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
- 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
- If the tumor is malignant or shows evidence of invasion into the lymph system or blood vessels, chemotherapy likely will be recommended.
- Tamoxifen usually does not have a good effect if the dogs has a malignant mammary tumor, as these usually have very low numbers of estrogen receptors. Side effects of this medication can include pyometra, vulvar enlargement, vulvar discharge, urinary tract infection, incontinence, low white blood count, death. Tamoxifen, in general, is not a good treatment for mammary tumors in dogs.
Mammary tumors in cats
- Spaying female cats before the first heat has a significant sparing effect on the development of mammary cancer.
- Mammary tumors in male cats
- About 80 to 90% of the mammary tumors are highly malignant in female cats
- Size matters: tumors less than 2 cm in size have a better prognosis than tumors greater than 3 cm in size
- Radical removal of both mammary chains should be performed if a large or multiple masses are present
- If only a single pea sized mass is present, lumpectomy (or mammectomy) is acceptable instead of radical surgery
- Chemotherapy should be considered to help control metastatic disease
- Anesthetic death – rare
- Infection – rare
- Break-down of the incision, which may require resuturing of the wound or leaving it to heal on its own
- Spread of cancer to other regions of the body (lymph nodes, lungs, bones)
- Recurrence of cancer
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