In the present study, we found marked differences in the age-specific incidence of papillary thyroid cancer according to birthplace among Chinese, Korean, Filipina, and Vietnamese women residing in the US. Among foreign-born women, rates increased until about age 70 and declined thereafter. In US-born women, incidence rates peaked during the reproductive years and, for Chinese women, plateaued during the menopausal period, mirroring the pattern seen in NH white women. The difference in age-specific patterns between immigrants and subsequent generations suggests that modifiable early and young-adult exposures, particularly those specific to immigration and acculturation of these Asian ethnic groups (e.g., goitrogenic exposures, diet, body size, and menstrual and reproductive events), may have a strong influence on the development of papillary thyroid cancer. In Japanese women, the incidence patterns were similar for US- and foreign-born women and followed the pattern observed for men and NH black women. Different incidence patterns between US-born Japanese and other US-born Asian women may reflect dissimilarities in migration patterns, acculturation, or culture-specific exposures and behaviors; however, we have no specific hypothesis at present as to specifically what these factors may be.
Differences in the prevalence of immigration- and acculturation-specific exposures and life events in young Asian American women and their correspondence with the observed incidence rates may provide clues as to the relative importance of various exposures in influencing disease risk. A case-control study of breast cancer among young (age<56) Chinese, Japanese, and Filipina women in California and Hawaii examined differences in risk factors among controls by birthplace and migration patterns. In that study, the prevalence of early (age ≤12) menarche was substantially higher in US-born Asian women (and young white women from a comparable study) than in foreign-born Asian American women (21
). Weight and body mass index (BMI) were also found to be significantly higher in US-born than foreign-born Asian women (22
). The relationship between age at menarche and papillary thyroid cancer risk has proven to be complex, with possible modification by race and age (6
); however, it appears to be an important thyroid cancer risk factor (6
). Greater BMI has been fairly consistently associated with increased risk (24
). Reproductive history has also been increasingly implicated in the etiology of thyroid cancer. Parous women are at greater risk of thyroid cancer than nulliparous women (6
). Among the Asian American controls in the previous breast cancer study, US-born women and those immigrating to the US before age 36 were more likely to be parous than older immigrants (21
). Age at first full-term pregnancy, has not been associated with thyroid cancer risk in most studies (23
), nor did it differ by nativity or age at immigration in the previous case-control study (21
). Age at last full-term pregnancy and recency of the last pregnancy have both been associated with thyroid cancer risk (6
); however, we have no data on how these factors differ by nativity and immigration status. Thus, it may be these later aspects of reproductive history that are influencing the birthplace-specific incidence patterns we observe.
Ethnic and nativity differences in incidence patterns may also be related to differences in sex steroid hormone levels. Populations which experience low thyroid cancer rates during early and mid-adulthood (i.e., men, black women, and foreign-born Asian women), tend to have higher androgen levels (overall, premenopausally, or during pregnancy) than white women and US-born Asian women (22
). In addition, estrogen levels have been observed to be 50% lower among the least westernized Asian women residing in the US compared to 3rd
generation US-born Asian women (22
). While a hypothesis of high estrogen levels being associated with the higher thyroid cancer rates among young women is not consistent with the higher total and free estradiol levels that have been observed among black compared to white women (33
), one study found no black-white difference in estrogen levels during pregnancy (32
), perhaps suggesting that a relatively greater transient increase in estrogen levels during this critical period may somehow play a role in the increase incidence rates. However, while the availability of these limited observations may provide clues to the role of hormones and menstrual and reproductive history in thyroid cancer etiology, further research is clearly needed to understand the complex relationships between these exposures and the observed differences in ethnic- and birthplace-specific papillary thyroid cancer incidence rate patterns.
We also found significantly higher age-adjusted incidence rates among US-born Chinese and Filipina women compared to their foreign-born counterparts, whereas the converse was true among Japanese. The only previous study to examine these associations found a similar pattern among Japanese (i.e., higher rates in foreign-born) and Chinese women (i.e., higher rates in US-born; albeit a much less pronounced difference than we observed) but the opposite among Filipina women (i.e., higher rates among foreign-born) (10
). This discrepancy may be explained by the different time periods and regional populations included in the studies. The present study covered a period, on average, 16 years later than the study by Rossing et al. (10
), and included a substantially larger and perhaps more homogeneous population in terms of immigration, whereas the study by Rossing et al. (10
) included populations from Hawaii, the San Francisco Bay Area, and western Washington. However, the discrepancies are also likely to result, at least in part, from methodologic differences. Rossing et al. (10
) randomly imputed birthplace for the 19% of Asian cancer cases who had unknown birthplace (compared to 2% random imputation in the present study), thereby assuming the same birthplace distribution for women with missing data as for those with known data, taking into account age, race, and area of residence. However, our research has shown that cancer cases with missing birthplace data are more likely to be alive, of younger age, and US-born than those with known data, and thus, random imputation of birthplace will underestimate rates in US-born and overestimate them in foreign-born persons (11
). Indeed, our rates for US-born Chinese and Filipina women were substantially higher than those observed in the previous study and our rates for foreign-born Japanese and Filipina women were substantially lower.
The state of California has the largest and fastest-growing Asian population in the nation (37
), as well as anywhere outside of Asia, making our enhanced population-based cancer surveillance data a powerful resource for examining patterns of cancer among racial/ethnic groups by immigration status. Understanding the factors influencing the differences in the ethnic- and birthplace-specific incidence patterns of thyroid and other cancers is important for cancer prevention efforts. The present study of these differences is based on a validated imputation method and uses a substantially larger population-based sample than was used in the previous report (10
), resulting in more precise rate estimates. Nonetheless, data are still imputed for 30% of cancer cases and, while an improvement over prior imputation methods, our method, with sensitivity and specificity of 84% and 80%, respectively, still results in some misclassification. In addition, despite the large population, some rates could not be precisely estimated due to the small number of cases in some age-specific US-born ethnic groups. These age-specific patterns, however, have not been previously reported by birthplace and provide important insight into the incidence of papillary thyroid cancer.
In summary, we used enhanced surveillance data to examine the influence of birthplace on papillary thyroid cancer incidence rates in Asian women residing in California. We observed that the pattern of age-specific rates is strongly influenced both by ethnicity and birthplace, with most US-born Asian women having a substantially increased incidence of this cancer during early and mid adult life. Further research is needed to elucidate how immigration and acculturation experiences influence other life events and exposures which may account for these elevated rates as this will be critical for cancer prevention efforts.