Osteoblastoma with cartilaginous matrix. An unusual morphologic presentation in 18 cases.

By definition, the occurrence of cartilaginous matrix in a bone-forming tumor eliminates the diagnosis of osteoblastoma and suggests a possible diagnosis of osteosarcoma. However, this rule is not absolute, as demonstrated by 18 cases of osteoblastoma that produced cartilaginous matrix. Hyaline cartilage was detected in four cases, and chondro-osteoid material was detected in 14 cases. In all patients, the tumor was in the appropriate patient age range and location for an osteoblastoma. The radiographic features as well as the histology, with the exception of the cartilaginous components, were in keeping with diagnosis of osteoblastoma. Intralesional excision (curettage or debulking) was performed in 13 patients and wide resection in five patients. Persistent tumor in one patient required a further intralesional excision. There were four recurrences, and no metastases were reported after follow-up of 1 to 23 years. The presence of cartilaginous matrix in a bone-forming lesion does not exclude the diagnosis of osteoblastoma and, more importantly, it is not always an indication that the tumor is an osteosarcoma.