Compared with conventional osteosarcoma, parosteal osteosarcoma differs in prevalence with regard to sex and age.
Medullary extension of parosteal osteosarcoma has been seen in 22% to 58% of patients, and this finding itself is not considered significant in the prognosis.
Grossly, parosteal osteosarcomas are hard ossified masses arising from the cortical surface.
Microscopically, parosteal osteosarcomas have a biphasic appearance composed of mature trabecular bone and fibroblastic spindle cell stroma.
Cartilage differentiation is encountered in parosteal osteosarcoma in up to 55% of cases.
Early studies (13,14) demonstrated characteristic cytogenetic change with supernumerary ring chromosomes or giant marker chromosomes carrying amplified DNA materials from the 12q13-15 region in parosteal osteosarcoma. Subsequent analysis (15) identified the amplified genes, including CDK4 and MDM2, which are important in cell cycle regulation.
Parosteal osteosarcoma is one of the most frequently misdiagnosed entities in bone and soft-tissue tumors.
High-grade surface osteosarcoma may be grossly similar to parosteal osteosarcoma; however, the discrimination is readily observed microscopically.
Grade 1 (Low Grade) Low-grade central osteosarcoma Parosteal osteosarcoma
Adamantinoma Grade 2 Periosteal osteosarcoma Grade 3 (High Grade) Ewing sarcoma/PNET Conventional osteosarcoma Telangiectatic osteosarcoma Mesenchymal chondrosarcoma Small cell osteosarcoma Secondary osteosarcoma High-grade surface osteosarcoma Dedifferentiated chondrosarcoma Dedifferentiated chordoma Malignant giant cell tumor Variable Grade