Juvenile myelomonocytic leukemia masquerading as Langerhans cell histiocytosis: An immuno-morphologic dilemma

Non-neoplastic, reactive proliferation of Langerhans cells is observed in leukemias, pseudo-lymphomas, carcinomas, etc. Juvenile myelomonocytic leukemia (JMML) is a rare myelodysplastic/myeloproliferative neoplasm with a few overlapping clinical features to Langerhans cell histiocytosis (LCH). JMML involving the lymph nodes may show the proliferation of Langerhans cells. A two year old boy presented with fever, hepatosplenomegaly and elevated total leukocyte count with monocytosis. Bone marrow revealed dysplasia in erythroids and myeloids. Axillary lymph node showed sheets of macrophages in sinuses and paracortical areas (S100, CD68 and CD 1a positive) admixed with eosinophils, mimicking LCH. Molecular analysis revealed somatic heterozygous, missense mutation in PTPN11 exon 3, a known pathogenic hot spot mutation in JMML. The treatment and prognosis of JMML and LCH are different. JMML may have a clinical and morphological overlap with LCH and hence can be misdiagnosed. This paper highlights the similarities and differences between the two diseases with a case illustration.