Population-based Breast Cancer Statistics in Korea during 1993-2002: Incidence, Mortality, and Survival

Breast cancer is the most common cancer in women worldwide regardless of the countries' level of development (1). Among Korean women, breast was the second most common cancer site following stomach in year 2001 (2). However, in 2002, it became the most common form of sporadic cancer in Korean women and comprised 16.8% of all female cancers (3). The Korean Ministry of Health and Welfare started a nationwide, hospital-based cancer registry called the Korea Central Cancer Registry (KCCR) in 1980. Details of the history, objectives, and activities of the KCCR were documented previously (3). In 1996, the Korea Breast Cancer Society (KBCS) initiated breast cancer on-line registration (4), which has been collecting data on breast cancer cases including stage information. The KCCR has been collaborating with the KBCS to improve completeness and validity of breast cancer registration. Breast cancer cases registered by the KCCR were merged with the cases registered by the KBCS. Also, through using data from 8 regional cancer registries and other sources including mortality data from the National Statistical Office (NSO), it became possible to produce national cancer statistics of Korea (2). In this paper, we present the trends of breast cancer incidence and mortality rates and the overall and stage-specific survival of breast cancer during 1993 and 2002 in Korea.

This is the first report, that examined breast cancer trends over a decade based on population-based data in Korea. Although breast became the most common cancer site in Korean women in 2002, the level of breast cancer incidence in Korean women is considered as being modest when compared with other countries' incidences reported in the Globocan 2002. ASR per 100,000 Korean women in 2002 was lower than the world average (26.2 vs. 37.4) and lower than that of Japan (32.7), which is thought to have similar ethnic background (1). In terms of the age-specific incidence pattern, the rates showed a peak around the perimenopausal period, which was also found in Japan and Taiwan. This pattern is different from in Western countries, whose age-specific rates increased sharply up to the ages of menopause, but at a slower rate above that age (12, 13). The median age at diagnosis of invasive cancer in Korean women was 46 yr, which almost did not change during the 10-yr period. However, average APC for incidence rates standardized to Korea 2000 population was 7.2% in women. It seems to be much higher than those of European countries, which ranged from 0.8% to 3.0%, even after considering the use of different standard population (14). This increase might be caused partly by changes in previously known risk factors such as early menarche, late menopause, and late first-delivery (15, 16). However, it is more likely to reflect increasing completeness of registration.

The breast cancer mortality rate was the 5th highest in Korean women in 2004 (5). Korean women still have relatively low breast cancer mortality rates than any other countries (4.6 vs. 13.2 per 100,000), and there was no significant increase or decrease during the period (1). Age-specific mortality rates of female invasive breast cancer seem to be different from age-specific incidence rates. Considering there was no significant difference in the distribution of disease stage between age groups (data not shown), this might reflect that cause of death for a large proportion of patients could be assigned as other competing diseases. For age-specific mortality curves, only deaths from breast cancer were considered.

Survival rates of Korean female breast cancer were reported as 2nd highest (5-yr RSR 77.5%), only second to thyroid cancer (95.1%) (17). Compared with survival of other developing countries using age-standardized 5-yr RSR, our survival is slightly lower than that of Hong Kong, China (82.5%), but higher than those of Turkey (77.2%), Singapore (76.4%), Thailand (62.9%), and India (51.6%) (18). For European countries, from EUROCARE-3 study, age-standardized 5-yr RSR for Europe, based on 256,464 female breast cancer patients diagnosed during 1990-1994 from 22 European countries, was 76.1% (19), which is similar to that of Korea during 1993-1997. However, there is almost a 5-yr gap in terms of the diagnosis period between these, which needs to be taken into consideration. In our study, when the study period was split into two (1993-1997 vs. 1998-2002) for comparing the survival experience during the period, 5-yr RSR during the latter period (84.3%, 95% CI 83.8, 84.8) is higher than that (79.1%, 95% CI 78.5, 79.8) of the former period by 5.4%. It is very likely that the survival of these European countries improved during those 5 yr as our survival did during the study period. Also, using crude 5-yr RSR among American women diagnosed during 1996-2002, survival (89.0%) (7) is higher than that of Korea during 1998-2002. The five-year RSR of breast cancer among Japanese women diagnosed during 1993-1996 was 83% (20), which was higher than that of Korea during the similar period. However, we should interpret these results with great caution, since there are differences in the diagnosis period and standardization methods among populations. For stage-specific survival, 50.4% (24,867 out of 49,304 cases) of cases were excluded due to missing stage information. There is a difference in overall 5-yr RSR between cases with stage information and cases without by 9.8%; 87.2% (95% CI 86.6, 87.7) vs. 77.4% (95% CI 76.8, 78.0). We checked the systematic differences between staged and un-staged cases using other available information such as the mean age at diagnosis, treatment (operation/non-operation), and residential areas (data not shown). We found that the proportion of staged cases was increasing during the study period, which could represent cases with more favorable prognosis with time. Also surgical treatment was more common in staged patients. This phenomenon was observed previously and been interpreted that stages of who had not undergone surgery were more likely to be advanced than cases with stage information, which could cause the worse overall survival in un-staged cases (21).

Completeness is an important issue in calculating and interpreting cancer incidence, which we checked indirectly using DCO% and M:I ratio (22). However, DCO% itself is not an indicator of completeness, with elevated DCO% being suggestive of incompleteness. DCO%, especially of female invasive breast cancer, kept decreasing from 9.9% in 1993 to 1.1% in 2002 (2). It made very difficult to interpret the secular trends of incidence, and the estimated APC is very much likely to be overestimated. The M:I ratio has been decreased, which suggested improved completeness of incidence DB and correctly reflected the changing pattern of survival during the period at the same time. Especially, an ad hoc medical record review survey covering data registered from 1999 has been performed since 2001, which increases completeness. Another important aspect of registry data is validity, which we checked using MV%. MV% has been steadily increased from 84.0% in 1993 to 96.9% in 2002.

For survival analysis, we achieved nearly 100% follow-up through the PIN, used in Korea, which is given at the time of birth and contains information about sex, date, and place of birth. The accuracy of cause of death for breast cancer was reported to be very high (23), and the proportion of cause of death certified by doctors has increased up to 86.5% (24) in 2005. Additionally, cause of death has become more accurate and complete by collaboration between KCCR and NSO since 2000.

In summary, the female breast cancer incidence in Korea seems to have been increasing, which is possibly due to the combined effects of various factors including completeness of registry data, population aging, and changes in risk factors. Contributing factors should be identified by further studies, which will enable to interpret the registry data properly and to plan effective protective measures. It is also necessary to examine the factors related to improved survival including patient characteristics, disease stage, and treatments. This report will be valuable as baseline data to plan and evaluate actions against breast cancer including evaluating the national breast cancer screening program.