Chemistry profile and urinalysis to evaluate function of the internal organs
• Chest x-rays to rule out evidence of spread to the chest
• Abdominal ultrasound
• Fine needle biopsy of enlarge lymph nodes and mammary masses

Surgery

• If a single gland is affected, then only that gland is removed
• If multiple glands on one side are affected, then the entire 5 glands on that side are removed
• If multiple glands have tumors on both sides then both mammary chains are removed (all 10 glands are removed)
• If the lymph nodes are within the resection zone, then they also are removed (and especially if they are enlarged)
• If the groin region is difficult to suture closed, a flap of skin from the flank may be needed to reconstruct the area
• The photo right shows a large mass (labeled M) in the mammary gland

Thank you,
Jackie

Article Written by Dr. Daniel A. Degner, Board-certified Veterinary Surgeon (DACVS)  Photo accompanied the article.