These preventable tumors are the most common tumor in female dogs as they age. They do occur in males, but this is a rare occurrence. Up to 50% of these tumors in dogs are malignant. The specific cause of this problem is unknown, although there is a very strong correlation to hormones, especially in the dog. It is an accepted fact, proven over many years, that if you spay (ovariohysterectomy) your dog prior to its first heat cycle there is a negligible chance your pet will get this cancer. The longer you wait once your pet starts its heat cycle the greater the chance it will get this problem.
Mammary tumors account for 17% of all cat tumors. They are the 3rd most common tumor in cats, after skin and blood cancer. Even though cats get this problem half as often as dogs, almost all of them are malignant. Intact females are at highest risk, although it does occur in males on rare occasion. Even though early spaying in the cat does not seem to yield as much protective effect as in the dog, you can still decrease the incidence of this tumor by up to 60% by spaying early.
Mammary tumors are very common in rats.
Some owners wait so long for medical care that by the time we see them they are as large as the rat.
We have two short Quicktime movies of surgery using the laser to remove a mammary tumor. They are graphic in nature, and not suitable for all viewers.
It is helpful to be exposed to several medical terms that will be used later in this page:
In dogs there are 5 sets (varies from 4-6) of mammary glands in a chain, for a total of 10 mammae. From top to bottom they are called:
- Cranial thoracic
- Caudal thoracic
- Cranial abdominal
- Caudal abdominal
From top to bottom you can trace the nipples. How many nipples do you count on this dog?
Nipples even show up on a radiograph. Can you see the three that are apparent on this radiograph?
They are the 3 small and circular white spots- there is one towards the top left, one towards the lower right, and one at the lower left.
The inguinal mammary tissue tends to the largest, and produces the most milk. Due to its size this area can look like it has a tumor when in reality it is normal. If you feel an enlargement here one of our doctors should check it to confirm it is nothing more than fat. This is the most common mammae for a tumor to form in the dog.
The upper mammary glands drain towards the auxillary (arm pit) lymph nodes. The inguinal mammary glands drain towards the inguinal (groin) lymph nodes. The middle mammary tissue (caudal thoracic and cranial abdominal) can drain in either direction. You can learn much more about lymph nodes by following this link.
In the cat there are 4 pair of mammary glands. The cranial gland are the most common ones for tumors to occur.
For a little comparative anatomy fun; manatees, and primates only have two mammary glands.
Mammary glands are modified sweat glands. They reside in the subcutaneous (SQ) fat, which is the fat just under the skin but above the muscle. The primary function of the mammary glands is to produce milk and hormones.
- Lactose (the milk carbohydrate)
- Fat- much higher in some animals than others, usually in the form of tryglycerides.
- Protein- Also varies quite a bit by species. The primary protein in milk is caled casein.
- Mineral, vitamins, and enzymes.
Whale and seal milk has 12x as much fat, and 4x as much protein, as cow’s milk. Cow’s milk has less protein and fat than cat and dog milk, which is why orphan kittens and puppies do not do well on it. It takes between 500 and 1000 liters of blood to make 1 liter of milk in the cow.
Numerous hormones are involved with the production of milk:
- Glucocorticoids (cortisone)
In the first week of lactation the milk that is produced is called colostrum. This milk contains antibodies to protect kittens and puppies from routine diseases like Distemper and Parvo.
Hormone receptors for estrogen and progesterone are present in the dog. In the cat there are usually progesterone receptors, the estrogen receptors are not very prevalent.
Mammary tumors can be malignant or benign. In dogs, up to 50% are malignant. In cats, almost all mammary tumors are malignant (adenocarcinomas). Although there are histologic variations on this, these are the main classifications. The more common ones are at the top of each list:
- Mixed tumors
- Tubular adenocarcinomas
- Papillary adenocarcinoma
- Anaplasric carcinoma
- Solid carcinomas
The exact cause of mammary tumors is unknown, although there is a strong correlation to hormones. It has to do with estrogen and progesterone receptors on the tumor. These receptors are present in up to 70% of canine mammary tumors, and 10% of cat tumors. For this reason we tend to stay away from estrogen and progesterone type drugs when treating other diseases.
