Case Report: Pediatric Erdheim-Chester disease with cerebral ventricular system infiltration and epiphyseal involvement.

Background: Erdheim-Chester disease (ECD) is an extremely rare non-Langerhans cell histiocytosis, a category of hematologic disorders characterized by the proliferation of foamy histiocytes. ECD predominantly affects adults, and pediatric cases with cerebral ventricular system involvement are exceptionally rare, with only a few previous reports. Moreover, epiphyseal involvement has not been characterized by magnetic resonance imaging (MRI) in children with ECD. Here we present a pediatric case of ECD involving both the cerebral ventricular system and epiphyses, highlighting diagnostic challenges and novel imaging findings relevant to pediatric hematologists. Case presentation: An 11-year-old boy presented with progressive gait disturbance, growth retardation, left knee pain, dysuria, and nocturia over two years. Imaging revealed a mass in the right lateral ventricle, patchy osteosclerosis of the tibial epiphyses, and bilateral cerebellar white matter lesions showing the "crab sign" (central cavitation within the dentate nuclei on Fluid Attenuated Inversion Recovery (FLAIR) sequences). The ventricular mass was surgically resected. Histopathology and immunohistochemistry (CD68+, CD163+, CD1a-) confirmed the diagnosis of ECD, and molecular analysis detected a BRAF V600E mutation (6.40%). The patient received dabrafenib (a BRAF inhibitor), interferon alpha-2b, and desmopressin. At the 32-month follow-up, the patient had become non-ambulatory and exhibited speech delay, partly due to poor medication adherence, highlighting that even brief interruption of dabrafenib therapy can lead to rapid disease progression. For pediatric hematologists, this case strongly demonstrates that continuous administration of dabrafenib is essential for controlling disease in children with BRAF V600E-mutated ECD. Conclusions: This report presents a rare pediatric case of ECD characterized by involvement of both the cerebral ventricular system and epiphyses. We suggest that persistent bladder wall thickening (≥1.5 cm) may serve as a novel imaging indicator of urinary tract involvement in ECD, although histological confirmation is required. The "crab sign" could aid in the early diagnosis of central nervous system involvement. Importantly, this case illustrates that continuous administration of dabrafenib is crucial for disease management in children with BRAF V600E-mutated ECD, as even a brief interruption in therapy can result in rapid and severe deterioration. Our findings highlight the necessity of multimodal imaging and molecular analysis for accurate diagnosis and targeted therapeutic strategies in pediatric ECD.