It has been projected that economically less developed countries are going to bear a greater burden of breast cancer than more developed countries (8). However, research on risk factors for and incidence of breast cancer in population among less developed countries have been scarce. In this study, by using well characterized registry-based data of the largest urban center of Vietnam, we have shown that over the past 20 years, the incidence of breast cancer had increased by 70%, and that the increase was mainly attributable to those age groups of 50 and 70. These very first results from Vietnam deserves further elaboration.
It seems clear that the age-standardized incidence of breast cancer in this study is lower than that in Caucasian populations. For instance, in the United States, during the period of 2009 and 2016, the age-standardized incidence rate was approximately 200 per 100,000 woman-years (9) which is higher than in Australia (~131 per 100,000 women) (10). In China, the age-standardized incidence of breast cancer was observed at 28.4 per 100,000 women (11). In Vietnam's neighboring country Thailand, the incidence rate was estimated to be 31.2 per 100,000 woman-years (12). In our study, the age-standardized incidence rate was 21.5 per 100,000 women over the period of 2011 and 2015, which can still be considered low relative to populations in more economically developed countries.
We found that the incidence of breast cancer in our cohort had been increasing over time, and this trend is consistent with previous observations (13-15). In 1996 - 2000, the age-standardized incidence rate was 12.4 per 100,000 women which is comparable with a previous estimate (16). However, 20 years later, this incidence was increased by almost two-fold. The increase in the breast cancer incidence rates has been found in China, Japan, and Thailand (17). This increase in the incidence plus the large population sizes in Asia imply that more cancer cases will be observed in Asian populations than in Caucasian populations. The increase in life expectancy and the aging of the population contribute three-fifths and the increased age-standardized rates contribute two-fifths of future trend (18). If this assumption holds, and given the improvement in life expectancy in Vietnamese women (~79.4 years) (19), it is expected that breast cancer will impose a heavy burden in Vietnam in the near future. Indeed, it has been projected that the incidence of all cancers in the two major cities of Vietnam will be increased by approximately 17% over the next 5 years (20).
The age structure of cancer cases in our cohort merits a comment. We found that the median age at diagnosis of breast cancer was 50 years (in women), which is almost identical to the median age at diagnosis of breast cancer among Singaporean (21) and Korean (22) women. In economically more developed countries, the average age at breast cancer diagnosis was ~54 years (23). Thus, our finding reaffirms the common 'law' that Asian women tended to have breast cancer at earlier ages than their Caucasian counterparts.
Almost 100% of breast cancer tumors in this study were in the invasive stage, only 0.3% was in situ, and 26% of cases were diagnosed at late stage III and IV. The proportion of in-situ tumors was 16% in the United States (24) and 14% in South Korea (22). Delayed diagnosis of cancer is a major factor that contributes to the increased risk of premature death, lower cancer survival and increase burden of cancer. Indeed, the overall 5-year relative survival rate is 99% for localized disease, 85% for regional disease and 27% for distant stage disease (25). The implication of our finding is that a more aggressive screening strategy for identifying cancer cases ealier is warranted.
Our finding should be interpreted in relation to a number of strengths and potential weaknesses. The data were ascertained from a well developed registry that could capture total cancer incidence in Ho Chi Minh City. It is worth noting that virtually all breast cancer cases are treated at hospitals within the City, with a very small proportion being treated overseas, and the data were therefore complete. However, a caveat of the study is that we could not follow individual patients to ascertain their survival status, and as a result, we could not analyze the rate of mortality among these patients. Moreover, our finding concerning incidence may not be generalized to rural or non-urban areas where the incidence is expected to be lower than that in urban areas.
In conclusion, our registry-based data suggest that although breast cancer incidence rates in a Vietnam's urban population remain relatively low compared to Caucasian populations, there was an increasing trend up to 70% over the past 20 years. Our data also confirm that Vietnamese women tend to have breast cancer at younger ages compared to Caucasian women. These findings imply that breast cancer screening should be targeted women of younger ages.