MuscSktf10-Osteosarcoma

Findings:

There is a hypointense lesion on the T1 Weighted images in the upper shaft of the right tibia in the metaphysis and also involving the region of the epiphysis posteriorly and extending up to the knee joint. This lesion is seen to turn heterogeneously hyperintense on the proton density and STIR images. This lesion is seen to extend over 10 cms from the level of the knee joint. There is a clear zone of transition between the normal marrow and the lesion.

Discussion: 

Osteosarcoma is the most common primary malignant bone tumor in childhood. The identifying feature of all osteosarcomas is the presence of malignant stromal cells that produce osteoid. There are two main categories of osteosarcoma, primary (de novo) and secondary.

PRIMARY OSTEOSARCOMA:
They often affect individuals younger than 20 years of age with a slight predominance in male patients. The lesion is usually metaphyseal and involves long bones It destroys trabecular and cortical bone, invades the soft tissues and may extend into the epiphysis.
SECONDARY OSTEOSARCOMA:
It arises in a setting of preexisting bone disease (e.g., Paget’ s disease, bone infarcts) or after exposure to irradiation . In osteosarcoma associated with Paget’ s disease, bones other than the long bones (e.g., the pelvis, skull, facial bones and scapula) are involved.

Pathology:

Several pathologic patterns of osteosarcomas have been identified.

• Telangiectatic osteosarcomas:
  They show hemorrhage and necrosis and may simulate an aneurysmal bone cyst. On conventional radiographs, the telangiectatic variant appears lytic because of the lack of demonstrable bone production. MRI shows an aggressive, destructive lesion, usually accompanied by an associated soft-tissue mass. The inhomogeneity of signal intensity, with both low and high signal intensity areas on T1W and T2W images, reflects the high degree of vascularity and the presence of large hemorrhagic cystic spaces.

• Osteoblastic osteosarcoma:
  They are rich in bone production and the bone and osteoid are deposited in massive solid amounts. The mineralization extends into the soft tissues. Blastic components remain low in signal intensity on T1W and T2W images, but associated peritumoral edema or nonsclerotic areas show increased signal intensity on T2W or STIR images.

• Chondroblastic osteosarcoma:
  The generation of chondroid elements, whether benign or malignant, contributes to increased signal intensity on T2-weighted images.

• Fibroblastic osteosarcoma:
  Osteosarcomas that produce large amounts of spindly fibroblasts are classified as fibroblastic osteosarcoma.

• Central, low-grade osteosarcoma:
  It is a rare type of osteosarcoma arising in medullary bone and is so well differentiated that it is frequently mistaken for a benign condition both radiologically and histopathologically. The lesion appears well circumscribed, simulating fibrous dysplasia. A search for small foci of cortical destruction may be essential for accurate diagnosis. The prognosis  is excellent. Metastasis occurs late, if ever; but local recurrences may be a serious problem if surgery is inadequate. On MRI, the low-grade central osteosarcoma may be difficult to distinguish from a conventional osteosarcoma, which has an identical radiographic appearance.

• Juxtacortical osteosarcomas:
       - Periosteal osteosarcoma:
  It favors the diaphyseal surface of long tubular bones and forms an irregular thickened cortex. Histologically, hyaline chondrosarcomatous elements are a prominent feature.
       - Parosteal osteosarcoma:
  They occur most frequently on the metaphyseal surface of long tubular bones within the parosteal soft tissue, and forms a lobulated mass. Histologically it is characterized by a fibrous proliferation surrounding parallel lamellar bony spicules. The fibrous component can appear externally bland, mimicking a fibrous dysplasia. Unlike periosteal osteosarcomas, chondrosarcomatous elements are minimal or absent in parosteal osteosarcomas. MR imaging performed in the early stages of developing myositis ossificans may display changes, which can be mistaken for a more aggressive process, such as parosteal or periosteal osteosarcoma. Rim enhancement with intravenous contrast injection and hyperintensity on T2W images mimics the appearance of an abscess or necrotic tumor.
       - High-grade surface osteosarcoma:

MRI:
 
The distal femur, proximal tibia, and humerus are common sites of involvement in conventional osteosarcoma. With MRI, intramedullary marrow involvement, soft-tissue extension, skip-lesion metastases and postoperative recurrence can be accurately assessed. Marrow infiltration can be mapped on MRI, even when standard radiographic results are negative. MRI can be used to differentiate exuberant bone, which is isointense with yellow marrow, from osteosarcoma, which causes both T1 and T2 prolongation. In addition, MR scans of aggressive, dedifferentiated osteosarcomas may demonstrate a necrotic, hemorrhagic fluid-fluid level within the more distal marrow cavity.
Osteosarcomas tend to demonstrate low signal intensity on T1Wsequences and high signal intensity on T2W sequences. Specific cellular constituents (e.g., fibrous, chondroid, blastic or telangiectatic components), however, can modify signal characteristics.
There is excellent correlation between the MR appearance of the extent of marrow, cortical bone and soft-tissue involvement and the gross pathologic specimen. Lesions that are primarily blastic and therefore sclerotic on plain film radiographs, demonstrate low signal intensity on both T1W and T2W sequences. However, even in lesions with extensive sclerosis, areas of increased signal intensity may be identified on T2W images. Hemorrhagic components within telangiectatic osteosarcoma demonstrate focal areas of high signal intensity on T1W and T2Wimages. Skip lesions and multiple sites of involvement are common in multicentric osteosarcoma.
In cases of parosteal osteosarcoma, the precise involvement of cortex and marrow can be assessed using sagittal or coronal images. Both edema and tumor extension may encase vessels and multiple axial images are usually necessary to distinguish edema that conforms and tracks along specific muscle groups.

1. A dark pattern of low to intermediate signal intensity areas on T1W and T2W images may correspond to calcified osteoid or cartilage matrix, dense granulation tissue and hemosiderin
2. A mottled or speckled pattern of intermediate signal intensity on T1W images and high signal intensity on T2W images, best appreciated on T2-weighted images may correspond to tumor matrix, edematous granulation tissue and hemosiderin deposits
3. A multicystic or bubbly appearance in the cystic pattern, of intermediate to high signal intensity on T1W images and high signal intensity on T2Wimages, is thought to correspond to either blood-filled cysts with viable tumor cells or tumor matrix mixed with edematous granulation tissue; however, foci of residual viable tumor cannot be specifically diagnosed
4. Skip metastases, a reduction in peritumoral edema, the development of a dark rim of collagenous capsule continuous with the periosteum, bone infarcts and intramedullary vascular channels can all be identified.