title: Pulmonary Langerhans Cell Histiocytosis: An Update From the Pathologists' Perspective.
abstract: BACKGROUND
  Pulmonary Langerhans cell histiocytosis (PLCH) is a rare histiocytic disorder that almost exclusively affects the lungs of smokers. PLCH is characterized by bronchiolocentric nodules and/or cysts in an upper and mid lung distribution with sparing of the costophrenic angles. The diagnosis can be challenging and often requires transbronchial biopsy or surgical lung biopsy. Pulmonary hypertension is a relatively common and sometimes severe complication of PLCH. The pathogenesis of PLCH is still debated. Recently, BRAF V600E mutation and BRAF expression have been identified in some patients with PLCH, suggesting that at least a subset of PLCH has a clonal proliferation. While smoking cessation is the first-line treatment of PLCH, some patients might require additional treatment and eventually transplant. Given that the lesional cells of PLCH express BRAF in some patients, MAPKinase pathway-targeted treatment might be useful for therapy-resistant patients.
  
  
  OBJECTIVE
  To present the more recently recognized clinical and pathologic aspects of PLCH, including pulmonary hypertension in PLCH, pathogenesis, and treatment, as well as the basic diagnostic approach to PLCH.
  
  
  METHODS
  Authors' own research, and search of literature database (PubMed) and UpToDate.
  
  
  CONCLUSIONS
  Despite the recent progress, more studies are needed to elucidate the biology of PLCH for identification of prognostic factors and appropriate treatment options, especially for therapy-refractory PLCH cases.
authors.affiliation: From the Department of Laboratory Medicine and Pathology, Mayo Clinic Rochester, Rochester, Minnesota.
authors.forename: Anja C
authors.initials: AC
authors.lastname: Roden
authors.forename: Eunhee S
authors.initials: ES
authors.lastname: Yi
mesh_terms: D019943:Amino Acid Substitution; D001706:Biopsy; D003131:Combined Modality Therapy; D003937:Diagnosis, Differential; D006646:Histiocytosis, Langerhans-Cell / Q000175:diagnosis* / Q000473:pathology / Q000503:physiopathology / Q000628:therapy; D006801:Humans; D006976:Hypertension, Pulmonary / Q000209:etiology; D007150:Immunohistochemistry; D008168:Lung / Q000000981:diagnostic imaging / Q000378:metabolism / Q000473:pathology*; D009154:Mutation; D011379:Prognosis; D048493:Proto-Oncogene Proteins B-raf / Q000235:genetics / Q000378:metabolism; D011859:Radiography; D020545:Respiratory Mucosa / Q000000981:diagnostic imaging / Q000378:metabolism / Q000473:pathology*; D012720:Severity of Illness Index; D016540:Smoking Cessation
chemical_list: C482119:BRAF protein, human; D048493:Proto-Oncogene Proteins B-raf