FINAL DIAGNOSIS: -- DEDIFFERENTIATED CHONDROSARCOMA
Dedifferentiated chondrosarcoma is defined as a high grade, nonchondroid sarcoma associated with a low grade cartilaginous tumor. They make up approximately 11% of all chondrosarcomas, and are among the most aggressive cancers described.
These tumors generally occur in older patients, typically over the age of 50. They present with localized pain, swelling, and paresthesias. Rarely, they are accompanied by pathologic fractures. The most common sites, in descending order of frequency, are: pelvis, proximal femur, proximal humerus, distal femur, and rib.
The radiologic appearance of dedifferentiated chondrosarcomas is fairly well-reported. They appear as ill-defined, lytic intraosseous lesions or extraosseous soft tissue masses devoid of calcification adjacent to a lesion possessing the classic features of a cartilaginous tumor. An abrupt transition between the two elements is evident.
There is a fair amount of controversy as to the origin of the dedifferentiated component of these tumors. It appears that the noncartilaginous element is derived from a separate noncartilaginous clone arising in the milieu of a cartilaginous tumor. This cartilaginous tumor is usually a central chondrosarcoma (defined as a chondrosarcoma arising in the medulla of bone). Grossly, these tumors are lobular with a biphasic appearance. One area is comprised of blue-gray cartilage. The other area usually appears as an ill-defined mass of brown, hemorrhagic, and necrotic soft tissue.
Microscopically, the chondrosaramatous region is a standard Grade I lesion consisting of chondrocytes with mildly atypical nuclei in a cartilaginous stroma. Few mitoses are present and necrosis is not seen. The dedifferentiated portion of the tumor usually represents a malignant fibrous histiocytoma. This region usually consists of spindle cells in a storiform pattern with frequent mitoses and necrosis. The junction between the two entities is typically sharp.
Multiple studies of the cytogenetics of dedifferentiated chondrosarcomas have been performed. These studies have not shown a common cytogenetic abnormality to be present in all cases. Many, but not all, of these cases did share a loss of a least a portion of 1p, trisomy 7, and breaks in the q11 region of chromosome 22.
The differential diagnosis consists of a variety of mesenchymal neoplasms. Both high-grade chondrosarcomas and mesenchymal chondrosarcomas can have cartilaginous regions and spindle-cell regions, but they are usually intimately intermixed with one another. Chondroblastic osteosarcomas may look similar to dedifferentiated chondrosarcomas except for the presence of malignant osteoid in the former lesion (which is absent in dedifferentiated chondrosarcoma).
Both malignant fibrous histiocytoma and fibrosarcoma can be comprised of spindle cells in a storiform pattern which are similar to the higher grade region of dedifferentiated chondrosarcoma. But neither of these two lesions contain a low grade cartiaginous portion.
The prognosis of dedifferentiated chondrosarcoma is poor. Widespread metastasis is the rule, with most patients dead within 2 years regardless of treatment modality. This patient's subsequent metastatic workup was negative. An above-knee amputation was performed, and he is currently in rehabilitation without recurrence approximately one month after initial diagnosis.
Contributed by Steven LaTulippe, MD and Martha R. Clarke, MD