Round and Mast cell tumors in dogs
Mast cells are cells that occur in the skin and other tissues, like the intestines and respiratory tract. They are also an integral part of the immune system. They consist of large amounts of histamine, heparin, and proteolytic enzymes (enzymes which break down protein). These have a toxic effect on foreign invaders, like parasites, and are released when the mast cell is triggered by the immune system. A mast cell tumor results from these mast cells. When mast cells are damaged, they pose health hazards. Large amounts of histamine, heparin, and enzymes are released into the body that usually have adverse effects on heart rate, blood pressure, and other body functions. Sites where the tumors are removed can sometimes refuse to heal and can become unmanageable. Normal mast cells are found in plenty in the lung and gastrointestinal tract. But tumors generally don’t originate in these sites. They rather develop in the dermis and sub-cutaneous tissues. They are most frequently found on trunks. The limbs account for generally ¼ the of the sites of these lesions. Mast cell tumors have been reported to develop infrequently in other sites like head, neck, conjunctiva, salivary gland, nasopharynx, larynx, oral cavity, gastrointestinal tract, ureter and spine.
Canine Mast Cell Tumors
Mast cell tumors (MCT) are cancerous proliferations of mast cells. Although they can and will spread throughout the body, the danger from mast cell tumors arises from the secondary damage caused by the release of chemicals that they produce. These chemicals can cause systemic problems that include gastric ulcers, internal bleeding, and a range of allergic manifestations. Clearly, mast cell tumors affect both lifespan and quality of life. Sometimes mast cell tumors are referred to as "the great imposters," as there is no way to definitively identify them without a biopsy and pathology report. Mast cell tumors vary widely in their size, shape, appearance, texture, and location. It can be difficult not only to recognize mast cell tumors but to predict their course. They may be relatively innocent or aggressively malignant. As mast cell tumors are very common in dogs, it is important for the regular pet owner to have at least a basic understanding of what they are and how they work.
Mast cells are specialized cells that normally are found distributed throughout the body and help an animal respond to inflammation and allergies. Mast cells can release several biologically active chemicals when stimulated, among them histamine, heparin, seratonin, prostaglandins and proteolytic enzymes. Although these chemicals are vital to normal bodily function, especially immune response, they can be very damaging to the body when released in chronic excess.
Other names: Histiocytic mastocytoma, mast cell sarcoma, mastocystosis (when there is systemic involvement).
Frequency/Location: Mast cell tumors are among the most common tumors in dogs and are the most common type of skin cancer found in dogs. Approximately 1/3 of all tumors in dogs are skin tumors, and up to 20% of those are mast cell tumors. The most common location to find mast cell tumors is, by far, the skin, followed by the spleen, liver, and bone marrow. Approximately half of all cutaneous (skin) MCT�s are found on the body proper, another 40% on the extremities (most frequently the hind limbs), and the remainder on the head or neck. Approximately 11% occur in more than one location.
Causes/Predispositions: No one fully understands what causes cancer. Mast cell tumors are very common in dogs, yet they occur far less frequently in cats and very rarely in human beings. They occur in dogs of all breeds, ages, and genders and can occur anywhere on the body. There appears to be a genetic component, as certain breeds are predisposed to developing MCT. Among the most common victims are beagles, Boston terriers, boxers, bulldogs, bullmastiffs, bull terriers, dachshunds, English setters, fox terriers, golden retrievers, Labrador retrievers, schnauzers, American staffordshire terriers, and weimaraners. Boxers are at the highest risk, yet mast cell tumors are often not as aggressive in this breed. There is some suggestion that mast cell tumor development may be associated with golden/red coat color and with chronic immune over-stimulation that occurs in dogs with allergies or other inflammatory conditions. There may be environmental factors, viruses, or other undetermined contributors. Mast cell tumors, as with all cancers, tend to be associated with age. Older dogs are more likely to develop cancerous growths, with the average age of a dog with MCT being 8-9 years.
Local symptoms: The most obvious sign of mast cell cancer is likely to be a tumor of some sort. Mast cell tumors can appear singly, in groups, lie on the surface of the skin or underneath it, crop up anywhere on the body, and defy easy description. You just don�t know it�s a MCT by looking at it. Most (at least half of) mast cell tumors are found in or under the skin on the trunk of the body itself, and the vast majority of the remainder are found on the extremities, especially the hind limbs. They are less commonly found on the head and neck, and less commonly still arise from tissues other than the skin. If they are very swollen or ulcerated, there may be pain, but most MCT�s are unlikely to be painful. It has been observed that higher-grade tumors may be more likely to be ulcerated in appearance and cause local irritation.
One characteristic quirk of mast cell tumors is the tendency for them to change in size, even on a daily basis. A tumor that gets bigger and smaller, seemingly on a whim, may be a MCT. Another idiosyncrasy is the potential of the tumor to produce "Darier�s sign" if poked and prodded. Handling these tumors - even a routine veterinary palpation or needle aspirate - can cause a heavy release of histamine that results in swelling, redness, itchiness, hives (wheal formation).
Systemic symptoms: Symptoms are variable, depending on the location of the tumor and the degree to which is has developed and/or spread. Signs of systemic involvement may include: loss of appetite, vomiting, bloody vomit, diarrhea, abdominal pain, dark or black feces, itchiness, lethargy, anorexia, irregular heart rhythm and blood pressure, coughing, labored breathing, various bleeding disorders, delayed wound healing, enlarged lymph nodes.
