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Background: Poor access to or inadequate health insurance contributes to disparities in cancer incidence and mortality. Cancer registry "payer source" data is collected by many cancer registries in the United States and has been used to compare cancer outcomes across insurance types., Objectives: We evaluated the validity of cancer registry data on patient Medicaid status against enrollment data from Medi-Cal, California's Medicaid program., Methods: Data from the statewide California Cancer Registry for persons under age 65 years diagnosed with 1) any cancer in 1998 and 1999 or 2) with invasive cervical cancer between 1996 and 1999 were obtained and linked probabilistically to Medi-Cal enrollment files. We compared registry Medicaid status, determined from payer source information, against linkage results and used crosstabulations to calculate sensitivity, specificity, and positive predictive value. These measures were compared across different hospital and patient characteristics and cancer types., Results: Cancer registry Medicaid status data had poor sensitivity (48%), good specificity (98%), and moderate positive predictive value (77%). Measures of validity did not vary substantially by cancer type, stage, patient age, sex, vital status, race/ethnicity, socioeconomic status, or diagnosing hospital size. Registry data undercounted the number of Medicaid patients by 52% and incorrectly assigned Medicaid as a payer to approximately 2% of patients., Conclusions: As a result of the poor validity of cancer registry Medicaid status data, caution should be used when interpreting cancer outcomes by insurance type calculated from registry payer source data. Linkage of registry data to Medicaid enrollment files represents a more accurate means of identifying Medicaid insurance status.

Public health surveillance systems relevant to cancer, centered around population-based cancer registration, have produced extensive, high-quality data for evaluating the cancer burden. However, these resources are underutilized by the epidemiology community due, we postulate, to under-appreciation of their scope and of the methods and software for using them. To remedy these misperceptions, this paper defines cancer surveillance research, reviews selected prior contributions, describes current resources, and presents challenges to and recommendations for advancing the field. Cancer surveillance research, in which systematically collected patient and population data are analyzed to examine and test hypotheses about cancer predictors, incidence, and outcomes in geographically defined populations over time, has produced not only cancer statistics and etiologic hypotheses but also information for public health education and for cancer prevention and control. Data on cancer patients are now available for all US states and, within SEER, since 1973, and have been enhanced by linkage to other population-based resources. Appropriate statistical methods and sophisticated interactive analytic software are readily available. Yet, publication of papers, funding opportunities, and professional training for cancer surveillance research remain inadequate. Improvement is necessary in these realms to permit cancer surveillance research to realize its potential in resolving the growing cancer burden.

Differences in cancer occurrence within a racial/ethnic group may be influenced by migrant status or level of acculturation, which can indicate variations in environmental exposures. When immigration and acculturation data on individual patients are not available, birthplace may serve as a proxy. As part of a larger study intended to assess the utility of the birthplace variable in the Greater Bay Area Cancer Registry (GBACR), part of the Surveillance, Epidemiology, and End Results (SEER) program, we measured its completeness, and determined whether missing birthplace is associated with patient and hospital reporting source characteristics. Subjects were persons diagnosed with cancer within the GBACR surveillance area during the period 1988-1996. Of the 204,553 subjects, 67% had birthplace recorded in the registry. Deceased persons were 10 times more likely than living persons to have data on birthplace. Racial/ethnic groups which included more foreign-born persons, such as Southeast Asians, tended to have more complete birthplace information than did groups with fewer foreign-born, such as Japanese and Hispanics. The effects of patient and reporting source characteristics on birthplace completeness differed across racial/ethnic groups. These data indicate that completeness of the birthplace variable in the GBACR is biased, and that investigators considering birthplace in analyses of SEER data should consider these biases. For birthplace data to be useful, completeness needs to be improved at the level of the diagnosing facility.