Can an osteoblastoma become malignant?

A rare case of an osteoblastoma in the clavicle is reported, that became malignant within 3 1/2 years, after two recurrences. The clinical course of the disease and a comparison of tissue sections removed during a total of four operations point to a secondary malignant transformation of an osteoblastoma, such as has been described by Schajowicz, among others. Six years after the first diagnosis was reached and 3 1/2 years after the last resection of the clavicle and the removal of a tumour from definitely healthy surrounding tissue, and after long-term chemotherapy, the now 17-year-old youth is free of metastases and there is no sign of a recurrence of the tumour. The problem of differentiating this kind of tumour from other types is discussed in light of international literature on the subject, and therapeutic recommendations are made.