In this large multiethnic population, population-based series of prostate cancer patients, increasing levels of SES were associated with higher incidence and lower mortality rates of prostate cancer. Furthermore, across all levels of SES, African-Americans had a substantially larger burden of prostate cancer deaths than other racial/ethnic groups, suggesting that SES alone cannot entirely account for the racial/ethnic differences in prostate cancer mortality.
The elevated incidence rate of prostate cancer associated with higher levels of SES is likely attributable at least in some part to variation in access and utilization of health services; in particular, prostate cancer screening through PSA testing. PSA testing greatly increases the detection of prostate tumors, which leads at the population-level to elevated incidence rates of prostate cancer. Studies have reported that men at higher levels of SES are more likely to undergo PSA testing [25
], ultimately influencing the amount of disease in the population. The peak in incidence rates of prostate cancer among non-Hispanics Whites and African-Americans at 65–74 years of ages are in agreement with previous SEER reports [2
] and may reflect heavier screening practices at earlier ages, while the later peak among Hispanics and Asian/Pacific Islanders at 75–84 years may reflect later adoption and lower utilization of PSA screening. Prior research has shown that Asians and Hispanics are less likely to receive physician discussions of PSA testing than higher risk Whites and African-Americans [27
A consistent racial/ethnic-specific pattern of incidence rates across SES levels was observed only among younger adult men (<65 years). To our knowledge, our findings of an increased incidence of prostate cancer among older Hispanics (75–85 years) at the higher levels of SES relative to non-Hispanic Whites have not been reported previously. In a national study of cancer among US Hispanics, Howe et al. [11
] reported that Hispanics are less likely to have health care coverage than non-Hispanics Whites, especially among those younger than 65 years. With more health care coverage for older Hispanics and better resources for those at higher levels of SES, such men may have improved access and utilization of screening services that otherwise may have not been available—this may account for the higher incidence rates among this particular group of Hispanics. Our findings are in line with two similar yet smaller studies in Los Angeles [5
] and the San Francisco Bay Area [10
] in which across all levels of SES a similar racial/ethnic-specific pattern in age-adjusted incidence rates were seen with the exception of greater incidence of prostate cancer among Hispanics than that of Whites for those at the higher levels of SES. Age-stratified effects as shown in our study were not examined in these previous reports [5
Because screening practices greatly influence the incidence rates of prostate cancer, we also examined mortality as it may serve as a better index of risk across groups and may reflect the most clinically relevant forms of disease. The lower mortality rates of prostate cancer seen with higher levels of SES are likely attributed to factors linked to a better health status by affording optimal use of medical services such as early detection and treatment regimens, acquiring pertinent health information and education, and avoiding high risk health behaviors [15
]. This overall inverse association between mortality and SES was largely driven by non-Hispanics Whites with the remaining racial/ethnic groups demonstrating no association. This could be explained due to insufficient power among the remaining racial/ethnic groups given their smaller numbers of deaths and fewer overall Census numbers at higher levels of SES. In addition, this could be due to inadequacy of our SES index in capturing SES parameters that are most relevant for certain non-White racial/ethnic groups (discussed below). Lastly, these findings may suggest that SES does not play a role in prostate cancer mortality among African-Americans, Hispanics, and Asian/Pacific Islanders.
For every level of SES, African-Americans had the highest burden of prostate cancer deaths in comparison to other three racial/ethnic groups. These findings are in agreement with three previous studies that reported that the measures of SES cannot account for the differences in mortality/survival between African-Americans and Whites [15
]. Similarly, in a multiethnic cohort study of African-Americans, Whites, and Asian-Americans, the disparity in prostate cancer survival and stage of presentation could not be eliminated by adjustment of SES and comorbidities [16
]. We conducted a comparable survival analysis of men in our study diagnosed with prostate cancer from 1998 to 2002, adjusting for SES, stage, and grade; hazard ratios (HR) for prostate cancer death confirmed such disparity in risk: African-Americans (HR = 1.20; 95% CI: 1.08–1.33), Asian/Pacific Islanders (HR = 0.59; 95% CI: 0.51–0.68), and Hispanics (HR = 0.89; 95% CI: 0.81–0.98) when compared to non-Hispanic Whites.
