TO THE EDITOR:

Erdheim-Chester disease (ECD) is a non-Langerhans cell histiocytosis characterized by tissue infiltration by foamy CD68+ CD1a histiocytes.1,2  ECD has a putative neoplastic and inflammatory nature. The neoplastic hypothesis is supported by the clonality of the infiltrating histiocytes, which harbor mitogen-activated protein kinase pathway mutations, of which BRAFV600E is the most common.3-5  Immune-mediated mechanisms contribute to histiocytic infiltration through a proinflammatory cytokine–chemokine network.6,7  Different treatments targeting these pathogenic mechanisms are considered first-line approaches for ECD, namely interferon-α (IFN-α), BRAFV600E, and MEK inhibitors. Their efficacy, however, is variable, depending on underlying mutations and organ involvement, and frequently limited by remarkable toxicity.8-16 

The mammalian target of rapamycin (mTOR) regulates cell growth, proliferation and apoptosis, and modulates immune responses. mTOR inhibitors (mTORi’s) (eg, sirolimus, everolimus) are used to treat several neoplastic and inflammatory...

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