Mast cell sarcoma with tissue eosinophilia arising in the ascending colon

Mast cell sarcoma is a rare disease. We report an unusual case of this neop lasm arising in the ascending colon of a 32-year-old Japanese woman who pre sented with abdominal pain. An ulcerating mass in the colon was resected, a long with enlarged mesenteric lymph nodes. Two years after surgery, the neo plasm recurred as left cervical lymphadenopathy and an intra-abdominal mass , Despite predonine and radiation therapy, the disease progressed, and the patient died. The tumor cells had abundant fine granular or clear cytoplasm , and oval, lobulated, or indented nuclei, Numerous mature eosinophils were intermingled with the tumor cells. Immunohistologic studies on paraffin se ctions demonstrated that the majority of the tumor cells were strongly posi tive for CD45RB, CD68, and mast cell tryptase. They were unreactive, howeve r, with a broad spectrum of antibodies against myelomonocytic and lymphocyt ic antigens. The mast cell nature of this rare type of tumor can be best id entifiable by immunostains for mast cell tryptase.