Although preoperative magnetic resonance imaging (MRI) can detect mammographically occult contralateral breast cancers (CBCs) among women with ductal carcinoma in situ (DCIS), the impact of MRI on the incidence of subsequent CBC events is unclear. We examined whether MRI use decreases CBC occurrences and detection of invasive disease among women who develop a CBC. Utilizing the Surveillance, Epidemiology, and End Results-Medicare dataset, we assessed overall, synchronous (<6 months after primary cancer diagnosis), and subsequent (≥6 months after diagnosis, i.e., metachronous) CBC occurrence in women aged 67-94 years diagnosed with DCIS during 2004-2009, with follow-up through 2011. We applied a matched propensity score approach to compare the stage-specific incidence rate of CBC according to MRI use. Our sample consisted of 9166 beneficiaries, 1258 (13.7 %) of whom received preoperative MRI. After propensity score matching, preoperative MRI use was significantly associated with a higher synchronous CBC detection rate (108.6 vs. 29.7 per 1000 person-years; hazard ratio [HR] = 3.65; p < .001) with no significant differences in subsequent CBC rate (6.7 vs. 6.8 per 1000 person-years; HR = 0.90; p = .71). The 6-year cumulative incidence of any CBC (in situ plus invasive) remained significantly higher among women undergoing MRI, compared with those not undergoing MRI (9 vs. 4 %, p < .001). Women undergoing MRI also had a higher incidence of invasive CBC (4 vs. 3 %, p = .04). MRI use resulted in an increased detection of synchronous CBC but did not prevent subsequent CBC occurrence, suggesting that many of the undetected CBC lesions may not become clinically evident.
Keywords: Contralateral breast cancer occurrence; Ductal carcinoma in situ; Early detection; Magnetic resonance imaging; Overdiagnosis.