A Study With Tovorafenib (DAY101) as a Treatment Option for Progressive, Relapsed, or Refractory Langerhans Cell Histiocytosis

Inclusion Criteria: * 180 days- \< 22 years (at time of study enrollment) * Patient must have a body surface area of ≥ 0.3 m\^2 * Patients with progressive, relapsed, or recurrent LCH with measurable disease at study entry * Patients must have had histologic verification of LCH (from either original diagnosis or relapse/progression) at the time of study entry * Tissue confirmation of relapse is recommended but not required. * Pathology report must be submitted for central confirmation of diagnosis within 7 days of enrollment. * Formalin-fixed paraffin-embedded (FFPE) blocks or unstained slides (initial diagnosis and/or subsequent biopsies) will be required for retrospective central confirmation of diagnosis and molecular studies * Patients with mixed histiocytic disorders (e.g. LCH with juvenile xanthogranuloma) may be included * Patients must have measurable disease * Patients must have progressive or refractory disease or experience relapse after at least one previous systemic treatment strategy * Pathogenic somatic mutation detected in genes encoding tyrosine kinase receptors (CSFR1, ERBB3 or ALK), RAS or RAF (may be from original or subsequent biopsy or peripheral blood/bone marrow aspirate). Clinical mutation reports may include quantitative polymerase chain reaction (PCR) (e.g. BRAFV600E) and/or Sanger or next generation sequencing. Immunohistochemistry (e.g. VE1 antibody for BRAFV600E) alone is not sufficient * Participant must be able to tak…