If your dog is spayed (ovariohysterectomy) before it goes into its first heat cycle, the chances this dog will get breast cancer later in life is virtually nil. A typical female dog will go into heat at 9 months of age, although this varies. If your dog is not spayed until after its first heat cycle the risk of breast cancer can be as high as 8% later in life. Another heat cycle prior to spaying gives a 26% chance of cancer later in life.
Another way to minimize the risk of mammary cancer is to keep your dog at its proper weight.
It is also beneficial to spay a cat early in life. This is especially important in cats because most of their breast tumors are malignant.
The beginning signs of breast cancer can be hard to detect because they are so subtle. Also, mammary tissue tends to hang down hiding any swelling or enlargement.
You should examine your dog or cat weekly while you are playing with it or petting it. Most pets love to have their bellies scratched, which is an ideal time to do your exam.
Run your hands along both chains of mammary tissue from top to bottom feeling for any difference in symmetry. Palpate each gland individually and gently for swelling, discharge, ulceration, hardness, extra warmth, nodules, or discomfort.
Look at each mammary gland, especially each nipple, for any signs of discharge, inflammation, or swelling. Any of the above symptoms are an indication to bring your pet in for us to perform an exam and even run some tests if we think a problem is present. Other symptoms to look for are lameness, swelling of the limbs, or difficulty breathing.
A thorough approach is needed for a correct diagnosis of mammary tumors. In every disease we encounter we follow the tenet’s of the diagnostic approach to ensure that we make an accurate diagnosis, and also so that we do not overlook some of the other diseases that are common in pets .
Diagnosing some cases of mammary tumors is straightforward, especially if the disease has been present for a significant amount of time before a diagnosis is made. Unfortunately, in these cases the disease can be well entrenched, and malignant tumors have had significant time to spread.
This tends to be a disease of middle-aged and older unspayed female dogs and cats. Even though it can occur it is rare in males.
Some breeds have a higher incidence:
- Hunting breeds- retrievers, pointers, and spaniels
- Terriers- Boston, fox, and Airedale
- German Shepherds
Some breeds have a low incidence:
Siamese cats have twice the risk as other cats, and their tumors tend to me more malignant than other cats. Domestic shorthair cats (DSH) have a higher incidence than other cats also.
Due to the location of the breast tissue it is easy for an owner to overlook this problem . Pets with early breast cancer do not show the usual symptoms of disease in general. They are usually active, eating well, maintaining normal weight, and have normal bathroom habits. A small tumor that is growing can easily be present for months before an exam is performed. This adds to the problem and can make treatment more complicated.
Some owners find a swelling, discharge, or a growth while bathing or petting their dog or cat. Any suspicious area should be checked by one of our doctors to determine if there is a growth, swelling, or just normal breast tissue.
When the tumor has already spread some pets might have difficulty breathing (dyspnea) due to buildup of the tumor in the lungs, or lameness due to spread of the tumor to the bones. In cats the dyspnea can be due to fluid buildup in the thorax (pleural effusion).
This cat has a mammary tumor at the nipple. This is the only sign of disease it had, and can easily be missed if you are not observant. This problem was easy to spot once we clipped the hair in preparation for surgery.
The tumor in this cat is more apparent. Unfortunately, when we see them at this stage they might have already spread since most cat tumors are malignant.
Intact female dogs can have a false pregnancy 2-4 weeks after their heat cycle that will cause the mammary tissue to swell and mimic a tumor. It is due to the progesterone that is produced during the heat cycle, and the problem will resolve on its own in a few weeks.
Other diseases that mimic breast cancer include an infection called mastitis, skin tumors, and an inguinal hernia. In cats the inguinal fat pad can be enlarged and mimic a tumor. Foreign bodies like BB’s (not all that uncommon for a cat to be found with a BB when we take an X-ray) feel like tumor nodules.
