A 28-year-old woman presents with a 6-month history of pain and swelling around the right knee. Imaging reveals a lytic lesion in the distal femur, eccentrically located in the metaphyseal-epiphyseal region, crossing the articular surface. Histology shows numerous multinucleated giant cells with hemosiderin deposits. Which feature best distinguishes giant cell tumor of bone from osteoblastoma?