Causes of Mast Cell Tumors in Dogs
The underlying causes of mast cells tumors are not known. Like other types of cancer, it’s likely they are caused by a mix of genetic and environmental factors, but we do not have enough information to recommend any changes to your dog’s environment to prevent mast cells tumors.
Certain breeds, however, are more likely to get mast cell tumors. These include:
What is a mast cell?
A mast cell is a type of white blood cell that is found in many tissues of the body. Mast cells are allergy cells and play a role in the allergic response. When exposed to allergens (substances that stimulate allergies), mast cells release chemicals and compounds, a process called degranulation. One of these compounds is histamine. Histamine is most commonly known for causing itchiness, sneezing, and runny eyes and nose – the common symptoms of allergies. But when histamine (and the other compounds) are released in excessive amounts (with mass degranulation), they can cause full-body effects, including anaphylaxis, a serious, life-threatening allergic reaction. ( via Wikimedia Commons / Joel Mills (CC BY-SA 3.0.)
Diagnosing and Staging Canine Mast Cell Tumors
Diagnosing a mast cell tumor can be done by your veterinarian with a quick procedure called a fine needle aspirate (FNA). Your vet will insert a needle into the lump and suck out some material, which is then put on a slide to examine under a microscope. Mast cell tumors are easy to identify because of their granules.
The next step is to stage the tumor, or determine how severe it is. This is often done by taking a biopsy and sending it out to a lab for a histopathology report. The pathologist will examine the biopsy under a microscope. Other things that can be useful for grading and determining the best treatment plan include lymph node aspirates, radiographs (x-rays), blood work, and a bone marrow biopsy, all to evaluate if the cancer has spread.
Dog Mast Cell Tumors: Symptoms, Diagnose, and Treatment of Dog Mast Cell Tumors with Dr. Dan
Up to 20% of dogs will get a malignant Mast Cell Tumor (MCT) in their lifetime, making it the most common tumor we encounter in dogs. Other names for MCT are mast cell sarcoma, histiocytic mastocytoma, and mastocytosis.
This disease has a cutaneous form (skin) and a systemic form (internal organs). The cutaneous form is more prevalent, and is sometimes referred to as the “great pretender” because the skin growth appearance is so variable. What might seem like just a minor swelling or inflamed area can be a highly malignant cancer. It might also be called the “great fooler” because most people underestimate the problem.
MCTs have the potential to be highly malignant and their treatment needs to be thorough and comprehensive in order to minimize the chance of spread and premature death of your pet. In almost every case the treatment of choice is surgery to remove the tumor. After surgery additional treatment might be indicated.
This page will give detailed information on MCT in the dog since that is the primary species that gets this disease. We occasionally see MCT in cats and ferrets. MCT are second most common tumor in cats. They behave somewhat differently in our feline friends. We will go over MCT in the cat at the end of this page.
Throughout this page are links to numerous other areas on the LBAH web site to give you additional information on that specific topic.
There are some mild surgery pictures in this page showing the open skin after a MCT has been removed. They are suitable for most viewers.
Mast cells, also known as mastocytes, originate in the bone marrow and live for weeks to months. Once produced by the bone marrow they circulate through the body ending up in large numbers in the connective tissue of the body, lymph nodes, under the skin, and in internal organs.
Mast cells surround nerves and blood vessels, and are oftentimes found at the interface between the environment and the pet. This means they tend to be found in higher numbers under the skin and the lining of the respiratory system and digestive systems. They play a role in wound healing and defense against invading bacteria, and are a big part of the immune system. They have binding sites on them for the immunoglobulin called IgE. This immunoglobulin is a big part of the immune system when it comes to allergies. You can learn more about it by linking to our allergy page.
Mast cells have the potential to release several different compounds, the most important of which are called vasoactive amines. Vasoactive amines are nitrogen-containing compounds that act on blood vessels. The more important vasoactive amines are histamine, heparin, serotonin and prostaglandin. Histamine is the most important one in relation to MCT.
Histamine goes to the stomach lining and stimulates the secretion of gastric acid. Histamine also dilates blood vessels and increases permeability of the blood vessels, leading to swelling (edema). This mechanism is part of the normal inflammatory process.
Release of large amounts of histamine from a mast cell can result in ulceration or perforation of the GI tract (stomach and intestines) causing pain, hemorrhage, delayed wound healing, shock, and dark blood in the stool (melena). A significant number of dogs with MCT that are necropsied (autopsied) after death have ulcerations in the stomach.
