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Case of the Month




March, 2021

75 year old female who presents with a history of indolent mastocytosis involving the bone marrow (diagnosed in 2004 per review of medical records), which has not required any treatment. Physical exam revealed no skin lesions or organomegaly. Tryptase level was 145. Radiological studies show possible skeletal involvement. Peripheral blood smears show no evidence of cytopenias or cytosis. Mast cells are not seen on peripheral blood smears. Bone marrow aspirate reveals increased mast cells (~6% by manual differential), which appear mature and spindle-shaped. Core biopsy demonstrates numerous, multifocal dense clusters of spindled mast cells (>30% marrow involvement) positive for CD117. Flow cytometry reveals a minute abnormal mast cell population (~0.05% of total cells) expressing CD117, CD25 and CD2 (partial). c-KIT mutation molecular analysis performed on core biopsy paraffin block detected a low level mutation at codon D816V exon 17. Overall, these findings are consistent with systemic mastocytosis. There is no evidence of mast cell leukemia or an associated hematologic neoplasm (i.e. MDS, MPN).

The diagnosis of systemic mastocytosis is made when the major criterion (multifocal dense aggregates of mast cells > or = 15 mast cells are detected in bone marrow core biopsy or organ) and at least 1 minor criterion are present, or when at least 3 minor criteria are met. Minor criteria includes: >25% mast cells in infiltrate with spindle-shaped/atypical morphology or <25% of all mast cells in aspirate smears appear immature or atypical, detection of point mutation at codon 816 of KIT, mast cells expressing CD25 with or without CD2 and serum total tryptase level greater than 20 ng/mL. There are five different variants of systemic mastocytosis and the prognosis depends on the variant the patient has. The five different variants include indolent, smoldering, aggressive, mast cell leukemia and systemic mastocytosis with an associated hematologic neoplasm.