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Introduction
Recent studies on global burden of disease have revealed the huge burden imposed by cancer worldwide [1], and in contrast with the improvements detected for other non-communicable diseases, a worsening trend for cancer burden has been identified [2]. To precisely evaluate the cancer burden and to appropriately manage cancer control plans or strategies at the level of nations, monitoring cancer statistics is of the utmost importance. In Korea, cancer is the leading cause of death, and the number of new cases increases each year [3]. This study reports the most recent nationwide statistics on cancer incidence, survival, prevalence, and mortality, and their temporal trends.
Materials and Methods
1. Data sources
The Korea National Cancer Incidence Database (KNCI DB) is a national, population-based database of cancer occurrence that includes information on patients diagnosed with cancer in hospitals across Korea and combined this with data provided by central and 11 regional cancer registries which included information regarding missing cancer patients; all this took 2 years to compile. Data from this database is used to examine cancer incidence, 5-year relative survival, and prevalence rates. The Korea Central Cancer Registry (KCCR) has compiled KNCI DB, and reported related nationwide statistics from 1999 onward. Detailed information on the KCCR and KNCI DB is provided elsewhere [3,4]. Completeness is an important indicator of data quality, and the 2017 KNCI DB was estimated to be 98.2% complete using the method proposed by Ajiki et al. [5].
Mortality data including cause of death and mid-year population data from 1983 to 2017 were obtained from Statistics Korea [6]. To verify the accuracy of individual vital statuses when calculating survival and prevalence rates, the KNCI DB was linked to both mortality data and population resident registration data, which were obtained from the Ministry of the Interior and Safety.
2. Cancer classification
All cancer cases had been registered in accordance with the International Classification of Diseases for Oncology, 3rd edition (ICD-O-3) [7], and were converted to the classification of the International Classification of Diseases, 10th edition (ICD-10) [8]. Only cases defined as malignant under the ICD-O-3 were included, with the exception of some hematopoietic diseases (myeloproliferative disorders/myelodysplastic syndromes). For mortality data, causes of death were coded according to the ICD-10. We classified cancers into 24 types, using a modified classification based on the taxonomy of 61 cancers used in Cancer Incidence in Five Continents, published by the International Association of Cancer Registries [9]. The summary staging system developed under the Surveillance, Epidemiology, and End Results (SEER) program (i.e., SEER summary staging) [10], was used to categorize the extent of tumor invasion or metastasis.
3. Statistical analyses
Incidence, mortality, and prevalence rates were expressed as crude rates (CRs) or age-standardized rates (ASRs) per 100,000 people. The CR was defined as the total number of newly diagnosed (for incidence) or deceased (for mortality) cases in a year divided by the mid-year population. The ASR, a weighted average of the age-specific rates in which the weights represent the proportions of people in the corresponding age groups in a standard population [11], was calculated to compare rates across different countries, regions, or time periods with different population age structures. ASRs were standardized using Segi’s world standard population [12]. The lifetime probabilities of developing cancer were assessed as cumulative cancer risks from birth to life expectancy, assuming no other cause of death (i.e., the sum of the age-specific rates from birth to life expectancy), as follows [11]:
Trends in ASRs were estimated using Joinpoint regression [13], and the results were summarized as an annual percentage change using a linear model on the natural log-transformed ASRs.
The 5-year relative survival rates, defined as the ratio of observed survival of cancer patients to expected survival in the general population, were adjusted for the effects of other causes of death using the standard population life table provided by Statistics Korea [6]. Survival rates in this report, including previously published rates, were calculated using the newly updated life table. Relative survival rates were estimated using the Ederer II method [14] with some minor corrections, based on an algorithm devised by Paul Dickman [15]. Trends in 5-year relative survival rates were evaluated as percent differences in survival rates between 1993-1995 and 2013-2017.
Prevalent cases were defined as the number of cancer patients alive on January 1, 2018 among all patients diagnosed with cancer between 1999 and 2017. Limited-duration prevalence was calculated using SEER*Stat 8.1.2 software (National Cancer Institute, Bethesda, MD). p < 0.05 was considered statistically significant. SEER*Stat, Joinpoint 4.7.0 (National Cancer Institute), and SAS ver. 9.4 (SAS Institute Inc., Cary, NC) were used for statistical analysis.
