Mast cell tumors are "graded" as to how likely they are to be malignant. The higher the grade, the more serious the tumor.
Grade I: Occur in the skin and are considered benign. Although they may be large and difficult to remove, they tend to not spread to other areas of the body. Most mast cell tumors are Grade I.
Grade II: Extend below the skin into the subcutaneous tissues. Their cells show some characteristics of malignancy and their response to treatment can be unpredictable.
Grade III: Invade areas deep below the skin, are very aggressive, and require more involved treatment.