Dedifferentiated chondrosarcoma can be observed in cases of recurrence and metastasis, as well as in primary tumors where different grades of malignancy coexist. In central chondrosarcoma, the progression from grade I central chondrosarcoma to grade II central chondrosarcoma is rare and, even when it occurs, it is very slow. Grade I central chondrosarcoma can maintain its malignancy unchanged after multiple recurrences and years of disease progression. However, the progression from grade II central chondrosarcoma to grade III central chondrosarcoma can be rapid and is not uncommon. Sometimes, a more malignant tumor can arise within grade I or II central chondrosarcoma, histologically presenting as malignant fibrous histiocytoma, fibrosarcoma, osteosarcoma, and angiosarcoma. These tumors are defined as "dedifferentiated chondrosarcoma." In 15% of dedifferentiated chondrosarcoma cases, the patients are older than those with central chondrosarcoma, typically occurring after the age of 50.

bubble_chart Pathological Changes

There are two distinct types of tumor tissues. One is a well-differentiated chondroid tumor, where the histopathology can vary between cases, and within the same case, histological manifestations ranging from a soft bone tumor to a grade III soft osteosarcoma can be observed. Most cases present as grade I or II central soft osteosarcoma. The other type is a highly malignant tumor tissue, typically malignant fibrous histiocytoma, osteosarcoma, and fibrous fleshy tumor, characterized by a high degree of malignancy. The process of transformation from one type of tumor tissue to another is not continuous.

Dedifferentiated soft osteosarcoma must be distinguished from grade III central soft osteosarcoma, where undifferentiated cells are present around the lobules of the central soft osteosarcoma, with a gradual transition between these cells and chondrocytes. Dedifferentiated soft osteosarcoma should also be differentiated from mesenchymal soft osteosarcoma, which contains small undifferentiated cells interspersed with differentiated cartilage islands.

The medical history can be extensive, presenting with grade II symptoms, followed by rapid progressive pain and swelling, which may extend into the soft tissues. In other cases, the initial symptoms may be those of a high-grade fleshy tumor without any warning signs; or they may manifest due to a pathological fracture.

bubble_chart Auxiliary Examination

X-ray findings: Primary chondroid lesions typically present as grade II bone expansion with cortical thickening and distinct bony margins, often containing characteristic calcifications within the tumor. Progressive lesions manifest as either predominantly osteolytic or, less commonly, osteoblastic changes, with disappearance of pre-existing calcifications, cortical destruction, and soft tissue invasion. In some cases, a new progressive tumor may be observed, whose radiographic appearance resembles high-grade osteolytic tumors seen in adults. Only by reviewing previous X-rays and/or conducting a histological examination of the entire tumor can traces of the original low-grade chondroid tumor be identified. In other cases, imaging and gross pathology may resemble conventional central osteosarcoma, with only a few limited areas on histological sections revealing the presence of a new high-grade fleshy tumor.

bubble_chart Treatment Measures

The prognosis for dedifferentiated central osteosarcoma is poor, with metastases often occurring early and frequently, even after wide or radical surgical resection. The efficacy of chemotherapy remains unclear, largely because it is rarely used in most patients due to their older age.

bubble_chart Differentiation

The diagnosis of dedifferentiated osteosarcoma must be approached with particular caution (malignant fibrous histiocytoma, osteosarcoma, high-grade fleshy tumor). Diagnosis should be based on the patient's medical history and radiological imaging, and a biopsy should be performed when the diagnosis is suspected. Sometimes, dedifferentiated osteosarcoma cannot be diagnosed preoperatively and can only be diagnosed postoperatively after a histological examination of the entire tumor.