While different classification systems exist for osteosarcoma staging, it is generally evaluated in three main stages:
* Stage I: Low-grade tumor; confined within the bone or with local extension to surrounding tissues.
* Stage II: High-grade tumor; confined within the bone or with local extension to surrounding tissues.
* Stage III: The tumor has metastasized (spread remotely) to another bone or organ in the body.

The treatment protocol to be applied varies according to the tumor's stage.

In Stage III osteosarcoma, i.e., when the tumor has metastasized to distant organs or bones, chemotherapy and surgical treatments are primarily applied. Metastatic tumors can also be treated with additional surgical interventions depending on the organ or bone to which they have spread.