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This dissertation examined the incidence and clinical presentation of select second primary cancers following a first primary breast or colorectal cancer (CRC) among Blacks and Whites in the United States. The incidence of hormonally-related second primary breast, endometrial, and ovarian cancers following a first primary cancer of the breast in Black and White women were quantified. We then compared the patho-epidemiological patterns of second primary contralateral breast cancers to those of the first primary tumors. We also explored the incidence of second primary CRC in Black and White female and male survivors of a first primary colorectal cancer. NCI's SEER Registry 9 database was used to follow 450,936 breast cancer survivors, and 299,096 colorectal cancer survivors, diagnosed at age 19 or older, for the occurrence of select second primary cancers between 1973 and 2007. Statistically appropriate bivariate tests were used to examine differences in demographic and clinical parameters by race and between the first and second primary cancers. Cumulative incidence curves, which account for competing events such as death, were generated to describe, among first primary cancer survivors, the probability of developing a second primary cancer. Pepe and Mori's test, a two-sample test of the equality of two cumulative incidence functions, was used to evaluate significant differences in cumulative incidence of second cancers by race. Results indicated that the incidence of second primary breast cancer was higher among Blacks, and that the incidence of second primary endometrial and ovarian cancers was higher among Whites. Further examination showed that second contralateral breast cancers presented at an earlier age in Black women, and were more likely to be larger, less differentiated, and had a comparable numbers of lymph nodes as Whites. Also, second contralateral breast cancers were more likely to have a concordant histology and a congruent estrogen receptor status as the first primary breast cancer. As for CRC, the incidence of second primary colorectal cancer increased with age and was significantly higher in men than in women only among patients diagnosed with a first primary CRC > 50. There was no significant difference in the cumulative incidence of second primary CRC by race. Patterns of second primaries do not necessarily follow the pattern of the first primary cancer, or that of a first primary cancer of the same anatomic site. This dissertation has established a successful framework for the study of second primary cancer incidence. Results contribute to the understanding of cancer disparities between Blacks and Whites by comparing incidence patterns of first and second primary cancers. Results also expand the current understanding of cancer pathogenesis by comparing the clinical parameters of second primary cancers between Blacks and Whites to those of the first primary cancer.

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