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Canine Mast Cell Tumors

Introduction
Mast cell tumors, also called mastocytomas, are the most common skin cancer in dogs.(1)  Mastocytomas develop most often in dogs seven and a half to nine years of age, but can occasionally be found in dogs as young as four to six months.(2)  Different breeds also have different rates of mast cell tumors.  Boxers and Boston terriers have the highest rates.(2)

Mast cell tumors originate from mast cells, immune system cells found in many tissues of the body.(3)   Because mast cells can be found almost anywhere, mastocytomas have the potential to appear in all different regions of the body.  They are most commonly located on the skin on hind legs, the region between the neck and abdomen (thorax), or genital regions.(4)    Mast cells contain a variety of biologically active substances, including histamine, heparin, serotonin, and prostaglandins.  These chemicals are released from the cells during an allergic reaction.(5)   These are the substances that cause the symptoms associated with allergies: redness, itching, swelling, blood pressure drops, tearing, nausea, wheezing, ect.  Normally these chemicals are highly regulated, but when mast cells become cancerous they no longer appropriately control the release of the chemicals. 

Risk Factors/Detection/Staging
The unregulated release of chemicals by a mastocytoma can cause a variety of symptoms.  The symptoms any particular dog shows is variable, but can include vomiting, ulcers, bloody excrement, abdominal pain, and blood-clotting difficulties; gastro-intestinal problems are the most common signs.(6) Mastocytomas appear as raised lumps that are often irritated or red in appearance.  Unfortunately, mast cell tumors have no characteristic form or color and are impossible to identify without laboratory testing.(1)   Often when a mastocytoma is touched the skin becomes red, itchy, and swollen.(7) (2)    This response is also called Darier's Sign.  It occurs because when the mast cells in the tumor are compressed they release the chemical histamine, and this causes irritation of the skin.

Although normal mast cells are typically fairly stationary, advanced mast cell tumors may spread (metastasize) to other places in the body.  Veterinarians classify (stage) mast cell tumors using a six tiered system (stages 0 to 5) based on the extent that the cancer has spread in the body.(1)  Staging takes into account the number and size of tumors, lymph node involvement, and recurrence rate.(1)  A higher stage indicates greater body involvement with Stage 5 representing distant metastasis including bone marrow or blood involvement.(2)  Another classification system, the histological grade (values from 1-3), is based on the physical appearance of the cells in the tumor.  More abnormal mast cells/tissues are associated with a higher histological grade and have a higher possibility of being malignant.(2)

Treatment
Surgical removal of the tumor is the most common treatment for mastocytomas of intermediate and lower stages.  Healthy tissue up to about one inch (2-3 centimeters) around the tumor must also be removed to prevent leaving any cancerous mast cells behind. For more advanced stages surgery is still recommended, but is often combined with radiation therapy or chemotherapy.(2)   If surgery is not thorough enough to remove all cells, external beam radiation therapy can be used to kill the remaining cells.  External beam radiation may also be used to reduce the size of a tumor before surgery.(2)

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Last Modified: 10/30/2012 Print Email Page
References for this page:
  1. Nash, DVM, MS, Holly. "Canine Mast Cell Tumors: A Common Cancer in Dogs." Dog, Cat, and Pet Care Tips, Health and Behavior Information by Veterinarians. Veterinary Services Department. Web. 2 June 2010. [http://www.peteducation.com/article.cfm?c=2+1638&aid=461]
  2. Welle MM, Bley CR, Howard J, Rufenacht S. Canine mast cell tumours: a review of the pathogenesis, clinical features, pathology and treatment. Veterinary Dermatology (2008) 19(6): 321-39. http://www3.interscience.wiley.com/cgi-bin/fulltext/121451889/HTMLSTART [http://www3.interscience.wiley.com/cgi-bin/fulltext/121451889/HTMLSTART ] [PUBMED]
  3. "Mast Cell." Encyclopædia Britannica. 2009. Encyclopædia Britannica Online. 2 June 2010 [http://www.britannica.com/EBchecked/topic/368641/mast-cell]
  4. "Cutaneous Mast Cell Tumors." The Merck Veterinary Manual. 2008. [http://www.merckvetmanual.com/mvm/index.jsp?cfile=htm/bc/72231.htm&word=canine%2cmast%2ccell]
  5. Dahm, Rebecca L., and Kenneth S. Latimer. "Mast Cell Disease in Dogs and Cats: An Overview." The UGA College of Veterinary Medicine. Web. 2 June 2010. [http://www.vet.uga.edu/VPP/clerk/Dahm/Index.php]
  6. Scott MA, Stockham SL: Basophils and mast cells. Schalm's Veterinary Hematology, 5th ed. Feldman BF, Zinkl JG, Jain NC (eds). Lippincott Williams & Wilkins, 2000, pp. 308-315. . [http://www.vet.uga.edu/VPP/clerk/Dahm/Index.php]
  7. Puttgen, Katherine MD. "Mastocytoma/Darier sign." Ed. Bernard MD A. Cohen and Christoph U. Lehmann. Dermatlas, Johns Hopkins University 2000-2009. Web. 2 June 2010. [http://dermatlas.med.jhmi.edu/derm/IndexDisplay.cfm?ImageID=-2031089971 ]
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