A 28-year-old woman presents with a 6-month history of progressive left knee pain and swelling. Radiographs show an eccentric, lytic lesion in the distal femur with a thin sclerotic rim, extending to within 1 cm of the articular surface. MRI confirms a multilocular appearance with hemosiderin deposits. Which single radiological feature best distinguishes giant cell tumor of bone from aneurysmal bone cyst in this location?