Racial and age differences in multiple primary cancers after breast cancer: a population-based analysis.

The occurrence of multiple primary cancers was evaluated among 17,944 white and black female residents of Metropolitan Detroit diagnosed with breast cancer between 1973 and 1983. Invasive second primary cancers were diagnosed among 1106 of these women, almost twice the expected number. Subsequent in situ cancers were detected four times more often than expected. Fifty-six percent of the subsequent invasive cancers were of the breast (Standardized Incidence Ratio, SIR = 3.80). Black women experienced higher risk of subsequent breast cancers (SIR = 5.30) than white women (SIR = 3.62). Highest risk was seen among women first diagnosed before age 40 (SIR for black women = 26.15, SIR for white women = 10.87) and within five years of initial diagnosis. These findings suggest that young breast cancer patients, especially black women, are at high risk of developing a second primary breast cancer soon after their initial diagnosis and should be under continued medical surveillance. The occurrence of multiple primary breast cancers among young women suggests a genetic component to risk. Identification of this subpopulation would be useful in the study of molecular and genetic markers for cancer. Subsequent colon (SIR = 1.24) and cervical (SIR = 1.54) cancers also were diagnosed significantly more often than expected, as were ovarian cancers among white women (SIR = 1.45). These findings are consistent with common etiologic factors associated with these cancers.