Transphyseal Proximal Humeral Aneurysmal Bone Cyst with Pathologic Fracture in a Child: A Case Report

Background and Clinical Significance: Aneurysmal bone cyst (ABC) is a benign but locally aggressive osteolytic lesion that typically arises in the metaphysis of long bones in children and adolescents. Extension across an open proximal humeral physis into the epiphysis is uncommon and creates a diagnostic challenge because unicameral bone cyst and telangiectatic osteosarcoma may show overlapping radiological features. Case Presentation: A 12-year-old girl presented with pain and restricted motion of the left shoulder after a bicycle-related fall. Radiographs demonstrated a large expansile lytic lesion of the proximal humerus with cortical thinning and pathologic fracture. CT showed cortical ballooning, septation-like internal architecture, and extension across the open proximal humeral physis into the epiphysis. MRI demonstrated a multiloculated cystic lesion with thin internal septa and thin peripheral/septal enhancement, without nodular solid enhancement or a bulky extraosseous soft-tissue component. Curettage, bone cement filling, and elastic intramedullary nailing were performed. Histopathology, immunohistochemistry, and USP6 FISH analysis supported the final integrated diagnosis of ABC, although USP6 rearrangement was not detected. At 40 months, the patient was pain-free, had essentially normal shoulder motion, had returned to normal school and daily activities, and required no further surgery; radiographs showed stable remodeling without aggressive re-expansion. Conclusions: A focused literature review supports that frank transphyseal ABC is rare, particularly in the proximal humerus. The diagnostic contribution of this case lies in the integration of radiography, CT, MRI, histopathology, ancillary testing, and long-term clinical and radiographic follow-up. In a skeletally immature patient with a proximal humeral cystic lesion, transphyseal extension should prompt careful multimodality assessment. Imaging can substantially narrow the differential diagnosis, but integrated radiologic-pathologic confirmation remains essential when malignant mimics are possible.