Osteosarcoma

+ -Text Size

Early Detection, Diagnosis, and Staging TOPICS

How is osteosarcoma staged?

Staging is the process of finding out how far a cancer has spread. The treatment and prognosis (outlook) for osteosarcoma depend, to a large extent, on the stage of the cancer when it is first diagnosed.

The stage of an osteosarcoma is based on the results of physical exams, imaging tests, and any biopsies that have been done, which were described in the section “How is osteosarcoma diagnosed?

A staging system is a standard way for the cancer care team to sum up the extent of the cancer. Two formal staging systems, described below, are sometimes used to describe the extent of an osteosarcoma. But doctors often use a simpler system that divides 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 started 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. Osteosarcomas that cannot be completely removed by surgery are called non-resectable (or unresectable).

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 organs.

About 1 out of 5 osteosarcoma patients has metastatic spread at the time of diagnosis. These patients are harder to treat, but 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.

Musculoskeletal Tumor Society (MSTS) Staging System

One 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 of the tumor (G)
  • The extent of the main (primary) tumor (T)
  • If the tumor has metastasized (spread) to nearby lymph nodes (bean-sized collections of immune system cells) or other organs (M)

The grade of a tumor 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).

The extent of the primary tumor 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.

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.

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). 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

T0: There is no evidence of a main (primary) tumor.

T1: The tumor is 8 cm (around 3 inches) across or less.

T2: The tumor is larger than 8 cm across.

T3: The tumor has “skipped” to another site or sites within the same bone.

N categories of bone cancer

N0: The cancer has not spread to regional (nearby) lymph nodes.

N1: The cancer has spread to nearby lymph nodes.

M categories of bone cancer

M0: There is no distant metastasis.

M1: Distant metastasis (spread of the cancer to tissues or organs far away from the original bone tumor).

    M1a: The cancer has spread only to the lung.

    M1b: The cancer has spread to other distant sites in the body.

Grades of bone cancer

Note: The grades used for the AJCC system are different from those in the MSTS 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.

G1, G2: Low grade

G3, G4: High grade

Stage grouping

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. 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 or G2: The tumor is 8 cm across or less and is low grade. It has not spread to nearby lymph nodes or to distant parts of the body.

Stage IB

T2-T3, N0, M0, G1 or G2: The tumor is larger than 8 cm across or has “skipped” to other sites in the same bone. It is low grade. It has not spread to nearby lymph nodes or to distant parts of the body.

Stage IIA

T1, N0, M0, G3 or G4: The tumor is 8 cm across or less and is high grade. It has not spread to nearby lymph nodes or to distant parts of the body.

Stage IIB

T2, N0, M0, G3 or G4: The tumor is larger than 8 cm across and is high grade. It has not spread to nearby lymph nodes or to distant parts of the body.

Stage III

T3, N0, M0, G3 or G4: The tumor has “skipped” to other sites in the same bone. It is high grade. It has not spread to nearby lymph nodes or to distant parts of the body.

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 might or might 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: 01/08/2013
Last Revised: 02/06/2014