B-Acute Lymphoblastic Leukemia Masquerading as Multifocal Langerhans Cell Histiocytosis: a Diagnostic Paradigm Shift.

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.