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Background: The cancer incidence for all sites has been reported to be lower in Native Americans than in White Americans. Concerns have been expressed, however, that the observed low incidence may be a result of inaccurate reporting of race. Purpose: The objective of this study was to investigate the extent to which racial misclassification may contribute to the observed low cancer incidence among Native Americans. Methods: A registry of individuals eligible to receive medical services funded by the Indian Health Service was linked by computer to the Puget Sound Surveillance, Epidemiology, and End Results (SEER) cancer registry. Results: Only 137 (60%) of the patients with invasive cancer registered with the Indian Health Service and for whom race was recorded were identified as Native Americans in the SEER registry. Similarly, 55 (69%) of 80 in situ cervical cancer case patients were classified as Native American. A strong association was observed between NativeAmerican blood quantum level and racial misclassification. Conclusion: The results of this study indicate that the observed low cancer incidence in Native Americans relative to Whites in the northwest United States is at least

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Vol. 84, No. 12, June 17, 1992

Received October 25, 1991; revised February 25, 1992; accepted March 24, 1992. F. Frost, Center for Health and Population Research, Lovelace Medical Foundation, Albuquerque, N.M. V. Taylor, E. Fries, Cancer Prevention Research Program, Fred Hutchinson Cancer Research Center. Seattle, Wash. We thank Dr. Emily White for her helpful review of this manuscript. *Correspondence to: Floyd Frost, Ph.D., Center for Health and Population Research, Lovelace Medical Foundation, 2425 Ridgecrest Dr., S.E., Albuquerque, NM 87108.

REPORTS 957

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Floyd Frost, * Victoria Taylor, Elizabeth Fries

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Influence of organ environment on extracellular matrix degradative activity and metastasis of human colon carcinoma cells. J Natl Cancer Inst 82:1890-1898, 1990 (54) FIDLER L): Orthotopic implantation of human colon carcinomas into nude mice provides a valuable model for the biology and therapy of cancer metastasis. Cancer Metastasis Rev 10:229-243, 1991

(35) KAIGHN ME. NARAYAN KS. OHNUKI Y, ET

Racial Misclassification of Native Americans in a Surveillance, Epidemiology, and End Results Cancer Registry

partially attributable to racial misclassification in the SEER cancer registry. [J Natl Cancer Inst 84:957-962, 1992]

tors associated with misclassification of race.

Methods The Indian Health Service and SEER Registries

958

A registry of individuals eligible to receive medical services funded by the Indian Health Service was initiated in mid-1984. The Indian Health Service had previously collected information concerning 1) health care services provided at facilities funded by the Indian Health Service and 2) reimbursement by the Indian Health Service for medical services provided at facilities that were not funded by the Indian Health Service. These databases were used to facilitate the initial formation of the Indian Health Service registry. The Portland area registry includes Idaho, Oregon, and Washington. To qualify for health care services funded by the Indian Health Service, a person must have documentation indicating membership in a federally recognized tribe. The majority of tribes require a total ancestry level of at least 25% for membership. Indian Health Service facilities also provide services to non-Native Americans in isolated areas who are without other medical facilities and to those in emergency situations. The Portland area Indian Health Service unit periodically updates the vital status of individuals in the registry. Both active and inactive records, however, are retained in the database indefinitely. The Indian Health Service registry provides information concerning total NativeAmerican blood quantum level, tribal ancestry, and Indian Health Service unit to which each person belongs. Blood quantum is defined by the proportion of an individual's ancestry which is Native American. For example, a person with two Native-American and two White grandparents would have a 50% blood quantum level. The Seattle-Puget Sound SEER cancer registry has collected data on newly diagnosed cancer cases which have occurred in 13 contiguous counties of western Washington since 1974. Cancer cases are ascertained from records of hospitals, pathology laboratories, surgical centers, oncology offices, and state death certificates. The SEER registry relies primarily on hospital admission

Data Linkage Procedure A linkage was performed between the 1989 Indian Health Service registry and the 1974-1989 SEER registry. Variables used for an initial computerized matching of individuals appearing in both databases were last name, first-name initial, and year of birth plus or minus one. This linkage resulted in the identification of 677 possible matches which, after manual review, yielded 360 actual matches. Two hundred seventy-six matches were made with an identical Social Security number, same or similar name, and same or similar birth date. Fifty-seven matches were made with the Social Security number missing in either the SEER data or in the Indian Health Service registry. For these case patients, matches were called if I) the first name, last name, and birth date matched or 2) the first name and last name matched, and the birth date was off by 1 day, 1 month, or I year. Twenty-seven matches were made with different Social Security numbers. These matches were called if I) the Social Security number was off by only one digit or was different because of a transposition of two digits or 2) the first name, last name, and middle name initials matched and the birth date matched or was off by only one digit. Two case patients were excluded because the Indian Health Service registry indicated they were not Native American. Data Analysis Individuals in the Indian Health Service registry who were entered as Native Journal of the National Cancer Institute

