Stereotactic radiosurgery for intracranial Langerhans cell histiocytosis: A multi-institutional case series with long-term follow-up.

Introduction: Intracranial Langerhans cell histiocytosis (iLCH) is rare and optimal local therapy remains undefined. Stereotactic radiosurgery (SRS) has emerged as a focused alternative to surgery or conventional radiotherapy for selected lesions. Research question: To report long-term outcomes of Gamma Knife radiosurgery (GKRS) for iLCH across multiple institutions. Material and methods: We retrospectively reviewed four patients (eight lesions) treated with GKRS between 2003 and 2015 at four institutions affiliated with the International Radiosurgery Research Foundation (IRRF). Inclusion required histopathologic LCH, single- or hypofractionated SRS to brain lesions, and ≥6 months radiological follow-up. Responses were assessed volumetrically (regression ≥20 %; progression ≥20 %). SRS-related toxicity was evaluated using the Radiation Therapy Oncology Group criteria for CNS toxicity. Results: The median age at GKRS was 27 years. Four patients were treated, three with single lesions and one with five lesions across three sessions; most affected the cerebellum (n = 5). Median dose was 13 Gy (range: 8-14 Gy) with a median tumor volume of 2.7 cc. All lesions regressed with no local failures. Side effects were minimal, limited to one case of grade 2 edema. No necrosis or major morbidity occurred. Discussion and conclusion: GKRS provided durable radiographic regression and local control of iLCH with minimal toxicity, using a median prescription dose of 13 Gy. This approach represents a valuable, minimally invasive option for localized iLCH, while endocrine dysfunctions typically persist. Further prospective and collaborative investigations are essential to standardize dosing, evaluate combination approaches with systemic regimens, and refine guidelines for patient selection.