In some cases a swelling or growth is found in the breast tissue during an exam for a different problem, or during a routine wellness exam.
Nodules might be small and solitary, or the whole mammary chain can be affected. Nodules that are adhered to the skin or underlying tissue, are ulcerated, painful, or swollen tend to be malignant. Nodules that are rapidly increasing in size also tend to be malignant. There might be a discharge from the nipple, and your pet might be running a fever.
Here are some typical lesions in a cat
Whenever your pet is placed under anesthesia we perform a thorough exam, including mammary tissue. This is an ideal time because your pet is not moving, it is commonly on its back and we have good access and visualization of the area, and the muscle relaxation allows us to thoroughly palpate small nodules. Your pet can have a malignant tumor and show minimal to no symptoms.
This dog is under complete general anesthesia just prior to her spay surgery. With the hair clipped away at her abdomen you can see the good access we have to the mammary tissue.This enables our doctor to perform a thorough exam of all the mammae
Enlarged lymph nodes due to spread of tumor might also be noted. A lymph node can contain the spread of tumor cells and still appear and feel normal. Cats frequently have the spread of their tumor to the lymph nodes.
One of the typical lymph nodes we will check during an exam are the axillary (arm pit)
In this picture that cat’s head is to the right, and we are checking the inguinal lymph nodes on the insides of the rear legs. If a malignant tumor has spread through the lymphatic system it can cause swelling of the rear legs due to blockage of the lymphatic drainage system.
Any pet suspected of having a mammary tumor needs routine tests as the first part of the diagnostic process.
A CBC (complete blood count) and biochemistry panel should be run on every dog or cat 8 years of age or more, especially if they have any of the symptoms of mammary disease
The CBC checks red and white blood cells. We are looking for signs of infection, cancer, anemia, or excess production of red blood cells. If your pet has mammary cancer it might also have inflammation or a secondary infection. We might get a clue of this from the CBC.
This cat might have an inflammation or infection as evidenced by the increase in the white blood cells. This is called leukocytosis. If the physical exam findings are consistent with an infection then we might put this cat on antibiotics before initiating any other treatment at the moment.
The next part of the blood panel is called the chem or biochemistry panel. It checks the internal organs, along with electrolytes and specific physiologic tests like blood sugar.
Dogs and cats with mammary tumors tend to be older, so Geriatric Diseases are more prevalent. Since surgery is usually a major part of treatment we need to make sure the internal organs are ready for anesthesia. This is particularly true of kidney and liver disease. Some pets with mammary cancer will have a high calcium level on this panel.
The same cat as above has a normal biochemistry panel
In some cases we might run a clotting panel looking for any signs of a disease called disseminated intravascular coagulation (DIC). This can occur when there is an inflammatory carcinoma.
A normal specific gravity in a dog should be around 1.025 or higher, cats should be higher than 1.035. There should be no or minimal protein, glucose, WBC’s, or bacteria, as a general rule.
In this test we insert a tiny needle with attached syringe in the mammary tissue. It is a tiny pin prick, and is the same kind of needle we use to give vaccines. Some cells are aspirated into the syringe and then gently pressed on to a microscope slide.
The pathologist looks at these cells to give us an idea of what we might be up against.
In some cases we use this test, especially if it is difficult to differentiate inflammation from an actual tumor. Its also gives us an idea of just how malignant or non-malignant the tumor is, so we can adjust our surgery accordingly. In cats we assume the tumor is malignant and usually skip this test and go right into surgery.
Even though it can be a useful test, it only looks at a small portion of the mammary tissue. So it is used only as ancillary information prior to surgery and not to make a final diagnosis. Also, multiple tumor types might be present, and you can make the wrong interpretation with just this test.
Aspiration of a local lymph node can also be helpful to detect evidence of any spread of a tumor. In cases of extensive mammary involvement, usually the whole chain, we might completely remove the lymph node that drains that area. This gives the pathologist much more tissue to work with to ascertain if there has been a spread of the tumor.
