Osteosarcoma multidisciplinary approach to the management from the pathologist's perspective.

Osteosarcoma is a primary malignant tumor of the bone in which proliferating neoplastic cells produce osteoid and/or bone, if only in small amounts. This histological principle defines a tumor that usually affects young males more frequently than females, and disproportionately involves the long bones of the appendicular skeleton. These tumors are generally locally aggressive and tend to produce early, lethal systemic metastases. However, osteosarcoma is not a single disease but a family of neoplasms, sharing the single histological finding of osseous matrix production in association with malignant cells. The majority (i.e., 75%) of cases are relatively stereotypical from the demographic, clinical, radiographic and histologic points of view. These tumors generally occur in the metaphyseal portion of the medullary cavity of the long bone and are referred to as "Conventional Osteosarcoma." The group is sub classified by the form of the dominant matrix present within the tumor, which may be bone, cartilage or fibrous tissue, and it is correspondingly referred to as osteoblastic, chondroblastic and fibroblastic osteosarcoma. The remaining 25% of cases have unique parameters that allow reproducible identification of tumors which are biologically different from conventional osteosarcoma and are referred to as "Variants." The parameters identifying Variants fall into one of three major groups: (1) clinical factors, (2) histologic findings and (3) location of origin--within or on the cortex. Because of their inherent biological difference from Conventional Osteosarcoma, the Variants identify cases which must be excluded from analysis of data pertaining to the treatment of the majority of cases: Conventional Osteosarcoma. The diagnostic parameters of osteosarcoma must be sufficiently inclusive to identify all the members of this potentially lethal tumor. Conversely, criteria for sub classification must be restricted to assure homogenous populations of tumors productively incorporating different biological behavior and the potential for development of unique treatment strategies which are different from those for Conventional Osteosarcoma. This can be designated "Classification Based Therapy" or "Therapy Based Osteosarcoma." With this background, we will discuss the highly disciplined approach to the management of osteosarcoma from the pathologist's perspective. Factors governing the assessment of the response to preoperative chemotherapy will also be reviewed.