Answer: Telangiectatic osteosarcoma

Histology: The criteria for the diagnosis of telangiectatic osteosarcoma are the following:
a) a radiolytic lesion without evidence of intralesional ratiodensity;
b) aneurysmal bone cyst histology on low power microscopy;
c) high-grade conventional osteoblastic osteosarcoma apparent on high-power microscopy.

Discussion: Telangiectatic osteosarcoma is a rare variant of osteosarcoma accounting for 3 to 4% of all osteosarcomas. Because this lesion is not associated with radiodensity, osteosarcoma is not often suspected on radiographic interpretation alone. This is unlike most osteosarcomas where the diagnosis can be made with reasonable confidence on radiographic study. Low-power examination of telangiectatic osteosarcoma shows areas of hemorrhage and dilated blood-filled lakes separated by bands of connective tissue. This pattern is identical to that of a conventional aneurysmal bone cyst. However, on high-power study there are sheets of frank sarcomatous tissue with atypical cells synthesizing osteoid. This is a pattern of conventional high-grade ostosarcoma. Aneurysmal bone cyst may present an identical radiographic picture as well as identical features on low-power study. However, aneurysmal bone cyst lacks areas of sarcoma.

Although some aneurysmal bone cysts are very cellular between the blood-filled lakes and mitotic figures may be present, they lack the cellular pleomorphism associated with a sarcoma. It is necessary to examine many slides of an aneurysmal bone cyst to rule out an underlying telangiectatic osteosarcoma.

Giant cell tumor is a lesion that may have radiographic features of aneurysmal bone cysts. In fact, aneurysmal bone cyst is frequently associated with giant cell tumor. Giant cell tumors are lytic lesions that involve both the epiphysis and the metaphysis near the articular surface when they occur in long bones. They may also involve other sites in the skeleton. Giant cell tumors may also be cellular and contain many mitotic figures. They also have areas of reactive bone. However, frankly sarcomatous tissue is absent. The osteoid deposition pattern in giant cell tumors is zonal and indicative of reactive bone. The lace-like osteoid of conventional osteosarcoma is absent.