Watchful Waiting in Children with Unifocal Osseous Langerhans Cell Histiocytosis.

BACKGROUND: Langerhans cell histiocytosis (LCH) is a rare inflammatory neoplasm most commonly involving bone. However, data on single-system, single-site LCH (SS-s-LCH), particularly with unifocal bone disease, remain limited. Management strategies for unifocal osseous LCH vary globally, and the safety of a watchful-waiting approach is not well-defined. This study aimed to evaluate whether patients with osseous SS-s-LCH can be managed with observation alone without compromising long-term outcomes. METHODS: In this retrospective, multicenter study, we enrolled children with biopsy-proven osseous SS-s-LCH from seven centers in the Chinese Children's Histiocytosis Group, excluding those with central nervous system (CNS)-risk bone lesions. The primary endpoint was 3-year event-free survival (EFS). Events were defined as disease progression or relapse. RESULTS: The cohort included 134 patients with a median follow-up of 36.0 months. Involved sites were cranial bones (n = 37), vertebrae (n = 33), and other bones (n = 64). At final follow-up, 16 patients had experienced an event, yielding a 3-year EFS of 88.1% (95% CI: 81.5-92.5%). Events included new lesions (n = 14) and primary lesion expansion (n = 2), with 93.8% (15/16) occurring within the first 12 months after diagnosis. EFS did not differ significantly by involved site or mutation status. However, patients staged by PET-CT at diagnosis had a significantly higher 3-year EFS (97.6%) than those staged by conventional imaging (83.7%) (p < 0.05). Long-term quality of life was satisfactory in all assessed patients. CONCLUSION: Watchful waiting is a safe and effective management strategy for children with unifocal osseous SS-s-LCH. Initial staging with PET-CT may be associated with superior event-free survival, highlighting its potential value in refining risk assessment.