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The incidence of specific cancers has been found to be higher among NativeAmerican than among White populations {1-12). The cancer rate for all sites combined, however, has been consistently reported to be lower in Native Americans than in White Americans. A decreased overall cancer incidence has been described among Native-American populations in Alaska, Canada, New Mexico, New York, and the state of Washington (6,9-13). Norsted and White (12) investigated the incidence of cancer among Native Americans in western Washington state, using data from the SeattlePuget Sound Surveillance, Epidemiology, and End Results (SEER) cancer registry, for the 1974-1983 time period. These authors reported a decreased agestandardized incidence ratio for all cancer sites in Native-American men and women. They expressed a concern, however, that the observed low incidence of most cancers among Native Americans may be a result of inaccurate reporting of race. Racial misclassification has been previously shown (14,15) to have contributed to the underestimation of mortality rates in Native-American populations. Using death certificates and corresponding linked birth certificates, Frost and Shy (14) examined the racial classification of infants who died in the state of Washington between 1968 and 1977. One third (33%) of the infants recorded as being Native American on their birth certificates were not recorded as such on their death certificates (14). A study linking 1960 census records and death records found that 17% of individuals who reported their race to be Native American in census returns were classified as White on their death certificates US). This report describes a data-linkage study using the Portland area Indian Health Service registry and the SeattlePuget Sound SEER cancer registry. The objectives of this study were 1) to investigate the extent to which racial misclassification may contribute to the observed low cancer incidence among Native Americans and 2) to define fac-

information and physician entries in patient charts for ascertainment of race. Individual physicians are contacted if racial information is not in the medical record. Additionally, the SEER Program uses racial data from death certificates if race is previously unknown. If an individual's race is recorded differently in medical records and on the death certificate, however, the Seattle-Puget Sound SEER registry uses preferentially the information in the medical records. The SEER registry contains data on age at diagnosis, race, vital status, cancer site, cancer histology, and county of residence. Five hundred forty-five individuals with invasive cancer were classified as Native American by the SEER Program between 1974 and 1989, and 139 women with in situ cervical cancer were recorded as being Native American.

American in the SEER database were compared with those who were not. Variables considered in comparative analyses were age at diagnosis, gender, total Native-American blood quantum level, year of diagnosis, and vital status. The chi-square test was used to determine the statistical significance of categorical data, and Student's t test was used for continuous data (16). Multivariate analyses used unconditional logistic regression to assess the independent effect of variables (77).

Histology

Racial Classification Native American = 137

Invasive =233

• • - "Misclassified" = 90 Unknown = 6

Unknown or benign = 12 Matches =358

Fig. 1. Characteristics and racial classifications of the SEER and the Indian Health Service registry matches.

In situ, site other than cervix =9

Characteristics of the unique matches between the SEER and the Indian Health Service registries are summarized in Fig. 1. Of the 358 matches, 233 were invasive cancer cases, and 113 were in situ cancer cases. The histology for the remaining 12 matches was benign or unknown. The uterine cervix was the site of 104 (92%) of the in situ cases. All but 14 (94%) of the 233 invasive cancer case patients and 98 (94%) of the 104 in situ cervical cancer case patients were known to be residents of the SEER counties at the time of diagnosis. Two hundred twenty-two (95%) of the invasive cancer cases and 97 (93%) of the in situ cervical cancers were registered with the Portland area Indian Health Service units located within the SEER counties. Invasive Cancer The characteristics of the 233 invasive cancer case patients are summarized in Table 1. Eighty-seven (37%) of these case patients were male, and 146 (63%) were female. The mean age at diagnosis was 57 years. Eighty-two (39%) of the invasive cancers occurred among individuals known to be full-blooded Native Americans. A further 73 (35%) of the case patients had at least 50% NativeAmerican ancestry, while 54 (26%) were less than 50% Native American. Nearly half (45%) of the invasive cancer case patients were diagnosed in 1986 or later, and 163 (70%) were recorded as being alive. Prostate cancer was the most commonly occurring invasive cancer among men in this study group, and breast cancer was the most frequent invasive malignancy in women. The racial classification of the invasive cancer case patients is summarized Vol. 84, No. 12, June 17, 1992

Native American = 55

In situ cervical = 104

General Observations

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Results "Misclassified" = 25 Unknown = 24

in Fig. 1. Race was entered as unknown in the SEER registry for six (3%) of the invasive cancer case patients registered by the Indian Health Service. These case patients were excluded from analyses

comparing individuals who were entered as Native American in the SEER registry with those who were "misclassified." Only 137 (60%) of the remaining 227 case patients registered by the Indian

Table 1. Characteristics, by gender, of invasive cancer case patients registered with the Indian Health Service Gender Male,* No. (%)

Female,t No. (9b)

Total,* No. (%)

Age at diagnosis, y

Racial misclassification of Native Americans in a surveillance, epidemiology, and end results cancer registry.

The cancer incidence for all sites has been reported to be lower in Native Americans than in White Americans. Concerns have been expressed, however, t...
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