Staging and Site Information
Historically, the Enneking staging system for skeletal malignancies was widely used. This system inferred the aggressiveness of the primary tumor by the descriptors intracompartmental or extracompartmental. The American Joint Committee on Cancer (AJCC) staging system for malignant bone tumors has updated this staging system, substituting compartmentalization with size (refer to Table 2). The AJCC classification is as follows:
|Stage||Tumor Grade||Tumor Size|
|aReprinted with permission from AJCC: Bone. In: Edge SB, Byrd DR, Compton CC, et al., eds.: AJCC Cancer Staging Manual. 7th ed. New York, NY: Springer, 2010, pp 281-90.|
|bSkip metastases: discontinuous tumors in the primary bone site.|
|III||Any tumor grade, skip metastasesb|
|IV||Any tumor grade, any tumor size, distant metastases|
For the purposes of treatment, there are only two stages of high-grade osteosarcoma. Patients without clinically detectable metastatic disease are considered to have localized osteosarcoma. Patients in whom it is possible to detect any site of metastasis at the time of initial presentation by routine clinical studies are considered to have metastatic osteosarcoma.
For patients with confirmed osteosarcoma, in addition to plain x-rays of the primary site that include a single plane view of the entire affected extremity to assess for skip metastasis, pretreatment staging studies should include magnetic resonance imaging (MRI) and/or computed tomography (CT) scan of the primary site or entire extremity. Additional pretreatment staging studies should include bone scan, postero-anterior and lateral chest x-ray, and CT scan of the chest. Positron emission tomography (PET) using fluorine-18-fluorodeoxyglucose is an optional staging modality.
Localized tumors are limited to the bone of origin. Patients with skip lesions confined to the bone that includes the primary tumor are considered to have localized disease if the skip lesions can be included in the planned surgical resection. Approximately one-half of the tumors arise in the femur; of these, 80% are in the distal femur. Other primary sites in descending order of frequency are the proximal tibia, proximal humerus, pelvis, jaw, fibula, and ribs. Compared with osteosarcoma of the appendicular skeleton, osteosarcoma of the head and neck is more likely to be low grade  and to arise in older patients.
Radiologic evidence of metastatic tumor deposits in the lungs, other bones, or other distant sites is found in approximately 20% of patients at diagnosis, with 85% to 90% of metastatic disease presenting in the lungs. The second most common site of metastasis is another bone. Metastasis to other bones may be solitary or multiple. The syndrome of multifocal osteosarcoma refers to a presentation with multiple foci of osteosarcoma without a clear primary tumor, often with symmetrical metaphyseal involvement. Multifocal osteosarcoma has an extremely grave prognosis.
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