The notable declines in late-stage diagnosis and mortality rates for breast cancer over the past twenty years have largely been attributed to improvements in early interventions among the population in general [¹,²]. However, over this same period, racial/ethnic disparities between late-stage diagnoses and mortality outcomes associated with breast cancer have widened due to disproportionate medical benefits that different racial/ethnic groups have received [³,⁴]. It is well recognized that African-American and Hispanic women are more likely to be diagnosed in the advance stages of breast cancer, a fact that has been attributed to the lower survival rates and higher mortality rates in the two groups [⁵]. According to Surveillance Epidemiology and End Results (SEER) data collected between 1995 and 2001, 42% of African-Americans and Hispanics overall were not diagnosed until the later stage for all breast cancer cases, compared to 33% of White women for the same measure [⁴]. Mortality rates were observed to be 50% higher for African-American women than non-Hispanic White women, while Hispanic women had the lowest relative mortality [⁶]. A similar study that utilized SEER data from 1992 to 2000 performed a comparison of adjusted mortality rates relative to non-Hispanic Whites and found an increased risk for Hispanic Whites with a relative risk of 1.22 (95% Confidence Interval (CI): 1.16-1.28) and for African-Americans with a relative risk of 1.75 (CI: 1.68-1.82) [⁷].

Causes and mechanisms of racial/ethnic disparities remain unclear in breast cancer [⁸]. The higher breast cancer mortality among African-American women results in part from the poor survival rates, which is highly associated with stage at diagnosis [⁵]. Thus, racial/ethnic disparities in late-stage diagnosis propagate their effects on existing racial/ethnic disparities in breast cancer mortality. Differences in stage at diagnosis depend on socioeconomic status to a great extent, which plays an important role in the quality and quantity of cancer treatments [⁹]. Both late-stage diagnosis and socioeconomic status place direct impacts on racial/ethnic disparities in breast cancer mortality. Cancer cases can be categorized based on how far the carcinoma spreads from an original point [¹⁰]. A localized cancer is constrained to the organ of the breast and has not spread further than the breast tissue; regional cancer extends beyond the limits of the breast organ; and distant stage cancer occurs when tumor cells have metastasized to other parts of the body[¹¹].

Few breast cancer studies have directly examined how racial/ethnic disparities at late-stage diagnosis and mortality vary across geographic regions, although some studies have been conducted to examine the geographic difference of stage at diagnosis and mortality [¹²,¹³]. A study of state-wide SEER data by stage has shown that Wyoming has the lowest rate of localized stage (56.5% of all diagnosed cases for breast cancer) and highest rate of distant (5.8%) breast cancer, with Alaska having the highest rate of regional breast cancer (33.2%) [⁷]. Among White women at age 50-64, the relative risk for breast cancer mortality decreased from 1.48 in 1950-1959 to 1.15 in 1990-1999 in the Northeast compared to that in the South, and African-American women experienced a downward trend of relative risk for the same age group from 1.13 in 1970-1979 to 1.00 in 1990-1999 [¹⁴]. This finding implies that White women experienced a larger proportion of decline in mortality than African-American women among the same age group.

Disparities between breast cancer late-stage diagnosis and mortality have not been compared at the census tract level across a large geographical area. Based on previous work by the authors [¹⁵], it was found that, in analysis results of racial/ethnic disparities at larger spatial scales (zip code and county level) relative to finer scales (e.g. census tract), there may be an obfuscation effect on racial/ethnic disparities across geographic regions due to aggregation. The study presented in this article, in contrast, intends to determine if there is any spatial relationship in racial/ethnic disparities between late-stage diagnosis and mortality for breast cancer as well as how socioeconomic status (SES) impacts the number of census tracts that are significant in both late-stage diagnosis and mortality and the spatial relationship between these two types of racial/ethnic disparities. An investigation of racial/ethnic disparities across different geographic regions may provide useful insights revealing the effects of unknown risk factors for late-stage diagnosis and higher breast cancer mortality among minority groups and facilitate the allocation of cancer prevention education and health care resources more effectively and efficiently. Investigation of SES influences on racial/ethnic disparities could provide some insights on the interaction between stage diagnosis and the continuous influence on survival rates of breast cancer. There may also be an opportunity to understand how socioeconomic status plays a role in determining cancer stage at diagnosis and mortality, especially in impoverished regions.

