Mast Cell Tumors

by Francisco J. Alvarez-Berger DVM, DACVIM (Oncology)

Mast cell tumors are common skin tumors in dogs, and can develop in any breed; however some breeds such as the Boxer, Golden Retriever and Pug are predisposed. The tumor originates from the mast cells (one type of normal cell in the body that plays an important role in inflammatory responses).

Since Mast cell tumors can look like other skin problems, therefore a microscopic evaluation of any skin lesion is always necessary. In most cases, only a small sample of skin is necessary to obtain a diagnosis. This sample is obtained by an aspirate using only a needle, and no sedation is needed. In most cases the sample is graded in order to try to predict the tumor’s behavior. Therefore, a biopsy of skin tissue for pathology is needed in all cases. This is usually done surgically and requires anesthesia.

Mast cell tumors are classified (graded) as well, moderately and poorly differentiated. Well differentiated (grade 1) and the majority of the moderately differentiated (grade 2) mast cell tumors in general have a low ability to spread to other sites in the body (low metastatic potential). Approximately >90% of the well differentiated tumors and approximately 80% of the moderately differentiated are considered cured with a complete surgical removal and no additional treatment is necessary. However, approximately 20% of the moderately differentiated and almost all of the poorly differentiated have a higher metastatic potential, and can spread to regional and/or distant lymph nodes and other organs such as spleen, liver and bone marrow; being necessary in these cases the use of systemic chemotherapy.

All patients with mast cell tumors should have a complete clinical evaluation including a physical examination, complete blood cell count, blood chemistry profile, urinalysis, abdominal ultrasound and sometimes chest X-rays when involvement of the lymph nodes inside the chest is suspected.

As previously mentioned, treatment for mast cell tumors depends highly on the grading and extent of disease. Usually a complete surgical removal is curative in dogs with most grade 1 (>90%) and grade 2 (>80%) mast cell tumors. However when surgical removal is incomplete (evidence of tumor cells that extend to the margins of the surgical site), if possible, a second surgery is recommended in order to achieve clear margins. In some cases a second surgery is not possible (due to the size of the surgical incision or anatomic location) and the use of radiation therapy or chemotherapy is recommended.

When surgery is not an option to remove the tumor, chemotherapy is recommended (and in some cases radiation therapy) to reduce the size of the tumor. In dogs with high-grade (grade 3 and approximately 20% of grade 2) mast cell tumors, chemotherapy is the best treatment option. In general, chemotherapy provides very good results and most patients tolerate chemotherapy very well. In a few cases, some patients may develop nausea, vomiting or diarrhea. Some patients may develop low blood cell counts after the treatment; however is not common to have to hospitalize a patient due to chemotherapy complications. Every time before each treatment, a complete physical exam and blood tests are performed to assess if the patient is able to receive the medication. Most of pet owners that decide to pursue chemotherapy find that the quality of life for dogs with mast cell tumors that respond to chemotherapy is very good to excellent.