Mast cell sarcoma: a rare and potentially under-recognized diagnostic entity with specific therapeutic implications

Mast cell sarcoma is a rare, aggressive neoplasm composed of cytologically malignant mast cells presenting as a solitary mass. Previous descriptions of mast cell sarcoma have been limited to single case reports, and the pathologic features of this entity are not well known. Here, we report three new cases of mast cell sarcoma and review previously reported cases. Mast cell sarcoma has a characteristic morphology of medium-sized to large epithelioid cells, including bizarre multinucleated cells, and does not closely resemble either normal mast cells or the spindle cells of systemic mastocytosis. One of our three cases arose in a patient with a remote history of infantile cutaneous mastocytosis, an association also noted in one previous case report. None of our three cases were correctly diagnosed as mast cell neoplasms on initial pathological evaluation, suggesting that this entity may be under-recognized. Molecular testing of mast cell sarcoma has not thus far detected the imatinib-resistant KIT D816V mutation, suggesting that recognition of these cases may facilitate specific targeted therapy.