Colorectal Cancer Incidence Rising in Young Adults: An Interview with Drs. Philip Rosenberg and William Anderson
March 23, 2017, by NCI Staff
The incidence of colorectal cancer has been increasing in the United States among individuals younger than age 50, according to a recent study led by researchers at the American Cancer Society.
For the analysis, the researchers used statistical methods developed by study coauthors Philip S. Rosenberg, Ph.D., and William F. Anderson, M.D., M.P.H., of NCI’s Division of Cancer Epidemiology and Genetics. In this interview, Drs. Rosenberg and Anderson describe the analysis, the findings, and some possible next steps.
What was the goal of this study?
Dr. Anderson: The goal was to do a very comprehensive analysis of a trend that other people had noticed—an increase in colorectal cancers among a younger generation in the United States—and to do it in a way that would help to define the problem as well as identify questions to explore through future analytic studies to address the problem.
How did you conduct the analysis?
Dr. Rosenberg: We used contemporary cancer registry data from NCI's Surveillance, Epidemiology, and End Results (SEER) program. SEER basically counts all cancers within defined geographic regions—in this case, we looked at data from the SEER 9 registries, which cover approximately 10% of the US population. We know the size of the population living within those areas, so we can calculate the absolute incidence rates for cancer.
Dr. Anderson: The total incidence rate for colorectal cancer is an average of the mix of increases and decreases in incidence among different age groups and across different segments of the population. To understand which subgroups are experiencing those increases and decreases in rates, we used a technique called Age-Period-Cohort analysis. This allowed us to disentangle the effects of age at diagnosis and year of birth, screening history, and risk factors that are occurring simultaneously.
What did you find?
Dr. Anderson: We found increases in cancers of the colon and rectum among young and middle-age adults in the United States. Overall, incidence rates of colorectal cancer have been going down, but that is because the overall rate is heavily influenced by the older cohort; the younger generation is not faring as well.
Can you please put the increases in historical context?
Dr. Rosenberg: During the first half of the twentieth century, the risk of a colorectal cancer diagnosis decreased. But beginning with people born in 1950, the level of risk increased, back to the level observed among individuals born in the late 1800s.
Dr. Anderson: Among adults ages 20 to 39 years old, the increases for rectal cancer started in the mid-1970s. The increases have been rising for a longer period of time, and more quickly, than the increases for colon cancer, which started to go up in the mid-1980s. Compared with individuals who were born around 1950, those born in 1990 have twice the risk of colon cancer and four times the risk of rectal cancer.
What is known about the reasons for the increasing rates?
Dr. Rosenberg: We really don’t know what’s causing the increased incidence in the younger population. Nor do we know why the incidence rates have increased faster for rectal cancer than colon cancer. But that is the nature of these kinds of studies—what we call descriptive studies—which observe patterns in the population and develop clues for future studies.
What are some of the limitations of descriptive studies?
Dr. Anderson: Descriptive studies look for patterns in the population. We have relatively limited information on individual people. For instance, we don’t know which individuals may have had risk factors for colorectal cancer.
Dr. Rosenberg: Descriptive studies often generate more questions than they answer. But they can lead researchers to develop follow-up studies to tackle etiologic questions.
What kinds of follow-up studies would you like to see?
Dr. Anderson: The current study looked at 10% of the U.S. population. It would be interesting to see if the risk varies for certain subgroups within this population. Looking into racial differences or geographic differences might turn up clues as to the causes of the increase and point to potential research areas for focusing on prevention.
Dr. Rosenberg: Yes, it would be important to do a study that focuses on certain ethnic groups, such as African Americans, who on average are at an increased risk of colorectal cancer compared with other ethnic and racial groups.
Do the findings have any immediate implications for prevention and screening?
Dr. Anderson: These findings raise a number of questions. More research is needed to identify appropriate strategies for prevention and screening in high-risk populations.
Dr. Rosenberg: We can, however, take meaningful steps toward reducing harm from this disease by increasing awareness of colorectal cancer and encouraging individuals who experience potential symptoms to follow up with their doctors.
Dr. Anderson: We should also promote the current screening guidelines. The younger generation—under age 50—is nearing the recommended screening age, and they’re going to benefit from screening at least as much as their parents’ generation.
What are the take-home messages of this study?
Dr. Rosenberg: This study confirms—using the most recent SEER data—a trend that had been reported previously. We do not yet know the reasons for the increases in incidence rates.
Dr. Anderson: This study on its own does not indicate that the screening age should be changed. But the results raise questions that people who conduct screening research can now investigate.