How is Rectal Cancer Different from Colon Cancer?

April 23, 2021

Medically Reviewed By: Benjamin L. Schlechter, MD

Cancers of the colon and rectum are often lumped together as “colorectal cancer” because they have many features in common. However, there are important differences in the structure of the colon and rectum and how cancers of each behave and are treated.

What is the difference between the colon and the rectum?

The colon, or large intestine, is about five feet long and is encased in a relatively rigid tissue or “wrapper,” says Benjamin L. Schlechter, MD, a medical oncologist at Dana-Farber.

The rectum, which is the last six inches or so of the colon leading to the anus, is more flexible and isn’t reinforced by any outer layer. This allow the rectum to expand and contract within the pelvis to allow for the storage of stool before it is eliminated.

What are the symptoms of colorectal cancer?

Colon and rectal cancer often don’t cause symptoms in their earliest stages but symptoms that might indicate cancer include:

  • A change in bowel habits, such as diarrhea, constipation, or narrowing of the stool, that lasts more than a few days.
  • A feeling of needing to have a bowel movement that isn’t relieved by having one.
  • Rectal bleeding with bright red blood.
  • Blood in the stool that may appear dark brown or black.
  • Cramping or abdominal pain that doesn’t improve.
  • Unintended weight loss.

Catching colon and rectal cancers early is of paramount importance for a good outcome, say specialists. They can be diagnosed during a screening colonoscopy, but many patients don’t heed the warning signs that should be followed up with a doctor, says Schlechter.

The main warning sign of rectal cancer, Schlechter says, is a change in bowel habits — such as a persistent pattern of constipation, bloating or diarrhea.

“You should have the same bowel habits over your entire life. If things change suddenly, or over a few months, that’s abnormal. You need to see a doctor,” Schlechter says. These changes could represent a number of diseases, not just cancer, but all should be evaluated in a timely manner.

How is colorectal cancer treated?

The main treatment for early stage colon cancers is surgery to remove the part of the colon with the cancer and nearby lymph nodes. Chemotherapy is another modality often used for colon cancer — sometimes after surgery to kill remaining cancer cells, or for advanced cancers that have spread to other organs.

The approach to treating rectal cancer is somewhat different. Some early rectal cancers can be removed surgically, sometimes minimally invasively using instruments inserted through the anus and rectum.

“Unfortunately, when there is a cancer that penetrates the muscle wall of the rectum, it can quickly get into the space around the rectum in the pelvis; rectal cancers spread sooner than colon cancers,” Schlechter says.

Because rectal cancers rapidly spread into the pelvis and threaten vital organs, the standard initial treatment is chemoradiation. That consists of 5 ½ to 6 weeks of daily radiation combined with a low dose of chemotherapy, either as a pill or an infusion, to eradicate the cancer cells that have gotten into the pelvis.

For cancers that have grown through the wall of the rectum but not spread to the lymph nodes, many people will have surgery after the initial treatment chemotherapy and radiation. Generally, there is additional chemotherapy either before or after surgery for four months.

Rectal cancers that have spread to distant organs like the liver or lungs are classified as stage IV. There are several options for treatment, including surgery followed by chemotherapy, or chemotherapy followed by surgery, and then chemotherapy and radiation.

In some cases, surgery for rectal cancer necessitates a colostomy — an artificial opening in the abdomen for excretion of solid waste. Surgeons are making great efforts to avoid performing a colostomy, says Schlechter.

“Rectal cancer is a curable malignancy if caught early. Don’t ignore changes in bowel habits,” Schlechter says. “And it’s important to be seen by a cancer doctor where there is high quality surgery and radiation therapy.” 

About the Medical Reviewer

Benjamin L. Schlechter, MD

Dr. Schlechter is a medical oncologist who specializes in gastrointestinal cancers including colorectal cancer, anal cancer, pancreatic cancer, and neuroendocrine cancers, among others. He is a former intern, resident, chief resident and fellow at Beth Israel Deaconess Medical Center as well as a member of the faculty at Harvard Medical School. In the past he was the Director of Inpatient Hematology and Oncology at Beth Israel Deaconess Medical Center as well as the Assistant Program Director of the Internal Medicine Residency Program. His work at Dana-Farber focuses on providing excellent patient care while trying to advance the treatment of gastrointestinal cancer patients.