OSTEOCHONDROMALIKE PAROSTEAL OSTEOSARCOMA - A REPORT OF 6 CASES OF A NEW ENTITY

OBJECTIVE. Our purpose was to describe a rare juxtacortical bone sarco ma with deceptively benign, osteochondromalike histologic characterist ics. We present criteria by which this low-grade malignant neoplasm ca n be distinguished from other benign and malignant surface lesions of bone with particular emphasis on the imaging features, MATERIALS AND M ETHODS. Six cases of a low-grade, chondroossifying parosteal sarcoma o f bone were reviewed. Patients included four males and two females 11 months to 66 years old. Histologic findings from initial tumors and fr om recurrent tumors were re viewed. Two musculoskeletal radiologists a nalyzed the imaging studies, which included plain films, CT scans, MR images, and a bone scan. RESULTS. Histologically, the lesions were cha racterized by a thin layer of proliferating, periosteally derived spin dle cells overlying a thin, low-grade malignant cartilage cap that und erwent calcification, neovascularization, and conversion into benign b one and marrow fat. These lesions were unique in that the malignant el ements were only at their periphery. All six cases were initially misd iagnosed as benign lesions on pathologic evaluation. In each patient, imaging revealed a ''pasted-on'' ossified surface lesion with an intac t underlying cortex and no medullary involvement. In three cases, recu rrent tumors had histologic appearances consistent with conventional p arosteal osteosarcoma. Dedifferentiation, metastases, and death occurr ed in one of these three cases. CONCLUSION. To our knowledge, this sur face lesion of bone has not been specifically described. Whether this tumor constitutes a distinct entity or is a specialized variant of par osteal osteosarcoma is unclear. Precise radiologic-pathologic correlat ion is essential for appropriate diagnosis and management.