Osteosarcoma Stages

After someone is diagnosed with osteosarcoma, doctors will try to figure out if it has spread, and if so, how far. This process is called staging. The stage describes how much cancer is in the body. It helps determine how serious the cancer is and how best to treat it. Doctors also use a cancer's stage when talking about survival statistics.

The stage of an osteosarcoma is based on the results of physical exams, imaging tests, and any biopsies that have been done, which are described in Tests for Osteosarcoma.

Localized versus metastatic

Doctors use formal staging systems (see below) to describe the extent of an osteosarcoma in detail. (A staging system is a standard way to describe the extent of the cancer.) But when trying to figure out the best treatment, doctors often use a simpler system that divides osteosarcomas into 2 main groups: localized and metastatic.

Localized osteosarcoma

A localized osteosarcoma is seen only in the bone it started in and possibly the tissues next to the bone, such as muscle, tendon, or fat.

About 4 out of 5 osteosarcomas appear to be localized when they are first found. But even when imaging tests don’t show that the cancer has spread to distant areas, there are likely to be very small areas of cancer spread that can’t be detected with tests. This is why chemotherapy is an important part of treatment for most osteosarcomas. If it isn’t given, the cancer is more likely to come back after surgery.

Doctors further divide localized osteosarcomas into 2 groups:

• Resectable osteosarcomas are those in which all of the visible tumor can be removed (resected) by surgery.
• Non-resectable (or unresectable) osteosarcomas can’t be removed completely by surgery.

Metastatic osteosarcoma

A metastatic osteosarcoma has clearly spread to other parts of the body. Most often it spreads to the lungs, but it can also spread to other bones, the brain, or other organs.

About 1 out of 5 osteosarcomas have spread already when they are first diagnosed. These cancers are harder to treat, but some can be cured if the metastases can be removed by surgery. The cure rate for these cancers improves markedly if chemotherapy is also given.

Musculoskeletal Tumor Society (MSTS) staging system

A system commonly used to stage osteosarcoma is the MSTS system, also known as the Enneking system. It is based on 3 key pieces of information:

• The grade (G) of the tumor, which is a measure of how likely it is to grow and spread, based on how it looks under the microscope. Tumors are either low grade (G1) or high grade (G2). 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.
• The extent of the primary tumor (T), which is classified as either intracompartmental (T1), meaning it has basically remained within the bone, or extracompartmental (T2), meaning it has extended beyond the bone into other nearby structures.
• If the tumor has metastasized (M), which means it has spread to other areas, either to nearby lymph nodes (bean-sized collections of immune system cells) or other organs. 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, using Roman numerals from I to III. Stages I and II are further divided into A for intracompartmental tumors or B for extracompartmental tumors.

Stage	Grade	Tumor	Metastasis
IA	G1	T1	M0
IB	G1	T2	M0
IIA	G2	T1	M0
IIB	G2	T2	M0
III	G1 or G2	T1 orT2	M1

In summary:

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

The TNM staging system

Another system sometimes used to stage bone cancers (including osteosarcomas) is the American Joint Commission on Cancer (AJCC) TNM system. This system is based on 4 key pieces of information:

• T describes the size of the main (primary) tumor and if it appears in different areas of the bone.
• N describes the extent of spread to nearby (regional) lymph nodes. Bone tumors rarely spread to the lymph nodes.
• M indicates if 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 describes 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 give more details about each of these factors.

Once the T, N, and M categories and the grade of the bone cancer have been determined, the information is combined into an overall stage. These stages (which are different from those of the MSTS system) are described using Roman numerals from I to IV (1 to 4), and are sometimes divided further. For more detailed information on the AJCC TNM staging system for bone cancers, see Bone Cancer Stages.

Osteosarcoma staging can be confusing. If you have any questions about the stage of the cancer, ask a member of the cancer care team to explain it to you in a way you understand.