In the current classification of the World Health Organization (WHO), bone marrow mastocytosis (BMM) is a provisional variant of indolent systemic mastocytosis (ISM) defined by bone marrow involvement and absence of skin lesions. However, no additional diagnostic criteria for BMM have been proposed. Within the registry dataset of the European Competence Network on Mastocytosis, we compared characteristics and outcomes of 390 patients with BMM and 1175 patients with typical ISM. BMM patients were significantly older, predominantly male, had lower tryptase and lower burden of neoplastic mast cells, and displayed a higher frequency of allergic reactions, mainly triggered by Hymenoptera, than patients with typical ISM. The estimated 10-year progression-free survival of BMM and typical ISM was 95.9% and 92.6%, respectively. In BMM patients defined by WHO-based criteria, the presence of one B-Finding and tryptase level ≥125 ng/mL were identified as risk factors for progression in multivariate analyses. BMM patients without any of these risk factors were found to have better progression-free survival (p < 0.05) and better overall survival (p < 0.05) than other ISM patients. These data support the proposal to define BMM as a separate SM variant characterized by SM criteria, absence of skin lesions, absence of B-Findings, and tryptase levels <125 ng/mL.

Allergy Unit Verona University Hospital Verona Italy

Biostatistical Service ASST of Mantova Mantua Italy

Department Hospital for Companion Animals and Horses University Hospital for Small Animals Internal Medicine Small Animals University of Veterinary Medicine Vienna Vienna Austria

Department of Allergology Medical University of Gdańsk Gdańsk Poland

Department of Allergology University Medical Center Groningen University of Groningen Groningen The Netherlands

Department of Dermatology and Allergy Biederstein Technical University of Munich Munich Germany

Department of Dermatology and Venereology Medical University of Graz Graz Austria

Department of Dermatology and Venerology Kepler University Hospital Johannes Kepler University Linz Austria

Department of Dermatology University of Cologne Cologne Germany

Department of Dermatology Venereology and Allergology Medical University of Gdańsk Gdańsk Poland

Department of Hematology and Internal Medicine Semmelweis University Budapest Hungary

Department of Hematology and Oncology Medical Center University of Schleswig Holstein Lübeck Germany

Department of Hematology University Medical Center Groningen University of Groningen Groningen The Netherlands

Department of Internal Medicine 1 Division of Hematology and Hemostaseology Medical University of Vienna Vienna Austria

Department of Internal Medicine 3 Hematology and Oncology Kepler University Hospital Johannes Kepler University Linz Austria

Department of Medical Sciences Uppsala University and Section of Hematology Uppsala University Hospital Uppsala Sweden

Department of Medicine 1 Medical Center University of Freiburg Faculty of Medicine University of Freiburg Germany and German Cancer Consortium Partner Site Freiburg Freiburg Germany

Department of Medicine Section of Hematology University of Verona Verona Italy

Department of Molecular Medicine University of Pavia Pavia Italy

Department of Oncology Haematology Haemostaseology and Stem Cell Transplantation University Hospital RWTH Aachen Aachen Germany

Division of Allergy and Clinical Immunology University of Salerno Salerno Italy

Division of Allergy Department of Dermatology and Department of Biomedicine University Hospital Basel and University of Basel Basel Switzerland

Division of Hematology Department of Medicine Stanford University School of Medicine Stanford Cancer Institute Stanford CA USA

Division of Hematology Fondazione IRCCS San Matteo Pavia Italy

Division of Hematology Istanbul Medical School University of Istanbul Istanbul Turkey

Faculty of Medicine and Health Sciences Department of Immunology Allergology Rheumatology University of Antwerp and Antwerp University Hospital Antwerp Belgium

Hämatologie und Onkologie 3 Medizinische Klinik Universitätsmedizin Mannheim Universität Heidelberg Mannheim Germany

Institut Imagine INSERM Unité 1163 and Centre National de la Recherche Scientifique ERL8654 Centre de Reference des Mastocytoses University of Paris Paris France

KU Leuven Department of Microbiology Immunology and Transplantation Allergy and Clinical Immunology Research Group and MASTeL University Hospitals Leuven Leuven Belgium

Laboratory of Hematology Pitié Salpêtrière Hospital Paris France

Ludwig Boltzmann Institute for Hematology and Oncology Medical University of Vienna Vienna Austria

Medical Clinic and Policlinic 1 Hematology and Cellular Therapy Leipzig University Hospital Leipzig Germany

Pediatric Dermatology Unit Department of Medicine University of Padova Padova Italy

University Hospital Department of Hematology and Oncology and Department of Medical Genetics and Genomics Faculty of Medicine Masaryk University Brno Czech Republic

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