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Manipulation of the tumor may result in the release of histamines from the tumor due to the mast cells releasing from the tumor into the blood stream. Antihistamines may be prescribed to alleviate some of the symptoms related to this effect. This same behavior can come into play as a result of surgical intervention; antihistamines will be used under the circumstances, as a large release of histamines on the body can have a drastic effect on the organs.
Aggressive surgical removal of the mast cell tumor and surrounding tissue is generally the treatment of choice. A microscopic evaluation of the surgically removed tissue is essential for determining the success of the surgical removal and for predicting the biological behavior of the tumor; if tumor cells extend too close to the surgical margins, your veterinarian will need to perform a more aggressive surgery as soon as possible. In case of lymph-node involvement with no generalized involvement in other parts of the body, aggressive surgical removal of the affected lymph node(s) and the primary tumor will be required; follow-up chemotherapy is useful for the prevention of further metastasis of tumor cells.
Mast cells are cells that reside in the connective tissues, especially those vessels and nerves that are closest to the external surfaces (e.g., skin, lungs, nose, mouth). Their primary functions include defense against parasitic infestations, tissue repair, and the formation of new blood vessels (angiogenesis). They are also associated with allergic reactions, since they contain several types of dark granules made up of various chemicals, including histamine and heparin, serving biologically to modify immune reactions and inflammation. Mast cells are derived from the bone marrow, and can be found in various tissues throughout the body.
Mast cell tumors (or mastocytomas) are graded according to their location in the skin, presence of inflammation, and how well they are differentiated. Grade 1 cells are well differentiated with a low potential for metastasis; Grade 2 cells are intermediately differentiated with a potential for locally invasive metastasis; and Grade 3 cells are poorly differentiated or undifferentiated with a high potential for metastasis. Differentiation is a determination of how much a particular tumor cell looks like a normal cell; the more differentiated, the more like the normal cell. In general, the more differentiated the mast cell tumor is, the better the prognosisis.
Boxers, bulldogs, pugs, and Boston terriers appear to be more susceptible to mast cell tumors than other breeds. Also at higher than average risk are the Shar pei, Labrador Retriever, Golden Retriever, Schnauzer, and Cocker Spaniel. The mean age for the development of this condition is eight years in dogs, though it has been reported in animals less than one year of age.
- Tumor on the skin or under the skin (subcutaneous), may have been present for days to months
- Tumor may appear to fluctuate in size
- Recent rapid growth after months of inactive or subtle growth is common
- Recent onset of redness and fluid build-up is most common with high-grade skin and subcutaneous tumors
- Extremely variable; may mimic or resemble other types of skin or subcutaneous tumors (benign and cancer); may resemble an insect bite, wart, or allergic reaction
- Primarily occurs as a single skin mass or subcutaneous mass, but may have multiple masses located throughout the body
- Approximately 50 percent of all mast cell tumors are located on the trunk and perineum (the area between the anus and vulva in females, or the anus and scrotum in males); 40 percent are found on the extremities, such as the paw; and 10 percent are found on the head and neck region
- Lymph nodes may be enlarged around the area of the tumor and may develop when a high-grade tumor spreads to the lymph nodes
- Masses may be itchy or inflamed due to the higher level of histamines in the tumor
- Enlarged liver and enlarged spleen are characteristic of wide-spread mast cell cancer
- Vomiting, loss of appetite, and/or diarrhea may occur, depending on the stage of the disease
- Stage 1 is characterized by a single tumor without metastasis
- Stage 2 is characterized by a single tumor with metastasis into the surrounding lymph nodes
- Stage 3 is characterized by multiple skin tumors, or by a large tumor that has invaded subcutaneously
- Stage 4 is characterized by the presence of a tumor, with metastasis to an organ or wide spread mast cell presence in the blood
You will need to give a thorough history of your dog's health, including a background history of symptoms. The history you provide may give your veterinarian clues as to which organs are being affected.
The most important preliminary diagnostic test will be an examination of the cells taken from one of the tumors. This will be performed with a fine needle aspirate and will determine the presence of an abnormal amount of mast cells in the blood. A surgical tissue biopsy will be necessary for definitive identification of both the grade of the cells occupying the mass, and the stage the disease is in. Additionally, your veterinarian may examine a sample from a draining lymph node, from the bone marrow, or from the kidney and spleen. X-ray and ultrasound images of the chest and abdomen will also be component of determining the exact location and stage of the tumor's development.
A basic blood panel is part of this evaluation process and should be obtained at this point if it has not already been obtained. This testing will help show any factors that limit kidney or liver function and thus determine what drugs of chemotherapy can or cannot be used. It also will show if there are circulating mast cells in the blood (a very bad sign) or if anemia (low red blood cell count) is present which might be related to the tumor.
The mast cell tumor releases histamine-containing granules that
lead to inflammation and increased stomach acid secretion.
These unpleasant symptoms may be alleviated with the use of H1 blockers (antihistamines such as Benadryl® and others) as well as
H2 blockers (antihistamines such as Pepcid AC® or Tagamet, and others).
These OTC store medications help palliate the inflammatory effects
of the spreading malignant mast cell tumor. But you also need to address the cancer.
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