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 mistagging 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