Erdheim-Chester Disease (ECD) is a rare form of histiocytosis characterized by the proliferation of blood cells, known as histiocytes, which infiltrate various organs and tissues, often causing irreversible damage. The causes of the pathology are still unknown. Although the disease typically affects adult individuals, cases of pediatric-onset ECD have been described. However, there is a lack of detailed information on the phenotypic characteristics of these patients, and reliable data on response to specific therapies and long-term outcomes are missing. Three patients referred to our reference center for Histiocytosis present a concomitant BRAF-mutated neoplasm. Such an association could be due to the presence of mosaicisms for the BRAF V600E mutation. Mosaicism is a biological event defined as the presence of more than one genetically dissimilar cell population in the same organism and is an increasingly studied field, both in normal and pathological conditions. If proven in ECD as well, this mechanism could contribute to providing answers to the still open questions regarding the development of this disease.

Inclusion Criteria:

• Prevalent and incident patients (adults and pediatric), with histologically confirmed diagnosis of Erdheim-Chester Disease according to the latest diagnostic recommendations;
• Signing of informed consent for study participation.

Exclusion Criteria:

• Patients for whom clinical and imaging data are not available and for whom it is not possible to retrieve histological samples."