This is a multicenter, retrospective and prospective, observational, no profit study including adult patients with LCH, diagnosed starting from January 2001 to two years after the first enrolled patient. Each patient will be followed up to one year after the last enrolled patient. Each patient will be followed up to one year after the last enrolled patient.This study plans to collect clinical information at the time of diagnosis and at various follow-ups to evaluate the efficacy of first-line therapies. Diagnostic and therapeutic data will be collected from routine clinical evaluations and laboratory and instrumental investigations carried out during clinical practice

This is a multicenter, retrospective and prospective, observational, no profit study including adult patients with LCH, diagnosed starting from January 2001 to two years after the first enrolled patient. Data from patients, diagnosed from January 2001 to the date of patient enrolment, is considered retrospective, while data collected after patient enrolment, and up to two years after the first enrolled patient, is considered prospective. At least one year of follow-up is expected for each patient. Each patient will be followed up to one year after the last enrolled patient. This study plans to collect clinical information at the time of diagnosis and at various follow-ups to evaluate the efficacy of first-line therapies. Diagnostic and therapeutic data will be collected from routine clinical evaluations and laboratory and instrumental investigations carried out during clinical practice. A sub-cohort of patients with available biological data on somatic mutations of the RAF-MEK-ERK genes will also be evaluated.

Inclusion Criteria:

• Confirmed histological and immunohistochemical diagnosis of LCH (CD1a+, S-100+, CD207+), from January 2001 up to two years after the first enrolled patient. Patients with isolated vertebra plana, not related to a malignancy, and without a soft tissue component, are included without a histological and immunohistochemical diagnosis;
• Age ≥18 years at the time of definitive diagnosis;
• Signed, written informed consent, according to ICH/EU/GCP, and national, local laws.

Exclusion Criteria:

• Age ≥18 years and presumptive diagnosis of LCH, or definitive diagnosis of non-Langerhans Histiocytosis (Juvenile xantogranuloma, Rosai-Dorfman disease, Erdheim-Chester disease, etc.).

adult patients with LCH, diagnosed starting from January 2001 to two years after the first enrolled patient.