LAUSR

The diagnosis is multi-system Langerhans cell histiocytosis (LCH). Water deprivation was consistent with central diabetes insipidus (DI) – paired serum osmolality of 305 mOsm/kg and urine osmolality of 113 mOsm/kg. The diagnosis of LCH was suspected based on central DI and the seborrhoeic scalp rash which was subsequently confirmed with immunohistochemistry. Staging investigations including a skeletal survey demonstrated lytic lesions in the parieto-occipital bones (Fig. 2). Cranial magnetic resonance imaging (Fig. 3) was performed which showed a thickened pituitary stalk and absent posterior pituitary bright spot. Histopathology examination of the skin biopsy showed coffee bean nucleoli (Fig. 4) and immunohistochemistry was positive for CD1a (Fig. 5) and CD207. Next-generation sequencing of the cells detected a BRAF mutation. LCH is an inflammatory neoplasm of myeloid origin characterised by infiltrating pathological CD1a/CD207 dendritic cells. An ongoing debate over the pathogenic classification of LCH was finally settled in favour of a neoplastic process after the discovery of the BRAF V600E mutation and the mitogen-activated protein kinase gene mutations in 2010. LCH is a clinically heterogeneous disease ranging from spontaneous self-resolving skin or single bone lesions to disseminated forms involving multiple organ systems. Currently, it is classified into two widely recognised disease extent categories: singlesystem LCH (involvement of a single organ or system) and multisystem LCH (involvement of two or more organ systems). Risk organ involvement includes the liver, spleen and bone marrow. The specific presenting diagnosis of LCH can be DI, present in 43% of the patients, with growth hormone deficiency being the second most common pituitary dysfunction. In children with LCH presenting with central DI, the predominant extracranial sites involved are bone (80%) and then skin manifestations (39%). Skin