Systemic mastocytosis: progressive evolution of an occult disease into fatal mast cell leukemia: unique findings on an unusual hematological neoplasm.

Systemic mastocytosis (SM) may be associated with a clonal hematopoietic non-mast cell-lineage disease (AHNMD). SM and AHNMD even may be clonally related. This report contributes to a better understanding of the different morphological aspects of SM by demonstrating that various AHNMDs can be detected in one patient during the course of disease. Routinely processed biopsy specimens of bone marrow and spleen removed from a 63-year-old man were investigated including a broad panel of immunohistochemical stainings. KIT codon 816 mutation analysis was carried out by melting point analysis of nested PCR products amplified from DNA of pooled microdissected mast cells. The histomorphological features of the initial bone marrow showed diffuse infiltration by hairy cell leukemia (HCL). Occult SM was only detected retrospectively by demonstration of a slight diffuse increase in loosely scattered, spindle-shaped mast cells carrying the activating point mutation KIT ( D816V ). In the second bone marrow, core biopsy removed about two years later HCL had been completely eradicated, while a diagnosis of SM-AHNMD with multifocal compact mast cell infiltrates associated with a myeloproliferative neoplasm (MPN) and significant increase in eosinophilic granulocytes was established. The third and last bone marrow biopsy specimen lacked the features of both MPN and HCL but showed progression into a secondary mast cell leukemia (MCL) with a focal sarcomatous component. To the best of the authors' knowledge, this is the first description of a case of SM-AHNMD with coexisting hematological neoplasms of lymphatic and myeloid origin initially presenting as occult disease and terminating as secondary MCL.