Mast Cell Tumors in Dogs: Grading

One of the strongest predictors of behavior of mast cell tumors is the grade of the tumor.  There Mast Cell Tumors in Dogs: Gradingare 3 grades of mast cell tumors, defined naturally as grade 1, grade 2 or grade 3 depending on a number of factors. The grade of the tumor can only be determined by removal of tissue and evaluation by a veterinary pathologist.

Grade 1 mast cell tumors in dogs

Grade 1 mast cell tumors are typically raised skin masses that are often hairless and confined to the superficial layer of skin.  They tend to be slow-growing solitary lesions (but can be multiple in number) that may be present for months.  They are usually cured by surgery if an area of normal skin tissue is removed around the tumor as well.  Although it is commonly recommended that mast cell tumors be removed with wide margins of 2-3 cm around the tumor itself, grade 1 mast cell tumors can often be removed completely with more narrow margins.  However, it is always best to remove a mast cell tumor with as much normal tissue as possible in the 2-3 cm range to increase the likelihood that the tumor is completely removed.

Grade 2 mast cell tumors in dogs

Grade 2 mast cell tumors are locally invasive tumors that have the potential to spread to other parts of the body.  The risk of spread increases if the tumor is not treated early in the course of its development or if initial treatment is incomplete and the tumor recurs. Grade 2 mast cell tumors fall in to the category between grade 1 and grade 3 mast cell tumors and will trend more towards one of those grades.  Low grade 2 mast cell tumors tend to have a mitotic index of < 5 per high-powered field, where as high grade 2 mast cell tumors tend to have a mitotic index of > 5 per high-powered field.  Mitotic index is defined as the total number of dividing cells observed in ten high powered viewing areas with the microscope.  It is important to try and get a sense of whether a grade 2 mast cell tumor is trending more towards the grade 1 or grade 3 side of the scale, as the potential behavior as well as treatment recommendations and prognosis varies greatly between the two.

Grade 3 mast cell tumors in dogs

Grade 3 mast cell tumors are the most aggressive type of mast cell tumors.  They often appear suddenly, grow rapidly and can be quite large in size.  Ulceration of the skin surface can often occur along with swelling and discomfort, however this is not always the case.  Grade 3 mast cell tumors are considered to be poorly differentiated and have a high likelihood of spreading to other parts of the body, most commonly the regional lymph node.  These types of mast cell tumors are rarely cured with surgery and systemic chemotherapy is commonly advised to help slow the progression of the disease.

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Dr Stephen Atwater

Stephen W. Atwater, DVM, MS, Diplomate ACVIM (oncology) is a board-certified veterinary oncology specialist. His professional interests include utilizing emerging therapies for difficult to treat cancers.

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