title: Incidence of Langerhans cell histiocytosis in children: a population-based study.
abstract: BACKGROUND
  Langerhans cell histiocytosis is a rare disease of unknown etiology. We wanted to assess the population-based incidence of LCH in a well-defined cohort of children.
  
  
  METHODS
  We identified all children <15-years old treated with LCH during the 10 years period 1992-2001 at the Department of Pediatrics, Karolinska University Hospital in Stockholm, the referral center for children with LCH in Stockholm County. We also contacted the Departments of Dermatology, Orthopedics, and Neurosurgery for possible additional patients.
  
  
  RESULTS
  Twenty-nine children (16 males) with LCH were identified, with a median age at diagnosis of 3.8 years (2 months-13.7 years). All children but one had a definitive diagnosis of LCH. The minimum incidence of LCH is estimated to 8.9/10(6) children per year. At diagnosis, 20 children (69%) had single system (SS) and 9 (31%) multisystem (MS) manifestations. Five of the 20 children with SS eventually developed MS disease, thus 14 (48%) had MS involvement at the maximal extent of disease (4.3/10(6) children per year). Interestingly, 22 children (76%) were diagnosed during the fall (September-November, n = 12) and winter (December-February, n = 10) seasons, as compared to seven children during the spring (March-May = 1) and summer (June-August = 6) seasons (P = 0.005, Chi-square).
  
  
  CONCLUSIONS
  The incidence of childhood LCH in our study is higher than previously reported. In our patient cohort, LCH was more commonly diagnosed during the fall and winter season as compared to the spring and summer season. Whether this seasonal variation can be confirmed in larger studies and whether it has relevance for LCH pathophysiology remains to be elucidated.
authors.affiliation: Childhood Cancer Research Unit, Department of Woman and Child Health, Karolinska University Hospital, Stockholm, Sweden.
authors.forename: Helen
authors.initials: H
authors.lastname: Stålemark
authors.forename: Evaldas
authors.initials: E
authors.lastname: Laurencikas
authors.forename: Jenny
authors.initials: J
authors.lastname: Karis
authors.forename: Désirée
authors.initials: D
authors.lastname: Gavhed
authors.forename: Bengt
authors.initials: B
authors.lastname: Fadeel
authors.forename: Jan-Inge
authors.initials: JI
authors.lastname: Henter
mesh_terms: D000293:Adolescent; D002648:Child; D002675:Child, Preschool; D015331:Cohort Studies; D015991:Epidemiologic Measurements; D005260:Female; D006646:Histiocytosis, Langerhans-Cell / Q000453:epidemiology*; D006801:Humans; D015994:Incidence; D007223:Infant; D008297:Male; D012621:Seasons; D013548:Sweden / Q000453:epidemiology