This release of excess histamine is called the paraneoplastic syndrome. It is serious and needs to be addressed prior to and after treatment. This is why any skin growth needs to be handled gently in case it is a MCT and histamine gets released in an excessive amount.
When excess secretion of histamine occurs and only affects the skin it is sometimes known as Darier’s sign. You will notice raised and red areas of the skin when this happens.
In young animals the disease might spontaneously resolve. This is not something you should count on though. This cancer in dogs is unpredictable and can be extremely aggressive, so waiting to see if it resolves is not a good option. Every MCT should be treated as potentially malignant. MCTs can mimic other tumors, making it impossible to identify them simply by appearance.
As a general rule, benign MCT will be slow growing, well circumscribed (their margins are easy to feel), non-painful, not attached to the underlying tissue, and exhibit minimal inflammation. They might be irritated if your dog or cat is licking at them.
Malignant tumors tend to grow more rapidly, might be ulcerated or painful, have poor margins to determine where they begin and end, are fixed to the tissue below, and may have spread to nearby blood vessels and lymphatic tissue as evidenced by enlarged peripheral lymph nodes.
If the MCT spreads to the internal organs (called systemic mastocytosis) it usually goes to the liver, spleen, lymph nodes, or even back to the bone marrow.
In dogs most MCT are usually found on the trunk and perineum, sometimes on the limbs, and occasionally on the head or neck. MCT of the perineal and inguinal regions tend to behave more aggressively than their tumor grade would predict. Some dogs get multiple tumors.
Mast cell tumors that are large (more than 4 cm), on the face, muzzle, and oral cavity tend to have a poor prognosis.
Mast cell tumors located in the perineal region (anus, scrotum and vulva) may be may be more difficult to remove completely. They might need additional surgery or radiation and chemotherapy.
Dogs that have signs of gastrointestinal ulcers (vomiting, poor appetite, lethargy, or black stools) generally have a poor prognosis. Tumors that recur soon after surgery have a poor prognosis.
You cannot tell definitively whether any skin inflammation is a tumor or something else, and if it is a tumor, whether it is benign or malignant, just by looking at it or feeling it. Microscopic analysis by a veterinary pathologist is the only way to confirm this diagnosis. When we take a tiny sample of cells with a needle (called a fine needle aspirate- FNA) we call this cytology. When we take a large piece of tumor, or even the whole tumor, we call this histopathology.
There are several factors that contribute to this problem that include genetics, hormonal, environmental, and immune influences. There is suggestion that MCT are related to red or golden hair coats, allergies, or viruses. None of this has been proven at this time.
As is the case with many cancers it is a genetic mutation that is the originating factor. A tumor suppressant gene (called p53) can mutate and is suspected to cause MCT in 50% of the canine cases. A gene named C-Kit can mutate, leading to excess production of a protein called Kit, which can cause excessive stimulation of mast cells.
MCT’s occur in both males and females, typically around 8 years of age or older, although they have been seen in dogs much younger.
This is one of the few diseases that might occur in mixed breed dogs as much as purebred dogs. Certain breeds have a higher incidence though:
In cats it is the Siamese breed that has a higher incidence. There is no difference in occurrence rates between males and females.
The most consistent symptom of pets with MCT is a growth somewhere on the skin when grooming or petting. Multiple growths are possible, but in most cases it is one skin growth that is found. It might be tiny and barely inflamed, or it might be large and firm. Some pets will be oblivious to the tumor, while others might lick or chew at it. Many MCT wax and wane, which can make pet owner’s think the growth is not important. This waxing and waning can go on for months.
Other symptoms depend on whether heparin or histamine is being released (the paraneoplastic syndrome). In this case a pet can be vomiting, have a bleeding disorder (coagulopathy), have stomach ulcers, blood in the stool (melena), poor appetite (anorexia) or no appetite, abdominal pain, or literally be in shock. Some symptoms will look as if your pet is having an allergic reaction and require immediate veterinary care. In pets that have spread of MCT to other parts of the body there might also be weight loss.
The following pictures are all MCT’s. As you will see they can vary considerably, so just looking at a skin tumor will not give you a diagnosis. Basal cell tumors, histiocytomas, soft tissue sacromas, and lipomas can all look like mast cells at times.
This large mast cell is on the back of a dog’s leg seen as it is being prepped for surgery
A large MCT on a dog’s upper eyelid. To help you get your orientation this is the left eye and the dog is facing to the left.
The black arrow points to an almost imperceptible area that is slightly red and raised. This is a MCT
A complete physical exam is performed on every pet brought to us for a skin growth. There are a multitude of reasons why check other body organs and not just the skin growth you noticed at home:
So we do not miss other skin growths that might also be present and that are small or hard to detect. These might be other MCT, lipomas, or squamous cell carcinomas, for example.
