The stage describes the growth and extent to which the cancer has spread at the time of diagnosis. The staging is crucial in deciding which treatment regimen to consider and plays a major role in prognosis. The two most common staging systems are the Surveillance, Epidemiology, and End Results (SEER) summary staging system and the TNM system. The SEER summary staging system is described below:
- Carcinoma in situ: cancer that has not spread beyond the layer of cells in which it began. They are pre-invasive lesions, as they have not yet begun to invade the wall of the colon or rectum.
- Localized: cancer that has grown into the wall of the colon or rectum. Although this is considered invasive cancer, it has not extended through the wall to invade nearby tissues.
- Regional: cancer that has penetrated through the wall of the colon or rectum and invaded nearby tissue, or spread to nearby lymph nodes.
- Distant: cancer that has spread to other parts of the body, such as the liver, lungs, lining of the abdomen (peritoneum), or ovaries.
The TNM staging system is most commonly used in clinical settings. The “T” stands for the primary tumor size, the “N” for whether the cancer has spread to lymph nodes, and the “M” for whether the cancer has spread to other parts of the body (metastasis). The T, N, and M are then combined to form the stage of the cancer (from stage 0 to stage IV).
Stage 0 is carcinoma in situ. At this stage, the polyp is removed via surgical resection (polypectomy) or local excision through a colonoscope. Some cases require resection of a segment of the colon if the tumor is too large to be removed by local excision alone.
Stage I and II are early stage, localized tumors. In these two stages, the tumor has penetrated through at least one layer of the colon or rectal wall. Some cancers in stage II may have grown to adjacent organs or structures. The tumor, a length of colon on either side of the tumor, and nearby lymph nodes are all surgically resected as the standard treatment.
Stage III includes cancers that have spread to multiple nearby lymph nodes. In this case, surgical resection of the segment of colon containing the tumor is the primary treatment, usually followed by chemotherapy. If the cancer has grown into adjacent tissues, radiation therapy may be recommended.
Stage IV (metastatic) colorectal cancer has spread to distant organs and tissues. Surgery is not recommended for all patients, and is usually only performed in order to relieve or prevent blockage of the colon, or to prevent other local complications. Chemotherapy, radiation, and immunotherapies may be given alone or in combination at this stage.