Despite of substantial advancements in the diagnosis and treatment technologies for cancer, cancer remains a prominent global burden, ranking among the most severe chronic illnesses worldwide. The global burden of disease study revealed that, in 2013, there were 14.9 million reported cancer incidence cases and 8.2 million cancer-related fatalities on a global scale [1]. In South Korea, in 2016, approximately 220,000 individuals were newly diagnosed with cancer, and around 78,000 died from cancer [2].

Several studies reported that there is socioeconomic inequity not only in the treatment of cancer but also in its incidence [3-10]. Several research conducted in Europe have indicated that individuals with low socioeconomic status tend to exhibit higher cancer incidence compared to individuals with high socioeconomic status [6-8].

South Korea is a nation with relatively low financial burdens regarding cancer diagnosis and treatment. The National Health Insurance (NHI) mandates that all citizens enroll in health insurance and provides financial assistance through the medical aid (MA) for those unable to afford medical insurance premiums, Consequently, South Korea achieved universal health coverage (UHC) [11]. Both the NHI and the MA in South Korea are encouraged to get regular cancer screening every 2 years. The out-of-pocket expenses for cancer screening are minimal, with the NHI covering up to 10% of the cost, and the MA offering screening at no cost to promote early cancer detection. Once a cancer diagnosis is confirmed, individuals with cancer are only required to bear 5% of their medical expenses.

The National Cancer Registry is very efficient and provides national cancer statistics. The National Cancer Registry provides highly accurate and detailed information on cancer incidence, mortality, and survival rates by cancer type, age group, sex, and region, including personal information such as patient’s social security number, address, and occupation, as well as various clinical information such as surveillance, epidemiology, and end results stage, metastasis, and differentiation, as well as date and cause of death and type of treatment.

However, this data does not include a variable to categorize the income level of patients, making it difficult to identify differences by economic level, which is one of the important factors in cancer incidence and mortality. The NHI claims data contains information on healthcare utilization related to the occurrence of cancer diseases and information on the type of health insurance coverage, which may indirectly reflect the income of cancer patients. Although more sophisticated results can be obtained by combining the National Cancer Registry data and the NHI claims data through additional work, we utilized the NHI claims data for efficient production of data and securing recent information. Because of the high correlation between the National Cancer Registry data and the NHI claims data on cancer incidence, the NHI claims data can be used to compare health disparities between the NHI beneficiaries and the MA recipients.

In previous studies utilizing the National Cancer Registry in South Korea, the nationwide cancer incidence is well-documented, but there is a lack of data regarding the socioeconomic status. Therefore, we aim to investigate the impact of socioeconomic status on cancer incidence using the NHI claims data. This study aims to investigate under the UHC whether: (1) there exists inequality in cancer incidence based on socioeconomic status, particularly insurance types and geographic regions; (2) whether such inequality is increasing; and (3) whether inequality exists in cancer treatment facilities based on socioeconomic status.

This study is the first paper to examine health inequities in relation to cancer incidence between the NHI beneficiaries and the MA recipients over the last decade in Korea. Cancer incidence has increased overall, and this trend was found in both the NHI group and the MA group. However, we found that cancer incidence was higher in the MA group than in the NHI group, and that the gap was widening. We also found that regional differences in cancer incidence still exist and are getting worse.

In addition, the risk of cancer incidence showed the same pattern as cancer incidence rates. The OR was higher in the MA group than in the NHI group. The OR of total cancer incidence was 1.79 (95% CI, 1.77 to 1.82) and 1.90 (95% CI, 1.88 to 1.93) in 2011 and 2021. Only the OR for thyroid cancer was conversely lower in the MA group, at 0.67 (95% CI, 0.63 to 0.72) and 0.68 (95% CI, 0.63 to 0.73) in 2011 and 2021, respectively. The reason why thyroid cancer tended to be different from other cancers is due to specific characteristic of thyroid cancer and thyroid cancer patients. Mortality of thyroid cancer is exceptionally low compared to other cancers, and thyroid cancer is known to be more affected by high socioeconomic factors such as income. People with high income tend to use more medical services than people with low income and are more at risk of overutilization [16-19]. High-income patients tend to be screened with more sensitive tests, and such tendency may lead to earlier diagnosis, increasing the possibility of overdiagnosis [19-21]. Such characteristics of thyroid cancer patients may have influenced the results that thyroid cancer has the highest number of patients of all cancers.

