Primary osseous kaposiform hemangioendothelioma confined to long bones: a retrospective imaging analysis of 12 pediatric cases

Kaposiform hemangioendothelioma (KHE) is a rare, locally aggressive vascular tumor predominantly affecting infants and children. While musculoskeletal involvement is common, cases originating in and confined exclusively to bone are exceptionally rare, and their specific imaging characteristics are not well-defined. This study aims to characterize the clinical and detailed radiological features of primary, intraosseous KHE in a pediatric cohort. We conducted a retrospective review of pediatric patients with biopsy-proven KHE admitted to our institution between January 2016 and December 2024. Inclusion criteria stipulated that lesions must be confined entirely within the bone on initial diagnostic imaging. The clinical data, computed tomography (CT), and magnetic resonance imaging (MRI) scans of 12 eligible patients were systematically analyzed by two senior musculoskeletal radiologists. Ki-67 proliferation index was analyzed to correlate with imaging patterns. The cohort included 12 patients (11 males, 1 female) with a median age of 5 years (range, 2–12 years). Common presenting symptoms were chronic pain, limping, and muscle atrophy. Notably, no patients presented with Kasabach-Merritt phenomenon (KMP) or cutaneous lesions. Two distinct and recurring radiological patterns were identified. The first, a permeative/moth-eaten pattern (n = 7/12), was characterized by dotted, infiltrative osteolytic lesions involving both the epiphysis and metaphysis, often with associated tubular formations. The second, a cortical excavation pattern (n = 5/12), presented as well-defined, saucer-like erosions of the bone cortex, primarily in the metaphysis. No cases exhibited significant periosteal reaction or a discrete soft-tissue mass. Histopathology confirmed infiltrative nodules of spindled endothelial cells forming slit-like vascular channels. The Ki-67 proliferation index was significantly higher in Pattern 1 compared to Pattern 2 (18.7% vs. 9.2%, P < 0.001, excluding one mixed-pattern case). Primary KHE confined to bone presents with two distinct imaging patterns: permeative/moth-eaten infiltration and cortical excavation. These patterns correlate with biological aggressiveness as indicated by Ki-67 expression. Recognition of these specific features, particularly in the absence of KMP or skin changes, is crucial for radiologists and clinicians. These findings likely represent the earliest detectable stage of osseous KHE and should prompt consideration of this rare entity in the differential diagnosis of pediatric lytic bone lesions, facilitating timely diagnosis and intervention.