Mast Cell Tumors in Dogs

Mast cell tumors (MCT) in dogs are very common, accounting for approximately 20% of all skin tumors diagnosed in dogs. Mast cell tumors can arise from any skin site on the body, and can have a variety of appearances.  We do not know what causes mast cell tumors in dogs. At this time, no carcinogenic agents or viral organisms have been found to directly cause MCT.

Symptoms of a Mast Cell Tumor

MCTs most commonly are seen as solitary lumps or masses in or underneath the skin; however, dogs can have multiple masses simultaneously.  Some dogs may have signs of systemic disease (vomiting, diarrhea, weight loss), which can be caused by the release of biologically active compounds found within mast cells.

Biological Behavior of Mast Cell Tumors

Tumor grade is considered the most reliable indicator of prognosis. Other criteria such as tumor location, presence of clinical signs, tumor stage (metastatic disease at the time of diagnosis), mitotic index, c-KIT status, and other indicators that your MedVet doctor will explain to you have also been reported to have predictive value.  It is important to note that cytology samples are often diagnostic for mast cell tumors, but a tissue biopsy is required to determine tumor grade and several other variables.

Clinical Staging

A needle aspirate sample will frequently provide the initial diagnosis of a mast cell tumor.  In addition to establishing a diagnosis, several other tests (abdominal ultrasound, regional lymph node aspirate, chest radiographs) are recommended prior to treatment.  These tests are used to establish the stage of the disease (i.e. how advanced the cancer is) and to more accurately determine the individual prognosis.  Additionally, blood tests, chest radiographs (X-rays) and urinalysis are used to assess the overall health of the pet.

Treatment Options

The recommended treatment plan for pets diagnosed with mast cell tumors is dependent upon clinical stage, tumor grade, and c-KIT analysis.

• Grade I mast cell tumors grow slowly and have a low risk of metastasis. The treatment of choice for grade I mast cell tumors is wide surgical removal. In cases where complete surgical excision is not possible, a combination of radiation and marginal surgical removal or radiation therapy alone are the next best options. Chemotherapy is generally unnecessary unless metastatic or tumors in multiple locations are disease present.
• Grade II mast cell tumors are locally invasive and may spread to other sites in the body.  The treatment of choice for a solitary grade II mast cell tumor is wide surgical excision.  When wide surgical excision is not a viable option, the next best treatment is a combination of marginal surgery and radiation therapy. Chemotherapy is generally utilized in patients with metastatic/ multi-centric disease or when other treatments (surgery/ radiation) are not viable options.
• Grade III (high grade) mast cell tumors are highly malignant. They are locally invasive and quickly metastasize to distant sites in the body.  Even with aggressive local therapy (wide surgical excision or radiation therapy), it is difficult to control these tumors long term. Because of the high metastatic potential, chemotherapy is recommended for dogs with grade III mast cell tumors. Most protocols utilize a variety of drugs to maximize effect. In addition to the chemotherapy agents, supportive measures such as antacids and antihistamines are recommended during the course of therapy.

Prognosis Notes

The prognosis for completely removed solitary grade I and grade II mast cell tumors is very good. The prognosis for incompletely removed grade I and II tumors treated with post operative definitive radiation therapy is also good, and approximately 90­95% of dogs have no evidence of local tumor recurrence within 3 years of radiation therapy. The prognosis for dogs with grade III mast cell tumors is guarded to poor as these tumors are difficult to control and the development of metastatic cancer is common.