Assuming that even within the same region there are differences in the choice of higher level of care for socioeconomic reasons, we analyzed whether there were differences by type of initial diagnosis. The proportion of the type of hospital where patients were first diagnosed with cancer was different between the NHI and the MA groups. More than 60% of cancer incidence in the NHI beneficiaries was diagnosed at tertiary hospitals and around 30% at general hospitals. On the other hand, general hospitals were where ≥ 60% of cancer cases of the MA recipients were diagnosed between 2011 and 2016. This represents that the NHI beneficiaries could afford to pay more medical expenses while the MA recipients are financially vulnerable. However, in the MA group, the proportion of cancer diagnosis at tertiary hospitals increased from 28.5% in 2011 to 54.0% in 2021, while the proportion at general hospitals decreased from 61.4% in 2011 to 38.7% in 2021. These changes have been significant since 2017 for most cancers. We could possibly interpret this result as enhanced cancer coverage by the NHI. Under the health insurance coverage enhancement policy implemented since 2017, the upper limit of health insurance deductibles paid by the lowest 50% of income earners was reduced to 10% of their annual income. In addition to expanding rebates for low-income earners, the catastrophic medical expense support program, which covers therapeutic out-of-pocket medical expenses, was expanded from four serious diseases to all diseases, and the maximum amount of support was raised from KRW 20 million to KRW 30 million (USD 15,000 to USD 24,000).

In Korea, when a person is diagnosed with cancer of any type, the NHI covers 95% of medical expenses. Cancer patients pay only 5% of total cancer treatment expenses regardless of their health coverage. Cancer patients burden 5% of treatment expenses from out-of-pocket, therefore we may assume that cancer patients in the MA group feel less burdened by treatment costs, resulting in an increased proportion of tertiary hospitals in the first diagnosis of cancer. Despite an increase in the proportion of the MA patients diagnosed with cancer at tertiary hospitals, it was still about 10% lower than in the NHI group, and disparity still existed.

This study has some limitations. First, we used only cancer incidence as the outcome variable in this study. Health inequity would be ideal if it did not exist. However, unfortunately, inequity exists in the real world. Accurately measuring the magnitude of inequity is important. Therefore, choosing appropriate health inequity indicators is inevitable to measure health inequity. Four perspectives are considered when conducting research regarding cancer: treatment, mortality, screening, and stage. In this study, we could not afford to take cancer treatment, cancer mortality, and stage as outcome variables due to data structure. However, cancer incidence may be appropriate outcome variable in Korea. As stated above, with the low cost of treatment, there would be less reason for newly diagnosed cancer patients not to have medical treatment. Further research would be necessary to evaluate health disparities in cancer using other outcome variables such as treatment, mortality, stage, etc. Second, our results cannot provide information regarding the impact of coronavirus disease 2019 (COVID-19) on cancer patients in 2020 and 2021. As no conclusive evidence of carcinogenesis of COVID-19 has been established, we ruled out the risk of COVID-19. Third, while it is common to use cancer registry data to study cancer incidence or mortality, we were unable to use a variable for type of health insurance, which is the minimum variable to distinguish the economic level that is the main interest of this study. Instead, we used health insurance claims data, which can be used to manipulatively define new cancers through healthcare use by the total population. By using the primary diagnosis and a special benefit code for cancer, the accuracy of the data can be improved and considered reliable. Seo et al. [22] investigated that cancer incidence rates between the National Cancer Registry and the NHI claims data were very similar, with 363.1 cases per 100,000 population in the NHI claims data and 361.9 cases per 100,000 population in the National Cancer Registry data from 2005-2008.

In terms of future policy recommendations, it would be ideal if the National Cancer Registry data included an indicator of income. Currently, there are no income indicators in the National Cancer Registry data, so we are forced to use the NHI claims data. If it is not possible to add an income indicator to the National Cancer Registry data, it is essential to cooperate with relevant organizations to link the NHI claims data.

In conclusion, this study investigated that socioeconomic disparities in cancer incidence have worsened by health coverage, region, and over the study period. Nevertheless, increased health coverage may have influenced the proportion of tertiary hospitals in the MA group. Understanding these disparities is crucial for implementing of public health policies to reduce health inequality.