Our findings suggest substantial influences of both innate and lifestyle factors in the differences in prostate cancer rates across groups. The consistent racial/ethnic disparity in incidence rates for all levels of SES among younger men (ages <55 years) indirectly support an important biological component to disease risk as early ages at diagnoses have been linked to biological contributors to disease. Exciting developments from recent genetic association studies, an admixture study of prostate cancer among African-Americans [28
] and a multiethnic fine-mapping study [29
], revealed a particular region on chromosome 8q24 that may contribute to the higher incidence of prostate cancer among African-Americans in comparison with non-Hispanic Whites. Compelling evidence provides strong support that genetic factors may account for at least part of racial/ethnic differences in disease. Lifestyle and contextual factors have yet to convincingly identify specific contributors; some studies have implicated dietary fat [30
], but the results are conflicting [32
]. Regardless, our data suggest that the ongoing search for environmental causes of prostate cancer continues to be warranted.
Differences in treatment practices are an important consideration in evaluating racial/ethnic differences in prostate cancer mortality. Cancer registry data do not include detailed information regarding treatment, so we were unable to account for potential differences in treatment practices across racial/ethnic groups. Previous studies have reported that African-Americans are more likely to undergo less aggressive treatment than Whites [14
], which may account for some of the observed differences in mortality. Recently, in a large California cancer registry study of differences in prostate cancer survival between African-Americans and Whites (n
= 109,270), adjustment for stage and treatment eliminated most of the racial difference in survival; and with additional adjustment for SES, grade, and year of diagnosis, the survival difference between African-Americans and Whites was eliminated (HR = 1.00; 95% CI: 0.93–1.08) [13
]. Although these findings indicate treatment differences are largely accountable for differences in survival, biological and environmental factors remain important contributors to racial/ethnic differences in the development of prostate cancer.
Our study has several limitations that warrant discussion. The use of a neighborhood-level index of SES is subjected to ecological fallacy such that incorrect inferences of individual levels of SES may have been made. In addition, by using overall census data to construct our index of SES, we may have overlooked factors that are particularly relevant for specific racial/ethnic groups. For example, Krieger et al. [35
] report that for homes of equal value, African-Americans pay higher taxes in comparison with Whites, and for a given level of education, the economic returns are higher for Whites in comparison with African-Americans and Hispanics. This suggests that certain racial/ethnic groups at the same level of SES may not share the same level of power, prestige, and opportunities—variables that can capture these factors may improve SES measurement [22
]. While we acknowledge SES may be measured with some error in our study, we have evidence that our index of SES is of sufficient quality to uncover important SES and cancer associations as seen in the literature (SES and breast cancer [22
] and Hodgkin-lymphoma [36
]), providing certain confidence that our SES index is valid.
There are several strengths to this study. Foremost, this is the largest and most diverse study of prostate cancer disparities to date, having 98,000 incident prostate cancer cases and 9,000 prostate cancer deaths with substantial numbers of cases from four major racial/ethnic groups. In addition, as a population-based study our findings may be generalized to the diverse population of California at large. While our use of census data to derive an area measure of SES may not completely reflect data at the individual level, area-based measures have been suggested to capture elements of the socioeconomic environment that may not be obtainable by individual-level data [37
In summary, the present study suggests that socioeconomic status alone does not appear to account for the differences in prostate cancer burden among African-Americans, non-Hispanic Whites, Hispanics, and Asian/Pacific Islanders. Large multiethnic studies with complementary individual- and area-level measures of SES are needed to corroborate our findings. The challenge remains to disentangle the complexities of racial/ethnic differences in screening, treatment, biological and environmental factors that contribute to differences in risk across groups. Such information will greatly aid the development of more targeted interventions to improve the social inequalities in prostate cancer incidence and mortality.