Imaging characteristics of primary osteosarcoma: nonconventional subtypes.

Osteosarcoma (OS) is a common primary malignant tumor of bone that produces osteoid matrix. According to the World Health Organization, OS of bone is classified into eight subtypes with distinct biologic behaviors and clinical outcomes: conventional, telangiectatic, small cell, low-grade central, secondary, parosteal, periosteal, and high-grade surface. Imaging plays a crucial role in the diagnosis of each subtype of OS and ultimately in patients' survival because the diagnosis is based on a combination of histopathologic and imaging features. Conventional OS is the most common subtype of OS and is readily identified at radiography as an intramedullary mass with immature cloudlike bone formation in the metaphyses of long bones. The imaging features of less common subtypes of primary OS are variable and frequently overlap with those of multiple benign and malignant entities, creating substantial diagnostic challenges. For accurate diagnosis, it is important to be aware of radiographic and cross-sectional imaging features that allow differentiation of each nonconventional subtype of OS from its mimics. © RSNA, 2010.