Coexistence of Multiple Myeloma, Mast Cell Hyperplasia, and Low-Level Myeloid Blastocytosis: A Report of a Rare Case

BACKGROUND Mast cell hyperplasia can present as systemic mastocytosis (SM) or reactive mast cell hyperplasia. Distinguishing between these 2 diseases is a critical challenge. Here, we report a rare case of concurrent mast cell hyperplasia, multiple myeloma (MM), and low-level myeloid blastocytosis. The potential mechanisms underlying the coexistence of the aforementioned 3 diseases were also analyzed. CASE REPORT A patient with muscle pain was initially diagnosed with monoclonal gammopathy of undetermined significance (MGUS). Three years later, she progressively developed pancytopenia and renal function impairment. Three populations of abnormal cells were found in the bone marrow (myeloma cells 10.5%, myeloid blasts 0.18%, and mast cells 10%). Bone marrow biopsy (BMB) showed a single focal mast cell aggregate (>15 cells) with no multifocal dense infiltrates. Tryptase testing and KIT D816V mutation analysis results were negative. CD25 and additional KIT gene sequencing were not performed. Finally, she was diagnosed with multiple myeloma (MM), mast cell hyperplasia, and low-level myeloid blastocytosis. CONCLUSIONS This association among MM, mast cell hyperplasia, and low-level myeloid blastocytosis could either arise from abnormal stem cells of a common origin or result from one disease indirectly inducing or accelerating the progression of the other. Comprehensive immunophenotyping (including CD117, CD2, CD30, and CD25) and KIT mutation analysis are necessary to differentiate reactive mast cell hyperplasia from both systemic mastocytosis (SM) and SM with an associated hematologic neoplasm (SM-AHN). Positive expression of CD2, CD25, or CD30 together with KIT mutation may indicate SM or SM-AHN, and additional diagnostic criteria are required to confirm the diagnosis. If all the above test results are negative, reactive mast cell hyperplasia can be considered. More mechanistic evidence and similar cases are needed to better understand complex multilineage dysplasia.