The question of what type of matrix is being made is fundamental to the diagnosis of primary mesenchymal tumors, especially benign and malignant bone and cartilage forming tumors. Besides the patterns of matrix observed with bone and cartilage tumors, a ground glass matrix pattern is often seen in fibrous dysplasia.
It may be difficult to differentiate calcified cartilage from ossification in tumor matrix. Calcium deposition in cartilage is typically less well organized than bone and usually has a punctate appearance, with small dots, circles, or whorls of calcification in an otherwise uncalcified background . The nodules of cartilage tumor matrix often undergo calcification at their periphery, leading to an arc-ring pattern of calcific matrix. Bone formation usually is recognized by the greater degree of organization of the matrix, with resultant higher radiographic density.
- The typical benign cartilage tumor is the enchondroma, which can be present in the metaphyseal or metaphyseal-diaphyseal bone and is geographic. The location of the enchondroma is similar to the usual appendicular chondrosarcoma, which also arises in the metaphyseal bone. However, the zone of transition in a chondrosarcoma is more permeative (ie, less geographic) than in the enchondroma, and there are frequently lytic areas interspersed between the calcified matrix. The other feature that differentiates the chondrosarcoma from the enchondroma is the relationship of the lesion to the overlying endosteal cortex. The characteristic cortical erosion observed in chondrosarcoma is described in the next question.
- Other benign cartilage tumors – chondroblastoma and periosteal chondroma – are less obviously calcified. However, small regions of punctate calcification may be observed, especially on CT imaging.
- Bone formation is observed in osteoid osteoma, osteoblastoma, and osteosarcoma. The bone formation seen in osteoid osteoma is mainly due to host bone response to the lesion (nidus) rather than true tumor bone formation. Osteoblastoma is often a lytic, expansile lesion, with little ossification observed on plain Xrays, although osteoid formation is observed at the microscopic level . Less frequently osteoblastomas may be entirely sclerotic lesions.
- Osteosarcoma often demonstrates bone formation within the intraosseous lesion or in the soft tissue mass that is usually associated with the tumor. However, absence of bone on radiographs does not preclude the diagnosis of osteosarcoma, since the osteoid formed by the malignant cells may not calcify and can only be appreciated in the histological sections. The ossification synthesized by the tumor should be differentiated from the periosteal reaction that may be observed in response to rapidly expanding non-osteogenic malignant tumors.
- Fibrous dysplasia produces matrix through the small trabeculae of woven bone scattered through the fibrous stroma. Depending on the extent and calcification of the matrix formed, the ground glass appearance associated with this lesion may illustrate a variable internal density.
In addition to the matrix formation observed with chondroid and bone-forming primary tumors, it should be remembered that metastatic disease, including breast and prostate cancer, can induce a marked sclerotic response in bone. The tumors themselves are not calcified, but they produce a substantial degree of reactive bone in surrounding trabeculae. Lymphoma may also generate a sclerotic response in bone.
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