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| Cancer staging | |
|---|---|
| Purpose | Determining the extent to which a cancer has developed |
Cancer staging is the process of determining the extent to which acancer has grown and spread. A number from I to IV is assigned, with I being an isolated cancer and IV being a cancer that has metastasized and spread from its origin. The stage generally takes into account the size of atumor, whether it has invaded adjacentorgans, how many regional (nearby)lymph nodes it has spread to (if any), and whether it has appeared in more distant locations (metastasized).[1]
Cancer staging can be divided into a clinical stage and a pathologic stage. In theTNM (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). This staging system is used for most forms of cancer, exceptbrain tumors andhematological malignancies.
Because they use different criteria, clinical stage and pathologic stage often differ. Pathologic staging is usually considered to be more accurate because it allows direct examination of the tumor in its entirety, contrasted with clinical staging which is limited by the fact that the information is obtained by making indirect observations of a tumor which is still in the body. However, clinical staging and pathologic staging often complement each other. Not every tumor is treated surgically, so pathologic staging is not always available. Also, sometimes surgery is preceded by other treatments such aschemotherapy andradiation therapy which shrink the tumor, so the pathologic stage may underestimate the true stage.
Correct staging is critical because treatment (particularly the need for pre-operative therapy and/or for adjuvant treatment, the extent of surgery) is generally based on this parameter. Thus, incorrect staging would lead to improper treatment.
For some common cancers the staging process is well-defined. For example, in the cases of breast cancer and prostate cancer, doctors routinely can identify that the cancer is early and that it has low risk of metastasis.[2] In such cases,medical specialtyprofessional organizations recommend against the use ofPET scans,CT scans, orbone scans because research shows that the risk of getting such procedures outweighs the possible benefits.[2] Some of the problems associated with overtesting include patients receiving invasive procedures,overutilizing medical services, getting unnecessary radiation exposure, and experiencing misdiagnosis.[2]
Pathologic staging, where a pathologist examines sections oftissue, 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 onecell 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 thelymph node happen not to be present in the slices of tissue viewed, incorrect staging and improper treatment can result.
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New, highly sensitive methods of staging are in development. For example, themRNA for GCC (guanylyl cyclase c), present only in the luminal aspect ofintestinal epithelium, can be identified using molecular screening (RT-PCR) with a high degree of sensitivity and exactitude. Presence of GCC in any other tissue of the body representscolorectal metaplasia. Because of its high sensitivity, RT-PCR screening for GCC greatly reduces underestimation of disease stage. Researchers hope that staging with this level of precision will lead to more appropriate treatment and betterprognosis. Furthermore, researchers hope that this same technique can be applied to other tissue-specificproteins.
Staging systems are specific for each type of cancer (e.g.,breast cancer andlung cancer), but some cancers do not have a staging system. Although competing staging systems still exist for some types of cancer, the universally-accepted staging system is that of theUICC, which has the same definitions of individual categories as theAJCC.
Systems of staging may differ between diseases or specific manifestations of a disease.
For solid tumors, TNM is by far the most commonly used system, but it has been adapted for some conditions.
Overall Stage Grouping is also referred to asRoman Numeral Staging. This system uses numerals I, II, III, and IV (plus the 0) to describe the progression of cancer.
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.
Stage migration is a change in the distribution of stages in a particular cancer population, induced by either a change in the staging system itself or else 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 ofMRI scans). Stage migration can lead to curious statistical phenomena (for example, theWill Rogers phenomenon).