Aggressive osteoblastoma with focal malignant transformation and development of pulmonary metastases. A case report with a review of literature.

We report the case of a 57-year-old woman with an unusually fast-growing and destructive osteoblastic tumor affecting the left humeral head. On histopathologic examination, most of the initial tumor revealed the characteristic morphologic features of a benign-appearing aggressive osteoblastoma. Based upon the presence of a few small scattered areas composed of atypical osteoblasts in abundant lace-like osteoid showing vascular permeation, the definitive diagnosis was that of an osteoblastoma with focal malignant transformation to well-differentiated osteosarcoma. Molecular biologic analysis revealed a splice mutation at the exon 5 donor site of the p53 gene, clearly indicating a malignant potential of the tumor. The proximal third of the humerus was resected en bloc and replaced by an uncemented modular endoprosthesis. Five months after surgery, an extensive local soft tissue recurrence occurred. Eight months postoperatively, a further massive recurrent tumor had developed an multiple pulmonary metastases became evident. Chemotherapy caused a marked decrease in the size of the soft tissue recurrences and the lung metastases showed no further increase of their number and size. Osteoblastomas with conversion to osteosarcoma should be considered a separate clinicopathologic tumor entity to be distinguished from genuine osteosarcoma. All cases of malignantly transformed conventional and aggressive osteoblastomas reported to date have shown a conversion to low- or high-grade osteosarcomas only in recurrent tumors. The present case supports the concept that osteoblastomas may primarily undergo early malignant transformation. Osteoblastomas with conversion to osteosarcoma require an aggressive surgical approach followed by chemotherapy in the hope of prolonging life expectancy or obtaining a cure.