In cats that have fluid buildup in their thorax we can submit this fluid for cytology also.
Radiography (X-Rays) are a very important test prior to surgery because up to 50% of the dogs with malignant breast cancer have spread of the disease to the lungs at the time of their exam. We need to confirm that there is no spread (metastasis) of the tumor to the lungs or else surgery might not be indicated. We take 3 different views of the chest to determine if the lungs are clean.
In this chest radiograph we have placed black arrows at some of the white and round areas that are the spread of the tumor in the lungs. Compare it to the normal dog radiograph below if you need to.
Normal dog chest radiograph
This is what a mammary tumor under the skin looks like on a radiograph. It is that round white object on the bottom.
Some cats will show signs of difficulty breathing. It can be subtle, so it behooves you to spend some time every day observing your pet for any changes that indicate a problem. This cat has fluid that has built up in its lungs.
The first one is from a normal cat. Notice the normal black lung area.
In this radiograph from a problem cat there is fluid throughout the thorax and you cannot see normal black lungs. The lungs have collapsed due to the fluid in the thorax. The only lung tissue you see is the slightly dark leaf-shaped structure towards the top of the thorax.
After oxygen therapy for stabilization we drained some fluid off the thorax. This caused an immediate improvement in breathing. A radiograph taken soon afterwards shows improvement as evidenced by the increase in the normal amount of black lungs visualized. When this fluid appears due to the spread of a malignant tumor from a mammary gland the prognosis is poor.
In some cases a radiograph of the bones will show spread of cancer. If a radiograph is taken of the abdomen some malignant cancers will show an enlarged sublumbar lymph node. Ultrasound is beneficial here in assessing local lymph nodes and abdominal spread from a malignant mammary tumor.
This abdominal radiograph shows the location of where the sublumbar lymph node is normally located. It is not apparent in this view, so it is not enlarged. The K stands for kidney and the B stands for urinary bladder. Ultrasound tends to be a more accurate way to assess abdominal lymph node enlargement when compared to radiography.
Response to Therapy
One of the tenets of the diagnostic process is whether or not a treatment that is instituted actually corrects the problem. Surgery is the main form of treatment, so response to treatment does not apply as much as to other diseases that are more medical in nature and treated with drugs.
Note: This section has links to two graphic movies during surgery to remove mammary cancer. They are not suitable for all viewers.
The treatment of choice for mammary tumors is surgery. Chemotherapy, radiation therapy, nor hormonal therapy have any proven benefit.
It is routine to spay (OVH) an intact female dog prior to or during surgery to remove an affected mammary gland. Even though at this usually late date it will not prevent more tumors from appearing, it will prevent uterine infection (pyometra) and uterine cancer, and might even prevent hormone influence on existing tumors.
If the gland is infected we might use antibiotics to reduce the swelling and inflammation. This will allow us to see the margins of the tumor more readily during surgery.
When your pet is relaxed under anesthesia, and the hair is clipped away prior to surgery, we will examine the mammary glands again. It is not uncommon to discover a small tumor that was missed during the routine exam.
Once our diagnosis and ancillary tests are complete we will remove the mass surgically. Depending on the location, size, duration, species, and physiologic status of your pet, we might do a lumpectomy or remove part or all of the chain. In extensive cases we might have to remove one chain of tissue in a first procedure, then the other chain several weeks later when the first chain has healed. Since cats usually get malignant tumors it is common to remove the whole chain on the affected side.
In all these surgeries we remove a wide margin of tissue to ensure we removed all of the tumor. In all cases our goal is to remove all of the tumor and get what are called “clean edges” by the pathologist. This means there is no microscopic signs of tumor cells in the tissue submitted for analysis. This makes for a much better prognosis.
This is a typical mammary tumor noted in a cat
After wide surgical incision this is the opening prior to suturing
What the suture site looks like when we are finished. The rubber tube is called a penrose drain tube.
