Pardanani, Animesh

ABSTRACT Overview Systemic mastocytosis (SM) results from clonal proliferation of mast cells (MC) in extra‐cutaneous organs. Diagnosis The major criterion is the presence of multifocal MC clusters in the bone marrow and/or extracutaneous organs. Minor diagnostic criteria include elevated serum tryptase level, MC CD25/CD2/CD30 expression, and the presence of activating KIT mutations. Risk Stratification Establishing SM subtype as per the International Consensus Classification/World Health Organization classification systems is an important first step. Patients either have indolent/smoldering SM (ISM/SSM) or advanced SM, including aggressive SM (ASM), SM with associated myeloid neoplasm (SM‐AMN), or mast cell leukemia. Identification of poor‐risk mutations (i.e., ASXL1, RUNX1, SRSF2, NRAS ) further refines the risk stratification. Several risk models are available to help assign prognosis in SM patients. Management Treatment goals for ISM patients are primarily directed towards anaphylaxis prevention/symptom control/osteoporosis treatment. Patients with advanced SM frequently need MC cytoreductive therapy to reverse disease‐related organ dysfunction. Tyrosine kinase inhibitors (TKI) (midostaurin, avapritinib) have changed the treatment landscape in SM. While deep biochemical, histological, and molecular responses have been documented with avapritinib treatment, its efficacy as monotherapy against a multimutated AMN disease component in SM‐AMN patients remains unclear. Cladribine continues to have a role for MC debulking, whereas interferon‐α has a diminishing role in the TKI era. Treatment of SM‐AMN primarily targets the AMN component, particularly if an aggressive disease such as acute leukemia is present. Allogeneic stem cell transplant has a role in such patients. Imatinib has a therapeutic role only in the rare patient with an imatinib‐sensitive KIT mutation.