Risk of metachronous contralateral breast cancer: Systematic review and meta-analysis

Introduction Over the last 2 decades, an increasing number of primary breast cancer (PBC) patients opted for a risk reducing contralateral mastectomy, to minimize risk of subsequent contralateral breast cancer (CBC). Therefore, accurate risk estimates of CBC are important for patient tailored counseling and decision making regarding treatment. Currently, CBC risk estimates are determined by BRCA1/2 mutation status, age at PBC diagnosis and family history. For other risk factors, results are inconclusive. We therefore aimed to quantify the association with CBC risk for patient, tumor and treatment related factors as reported in the literature. Methods Medline was searched for publications on CBC risk by one reviewer (DA). We focused on associations between CBC risk and BRCA1/2 and CHEK2∗1100delC mutations, SNPs, risk-reducing salpingo-oophorectomy and various factors at PBC diagnosis: family history of breast cancer (BC), age, BMI, menopausal status, mammographic density, TNM-stage, receptor status, morphology, administered radiotherapy and adjuvant systemic treatment. Eligible papers were published in English between 01-01-1990 and 01-04-2015, investigated female patients with invasive early BC and reported relative risk (RR) estimates (i.e., hazard ratios, relative risks or odds ratios). We combined RR estimates using a random effects model. Heterogeneity was assessed using the I2 statistic. Forest plots for crude and adjusted estimates were generated stratifying for mutation status (i.e., BRCA1, BRCA2, CHEK2∗1100delC), non-carriers and unselected patients (i.e., population/hospital based cohorts). Results After screening of 1,423 papers for title and abstract, 173 eligible papers were fully read, and 96 papers fulfilled the inclusion criteria. Both in the unselected group and in the BRCA1 and BRCA2 groups, administration of adjuvant endocrine therapy (vs. not), was associated with decreased CBC risk (RR, 0.62 (95% CI 0.55-0.69), 0.55 (95% CI 0.39-0.77), and 0.62 (95% CI 0.40-0.95), respectively). Adjuvant chemotherapy was associated with reduced CBC risk in unselected patients (RR 0.73; 95% CI 0.62-0.86). CBC risk was increased in unselected patients who received radiotherapy at age30 (vs.