The epiphysis develops separately from the growth plate as a secondary ossification center. Thus, the range of tumors seen in the epiphysis is unique. Other secondary ossification centers, such as the apophyses and sesamoids, should be considered as epiphyseal locations in terms of the tumor differential diagnosis. Due to the intraarticular location of the epiphysis, inflammatory and degenerative joint conditions can also affect the epiphysis, while giant cell tumors frequently extend into the epiphysis from their metaphyseal origin.
A typical giant cell tumor in the distal radius, presenting as a lytic metaphyseal lesion extending into the epiphysis.
The differential diagnosis for epiphyseal lesions also includes subchondral cysts. As illustrated in this subchondral tibia, these are most common in the setting of degenerative arthrosis and typically demonstrate geographic lysis with sclerotic borders.
Tumors with a predisposition to the diaphysis include fibrous dysplasia, osteoid osteoma, and the round cell tumors such as Ewing sarcoma, Langerhans cell histiocytosis, myeloma, and lymphoma. Osteofibrous dysplasia and adamantinoma, as intracortical tumors, are exclusively seen in the diaphysis. Enchondromas and unicameral bone cysts can extend from the metaphysis into the diaphysis and occasionally are found entirely within the diaphysis.
Unicameral bone cysts in the long bones originate in the metaphysis from the growth plate and can extend into the metadiaphysis. However, with patient growth, the lesion can completely migrate into the diaphysis, as illustrated here.
The metaphysis is the most metabolically active portion of bone and therefore is host to the largest variety of tumors. Among these are nonossifying fibroma, aneurysmal bone cyst, and osteosarcoma. Giant cell tumor, while centered in the metaphysis, often extends into the epiphysis.
Osteosarcoma in the proximal humerus metaphysis is illustrated, with osteoid matrix and soft tissue extension. The large majority of osteosarcomas (90%) occur in the metaphysis.
Medullary (Central and Eccentric), Intracortical, and Juxtacortical
MEDULLARY CENTRAL AND ECCENTRIC TUMORS
Whether a medullary lesion originates centrally or eccentrically can help in distinguishing between tumors such as unicameral bone cyst versus aneurysmal bone cyst, enchondroma versus chondromyxoid fibroma, and fibrous dysplasia versus nonossifying fibroma. Chondrosarcoma is most commonly located centrally, as are the round cell marrow tumors such as Ewing sarcoma, leukemia, lymphoma, and myeloma. While osteosarcoma and malignant fibrous histiocytoma (MFH) of bone tend to originate eccentrically, they are often so large as to make the precise location of their origin difficult.
Unicameral Bone Cyst
T2 MRI demonstrates the typical central location of this proximal humerus unicameral bone cyst.
Larger osteosarcomas can fill the entire medullary canal and appear central, as in this proximal tibial lesion. The most common locations for osteosarcoma are, in decreasing frequency, distal femur, proximal tibia, and proximal humerus.
Osteosarcoma can appear eccentric, as in this distal femur lesion, which also demonstrates osteoid matrix and soft tissue extension.
The range of intracortical lesions is narrow. When confronted with a painful radiodense lesion, consider osteoid osteoma or stress fracture. Osteofibrous dysplasia and adamantinoma have a somewhat similar appearance, although the latter more frequently has intramedullary involvement and may have a soft tissue component. Cortical desmoids are lytic intracortical lesions. Aneurysmal bone cysts can be medullary or intracortical, in which case they can demonstrate marked cortical expansion, medullary involvement, or even simulate soft tissue extension.
This intracortical osteoid osteoma in the proximal diaphyseal humerus demonstrates a solid periosteal reaction and a small central lucency, representing the nidus ( arrow ).
In this example of a tibial stress fracture, a focal area of cortical thickening is noted. However, the accompanying band of transverse sclerosis ( arrow ), not always seen early in the process, helps to differentiate this from osteoid osteoma.