Osteosarcoma of the flat bone.

OBJECTIVE We aimed to understand the biology of osteosarcoma of the flat bone, which is a rare tumor entity. METHODS Cases with osteosarcoma of the flat bone were compared with those of the extremity in order to evaluate their clinicopathologic characteristics. And the influences of heterogeneous treatment modalities on outcome were analyzed. RESULTS Tumors of the flat bone comprised 91 (11.3%) of 806 osteosarcoma cases. Eight cases were secondary osteosarcoma associated with previous radiotherapy. Patients with a flat bone tumor were significantly older and more likely to present with metastases at diagnosis than extremity tumor. The proportions of female sex and chondroblastic subtype were higher among flat bone tumors than among extremity tumors. The 5-year overall survival and event-free survival rates were 35.2 +/- 5.4% and 24.7 +/- 5.5%, respectively. Although age and histologic response to pre-operative chemotherapy were not related to outcome of flat bone tumors, treatment modality influenced the survival. Patients treated surgically had better outcomes than those treated by another means. Radiation therapy did not appear to be an effective local control measure as surgery. CONCLUSIONS Treatment outcome of the tumor of the flat bone was worse than extremity tumors. Further studies are needed to identify effective local control measures that can substitute for surgery and to determine the biologic characteristics of osteosarcoma of the flat bone.