RADIOLOGY: MUSCULOSKELETAL: Case# 33635: OSTEOSARCOMA. 13 year old female with knee pain. There is a mixed lytic and sclerotic lesion seen in the metaphyseal region of the distal femur. T1 weighted sagittal views of the knee without and with contrast confirm the metaphyseal origin of this lesion and show soft tissue extent. Multiple areas with fluid/fluid levels are identified compatible with blood filled spaces. The findings on MR combined with the plain film are compatible with a telangiectatic osteosarcoma. The differential diagnosis for multiple fluid levels on MR includes a number of entities such as aneurysmal bone cyst, giant cell tumor, angiosarcoma and telangiectatic osteosarcoma. A telangiectatic osteosarcoma has a worse prognosis than a conventional osteosarcoma. Telangiectatic osteosarcoma is 3-11% of all cases of osteosarcoma. Though the metaphysis is the usual site of involvement, 10% of cases can be seen centered in the diaphyseal region.