Cancer is the leading cause of death in many countries including South Korea [1]. Currently, there are 81,203 annual mortalities due to cancer which is 27.5% of the total deaths in Korea [2].

To reduce the burden of cancer on society, Korea has implemented a nationwide colorectal screening program as part of the National Cancer Screening Program since 1999 and has included colorectal cancer as part of this program since 2004 [3]. The program primarily focuses on low-income groups and provides an annual fecal occult blood test for adults aged 50 years or older for Medical Aid recipients and National Health Insurance beneficiaries within the lower 30% (broadened to lower 50% in 2005) income bracket. It also covers a subsequent colonoscopy or doublecontrast barium enema study for those with a positive occult blood test. The program indirectly affected the higher income bracket by promoting the need for screening through advertisements to the general public. All procedure-related costs are paid by the individuals in these instances; however, the fees were kept affordable by the government for ease of accessibility. This has markedly increased survival outcomes through early detection and better treatment strategies. Currently, colorectal cancer is the fourth leading cause of cancer in Korea with 29,030 new cases each year [2]. And it is also one of the leading groups in the world in terms of 5-year relative survival [4].

Despite these efforts, colorectal cancer is still the 3rd leading cause of mortalities among cancer patients [2]. And recent data of the past 15 years showed that 82.1% of all colorectal cancer patients have still died due to the primary cancer [5]. This discrepancy in treatment outcomes and mortalities shows there are still obstacles to overcome. To approach this matter, we sought to find trends in the mortality rate among colorectal cancer patients in Korea with respect to the year of mortality, primary tumor loci, and age group.

Our study reported trends in colorectal cancer mortalities over a 20-year period with respect to sex, primary loci, and age group. Our findings showed a biphasic increase in crude mortality rates where the second decade was slower in increments compared to the first decade. ASMR showed a decrease over the second decade after an initial increase in the first decade. The decrease was primarily from lowering of ASMR for rectosigmoid cancers contrary to right- and left-sided colon cancers remaining generally leveled out during the second decade. Age group analysis showed lowering of AMSR mainly in the 45–59-year, 60–74-year, and 75 years and older age groups; however, the 0–29-year and 30–44-year age groups showed generally unchanged ASMR over the total period.

Despite advancements in treatment and implementation of a nationwide screening program, the crude mortality rate has steadily increased during the observed period. Korea is one of the fastest aging countries in the world earning the title of aged society by World Health Organization definitions in 2017 (14% of the total population over 65 years of age) and is projected to reach superaged society (21% of total population over 65 years of age) within 10 years [9]. The total mortalities of ages 60 years and older comprised 83.5% (7,407 of 8,869) of the total colorectal cancer mortalities in 2020. This was up from 71.2% (2,970 of 4,174) in 2000 and 79.1% (6,049 of 7,645) in 2010. The shift toward an aging population was the primary driver of higher crude mortality rates in the observed period and may continue in the years to come.

The ASMR showed an increase up to 2003, leveled out from 2003 to 2010, and decreased mainly during the second decade. This latent decrease has caused some concerns about the effect of the nationwide screening program in Korea in terms of cancer mortality prevention during the first decade of implementation [10]. However, numerous studies have shown the effect of active screening programs on lowering mortality from colorectal cancer in the long term [11-13]. Additionally, the change was not instantaneous and required leading time until the decrease in mortality was achieved [13, 14]. This trend was also evident in the Korean population and eventually ASMR decreased during the second decade. While the initiation of a nationwide screening program may have a primary role in lowering colorectal cancer ASMR through early detection and treatment of cancerous lesions in their earlier state, there are additional policy-induced contributing factors to the decrease in ASMR over the years including increasing awareness for higher adherence rates of screening [3, 15], better treatment options available through insurance coverage [16], and nationwide quality assessment of the treatment given to colorectal cancer patients [17]. This has in turn led to a higher standard of care where Korea is now one of the leading countries in terms of colorectal cancer treatment and survival rates [18, 19].

Despite these strengths, our findings show areas where improvement can be made. While the decrease in AMSR of rectosigmoid colon cancers started early and persisted throughout the second decade, the ASMR of right- and left-sided colon cancers has remained generally unchanged during the second decade. Additionally, mortalities and AMSR were higher for females than it was for males for right-sided colon cancer. Right-sided colon cancers are known to be predominantly female and have a higher proportion of genetic predisposition [20]. A better understanding of the pathogenesis and pathophysiologic properties of right- and left-sided colon cancers vs. rectosigmoid cancers will provide insight into the stagnation of ASMR in recent years.

Equally important regarding sexual differences, is that there is still a male predominance in left-sided and rectosigmoid colon cancer mortalities. It has been presumed that differences in risk factor exposure among sexes may have contributed to incidence and mortality differences. Bridging this gap among the sexes is also of importance. Further studies regarding the incidence and mortality per primary loci and age group among sexes may provide insight toward this matter.

Another feature is the relatively unchanged mortality trend of early-onset colorectal cancer. The incidence of young colorectal cancer is increasing in recent decades [21] and recent changes in recommendations have been made for the starting age of screening for colorectal cancer in various societies [22, 23] including Korea [24]. However, there are concerns that ever lowering the screening age may not be an economical choice and individual approaches should be made for screening those at high risk.

Limitations of this study were first, the lack of lead time or cancer stage information when the patient was first diagnosed. As discussed, the lead time was the primary cause of the delay in screening program initialization and its effects on lowering ASMR. Without this data, it is hard to distinguish mortalities that were from initially late-stage colorectal cancer from patients who received all treatment but eventually recurred or metastasized. Second is the high proportion of primary site unspecified colorectal cancer in the first decade. It is unclear whether this was the result of high proportion of initially late-stage colorectal cancers or masked results from the practice habits of physicians. Nonetheless, the proportion was lowered and was a downward trend during the second decade. Third is the usage of 2005 mid-year population of Korea as the standard population prohibiting direct comparison of ASMR rates to other countries. Although the standard population may be an arbitrary population, many international age-specific rates are derived from the Segi standard population [7] to provide direct comparison among populations in a temporal and geographical sense. However, it is criticized that this reference population does not accurately reflect the population in the present especially in the elderly. We thus used our national census data to better reflect a rapidly aging society.

After a brief incline of age-specific mortality of colorectal cancers during the early 2000s, colorectal cancer mortality has gradually been decreasing in the past decade. This was mainly due to decreased mortalities in rectosigmoid colon cancers, especially in the age groups that were the target of early screening.