Slide 1
The epiphyseal location of this lytic lesion of bone with extension to the subchondral bone under the articular cartilage should elicit strong suspicions about the nature of this lesion. In the fresh specimen, the contents of the lytic lesion would have been hemorrhagic along with areas of pinkish-tan soft tissue that is still present in the specimen. It is the latter that contains some multinucleated giant cells and the neoplastic stroma you have seen microscopically.

Slide 2
The radiograph demonstrates a "bubbly" lytic lesion involving the proximal tibia, with numerous septations. The somewhat eccentric position and site of the lesion, extending from the metaphysis to the subchondral location, is characteristic of a giant cell tumor.