MRI and CT scan are generally better at showing typical soft tissue masses than plain radiographs. However, it is often possible to visualize abnormalities in the soft tissues surrounding bone on conventional radiographs, especially when there is ossific or calcific matrix formation within the soft tissue lesion. The presence of a soft tissue mass usually indicates that the lesion has extended through the cortex, which is an indicator of an aggressive or malignant lesion.
- Aggressive lesions tend to have smaller, more contained soft tissue masses. The process of a periosteal neo-cortical response (described in question 4) is an example of the host bone responding to the development of a soft tissue mass in an aggressive tumor. Generally the soft tissue mass is contained by the development of this neo-cortex. Soft tissue masses are certainly found with aggressive tumors, particularly in regions of metaphyseal bone where the periosteum is deficient. However, the soft tissue extension is usually smaller than that encountered with a malignant lesion.
- The other feature that discriminates between the soft tissue mass in aggressive and malignant tumors is the relationship of the mass to the cortex.
- Aggressive tumors almost always develop a soft tissue component by destroying the cortex and expanding into the surrounding tissues.
- Malignant tumors may extend by this route or, alternatively, they may grow directly through the Haversian canal system of the cortex, leaving the cortex structurally intact.
- The hallmark of a soft tissue mass associated with bone sarcoma is that it lies on top of the intact cortex, with sarcoma present inside and outside the cortex. Although this feature of tumor spreading through the cortex to form a soft tissue mass is occasionally observed in metastatic tumors, it is more commonly observed in primary mesenchymal bone tumors, particularly high-grade sarcomas (osteosarcoma, Ewing's sarcoma, malignant fibrous histiocytoma of bone) or lymphoma of bone. In general, metastatic bone tumor are more likely to extend to the soft tissues by eroding through bone in a fashion similar to the aggressive tumor.
- The presence of "malignant" matrix in the soft tissue mass is almost always a sign of a mesenchymal malignancy. This finding is most common in osteosarcoma, where ossification will be evident in the soft tissue mass, or occasionally in chondrosarcoma. The calcified soft tissue mass in chondrosarcoma is found most frequently in secondary surface cartilage lesions that have undergone malignant degeneration or, alternatively, in high-grade central chondrosarcomas that have either eroded through the cortex or have extended through an intact cortex into the soft tissues.
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