Chronic myelomonocytic leukaemia associated with pulmonary Langerhans cell histiocytosis

<bold>Introduction</bold>. Langerhans cell histiocytosis (LCH) and chronic myelomonocytic leukaemia (CMML) are two rare clonal myeloid neoplasms. Patients with cutaneous or multisystem LCH may rarely present concomitant CMML. Only one patient with pulmonary LCH (PLCH) and CMML has been reported. <bold>Objectives</bold>. To describe PLCH patients with CMML seen at our reference centre for histiocytoses. <bold>Results</bold>. We identified 6 (1.5%) PLCH patients with CMML (5 males, median age 68 years, IQR [66-70], current n=3, former smokers n=3). PLCH was isolated in 4 cases. Two patients had extra pulmonary LCH involvement (liver n=2, skin n=1). HRCT showed typical nodulo-cystic in 5 cases (with ground glass opacities n=1), and a cystic pattern in one patient. The median age at CMML diagnosis was 70 years, [66-72]. Tissue biopsy was performed in 2 cases (<italic>BRAF</italic>^V600E mutation in one). At the time of PLCH diagnosis, 5 patients had elevated monocyte counts (median 3.5 G/L, [1.9-6.3]). CMML diagnosis was made after a median time of 6 months [1-9]. No patient needed CMML treatment. Blood and/or bone marrow <italic>TET2</italic> mutations were present in 5 cases (with other myeloid mutations in 4, including <italic>JAK2</italic> in 2 cases). One patient developed Polycythaemia Vera requiring hydroxyurea therapy. One patient developed cytopenias under cladribine therapy for her PLCH. Three patients died after 14, 35 and 38 months, respectively (related to LCH n=2, unknown cause n=1). <bold>Conclusion</bold>. The occurrence of CMML with PLCH suggests a possible common clonal origin of these myeloid disorders in this circumstance. Physicians should pay careful attention to blood monocyte counts of PLCH patients to early diagnose concomitant CMML for appropriate management of these patients.