Results
1. Incidence
In 2017, there were 232,255 newly diagnosed cancer cases in Korea, of which 122,292 (52.7%) were reported in men and 109,963 (47.3%) were reported in women (Table 1). Stomach, colorectal, and lung cancers were commonly diagnosed in both sexes; these were followed by prostate cancer and liver cancer in men, whereas breast cancer and thyroid cancer were more commonly diagnosed in women. Collectively, the five most common cancers in men and women accounted for 65.0% and 65.5% of all cancer cases, respectively. Table 2 presents the CRs and ASRs of overall incidence and incidence of each cancer in 2017. The ASR of thyroid cancer, which occurs relatively frequently in younger age groups, was the highest. The overall lifetime probability of being diagnosed with any cancer was 37.0%, if one survives to reach the age that matches the life expectancy of the Korean population. That probability was higher in men (39.6%) than in women (33.8%) (data not shown).
In terms of age-specific incidence rates, leukemia (0-14 years) and thyroid cancer (15-34 years) were the most common cancers in both sexes among the childhood and ‘adolescent and young adult’ populations (Fig. 1). Brain and central nervous system (CNS) cancer and non-Hodgkin lymphoma were ranked next in the 0-14-year age group (both sexes), while breast cancer (women) and leukemia (men) were the next highest in terms of incidence in the 15-34-year age group. In the 35-64-year age group, men were most commonly diagnosed with stomach cancer and colorectal cancer, while breast cancer and thyroid cancer were most common in women. For those aged 65 years and above, lung cancer and stomach cancer were most common in men, while colorectal cancer and lung cancer were most common in women. The incidence rates of all major cancers increased with age; therefore, cases diagnosed at over 60 years accounted for the majority of incidences (Fig. 2). The only exceptions to this were the incidences of thyroid and breast cancer in women, which were the highest in women in their 40s and 50s.
2. Mortality
A total of 78,863 people (62.0% men and 38.0% women) died of cancer in 2017, accounting for 27.6% of all deaths ccurring in Korea (Tables 1 and 3). The overall CR for cancer deaths was 153.9 per 100,000, and the cancer mortality rate was higher in men than in women (Table 4). The top five cancers in terms of mortality rates were lung, liver, colorectal, stomach, and pancreatic cancers. Lung cancer was the leading cause of death in both sexes, followed by liver cancer and stomach cancer in men, and colorectal cancer and stomach cancer in women. When stratified based on age at time of death, the most common causes of cancer mortality were as follows: leukemia at ages 10 and 20 years, stomach cancer at 30 years, liver cancer at 40 and 50 years, and lung cancer at 60 years or more (data not shown).
3. Trends in cancer incidence and mortality
Fig. 3 illustrates the trends in cancer incidence rates from 1999 to 2017 and mortality rates from 1983 to 2017. The overall cancer incidence rates increased by approximately 3.5% per year until 2011, and thereafter declined by approximately 2.7% per year (Table 5). These trends were observed in both men and women, although the slopes of change were much more pronounced in women. Breast, prostate, kidney, and pancreatic cancer incidences have been continuously increasing since 1999; showing a steady increase for pancreatic cancer throughout the entire period, while the rest of those cancers revealed initially displaying rapid increases and then moderate increases after 2002 or 2009. The incidences of colorectal cancer, thyroid cancer, and bladder cancer demonstrated increasing trends that have recently reversed significantly, with decreases starting in 2010, 2011, and 2004, respectively. The rapid increase and decrease in the incidence of thyroid cancer ASRs were similar to overall trend of cancer incidence (i.e., the timing and pattern of changes), appears that the former has a major influence on the latter. In contrast, stomach, liver, and cervical cancers have displayed steady decreases in incidence since 1999 (Fig. 4).
The ASRs for the mortality of all cancers increased until 2002, then continuously decreased thereafter (Table 6, Fig. 3). More rapid changes were observed in decreasing phase compared to the preceding increasing phase, and the slopes of change were much more pronounced in men than in women. Continuous decreasing trends were evident throughout the observed period for mortality associated with stomach, liver, and uterine cancer, whereas the mortality rates associated with colorectal cancer and lung cancer (men) began to decline in the early or mid-2000s, after an initial increasing trend. Prostate cancer and breast cancer (women) displayed steadily increasing mortality trends; however, attenuation of the increasing trend in prostate cancer has been observed in recent years (Fig. 5).
