Biopsy was performed.

Microscopic description:

Core biopsies show variably cellular proliferation of osteoblastic cells set in variable stroma with some areas of osteoid production. Some fibromyxoid stroma is noted and an occasional figure is seen.

Opinion:

Core biopsies from left iliac crest lesion - the features are of conventional type osteosarcoma.

Osteosarcoma is an aggressive osseous malignancy, with osteoid matrix. The vast majority are of unknown etiology and considered primary (like this case). A small proportion of secondary osteosarcoma are due to genetic mutation, such as retinoblastoma gene, or other disease processes such as Paget disease.

Note that in the initial radiograph and CT, this case lacked many of the stereotypical aggressive features - periosteal reaction, broad zone of transition, soft tissue mass.