Editor-In-Chief: C. Michael Gibson, M.S., M.D. 
The stage of a cancer is a descriptor (usually numbers I to IV) of how much the cancer has spread. The stage often takes into account the size of a tumor, how deep it has penetrated, whether it has invaded adjacent organs, how many lymph nodes it has metastasized to (if any), and whether it has spread to distant organs. Staging of cancer is important because the stage at diagnosis is the most powerful predictor of survival, and treatments are often changed based on the stage.
The TNM Staging system
Cancer staging can be divided into a clinical stage and a pathologic stage. In the TNM (Tumor, Node, Metastasis) system, clinical stage and pathologic stage are denoted by a small 'c' or 'p' before the stage, e.g. cT3N1M0 or pT2N0.
- Clinical stage is based on all of the available information obtained before a surgery to remove the tumor. Thus, it may include information about the tumor obtained by physical examination, radiologic examination, and endoscopy.
- Pathologic stage adds additional information gained by examination of the tumor microscopically by a pathologist.
Because they use different information, clinical stage and pathologic stage are often different. Pathologic staging is usually considered the "better" or "truer" stage because it allows direct examination of the tumor and its spread, contrasted with clinical staging which is limited by the fact that the information is obtained by making indirect observations at a tumor which is still in the body. However, clinical staging and pathologic staging should complement each other. Not every tumor is treated surgically, so sometimes pathologic staging is not available. Also, sometimes surgery is preceded by other treatments such as chemotherapy and radiation therapy which shrink the tumor, so the pathologic stage may underestimate the true stage.
Considerations in staging
Correct staging is critical because treatment is directly related to disease stage. Thus, incorrect staging would lead to improper treatment, and material diminution of patient survivability. Correct staging, however, can be difficult to achieve. Pathologic staging, where a pathologist examines sections of tissue, can be particularly problematic for two specific reasons: visual discretion and random sampling of tissue. "Visual discretion" means being able to identify single cancerous cells intermixed with healthy cells on a slide. Oversight of one cell can mean misstaging and lead to serious, unexpected spread of cancer. "Random sampling" refers to the fact that lymph nodes are cherry-picked from patients and random samples are examined. If cancerous cells present in the lymph node happen not to be present in the slices of tissue viewed, incorrect staging and improper treatment can result.
New, highly sensitive methods of staging are in development. For example, the mRNA for GCC (guanylyl cyclase C), present only in the luminal aspect of intestinal epithelium, can be identified using molecular screening (RT PCR) with an astonishing degree of sensitivity and exactitude. Presence of GCC in any other tissue of the body represents colorectal metaplasia. Because of its exquisite sensitivity, RT PCR screening for GCC nearly eliminates the possibility of underestimation of true disease stage. Researchers hope that staging with this level of precision will lead to more appropriate treatment and better prognosis. Furthermore, researchers hope that this same technique can be applied to other tissue-specific proteins.
Systems of staging
Staging systems are specific for each type of cancer (e.g. breast cancer and lung cancer). Some cancers, however, don't have a staging system. Often competing staging systems exist for the same type of cancer; however, the universally-accepted staging system is that of the UICC, which has the same definitions of individual categories as the AJCC.
Systems of staging may differ between diseases or specific manifestations of a disease. (In cases where the main Wikipedia article has a specific section on staging, that section has been linked below.)
- Lymphoma: uses Ann Arbor staging
- Hodgkin's Disease: follows a scale from I-IV and can be indicated further by an A or B, depending on whether a patient is unsymptomatic or has symptoms such as fevers. It is known as the "Cotswold System" or "Modified Ann Arbor Staging System". 
For solid tumors, TNM is by far the most commonly used system, but it has been adapted for some conditions.
- Breast cancer: Uses TNM 
- Cervical and ovarian cancers: uses "FIGO" (similar to TNM). For premalignant dysplastic changes, the CIN (cervical intraepithelial neoplasia) grading system is used. 
- Colon cancer: originally consisted of four stages: A, B, C, and D (the Dukes staging system). More recently, colon cancer staging is indicated either by the original A-D stages or by TNM. 
- Kidney cancer: uses TNM 
- Cancer of the larynx: Uses TNM 
- Liver cancer: uses Stages I-IV 
- Lung cancer: uses TNM  or 
- Melanoma: TNM used. Also of importance are the "Clark level" and "Breslow depth" which refer to the microscopic depth of tumor invasion ("Microstaging"). 
- Prostate cancer: outside of US, TNM almost universally used. Inside US, Jewett-Whitmore sometimes used. .
- Non melanoma skin cancer: uses TNM 
Overall stage grouping
Overall Stage Grouping is also referred to as Roman Numeral Staging. This system uses numerals I, II, III, and IV (plus the 0) to describe the progression of cancer.
- Stage 0 carcinoma in situ.
- Stage I cancers are localized to one part of the body.
- Stage II cancers are locally advanced, as are Stage III cancers. Whether a cancer is designated as Stage II or Stage III can depend on the specific type of cancer; for example, in Hodgkin's Disease, Stage II indicates affected lymph nodes on only one side of the diaphragm, whereas Stage III indicates affected lymph nodes above and below the diaphragm. The specific criteria for Stages II and III therefore differ according to diagnosis.
- Stage IV cancers have often metastasized, or spread to other organs or throughout the body.
Within the TNM system, a cancer may also be designated as recurrent, meaning that it has appeared again after being in remission or after all visible tumor has been eliminated. Recurrence can either be local, meaning that it appears in the same location as the original, or distant, meaning that it appears in a different part of the body.
TNM Staging is used for solid tumors, and is an acronym for the words Tumor, Nodes, and Metastases. Each of these criteria is separately listed and paired with a number to indivate the TNM stage. A T1N2M0 cancer would be a cancer with a T1 tumor, N2 involvement of the lymph nodes, and no metastases (no spreading through the body).
- Tumor (T) refers to the primary tumor and carries a number of 0 to 4.
- N represents regional lymph node involvement and can also be ranked from 0 to 4.
- Metastasis is represented by the letter M, and is 0 if no metastasis has occurred or 1 if metastases are present.
Stage migration describes change in the distribution of stage in a particular cancer population induced by either a change in the staging system itself or a change in technology which allows more sensitive detection of tumor spread and therefore more sensitivity in detecting spread of disease (e.g. the use of MRI scan). Stage migration can lead to curious statistical phenomena. See Will Rogers phenomenon.
- www.cancerstaging.info - A free and user tailored online staging program to facilitate quick and accurate cancer staging
- "Staging: Questions and Answers" at the National Cancer Institute
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