LAUSR

Erdheim-Chester disease (ECD) is a rare neoplasm from nonLangerhans cell (CD1a -, CD68+, S100-) histiocytes most commonly reported in elderly males. Patients with ECD have been reported to harbor several somatic mutations activating MAPK and PI3K pathways as BRAF+ (38−100%), BRAF wildtype, MAP2K1/K2, and PI3KCA mutations. Other MAPK mutations involving GTPases have also been described via RAS/ RAF/MEK/ERK pathway in NRAS/ KRAS (Ras family) and ARAF (Raf family). These oncogenes are thought to promote early progenitor cells self-renew, increased foamy histiocyte infiltration, and chronic inflammation. Given the ubiquitous presence of monocytic-originated cells, ECD may vary from one indolent spectrum to another with poor prognosis, representing a complex diagnosis as coexisting mixed Langerhans (CD1a + S100 +CD 207 +) and nonLangerhans (CD 1a − CD 68+ CD 163+) histiocytosis may develop. Most cases involve infiltration of long bones, retroperitoneum, orbits, skin, “hairy kidneys”, tests, and cardiovascular lesions. The central nervous system (CNS), gastrointestinal system (GIS), and skeletal involvement indicate a poor prognosis. Blood disorders have been reported before and after ECD onset, which are mostly myeloproliferative (MPS) disorders such as polycythemia vera, Essential Thrombocythemia, and myelofibrosis. Among MPS disorders, polycythemia vera (PV) is the most commonly reported with up to 20-year time lapse between PV diagnosis and ECD initial manifestation. The aim of this report is to describe a rare case of essential thrombocytosis (ET) who evolved, after 5 year-follow up, to ECD with extensive infiltration of long bones and to review the associations between MPS disorders and ECD.