4. Survival rates
The 5-year relative survival rates for all patients diagnosed with cancer in the recent 5 years, from 2013 to 2017 were 63.5% in men and 77.5% in women, for a combined overall survival rate of 70.4% (Table 7). The temporal trends in survival rates demonstrated remarkable improvement in both sexes, from 42.9% in 1993-1995 to 70.4% in 2013-2017. Such findings were maintained even after excluding thyroid cancer, which has excellent prognosis and a 5-year relative survival rate of 100.1%.
After thyroid cancer, survival was the highest for prostate and testicular cancers in men (94.1% and 95.2%, respectively) and for breast cancer in women (93.2%); the survival was lowest for cancers of the pancreas (12.2%), gallbladder plus other and unspecified parts of the biliary tract (28.9%), lung (30.2%), and liver (35.6%). Stomach cancer (both sexes), prostate cancer (men), and lung cancer (women) were associated with outstanding improvements in survival rate over the time period studied. In contrast, advances have been slow for pancreatic cancer, as well as cancers of the brain and CNS.
Fig. 6 shows the 5-year relative survival rates (2013-2017) based on SEER summary stage and stage distribution at diagnosis of the top 10 most common cancers for each sex in 2017. In men, 63.9%, 71.3%, and 73.1% of the stomach, kidney, and bladder cancers, respectively, were diagnosed at the localized stage, with 5-year survival rates of 97.4%, 97.5%, and 87.9%, respectively. However, these rates decreased to 6.0% (10.9% of cases), 13.9% (12.3% of cases), and 9.4% (4.0% of cases), respectively, for cases diagnosed at the distant metastatic stage. In women, 58.1%, 61.9%, and 71.5% of the breast, stomach, and uterine corpus cancers, respectively, were diagnosed at the localized stage, with 5-year survival rates of 98.7%, 95.3%, and 95.7%, respectively. These rates decreased to 39.9% (4.8% of cases), 4.9% (10.8% of cases), and 32.6% (6.9% of cases), respectively, for cases diagnosed at the distant metastatic stage. In both sexes, relatively large proportions (> 40%) of lung and pancreatic cancers were diagnosed at the distant metastatic stage, resulting in poor prognosis.
5. Prevalence rates
A total of 1,867,405 cancer cases were identified as of January 1, 2018 (Table 1), suggesting that 1 in 28 people, i.e., 3.6% of the Korean population (3.2% of men and 4.1% of women) has a history of being diagnosed with cancer. Of these, 826,103 (44.2% of all prevalent cases) were aged ≥ 65 years, indicating that 1 in 9 people in that age group (15.2% of men and 9.1% of women) will have experienced cancer (data not shown).
Table 8 presents the CRs and ASRs of prevalence for all cancers combined and for specific cancers. In the total population, thyroid cancer had the highest prevalence (CR, 790.6 per 100,000, derived from 277.7 per 100,000 for men and 1,302.0 per 100,000 for women), followed by stomach cancer (CR, 564.6 per 100,000) and colorectal cancer (CR, 490.1 per 100,000). Prostate cancer and breast cancer (women) also revealed high prevalence rates (CR, 337.9 and 843.5 per 100,000, respectively).
Fig. 7 depicts the number of prevalent cases for common cancers, categorized according to time since cancer diagnosis. In total, 1,039,659 (55.7% of all prevalent cases) had survived > 5 years after cancer diagnosis, including high proportion of patients diagnosed with thyroid, stomach, colorectal, breast, and cervical cancers. Another 446,428 (23.9% of all prevalent cases) were alive 2-5 years after their cancer diagnosis, leaving 381,318 (20.4% of all prevalent cases) who had been diagnosed for < 2 years and who may still be undergoing active cancer treatment.
Conclusion
In summary, the numbers of newly diagnosed cancer cases and cancer-related deaths in 2017 were 232,255 and 78,863, respectively. Although overall cancer incidence and mortality have declined since 2011 and 2002, respectively, some cancers, such as breast and prostate cancer, demonstrate increasing trends with respect to both incidence and mortality. The 5-year relative survival rates have continuously improved, reaching 70.4% for all patients diagnosed with cancer between 2013 and 2017. Accordingly, cancer prevalence continues to increase, reaching approximately 1.87 million by the end of 2017. Although overall cancer incidence and mortality rates are on decreasing trend in Korea, improved survival is rapidly increasing cancer prevalence. These results suggest that comprehensive cancer control strategies and efforts should continue, based on the changes of cancer statistics.
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