Rectal cancer is a cancer that starts in the rectum, the final 6-inch segment of the large intestine that ends at the anus. It develops when cells in the rectal lining grow out of control, typically starting as small growths called polyps that can become cancerous over time. While often grouped with colon cancer under the umbrella of “colorectal cancer,” rectal cancer is increasingly recognized as a distinct disease with its own biology, treatment approach, and surgical challenges.
How the Rectum Differs From the Colon
The rectum spans roughly 16 centimeters (about 6 inches) from the anal opening upward into the pelvis. Doctors divide it into three sections: the lower third (closest to the anus), the middle third, and the upper third. The lower two-thirds sit deep in the narrow confines of the pelvis, surrounded by the bladder, prostate or uterus, and a network of nerves that control urinary and sexual function. This tight space is what makes rectal cancer fundamentally different from colon cancer to treat.
The rectum also has a different blood supply and drainage system than the colon. Colon cancers primarily spread to the liver through veins that drain directly there, while lower rectal cancers can spread through a separate set of blood vessels connected to the internal iliac arteries. The molecular pathways that drive tumor growth also appear to differ between the two locations. These distinctions are why rectal cancer requires its own treatment strategies rather than being treated as simply another form of colon cancer.
Symptoms to Watch For
The most common warning sign of rectal cancer is blood in the stool. Because the rectum is close to the anus, this blood is usually bright red rather than the dark, tarry color associated with bleeding higher in the digestive tract. Many people dismiss this as hemorrhoids, which can delay diagnosis.
Other symptoms include:
- Thin or ribbon-shaped stools lasting more than a few days, caused by a tumor narrowing the rectal passage
- A feeling of incomplete emptying after a bowel movement, as if you still need to go
- Changes in bowel habits such as new diarrhea or constipation that persists beyond a few days
- Unexplained weight loss or fatigue
Early-stage rectal cancer often produces no symptoms at all, which is why screening matters even when you feel fine.
Who Is at Risk
Several factors raise your chances of developing rectal cancer. Some you can change; others you cannot.
On the lifestyle side, obesity is a significant risk factor. For every five-point increase in BMI, colon cancer risk rises by 12% in women and 30% in men, and the risk climbs steeply at higher weights. Diets high in red and processed meat also increase risk, with relative risk estimates ranging from about 1.2 to 1.8 depending on the study. Physical activity appears to protect against colon cancer, though interestingly it doesn’t seem to reduce rectal cancer risk to the same degree.
On the genetic side, about 5 to 10 percent of colorectal cancers are inherited. The two most well-known hereditary conditions are familial adenomatous polyposis (FAP) and Lynch syndrome. FAP causes hundreds to thousands of polyps throughout the intestine, usually appearing by age 15, and leads to cancer in nearly all affected individuals by age 40 if not treated surgically. Lynch syndrome accounts for 3 to 4 percent of all colorectal cancers and carries a 70 to 80 percent lifetime risk of developing the disease. People with Lynch syndrome also face elevated risks of cancers in other organs, including the uterus and kidneys. Having a family history of colorectal cancer, even without a named genetic syndrome, also raises your risk.
How It’s Diagnosed
Diagnosis begins with a physical exam and a look inside the rectum using a scope. A tissue sample (biopsy) taken during this exam confirms whether cancer is present. But determining how far the cancer has grown requires imaging.
Pelvic MRI is the gold standard for local staging of rectal cancer. It provides detailed images of the tumor’s depth, whether it has reached nearby structures, and how close it comes to critical surgical margins. MRI can also detect signs that the cancer has invaded blood vessels outside the rectal wall, a finding that influences treatment decisions. A CT scan of the chest and abdomen checks for spread to distant organs like the lungs or liver. Together, these tools create a full picture of the cancer’s extent.
Staging and What It Means
Rectal cancer is staged using a system that evaluates three factors: how deeply the tumor has penetrated the rectal wall (T), whether cancer has reached nearby lymph nodes (N), and whether it has spread to distant organs (M).
