A Dose-Response Relationship to Radiotherapy for CutaneousLesions of Langerhans Cell Histiocytosis

Langerhans cell histiocytosis (LCH) is a rare disease, afflicting approximately 4.6 and 1-2 per 1 million children and adults,respectively. While LCH can involve numerous organ systems such as the lung or bone, it is uncommon for the disease to belimited to the skin. Radiotherapy has an established role for osseous lesions. However, the efficacy and dose for nonosseousmanifestations of the disease are not well described. In the current case report, we detail a 49-year-old adult male with skin-limited LCH requiring palliative radiotherapy (RT) to numerous sites for pain control. The patient was initially diagnosed andtreated with single agent cytarabine for approximately 6 months. Despite treatment, he had little symptomatic response of hiscutaneous lesions. We delivered a single dose of 8 Gray (Gy) to 3 separate skin lesions, including the bilateral groin, rightpopliteal region, and right axillary lesion, which resulted in pain reduction and partial response at four-month follow-up.Subsequently, we decided to treat the left axillary untreated lesion to a higher dose of 24 Gy in 12 fractions. At four-monthfollow-up, the left axilla RT resulted in complete clinical response and improved pain control compared to the right axilla.Following RT treatments, the patient was found to have a BRAF mutation, and vemurafenib was initiated. Further follow-upwith positron emissions tomography demonstrated complete metabolic response in numerous disease areas, including bothaxillae. Based on this case report’sfindings, a higher radiotherapy dose may be more effective for treating cutaneous LCH