Staging is a process the doctor uses to determine how far the cancer might have spread. The treatment and prognosis (outlook) for osteosarcoma depend, to a large extent, on the tumor's stage when the patient is first diagnosed.

The stage of an osteosarcoma is based on the results of imaging tests and biopsies of the main tumor and possibly other tissues, which were described in the section "How are osteosarcomas diagnosed?"

A staging system is a standardized way in which the cancer care team describes the extent of the cancer. There are 2 formal staging systems sometimes used to describe the extent of an osteosarcoma, which are described below. But in practice, doctors often divide osteosarcomas into 2 groups -- localized and metastatic -- when deciding on the best course of treatment.

Localized versus metastatic osteosarcoma

Localized osteosarcoma

A localized osteosarcoma is seen only in the bone it starts in and possibly the tissues next to the bone, such as muscle, tendon, or fat. About 4 out of 5 osteosarcomas are thought to be localized when they are first found. But even when imaging tests do not show that the cancer has spread to distant areas, most patients are likely to have micrometastases (very small areas of cancer spread that can't be detected with tests), which is why chemotherapy is an important part of treatment for most osteosarcomas. If it isn't given, the cancer might be more likely to come back after surgery.

Doctors further divide localized osteosarcomas into 2 groups. Resectable cancers are those in which all of the visible tumor can be removed by surgery. Non-resectable (or unresectable) osteosarcomas cannot be completely removed by surgery.

Metastatic osteosarcoma

A metastatic osteosarcoma has clearly spread to other parts of the body such as the lungs or to other bones not directly connected to the bone the tumor started in. Most often it spreads to the lungs, but it can also spread to other bones, the brain, or other internal organs. About 1 out of 5 osteosarcoma patients has metastases at the time of diagnosis.

Patients who already have visible metastases when they are first diagnosed are harder to treat, although some can be cured if the metastases can be removed by surgery. The cure rate for these patients improves markedly if chemotherapy is also given.

Surgical Staging System

One system commonly used to stage osteosarcoma is the Surgical Staging System, also known as the Enneking system. It is based on 3 key pieces of information:

• The grade (G) of the tumor
• The extent of the original (primary) tumor (T)
• If the tumor has metastasized (spread) (M) to nearby lymph nodes (bean-sized collections of immune system cells that fight infections and cancers) or other organs

The grade (how likely the cells are to grow and spread, based on how they look under the microscope) is divided into low grade (G1) and high grade (G2).

The extent of the primary tumor is classified as either intracompartmental (T1), meaning it has basically remained in place, or extracompartmental (T2), meaning it has extended beyond the bone into other nearby structures.

Tumors that have not spread to the lymph nodes or other organs are considered M0, while those that have spread are M1.

These factors are combined to give an overall stage, designated in Roman numerals from I to III. Stages I and II are further divided into A for intracompartmental tumors or B for extracompartmental tumors.

In summary:

• Low-grade, localized tumors are stage I.
• High-grade, localized tumors are stage II.
• Metastatic tumors (regardless of grade) are stage III.

AJCC staging system

Another system sometimes used to stage bone cancers is the American Joint Commission on Cancer (AJCC) system. The AJCC uses one system to describe all bone cancers, including osteosarcomas. The AJCC staging system for bone cancers is based on 4 key pieces of information:

• T describes the size of the main (primary) tumor and whether it appears in different areas of the bone.

• N describes the extent of spread to nearby (regional) lymph nodes (small bean-shaped collections of immune system cells that are important in fighting infections). Bone tumors rarely spread to the lymph nodes.

• M indicates whether the cancer has metastasized (spread) to other organs of the body. (The most common sites of spread are to the lungs or other bones.)

• G stands for the grade of the tumor, which is a description of how the cells look under a microscope. Low-grade tumor cells look more like normal cells, and are less likely to grow and spread quickly, while high-grade tumor cells look more abnormal.

Numbers after T, N, M, and G provide more details about each of these factors.

T categories of bone cancer

N categories of bone cancer

M categories of bone cancer

Grades of bone cancer

Note: The grades used for the AJCC system are different from those in the Surgical Staging System. There are other differences between the systems as well. To avoid confusion, it may help to ask your (child's) doctor which staging system he or she uses.

Stage grouping

Once the T, N, and M categories and the grade of the bone cancer have been determined, the information is combined and expressed as an overall stage. The process of assigning a stage number is called stage grouping. The stages are described in Roman numerals from I to IV (1 to 4), and are sometimes divided further.

Stage IA

T1, N0, M0, G1-G2: The tumor is confined to the bone, is 8 cm across or less, and is low grade.

Stage IB

T2-T3, N0, M0, G1-G2: The tumor is confined to the bone. It is larger than 8 cm across or has "skipped" to other sites in the same bone. It is low grade.

Stage IIA

T1, N0, M0, G3-G4: The tumor is confined to the bone, is 8 cm across or less, and is high grade. Stage IIB

T2, N0, M0, G3-G4: The tumor is confined to the bone, is larger than 8 cm across, and is high grade. Stage III

T3, N0, M0, G3-G4: The tumor is confined to the bone but has "skipped" to other sites in the same bone. It is high grade. Stage IVA

Any T, N0, M1a, any G: The tumor has spread only to the lungs. It has not spread to the lymph nodes or to other distant sites. (It can be any size or grade.) Stage IVB (if either of these applies)

Any T, N1, any M, any G: The tumor has spread to lymph nodes. It can be any size or grade, and may or may not have spread to other distant sites.

Any T, any N, M1b, any G: The tumor has spread to distant sites other than the lung. It can be any size or grade.

Last Medical Review: 01/14/2009
Last Revised: 01/14/2009