A study of 15,104 patients who were diagnosed with breast cancer and had one breast surgically removed looked at which inherited cancer gene variants increased the risk of developing cancer in the contralateral breast. Among the patients, breast cancer patients with genetically significant pathogenic variants in the BRCA1, BRCA2, and CHEK2 genes had twice the risk of developing contralateral breast cancer compared to patients with normal breast cancer. And patients with estrogen hormone-negative breast cancer who had a genetically significant variant in the PALB2 gene had a threefold increased risk of developing contralateral breast cancer compared to patients with typical breast cancer. In contrast, patients with breast cancer who had a genetically significant pathogenic variant in the ATM gene had no difference in risk of developing contralateral breast cancer compared to patients with typical breast cancer. The 10-year cumulative contralateral breast cancer rates for each patient with a genetically significant variant were 33% for BRCA1, 27% for BRCA2, and 13% for CHEK2 in premenopausal breast cancer patients, and 35% for PALB2 gene carriers who were estrogen hormone negative. For postmenopausal breast cancer patients, the rates were 12% for BRCA1, 9% for BRCA2, and 4% for CHEK2.

Having a genetically significant variant can mean a lot, not only to the patient, but also to their family. Patients with a mutation associated with hereditary cancer are at an increased risk of developing cancer in the opposite breast, even after being diagnosed with breast cancer and having one breast surgically removed, so they need a cancer prevention strategy that includes aggressive cancer screening.