Taking part in routine colorectal cancer screening may greatly reduce the chance of dying from the disease, according to a large study from Sweden.
Researchers estimated that people who actually participated in screening had as much as a 43% lower risk of colorectal cancer death. The findings show why responding to a screening invitation may matter even when a person feels completely healthy.
Colorectal cancer develops in the colon or rectum, which are parts of the digestive system. It is one of the most common cancers worldwide and becomes more common with age. The disease can often be treated successfully when it is found early, before it has spread to other parts of the body.
One reason screening can be so useful is that colorectal cancer may grow quietly for years. Early tumors and precancerous growths can exist without causing pain or obvious changes in bowel habits. Waiting for symptoms can therefore allow the disease to reach a more advanced stage.
Many colorectal cancers begin as small growths called polyps in the lining of the bowel. Most polyps never become cancer, but some can slowly change over time. Finding and removing these growths during a colonoscopy can sometimes prevent cancer from developing at all.
Sweden now offers nationwide colorectal cancer screening to people between the ages of 60 and 74 every two years. Participants receive a test kit at home and provide a small stool sample. The sample is checked for tiny amounts of blood that cannot normally be seen.
Hidden blood in stool does not automatically mean someone has cancer. Bleeding can have many causes, including hemorrhoids and other bowel conditions. However, a positive result means further investigation is needed, usually with a colonoscopy.
During a colonoscopy, a doctor uses a thin flexible tube with a camera to examine the inside of the colon. Suspicious tissue can be sampled, and many polyps can be removed during the same procedure. This gives doctors a chance to detect cancer early or remove changes that might later become dangerous.
Stockholm and Gotland introduced routine screening earlier than many other parts of Sweden, beginning in 2008. That gave researchers an unusual opportunity to study what happened to people over a long period. Scientists from Karolinska Institutet and Umeå University followed participants for as long as 14 years.
The new analysis included 376,511 people. During the follow-up period, 1,668 participants died from colorectal cancer. The researchers compared people invited to screening between 2008 and 2012 with people who were invited later or were not initially invited.
Earlier evaluations had suggested that simply receiving a screening invitation was associated with a 14% lower risk of dying from colorectal cancer. With longer follow-up and more detailed statistical analysis, the researchers now estimated a larger benefit. Being invited was associated with a 26% lower risk of death from the disease.
The difference was even greater among people who actually completed the screening. Active participation was associated with a 43% lower risk of colorectal cancer death. The study was published in JAMA Network Open by researchers at Karolinska Institutet and Umeå University.
This distinction between being invited and actually participating is important. A screening program cannot find hidden cancer if the test kit remains unused at home. According to corresponding author Johannes Blom, around one-third of people offered the free test do not return a sample.
There are many possible reasons people avoid bowel cancer screening. Some may feel embarrassed about collecting a stool sample, while others may believe they do not need testing because they have no symptoms. People may also worry about the possibility of a colonoscopy or simply forget to complete the test.
However, having no symptoms is exactly why screening exists. Screening is intended for people who appear healthy, with the goal of finding disease before it becomes noticeable. Detecting cancer earlier can mean simpler treatment and a better chance of survival.
The researchers used statistical methods to deal with a complicated real-world problem. Some people who were originally in the comparison group later became eligible for screening, while some people who received invitations chose not to participate. Without accounting for these differences, the true effect of screening could be difficult to estimate.
The study has several important strengths. More than 376,000 people were included, follow-up lasted as long as 14 years, and Swedish health registers allowed researchers to track cancer deaths over time. These features provide a much stronger picture than a small or short-term study could offer.
Still, the reported 43% reduction should be interpreted carefully. People who choose to participate in screening may also differ from nonparticipants in other ways, such as health habits, use of medical care or socioeconomic circumstances. Statistical adjustments can reduce this problem but cannot guarantee that every difference has been removed.
This means the study cannot prove that completing the stool test alone caused the entire 43% reduction in mortality. Even so, the findings are consistent with the established principle that colorectal cancer screening can save lives through earlier detection and removal of precancerous growths. The large size and long follow-up make the results particularly noteworthy.
The study’s most practical message is simple. Receiving a screening invitation is useful, but completing the test is what allows the screening system to work. Increasing participation among the roughly one-third of eligible people who do not return their samples could potentially prevent additional deaths.
Colorectal cancer screening programs vary between countries, including the ages at which testing begins and how often it is offered.
People should follow the screening recommendations where they live and discuss symptoms or a strong family history with a health professional rather than waiting for routine screening. Screening is not perfect, but this study suggests that participation can make a substantial difference.
Source: Karolinska Institutet and Umeå University.
