Mast cell tumors (MCTs) are the most common skin tumors in the dog, representing 16–21% of all skin tumors. They have a wide range of appearance and behavior, which can make these tumors challenging to diagnose and treat. Mast cell tumors are seen predominantly in middle-aged dogs, but have been recorded in ages ranging from 3 weeks to 19 years. It appears that the Boxer, Labrador Retrievers, Golden Retriever, Weimaraner and Miniature Schnauzer are more commonly affected than other breeds. There is no sex predilection reported.
Mast cell tumors are known as "the great pretenders" as they can mimic many other lesions. Classically the well differentiated mast cell tumor will appear as a raised solitary hairless lesion, whilst the poorly developed tumor may be ulcerated, bleeding and swollen, with satellite lesions surrounding the primary mass. Acral lick dermatitis ("lick granuloma") in dogs is often thought to have a behavioral etiology. However, other diseases may cause lesions on the dog's legs, mimicking acral lick dermatitis including lymphoma, deep pus-forming skin infection (pyoderma), mast cell tumor, protozoan parasitic infections (leishmaniasis), and sporotrichosis. Mast cell tumors may appear anywhere on the body surface as well as in internal organs, but the limbs (especially the back of the upper thigh) lower abdomen, and chest are the most common sites.
Tumors located on the mucous membranes are more likely to spread than mast cell tumors in the other areas. These tumors vary greatly in size and rate of growth. Well differentiated tumors show distinct boundary between the tumor and normal tissues. If a lesion seems to be appearing "from nowhere", a mast cell tumor should be suspected. The released histamine may also have may cause vomiting and gastrointestinal tract ulceration. Some dogs will have with vomiting as their primary sign of the disease. The majority of well differentiated tumors will not spread to other areas of the body and if treated properly can be cured. More than 75% of poorly differentiated tumors will spread to other organs such as lymph nodes, spleen, liver, bone marrow and lung (rare). Mast cell tumors of the internal organs and bone marrow are usually fatal.
The treatment of choice to control the local tumor is surgery, especially in the case of well differentiated tumors, which can be curative in about 77% of cases. Chemotherapy is much less effective in treatment of both well- and poorly differentiated mast cell tumors. None of the drugs used so far has shown compelling evidence of increased survival rates, although some of the drugs are investigated for control of macroscopic metastizes. Corticosteroids are used to control the inflammation and make the dog more comfortable. H-2 blockers are sometimes used as adjunctive therapy to protect the gut against ulceration.
Development of cancer (carcinogenesis) is a multistep process that converts normal cells into malignant cells. Once transformed, malignant cells acquire the ability to invade and metastasize, leading to clinically evident disease. During this process from normal to metastatic cells, carcinogenic steps can be arrested or reversed through pharmacological treatments, known as cancer chemoprevention. Chemoprevention strategies represent therapeutic interventions at early stages of carcinogenesis, before the onset of invasive cancer. Retinoids are natural and synthetic derivatives of vitamin A (retinol). They have diverse structures, pharmacological profiles, biologic activities and specific toxicities. There is some evidence that retinoids may be highly effective adjunctive chemotherapeutic agent for the treatment of canine MCT at early stages. Animals with rapidly growing tumors and those having vomiting, loss of appetite, widespread swelling and reddness have a poorer prognosis. Those with tumors growing over months and even years have a much better prognosis.
- Mast cell tumours. Sue Murphy.
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- The Retinoids and Cancer Prevention Mechanisms. Konstantin H. Dragneva, James R. Rigasa, Ethan Dmitrovskya. The Oncologist, Vol. 5, No. 5, 361-368, October 2000