INCIDENCE OF CONTRALATERAL BREAST CANCER IS FALLING!
Contralateral breast cancer (CBC) has declined annually in the USA by approximately 3% from the mid-1980s to 2006. Multiple explanations have been put forth, including the use of tamoxifen in the 1980s and 90s, aromatase inhibitors in the early 2000s, a shift toward taxanes in chemotherapy regimens, and the use of trastuzumab for HER2 positive cancers. Furthermore, contralateral prophylactic mastectomy usage has tripled to over 10% of those requiring unilateral mastectomy for cancer treatment. Researchers from the National Cancer Institute (Ramin et al.) used SEER data from 1992-2016 to compare the risk of CBC to the risk of breast cancer in the general population. The researchers also tracked the incidence of CBC from 2004-2016 based on the first cancer characteristics. The authors used standardized incidence ratios (SIRs), a relative risk measure, to report their results.
Results: The authors identified 12,986 cases of CBC during the study period. There was a 2-fold increase in CBC incidence in breast cancer patients than in the general population (SIR 2.21). Relative risk was higher for women initially diagnosed <40 years of age (SIR 6.41). The risk was also higher following an initial ER-negative tumor (SIR 3.05). CBC also was found to differ based on initial staging, with risk higher after stage 1 (SIR 2.08) than stage 3 (SIR 2.97). SIRs were slightly higher in women treated with chemotherapy (SIR 2.49) than those treated with hormone therapy (SIR 1.88). There was no significant difference in the risk of CBC when comparing women with HERE2 negative and positive breast cancers (SIR 1.45, 1.58). CBC risk was highest after an initial triple-negative cancer (SIR2.43).
Over the calendar period of diagnosis, the risk of CBC significantly declined. The SIR for CBC decreased 2.49 in 1992-1997, to 1.57 in 2010-2015 (p-trend<0.0001). This risk of CBC decline over time was noted regardless of age at diagnosis after an ER-positive tumor but not after an ER-negative tumor. The authors noted similar temporal trends even when excluding patients having contralateral prophylactic mastectomies.
The data suggest that 5- year CBC risk declined over time among women treated with chemotherapy, particularly in women <50. In the recent treatment era of 2004 to 2015, breast cancer patients had an incidence of CBC of 1.31. The highest risk noted was in women <50 with ER-negative tumors and the lowest in women <50 with ER-positive tumors (SIR 1.89). The 5-year cumulative incidence of CBC did not differ by HER2 status during this latter 5 year time period. During this period, the highest 5-year cumulative incidence was noted after a first triple-negative breast cancer (SIR 1.71)
Conclusions: The researchers found that the risk of CBC continued to fall in the latest treatment era examined. CBC risk continued to decline past 2006, potentially owing to the more aggressive use of aromatase inhibitors. The authors noted the first decline in CBC after initial triple-negative breast cancer in 2004-2009, with further decreases noted through 2016, particularly in women <50. It might be convenient to believe that this is due to advances in chemotherapy with a shift towards taxanes-based therapy. No matter the reason, this data from the NCI further informs the breast surgeon to guide treatment decisions.