The results of this study indicate that the spatial relationships of racial/ethnic disparities between breast cancer late-stage diagnosis and mortality were complex and non-stationary across Texas from 1995-2005. The dichotomous results from the significance test indicate that about 40% (109/278 or 130/328) of the census tracts with significant racial/ethnic disparities in breast cancer mortality also displayed significant disparities in late-stage diagnosis for both African-Americans and Hispanics, respectively. On the other hand, 58% of the census tracts tested significant in racial disparities of late-stage diagnosis displayed significant in breast cancer mortality as well for African-Americans, and the similar result (49%) were observed for Hispanics.

More census tracts were identified as having significant mortality disparities compared to census tracts having late-stage disparities. This implies that some other factors impact racial/ethnic disparities in breast cancer mortality. While stage of breast cancer at diagnosis is a critical factor in determining breast cancer survival, other factors such as treatment options and financial support are also crucial in ultimately saving women's lives [³²,³³]. Linking Detroit SEER registry with Medicaid enrollment data in Michigan, Bradley et al. in 2002 revealed that African-American women had less likelihood to undergo cancer surgery and radiation that contributed to the elevated mortality risk. In this study, there were more census tracts that tested significant in both late-stage and mortality disparities for Hispanics than for African-Americans due to the much higher Hispanic population resided in Texas [³⁴].

Using regression analysis, this study provides additional evidence, in the form of a spatial perspective, of the strong and positive relationship between late-stage diagnosis and mortality of breast cancer with regard to racial/ethnic disparities. If a census tract tested significant in racial/ethnic disparities of late-stage diagnosis, this tract is more than 30 times more likely to be significant in racial/ethnic disparities of breast cancer mortality. Similarly, using Surveillance Epidemiology and End Results (SEER) data, Li and colleagues (2003) found that African-Americans were 2.3 and 2.5 times more likely to have the elevated risk of advance stage of breast cancer compared to non-Hispanic Whites. On the other hand, Hispanic women tested 1.8-fold in diagnoses at the late-stage of breast cancer. Furthermore, African-Americans and Hispanics were found to have 1.5 and 1.1 greater in mortality risk than Whites, even after adjusting age, SEER registry, stage, ER and PR status, surgical treatment and radiation therapy [⁵].

A large number of census tracts were found to have significantly higher breast cancer late-stage diagnosis and mortality rates for African-American and Hispanic women in comparison with non-Hispanic White females. This result strongly agrees with the argument that minority women tend to be diagnosed in the advanced stage of breast cancer relative to non-Hispanic White women. Richardson and colleagues [³⁵] found that African-Americans and Hispanics were at 1.29- and 1.32-fold greater risk to be diagnosed in the advance stage of breast cancer in Los Angeles, California than White non-Hispanics, when SES was controlled. Hispanics are often reported to have lower incidence rates than non-Hispanic Whites [⁶]. However, the incidence rate ratio for Hispanics has been observed to be significantly lower in the early detection of breast cancer relative to non-Hispanic Whites, especially within the age group under 50 [³⁶].

Census tracts that had significantly elevated mortality rates were associated with lower SES characteristics, mostly situated within the three metropolitan areas and the Southwest border with Mexico. This may be due to the fact that medical services, including access to health care, sufficient physicians and cancer treatment are more likely to be underserved in deprived neighborhood communities [³²,³⁷]. Lower socioeconomic status is an important determinant for the advanced stage at diagnosis and cancer treatment follow-ups that lead to lower survival and higher mortality rates, especially among susceptible minority groups [³⁸].

