Prognosis of metachronous contralateral breast cancer according to stage at diagnosis: The importance of early detection

Detection of metachronous contralateral breast cancer (MCBC) is an important aspect of follow-up among the many women previously diagnosed with first primary breast cancer (FPBC). While randomized studies have demonstrated the efficacy of early detection of FPBC, such findings cannot be generalized to women previously diagnosed with breast cancer since they were specifically excluded from participation. In this study, we determined if detection of MCBC at stage 0-I improves prognosis among women diagnosed with stage 0-III FPBC. Using data from the 1990-2000 Surveillance, Epidemiology, and End Results program, we defined MCBC as contralateral breast cancer that occurred at least 6 months after stage 0-III FPBC diagnosis. Women who developed a MCBC were categorized as having stage 0-I versus stage II-IV. To avoid lead-time bias, survival of MCBC was measured from the date of FPBC. Among 170,453 women who had been diagnosed with stage 0-III FPBC, 2904 were subsequently diagnosed with MCBC. Of these 2904 women, 329 died from breast cancer and 194 died from other causes. Seventy percent of women were diagnosed with stage 0-I MCBC. An 81% survival benefit existed for stage 0-I MCBC relative to those with stage II-IV MCBC in multivariable analysis (adjusted hazard ratio: 0.19; 95% confidence interval: 0.15-0.25). The findings are robust for various subpopulations. Unmeasured confounding was unlikely to explain the observed results in sensitivity analysis. For women with stage 0-III FPBC, diagnosis of stage 0-I MCBC is associated with an 81% reduction in risk of breast cancer death.