This study aims to (1) clinically and radiographically characterize a series of unifocal (single-system singlesite) and multifocal (single-system multiple-site) langerhans cell histiocytosis (LCH) lesions in the vertebra and (2) determine the success and recurrence rates with different treatment modalities in a pediatric population at a tertiary children's hospital. Patients younger than 18 years old with a diagnosis of LCH before 1 June 2021 at our institution were reviewed. The inclusion criteria were a unifocal or multifocal vertebral lesion without systemic disease. Clinical presentations, lesion sites, radiographic findings, treatments, complications, recurrence rates, and length of follow-up were reviewed and recorded. Thirtynine patients had unifocal (36%) or multifocal (64%) vertebral lesions. 44% of patients had vertebral lesions only. The most common clinical presentation was neck or back pain (51%) and difficulty or inability to ambulate (15%). 70 vertebrae were involved in total; 59% cervical, 62% thoracic, 49% lumbar, and 10% sacral. 88% of multifocal patients underwent chemotherapy compared to 60% of unifocal patients. The recurrence rate in the entire cohort was 10%. The median length of follow-up was 5.2 years (0.6-16.8). Chemotherapy is often utilized as a treatment for vertebral LCH lesions regardless of unifocal or multifocal osseous presentation, with good outcomes and low recurrence rates. However other treatments such as observation only and steroid injections may be a better option with smaller and less widespread lesions due to side effects and length of treatment with chemotherapy. Determination of more invasive treatments including surgical excision or fixation will need to be considered on a case-by-case basis. Level of evidence: IV.

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Introduction Langerhans cell histiocytosis (LCH) is a rare, clonal disorder characterized by proliferation and tissue infiltration by myeloid dendritic cells, most commonly occurring in pediatric populations. It often manifests as skeletal lesions with possible pelvic involvement. Few studies have characterized and reviewed outcomes after treatment of isolated pelvic LCH lesions. Methods A retrospective single-institution review was conducted on diagnoses of patients younger than 18 with a diagnosis of unifocal or multifocal skeletal LCH lesions involving the pelvis. Clinical presentations, lesion sites, focal classification, radiographic findings, treatments, complications, and recurrence rates were reviewed. Results Twenty patients had unifocal or multifocal LCH pelvic lesions (11 males, nine females). The median age at diagnosis was 3.5 years (0.8-21.6). Eight cases (40%) involved unifocal lesions, and twelve (60%) involved multifocal lesions, with the most common associated skeletal disease occurring at the ilium. 100% of cases had a lytic bone lesion with no pathologic fractures. All cases were treated nonoperatively with chemotherapy medications, corticosteroids, or observation alone. 75% of cases were treated with chemotherapy with a 100% resolution rate. The median length of follow-up was 4.5 years (0.4-16.7). Conclusion Our study found that chemotherapy alone or chemotherapy with corticosteroid supplementation are appropriate options for unifocal pelvic LCH lesions. In contrast, pelvic lesions that are part of a multifocal presentation may be managed adequately with varied chemotherapy regimens. Corticosteroid therapy and observation alone may also be reasonable for a single organ system, multifocal, skeletal lesions that are anatomically accessible for biopsy and small in number or size.