Results of this study reveal that SES considerably interrelates with racial/ethnic disparities between stage diagnosis and mortality of breast cancer. Racial/ethnic disparities are widening, in part due to the greater decrease of advanced stage diagnosis and corresponding increase of survival rates for non-Hispanic White women compared to disadvantaged minority groups within same regions [³⁹]. Women with lower socioeconomic status (SES) are more likely to be diagnosed at late-stage, which therefore may lead to higher mortality for these groups [⁴⁰]. After adjusting SES cofounders, the odds ratio of disparities in late-stage diagnosis and breast cancer mortality were reduced dramatically with a tighter confidence interval. However, stage of diagnosis is still a critical factor in determining if a census tract is significant in racial/ethnic disparities of breast cancer mortality. The underlying factors including health care access, mammography screening and education etc., warrant the further investigation in order to help policy makers to allocate health-related resources with the ultimate purpose of reducing disparities in late-stage diagnosis and breast cancer mortality. Nevertheless, SES at aggregate levels may place some limitations on this study in that this contextual measure may not reflect the accurate SES for different racial/ethnic groups at the same geographic area because SES was measured by all people under the poverty level instead of each racial/ethnic group separately [⁴¹].

Compared to non-Hispanic Whites, African-American and Hispanic women are subject to elevated mortality rates in breast cancer. This disparity can be partially explained by the prevalence of late-stage diagnosis for minority breast cancer patients. Lannin et al. [⁴²] found that African-American women with low-income had 3.0 higher odds of being diagnosed in late-stages. In addition, lack of private health insurance, delay in seeing a physician, lack of access to transportation, less utilization of mammography screening, and cultural beliefs may be important social barriers to preventing minorities from early diagnosis of breast cancer [⁴³,⁴⁴]. Despite lower breast cancer incidence for Hispanics, early stage diagnosis is less likely found among these women than non-Hispanic Whites [⁴⁵].

The correlations between the racial/ethnic disparities of breast cancer late-stage diagnosis and mortality were shown to vary by region. More than 90% of census tracts were identified as having a positive association between the two racial/ethnic disparities. The strength on the variability in the association between the two racial/ethnic disparities may indicate different underlying risk factors impacting the stage diagnosis and mortality in breast cancer across space. For some regions that had a stronger relationship between the two racial/ethnic disparities, early intervention programs, such as mammography screening, may help to reduce late-stage diagnoses, in effect attenuating racial/ethnic disparities in mortality. For regions that have a weaker relationship, identifying other risk factors beyond stage diagnosis might be critical in closing racial/ethnic disparities in breast cancer mortality.

The results reported here indicated a 40% overlap in census tracts that tested significant for both racial/ethnic disparities of breast cancer late-stage diagnosis and mortality. The socioeconomic level for each census tract is a fundamental determinant in the outcomes of the significance tests of both types of disparities in breast cancer outcomes. Based on the results of a regression analysis using data at the census tract level, this study found that there existed a positive spatial association of these racial/ethnic disparities for both African-American and Hispanic women. The regions with a weaker relationship between the two racial/ethnic disparities suggest that further investigation is warranted to understand underlying risk factors including cancer treatment and cultural factors, which may be responsible for these specific results between stage diagnosis and breast cancer mortality. The discrepancies in racial/ethnic disparities reported in this paper imply that different risk factors have an influence on racial/ethnic disparities between diagnosis at advance stages and mortality of breast cancer in different geographic regions. Future work should concentrate on identifying risk factors contributing to racial/ethnic disparities between late-stage diagnosis and mortality as well as better understanding how intervention programs would improve the rates of early detection and breast cancer survival for all women.

The authors declare that they have no competing interests.

NT designed and performed the statistical analysis as well as wrote the main body of the article. JGW significantly contributed the interpretation of the results and revised the article. FBZ helped the study design, interpreted the results, and participated in the preparation of the manuscript. All authors have read and approved the final version of the manuscript.

This work was based on one chapter of Nancy Tian's dissertation research completed at Texas State University-San Marcos under the supervision of F. Benjamin Zhan. Benjamin Zhan's work was in part supported by Wuhan University and the Chang Jiang Scholar Awards Program. The Chang Jiang Scholar Awards Program is jointly sponsored by China Ministry of Education and the Li Ka Shing Foundation (Hong Kong, China). Jeff Wilson served as one of Tian's committee members. Thanks are due to BioMedare for software support and Dr. Pierre Goovaerts, who was instrumental in helping interpreting the regression results. The authors greatly acknowledge the Texas Department of State Health Services and the Texas Cancer Registry for providing the data used in the analyses.