AJCC staging system
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The AJCC staging system is a classification system developed by the American Joint Committee on Cancer for describing the extent of disease progression in cancer patients. It utilizes in part the TNM scoring system: Tumor size, Lymph Nodes affected, Metastases.
Cancers are classified at different stages depending on how advanced tumours have progressed. Staging describes the extent or severity of an individual's cancer based on the extent of the original (primary) tumor and the extent of spread in the body. Knowing the state of the disease helps the doctor plan a treatment and determine a prognosis (likely outcome or course of the disease). Staging provides a common language with which doctors can communicate about a patient's case. Knowing the stage is important in identifying clinical trials that may be suitable for a particular patient.
Common elements of staging systems
Staging is based on knowledge of the way cancer develops. Some staging systems cover many types of cancer; others focus on a particular type. For most cancers, the stage is based on three main factors:
- Location of the primary (original) tumor
- Tumor size and number of tumors
- Lymph node involvement (whether or not the cancer has spread to the nearby lymph nodes)
- Presence or absence of metastasis (whether or not the cancer has spread to distant areas of the body)
Cancers of the brain and spinal cord are classified according to cell type and grade. Different staging systems are used for many cancers of the blood or bone marrow such as lymphoma.
Doctors gather different types of information about a cancer to determine its stage. The various tests used for staging depend on the type of cancer. Tests include the following:
- Physical exams provide clues as to the extent of the cancer. The physical exam may determine the location and size of the tumor(s) and the spread of the cancer to the lymph nodes and/or to other organs.
- Imaging tests such as x-rays, CT scans, and MRI scans produce pictures of areas inside the body and can show the location of the cancer, the size of the tumor, and whether the cancer has spread. These studies are important in determining stage.
- Laboratory tests are studies of blood, urine, other fluids and tissues removed from the body. These tests can also provide information about the cancer.
- Pathology reports may include information about the size of the tumor, the growth into other tissues and organs, the type of cancer cells, and the grade of tumor (how closely the cancer cells resemble normal tissue). Removing tumors or pieces of tumors and looking at them under the microscope often confirms the diagnosis of cancer, and can also stage the cancer.
- Surgical reports tell what is found during surgery. Samples are removed during surgery to determine the size and appearance of the tumor and often include observations about lymph node and organ involvement.
Different types of staging
There are three different types of staging:
- Clinical Staging determines how much cancer there is based on the physical examination, imaging tests, and biopsies of affected areas.
- Pathologic Staging can only be done on patients who have had surgery to remove or explore the extent of the cancer. This type of staging combines the results of both the clinical staging (physical exam, imaging test) with the results from the surgery.
- Restaging is used to determine the extent of the disease if a cancer comes back after treatment. This is done to determine what the best treatment option would be at the time. This type of staging is not common.
TNM Staging System
The TNM Staging System is one of the most commonly used staging systems. This system was developed and is maintained by the American Joint Committee on Cancer (AJCC) and adopted by the Union for International Cancer Control (UICC). The TNM classification system was developed as a tool for doctors to stage different types of cancer based on certain standard criteria.
The TNM Staging System is based on the extent of the tumor (T), the extent of spread to the lymph nodes (N), and the presence of metastasis (M).
The T category describes the original (primary) tumor.
TX - Primary tumor cannot be evaluated
T0 - No evidence of primary tumor
Tis - Carcinoma in situ (early cancer that has not spread to neighboring tissue)
T1–T4 - Size and/or extent of the primary tumor
The N category describes whether or not the cancer has reached nearby lymph nodes.
NX - Regional lymph nodes cannot be evaluated
NO - No regional lymph node involvement (no cancer found in the lymph nodes)
N1-N3 - Involvement of regional lymph nodes (number and/or extent of spread)
The M category tells whether there are distant metastases (spread of cancer to other parts of the body).
MO - No distant metastasis (cancer has not spread to other parts of the body)
M1 - Distant metastasis (cancer has spread to distant parts of the body)
Each cancer type has its own classification system, so letters and numbers do not always mean the same thing for every kind of cancer. Once the T, N, and M are determined, they are combined, and an overall "Stage" of I, II, III, IV is assigned. Sometimes these stages are subdivided as well, using letters such as IIIA and IIIB.
Stage I cancers are the least advanced and often have a better prognosis (outlook for survival). Higher stage cancers are often more advanced, but in many cases can still be treated successfully.
Changes in cancer stage
The formal "stage" of a cancer does not change over time, even if the cancer progresses. A cancer that returns or spreads is still referred to by the stage it was given when it first diagnosed. Sometimes, after a period of remission (cancer being undetectable) for certain cancers, if more treatment is planned, a doctor might restage the cancer. The same process that was done when the cancer was first diagnosed will be repeated: exams, imaging tests, biopsies, and possibly surgery to restage the cancer. If the cancer is restaged, the new stage will be recorded with a lower case "r" before the restaged designation. As previously stated, this is not done often. 
-  AJCC official website
AJCC staging system 7 edition 2011
- AJCC staging system entry in the public domain NCI Dictionary of Cancer Terms