Stages and subtypes of colorectal cancer

Doctors use the stage and subtype of colorectal cancer to create a plan to treat it.

Colorectal cancer stages

Doctors measure the stage of colorectal cancer by looking at the following: 

• how far it has grown into the wall of the colon or rectum. Most colon cancers start in the inner lining of the colon/rectum, known as the mucosa.
• whether it has spread to nearby lymph nodes and, how many lymph nodes contain cancer cells.
• whether it has spread to other organs beyond the colon/rectum and nearby lymph nodes and the location and number of sites in the body where it has spread.

The different stages are: 

• Stage 0 cancers are non-invasive, also called "in situ." The abnormal cells have not grown beyond the innermost layer of the colon/rectum. Cancers caught and removed at stage 0 are very treatable.
• Stage 1 cancers have grown beyond the inner layer of the colon or rectum, but have not penetrated through the muscle layer of the large intestine and have not spread to nearby lymph nodes or elsewhere in the body.
• Stage 2 cancers have grown through the muscle layer of the colon or rectum, but have not spread to nearby lymph nodes or elsewhere in the body. More advanced stage 2 cancers may grow through all the layers of the colon and attach to nearby tissue or organs.
• Stage 3 cancers have spread to one or more nearby lymph nodes or the tissue near the lymph nodes, but have not spread elsewhere in the body. More advanced stage 3 cancers have spread to a larger number of lymph nodes or have spread to nearby organs.
• Stage 4 or metastatic cancers have spread to distant organs or the distant lining of the abdomen, known as the peritoneum. The most common site for metastasis is the liver. 

Doctors use imaging of the chest, abdominal and pelvis, using CT, MRI, and PET scans to learn if the cancer has spread beyond the colon or rectum. A pathologist looks closely at the types of cells in the biopsy sample for additional clues on how to best treat it. 

Colorectal cancer subtypes

• Adenocarcinomas are the most common subtype of colorectal cancer, making up 95 percent of all colorectal cancers. These cancers usually start as an abnormal growth known as a polyp. Some, but not all polyps  can become cancerous if they are not removed. Polyps found during colonoscopies are removed and examined to see if they are cancerous or if they were likely to become cancerous. Adenocarcinomas may be found in people with Lynch syndrome, and other genes linked to hereditary colorectal cancer. 

Less common types of colorectal cancer include :

• Mucinous adenocarcinoma is a subtype of colorectal adenocarcinoma. When doctors look at these tumors under a microscope, about half will be made up of mucus. Mucinous adenocarcinomas may be more common in people with Lynch syndrome.  
• Gastrointestinal carcinoid tumors are a type of cancers called neuroendocrine tumors (NETs). Carcinoid tumors make up 1 percent of all colorectal cancers. Inherited mutations in the MEN1 gene or the NF1 gene have been linked to neuroendocrine cell cancers. NETs can also develop elsewhere in the GI tract (e.g., stomach, small intestine, pancreas) and in other organs (e.g., lung).
• Gastrointestinal stromal tumors (GISTs) are a rare type of colorectal cancer that forms in the cells that line the gastrointestinal tract. Most GISTs begin in the stomach, but they can also be found in the rectum, small intestine or colon.  

Additional tests such as biomarker tests and genetic tests can help guide the choice of immunotherapy or targeted therapy.