Answer: Synovial chondromatosis

Histology: The ringed and stippled pattern of radiodensity is diagnostic of a cartilage lesion. Histologically, synovial chondromatosis is characterized by well-defined lobules of hyaline cartilage. Occasionally, chondrocyte atypia may be present.

Discussion: To make the diagnosis of synovial chondromatosis radiologically, the ringed and stipple pattern of radiodensity must be placed in a setting of either a surface or an intramedullary lesion. Often a CT scan is necessary to answer this question. In this case, the lesion is in the soft tissues adjacent to the shoulder. The lesion is sharply marginated and uniform, a pattern typical of synovial chondromatosis. A chondrosarcoma is usually poorly circumscribed and has an irregular pattern of mineralization. An osteochondroma has a bony stalk in which the medullary canal of the bone is continuous with the medullary canal of the stalk.

Despite chondrocyte atypia almost all cartilage lesions in the soft tissue represent chondrometaplasia of the synovial membrane. This process may occur within joints, within bursae, or within tendon sheaths. A chondrosarcoma will have its origin from the surface of the bone. The pattern of cartilage growth is irregular and there may be infiltration of soft tissues. A periosteal chondroma is a single lobule of cartilage confined to the subperiosteal region. These are rarely larger than the size of a large olive. An osteochondroma may have a cellular cartilage cap. However, the bony stalk of the osteochondroma clearly identifies it as a developmental growth rather than chondrometaplasia.