This study looks at how a new drug, tovorafenib (DAY101), can help people with a disease called Langerhans Cell Histiocytosis (LCH). LCH happens when too many immature white blood cells called Langerhans cells build up and form tumors. These tumors can damage bones, skin, and other organs. The study is for people whose LCH is getting worse or hasn't responded well to other treatments.

Participants will take tovorafenib by mouth once a week for about a year. Researchers want to see how well the drug works and what side effects it might cause. They will also check how the disease changes with treatment. To be eligible, you must be under 22, have a body surface area of at least 0.3 m², and meet other health criteria.

• The study lasts for up to a year with visits every month.
• Participants will have regular scans and blood tests.
• There may be risks like side effects from the drug.

After the study, there will be follow-ups for up to two years to monitor health changes.