In simplified terms, early-stage tumors (T1 and T2) are confined to the inner layers of the rectal wall. T3 tumors have grown through the wall into surrounding fat tissue, and T4 tumors have invaded adjacent organs. Lymph node involvement is classified by how many nodes contain cancer cells: one to three (N1) or four or more (N2). Distant spread (M1) most commonly goes to the liver or lungs.
These staging details directly affect survival. Based on data from people diagnosed between 2014 and 2020, the five-year relative survival rate for rectal cancer caught while still localized (confined to the rectal wall) is 90%. When cancer has spread to nearby lymph nodes or tissues, that rate drops to 74%. For cancer that has reached distant organs, it falls to 18%. Across all stages combined, the five-year survival rate is 67%.
Treatment Approach
Rectal cancer treatment is more complex than colon cancer treatment, largely because of the rectum’s location deep in the pelvis. The standard approach for locally advanced cases (those that have grown through the rectal wall or reached lymph nodes) involves radiation and chemotherapy before surgery, not after. A landmark German trial found that giving radiation and chemotherapy before surgery cut five-year local recurrence rates roughly in half compared to giving the same treatment afterward, while also causing fewer side effects.
Two radiation schedules are commonly used before surgery. In the United States, the standard is a longer course of about five and a half weeks of daily radiation combined with chemotherapy. In Europe, a shorter course of five days of radiation is more common. Both aim to shrink the tumor and reduce the chance of cancer returning in the pelvis after surgery.
Surgery
The primary surgery for rectal cancer is called total mesorectal excision, which removes the rectum along with the surrounding envelope of fat and lymph nodes as a single, intact unit. This precise technique reduced local recurrence rates to 5 to 10 percent and became the surgical standard of care.
The specific operation depends on where the tumor sits. If the cancer is far enough from the sphincter muscles (the muscles that control bowel movements), surgeons perform a low anterior resection, removing the cancerous section and reconnecting the remaining bowel. Most patients retain normal bowel function, though it can take months for regularity to return. When the tumor involves the sphincter muscles or sits very close to the anus, an abdominoperineal resection is necessary. This removes the rectum and anus entirely and requires a permanent colostomy, where a portion of the colon is brought through the abdominal wall into a bag. High-quality MRI imaging before surgery helps determine which procedure is needed by showing exactly how close the tumor is to the sphincter complex.
Watch and Wait for Complete Responders
A growing area in rectal cancer treatment involves skipping surgery entirely for select patients. Some people respond so well to radiation and chemotherapy that no visible cancer remains, a state called a complete clinical response. These patients may be offered a “watch and wait” approach instead of surgery, with close monitoring through regular exams and imaging.
About 25 to 30 percent of patients managed this way will see the cancer regrow, most commonly within the first three years. When regrowth happens, surgery at that point still provides excellent disease control, with only about 5 percent experiencing a second recurrence. The risk of the cancer spreading to distant organs after a complete response is very low, particularly once five years have passed. This approach is best suited for tumors in the lower rectum that are accessible to physical examination, and careful imaging must confirm there is no residual cancer in lymph nodes or blood vessels.
Screening Catches It Early
The U.S. Preventive Services Task Force recommends colorectal cancer screening starting at age 45 for people at average risk. This was lowered from 50 in response to rising rates of colorectal cancer in younger adults. Screening continues through age 75, with the method and interval depending on the test. A colonoscopy, the most thorough option, is typically repeated every 10 years if results are normal. Stool-based tests that detect blood or abnormal DNA are done more frequently, usually every one to three years.
People with a family history of colorectal cancer, a personal history of inflammatory bowel disease, or a known genetic syndrome like Lynch syndrome or FAP often need to start screening earlier and be screened more frequently. The goal of screening is to find and remove polyps before they ever become cancerous, or to catch cancer at the localized stage when the survival rate is 90%.