It is used to decrease swelling during healing, and is removed in 3-5 days. When the sutures are removed in 10-14 days their is minimal scar and its hard to tell surgery was even performed
Since cats frequently get malignant tumors we commonly remove the whole chain of mammary tissue on the affected side. We might also remove the closest lymph node to look for metastasis.
We routinely use our laser for this surgery. This dramatically minimizes post operative bruising, discomfort and swelling. We can’t emphasize enough how important the use of the laser is in this surgery. Prior to our laser these dogs and cats would have extensive bruising and swelling of the sensitive mammary tissue. We would place many sutures under the skin to prevent fluid buildup and discomfort. We no longer need to with the laser. Dogs and cats that have this surgery, even when a radical surgery is performed, routinely go home the same day and have minimal discomfort. When laser is used with routine pain medication your pet will usually be eating and resume normal activity within 12-24 hours.
When we remove the mammary tissue on one whole chain there is a long incision. With the use of the laser and routine surgical and post surgical pain medication these cats recover rapidly from surgery.
Click on the link below to see a laser lumpectomy surgery on a cat. Notice how little bleeding there is when the laser is used.
This next cat has more gland involvement and requires more surgery. Notice how diseased the tissue appears and the lack of bleeding when using the laser. As the surgery progresses you can see milk coming from the gland.
Click here to learn more about the laser and how it is used in many types of surgeries at our hospital.
Diseased mammary tissue that is removed during surgery is submitted for histopathic analysis. The pathologist will determine the type of tumor and will also stage it. Stages usually go from 0 – 3.
- Stage 0- Tumor cells are limited to the ducts within the mammary tissue
- Stage I- Tumor cells are in the ducts and the supportive or framework tissue of the mammae (called stroma)
- Stage II- Tumor cells are in the blood vessels, lymphatic tissue, or regional lymph node
- Stage III- Tumor cells have spread through the body- usually lungs or bones
Chemotherapy is used when we cannot remove all of the tumor of if your pet has inflammatory carcinoma. Chemotherapy for mammary cancer tends to be unrewarding. Some of the drugs we use, which should be under the direction of a veterinary oncologist, include:
Tamoxifen, a product commonly used for human breast cancer, is ineffective and has the potential for serious side effects in dogs.
In some dogs and cats the tumor is not resectable. This occurs in inflammatory carcinomas. In these cases we used what is called palliative therapy. We attempt to keep them comfortable with antibiotics, pain medication, fluids, assist feeding, good nutrition, and lots of TLC.
If your pet’s tumor is benign and completely removed, then it will be cured of the disease. Benign tumors can appear in other glands though when only a nodule is removed and the gland is left intact.
If the tumor is malignant then it is hard to predict due to the variable nature of the malignancy. Low grade malignancies can be cured with surgery. Those tumors with a higher stage of malignancy or ulceration can recur and spread within the first year of surgery. Some of the factors that influence prognosis are:
- Tumor size- tumors smaller than 2-5 cm have the best prognosis for malignant tumors.
If the tumor is smaller than 2-3 cm many pets will live up to 3 years.
If the tumor is larger than 2-3 cm most pets will live for 6 months
- Tumor histology- Sarcomas, carcinomasarcomas, and malignant mixed tumors have a poorer prognosis than carcinomasadenocarcinomas. Inflammatory carcinomas have a very bad prognosis.
- Tumors classifed in histologic stage II or III carry a worse prognosis.
Cats with malignant tumors usually do not survive more than a year. Their tumors grow rapidly and spread to the lungs early, usually before a pet owner is aware and brings them in for diagnosis and treatment. Prognosis for a cat depends on 5 factors:
If there is spread to the lymph nodes
Stage I- > 24 months
Stage II- 12-24 months
Stage III- 4-12 months
Stave IV- 1 month
Invasion of the lymphatics that drain the gland
Siames cat has a poorer prognosis than domestic cats
This is the report on the cat with the nipple that was inflamed. We showed you the picture of this tumor earlier when we talked about how easy it is to miss some of these tumors.