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This is the most important question to ask when developing a differential diagnostic list: The particular bone involved as well as the portion of the bone affected must be determined.
The Affected Bone
Features important to the bone involved by tumor include the following:
- Symptomatic metastatic tumors are usually found proximal to the knees and elbows.
- The metastatic lesion most commonly found distal to knees and elbows results from metastatic lung cancer, followed by renal cell cancer and melanoma.
- The phalanges are a common site for enchondromas, which may be quite active and destructive in the hand but are rarely malignant.
- The tibia is a common site for two rare tumors: osteofibrous dysplasia and adamantinoma. Adamantinoma is rarely found, but when it is, the lesions is in the tibia and virtually never elsewhere.
- Aggressive and malignant primary bone tumors most frequently occur in the regions of greatest longitudinal bone growth. These sites include the metaphyses of the distal femur, proximal tibia, proximal femur, proximal humerus
- Chondrosarcoma is more frequently found in the proximal skeleton (pelvis, scapula, proximal humerus and femur) than in the distal extremities.
- Chordoma is most commonly found in the sacrum and at the base of skull. Much less frequently, chordoma arises in the remainder of the spinal column.
- The anterior spine is frequently involved with metastatic disease. The primary tumors found most frequently in the spine include giant cell tumor and aneurysmal bone cyst (anterior spine) and osteoid osteoma and asteoblastoma (posterior elements).
The Affected Portion of the Bone
After assessing the bone that is involved, next determine the portion of the bone involved. The skeletal anatomic regions that may be affected by bone tumors are defined as follows:
- Peri-articular. Changes are present on both sides of the joint or present in the soft tissue bursae surrounding the joints. These changes are evident in pigmented villonodular synovitis and synovial chondrometaplasia. Infectious, inflamatory, and metabolic (gout) joint disease may present with peri-articular erosions of bone. Lytic lesions that extend across joints to involve adjacent bones suggest a vascular tumor, such as disappearing bone disease (Gorham's disease- angioma or lymphangioma of bone) or angiosarcoma of bone.
- Epiphyseal. These lesions in adults may extend across the growth plate scar but are clearly centered in the epiphysis. This is typical of a chondroblastoma.
- Epiphyseal-metaphyseal. This is a classic location for locally aggressive tumors such as giant cell tumor of bone.
- Metaphyseal or metaphyseal-diaphyseal. Lesions that do not cross the growth plate and tend to grow away from the physis (enchondroma, unicameral bone cyst) are described by this location. Bone abscesses (Brodie's abscess) may be metaphyseal but may extend across the physis or its scar. The metaphysis is the most common site for primary mesenchymal malignancies (osteosarcoma, chondrosarcoma).
- Diaphyseal. This is a relatively uncommon location for bone tumors. Infections and fractures may cause tumor-like changes in the diaphysis. Ewing's sarcoma, eosinophilic granuloma, osteoid osteoma, and metastases are found (not exclusively) in this region.
- Parosteal or cortical. This location describes lesions that are attached to the surface of the bone. Examples include osteochondroma and parosteal osteosarcoma. Other lesions may grow directly within the cortex (osteoid osteoma, periosteal chondroma). It is often necessary to obtain cross-sectional imaging to delineate the relationship of the parosteal lesion or cortical lesion to the surface of the bone. CT and MRI are valuable for analysis of the relationship between the lesion and the underlying cortex. MRI is superior for determining if the lesion has extended within the marrow cavity.
- Soft tissue. Some lesions with calcified or ossified matrix may be present in the soft tissues but are well visualized on conventional radiographs. Examples include myositis ossificans and soft tissue sarcoma with ossification that might include malignant fibrous histiocytoma and synovial sarcoma.
- Central or eccentric? Central lesions include enchondroma and fibrous dysplasia; typical peripheral eccentric peripheral lesions include non-ossifying fibroma and chondromyxoid fibroma.
The Other Questions to Ask
This first question, related to location, is critical in developing a differential diagnostic list. The other six questions define the biological activity demonstrated by the lesion on conventional radiographs and help to define the most likely diagnostic possibilities within the differential diagnostic lists.
