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Background: This report describes a diagnostically challenging case of B-cell acute lymphoblastic leukemia (B-ALL) perfectly mimicking multifocal Langerhans cell histiocytosis (LCH), revealing a critical diagnostic pitfall in pediatric oncology.
Methods: A 6-year-old girl presented with progressive back pain. MRI showed multilevel vertebral collapse (T11-L2) with classic LCH features, while PET-CT revealed disseminated hypermetabolic bone lesions (SUVmax 2.7). Comprehensive pathology included immunohistochemistry (CD79a, TdT, CD1a, Langerin) and next-generation sequencing.
Results: Despite typical LCH imaging, immunohistochemistry demonstrated CD79a⁺/TdT⁺ B-lymphoblasts without CD1a/Langerin expression. Molecular analysis identified a pathogenic KRAS p.Gly13Asp mutation (VAF 1.5%), confirming B-ALL. This represents the first molecularly confirmed case of KRAS-mutated B-ALL mimicking LCH radiographically.
Conclusions: This case mandates: 1) routine TdT staining for LCH-like lesions, 2) recognition of KRAS-driven osteolysis as a novel B-ALL mechanism, and 3) implementation of molecular profiling in atypical osteolytic cases. It highlights the need for integrated diagnostic approaches combining imaging, pathology, and molecular techniques in pediatric bone lesions.
DOI: 10.7754/Clin.Lab.2025.250657
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