To ascertain any spread of tumor to lymph nodes. We check the following peripheral lymph nodes during our exam:
For more information on where these lymph nodes are located and their role in the immune system follow this link. Our doctors will gladly show you how to palpate them for an in-home exam.
Care must be taken not to palpate a suspected tumor too vigorously because if it is a MCT it can degranulate and release histamine. At the least it might cause the skin growth to become more inflamed (Darier’s sign), and in the worst case it will cause your pet to go into an allergic reaction and shock if excess histamine is released from the MCT. This is a rare occurrence but something to keep in mind as you touch any specific growth on your pet’s skin.
Routine blood panel, urinalysis, and fecal exam are needed in every case prior to any treatment. These tests give us a detailed picture of your pets overall health status and look for other problems that are not obvious during the physical exam. Finding other problems in pets that present for just a skin growth is a situation we encounter more often than you might think. Heartworm and FeLV/FIV status needs to be know also.
The results of a normal blood panel on a dog that prior to mast cell tumor removal. Sometimes we will see anemia due to blood loss from an ulcer in the GI tract. We might also see an increase in liver enzymes if the MCT has spread to the liver, and also an increased BUN (Blood Urea Nitrogen) if there is GI bleeding.
We take special pre-anesthetic precautions on older dogs undergoing mast cell surgery. One of those precautions is an EKG to make sure the heart is ready for anesthesia. This is the same dog as the blood panel above, and is a normal EKG.
A very important test to diagnose any skin tumor is called a final needle aspirate (FNA). In this test we take a tiny needle and syringe and obtain some cells from the mass. The test is easy to perform, feels like a pinprick, and does not require anesthesia. It is similar to giving a vaccine, but in this case we are not giving something, we are taking something.
The small number of cells obtained from an FNA are put on a microscope slide and sent to our pathologist. They know why we are sending the sample to them and they specifically look for any signs of a benign or malignant tumor. Due to the high incidence of MST they look for these cells specifically. Pathologists are experts at analyzing cells under a microscope and they will come to one of the following conclusions:
This is what a pathologist is looking for under the microscope for a FNA. The arrow points to a clump of 3 mast cells that have been stained to stand out.
Even though no guarantee can be given to the usefulness of the FNA, it is worthwhile in almost all cases because the potential to give us useful information is high, especially when you consider the ease of obtaining the sample.
Knowing whether we are dealing with a MCT ahead of surgery is important because these tumors need aggressive surgery to be certain we remove the complete tumor. If we know going into surgery that we have a MCT we will take at least 3 cm margins around the visible mass. We will also dissect down to a deeper fascial plane to make sure no tumor remains after surgery. This type of aggressive approach is usually not needed in other skin tumor surgeries.
A pre-surgical radiograph is indicated in many cases to look at the thorax and abdomen looking for any spread of the MCT from the skin to the internal organs (systemic mastocytosis). In addition, a radiograph is indicated any time we anticipate anesthesia and surgery.
We might seen an enlarged sternal lymph node in the chest if the MCT has spread. The white arrow points to the location where the sternal lymph node resides.
This crescent shaped organ under the black arrow is a big liver, called hepatomegaly, on a radiograph. A MCT is one of several causes to an enlarged liver.
If an abnormality is found on the physical exam, or any diagnostic test, an ultrasound might also be needed. If we see an enlarged liver or spleen prior to surgery we will perform an ultrasound. This is what a spleen looks like during ultrasound
Here is the report on the normal appearing dog spleen above. There is no sign the MCT has spread to the liver or spleen.
Cellular slides consist of a heterogenous lymphoid population and a few scattered aggregates of stromal connective tissue/splenic trabeculae against a hemodiluted background. Small mature lymphocytes predominate followed by a low number of intermediate lymphocytes and large lymphoblasts. A few metarubricytes, mature plasma cells, and macrophages noted. Nor organisms or atypical cells seen.
This is an enlarged spleen with nodules that could be caused by a MCT, although there are other causes to this also
If a regional lymph node is enlarged or draining we will perform an aspirate to determine if the tumor has spread. We can also aspirate the bone marrow looking for mast cells. Information from all these tests is used as part of the surgical plan. It is also used to determine prognosis after surgery once the final tissue sample has been analyzed. If it looks like there has been the spread of tumor to an internal organ then chemotherapy or radiation therapy might be indicated in addition to surgical removal of the mass.