Characterization of an undifferentiated malignancy as a mast cell tumor using mutation analysis in the proto-oncogene c-KIT

A 6.5-year-old female Boxer was euthanized and presented for necropsy follo wing rapid clinical decline concomitant with the development of numerous tu mor masses. The largest of these masses was in the same location as a mast cell tumor that had been previously removed from this dog. Gross examinatio n revealed the presence of nodules 5-200 nim in diameter throughout the bod y, including the lymph nodes. Histologic analysis showed an influx of round cells with no granules, leading to the provisional diagnosis of systemic l ymphosarcoma. Immunohistochemical staining for B- and T-lymphocyte antigens was negative. Molecular tests were used to identify a tandem duplication i n the c-KIT proto-oncogene from both the earlier mast cell tumor and the cu rrent nodules, implicating a common origin. Addition of molecular testing t o conventional necropsy evaluations allowed a definitive diagnosis of mast cell tumors.