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.
Chondroblastoma is seen in the proximal humerus epiphysis. The most common locations for chondroblastoma are the proximal femur (25%) and distal femur (20%), followed by the proximal humerus (17%).
Chondroblastoma also occurs in epiphyseal equivalents, such as apophyses and sesamoids, of which the patella is an example. Note the chondroid mineralization, which adds specificity to the diagnosis.
Trevor disease is an osteochondroma arising from the epiphysis, seen here in the distal femoral medial condyle. Multiple bone involvement is seen in 65% of patients.
This is a rare subtype (2%) of chondrosarcoma that almost always (90%) occurs in the epiphysis. The most common sites are the proximal femur and the humerus (75% to 80% of lesions).
A typical giant cell tumor in the distal radius, presenting as a lytic metaphyseal lesion extending into the epiphysis.
Subchondral Cyst
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.
DIAPHYSIS
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.
Langerhans cell histiocytosis appearing as a lytic lesion, centrally located in the diaphyseal femur. The thick periosteal reaction implies an indolent process, despite the wide zone of transition.
Enchondromas appear to originate from the physis and can be found in the metaphysis, in the metadiaphysis, or entirely within the diaphysis, as seen in this distal femur lesion.
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.
METAPHYSIS
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.
Nonossifying fibroma with typical metaphyseal location is seen in the distal tibia. Note the ossification beginning in the metadiaphyseal zone, farthest from the physis.
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.
Axial Location
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.
Central Medullary
Eccentric Medullary
Unicameral Bone Cyst
T2 MRI demonstrates the typical central location of this proximal humerus unicameral bone cyst.
Aneurysmal bone cysts are typically located eccentrically in the medullary cavity, as illustrated in this proximal tibial lesion, or can be intracortical.
Enchondroma
The typical central location of enchondromas is demonstrated in this T2 MRI of a distal femur lesion.
Chondromyxoid Fibroma
In contrast to enchondromas, chondromyxoid fibromas originate eccentrically or intracortically, often with thinning or expansion of the cortex, as seen in this proximal tibial lesion.
Fibrous Dysplasia
Femoral lesion demonstrating typical central location. Fibrous dysplasia distribution is as follows: femur, 35% to 40%; tibia, 20%; skull and facial bones, 10% to 25%; and ribs, 10%.
Nonossifying fibroma is typically found eccentrically in the metaphysis, as illustrated in this distal femur lesion, but likely originates in the cortex.
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
Osteosarcoma can appear eccentric, as in this distal femur lesion, which also demonstrates osteoid matrix and soft tissue extension.
INTRACORTICAL TUMORS
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 ).
Stress Fracture
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.
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