Most women treated for breast cancer in one breast never develop a new cancer in the other. For the typical woman the chance is about 1 in 100 over five years and about 1 in 10 over 25 years. The risk is real but builds slowly, a small amount each year over decades. Inherited gene changes and certain tumor types can multiply it several times over, and in women with BRCA1 or BRCA2 mutations it can approach 1 in 2 over 25 years. Standard treatments for the first cancer often lower the chance of a second one as a side benefit.
The Odds of Cancer in the Other Breast
A new cancer in the other breast is called contralateral breast cancer. It is a separate cancer, not spread from the first one. In recent years about 1.3% of women developed it within 5 years of their first diagnosis.1PubMed Central. Risk of contralateral breast cancer according to first breast cancer characteristics among women in the USA, 1992–2016 The yearly risk is about 0.37%, which adds up to about 9.9% over 25 years.2PubMed Central. The risk of contralateral breast cancer: a SEER-based analysis After DCIS, an early non-invasive cancer, the 25-year figure is about the same, at 10.1%.3PubMed Central. The risk of contralateral breast cancer: a SEER-based analysis
Cancer found in both breasts at the same time is rare, about 1% of breast cancer cases.4PubMed. Bilateral breast cancer, synchronous and metachronous; differences and outcome Cancer appearing in the second breast months or years later is more common, about 7% in one large group followed long term.5PubMed. Bilateral breast cancer, synchronous and metachronous; differences and outcome Depending on the definition used, doctors usually call cancers found within 3 to 6 months of each other simultaneous, and later ones a separate later cancer.6PubMed Central. Synchronous and Metachronous Breast Malignancies: A Cross-Sectional Retrospective Study and Review of the Literature7PubMed. Bilateral breast cancer, synchronous and metachronous; differences and outcome
A cancer in the other breast is usually a new primary cancer, not the first one coming back, and it gets its own workup and its own stage. Spread from the first cancer is metastatic disease, which is treated as a different and more serious situation, so the distinction changes the treatment plan. Pathologists compare the two tumors under the microscope, looking at type, grade, and receptor status, to judge whether the second is new or related.
How Inherited Gene Changes Raise the Risk
BRCA1 and BRCA2 are genes that raise breast cancer risk when inherited in a faulty form. Women with these mutations have about 3.4 to 4.5 times the usual risk of cancer in the other breast.8PubMed Central. Population-based study of the risk of second primary contralateral breast cancer associated with carrying a mutation in BRCA1 or BRCA2 Within 10 years, about 17% of BRCA1 carriers and 7% of BRCA2 carriers develop a second breast cancer.9PubMed Central. Second primary breast cancer in BRCA1 and BRCA2 mutation carriers: 10-year cumulative incidence in the Breast Cancer Family Registry For BRCA1 carriers diagnosed before age 40, the 10-year chance rises to about 22%.10PubMed Central. Second primary breast cancer in BRCA1 and BRCA2 mutation carriers: 10-year cumulative incidence in the Breast Cancer Family Registry In families with these mutations, the 25-year risk reaches about 47%.11PubMed. Contralateral breast cancer risk in BRCA1 and BRCA2 mutation carriers Younger age at the first diagnosis raises risk further, especially with BRCA1.12PubMed. Contralateral breast cancer risk in BRCA1 and BRCA2 mutation carriers
Other genes matter too. A faulty CHEK2 gene roughly doubles the risk, about 1.9 times.13PubMed Central. Contralateral Breast Cancer Risk Among Carriers of Germline Pathogenic Variants in ATM, BRCA1, BRCA2, CHEK2, and PALB2 PALB2 raised risk mainly when the first tumor was not hormone-sensitive, about 2.9 times.14PubMed Central. Contralateral Breast Cancer Risk Among Carriers of Germline Pathogenic Variants in ATM, BRCA1, BRCA2, CHEK2, and PALB2 ATM mutations did not clearly raise the risk.15PubMed Central. Contralateral Breast Cancer Risk Among Carriers of Germline Pathogenic Variants in ATM, BRCA1, BRCA2, CHEK2, and PALB2
Testing results can change surgery choices. A woman with a BRCA mutation may weigh removing both breasts differently, so ask about testing before deciding on surgery. Common reasons to be offered testing include a family history of breast or ovarian cancer, a diagnosis at a young age, and a triple-negative tumor. Guidelines on who qualifies have been widening, so a woman who was told no years ago may want to ask again.
A result can come back positive, negative, or as a variant of uncertain significance, meaning a change was found but its effect on cancer risk isn’t known. That kind of variant shouldn’t be treated as a positive result or drive surgery decisions by itself, and its classification can change as research grows. A negative result lowers concern about an inherited cause but doesn’t erase the average risk every survivor carries.
Tumor Type, Age, and Race
Your pathology report lists the tumor’s ER status. ER-positive tumors grow with estrogen; ER-negative ones don’t. An ER-negative first tumor carries about 30% higher risk of cancer in the other breast.16PubMed Central. Hormone receptor status of a first primary breast cancer predicts contralateral breast cancer risk in the WECARE study population Over 25 years the risk was about 11.2% after an ER-negative cancer versus 9.5% after an ER-positive one.17PubMed Central. The risk of contralateral breast cancer: a SEER-based analysis After triple-negative cancer, the 5-year risk was about 1.7%.18PubMed Central. Risk of contralateral breast cancer according to first breast cancer characteristics among women in the USA, 1992–2016 Lobular cancer carried a 5-year risk of 2.6%, versus 1.5% for the more common ductal type.19Journal of Clinical Oncology. Risk of contralateral breast and other cancers in patients with invasive lobular breast cancer
Under age 50, the 5-year risk ranged from about 1.0% after hormone-sensitive tumors to 1.9% after tumors that were not.20PubMed Central. Risk of contralateral breast cancer according to first breast cancer characteristics among women in the USA, 1992–2016 Black women had about 44% higher risk than white women, and Hispanic women 11% higher.21PubMed Central. Race, Ethnicity and Risk of Second Primary Contralateral Breast Cancer in the United States Diagnosed before 40, Black women had about 2.5 times the risk of white women.22PubMed Central. Race, Ethnicity and Risk of Second Primary Contralateral Breast Cancer in the United States Over 25 years the risk was about 12.7% for Black women versus 9.7% for white women.23PubMed Central. The risk of contralateral breast cancer: a SEER-based analysis
You can ask your oncologist’s office for a copy of your pathology report. It lists tumor type, such as ductal or lobular, along with ER, PR, and HER2 status. A tumor negative for all three is called triple-negative, one of the groups with higher risk in the other breast. Study numbers describe groups rather than individuals, so ask how your own mix of features compares with the average. Several factors together, such as young age and a tumor that isn’t hormone-sensitive, can matter more than any one alone.
Treatments and Habits That Lower the Risk
Chemotherapy and tamoxifen each lower the risk of cancer in the other breast, for at least 10 and 5 years respectively.24JNCI: Journal of the National Cancer Institute. Chemotherapy and Tamoxifen Reduce Risk of Second Breast Cancer In BRCA carriers, tamoxifen cut the risk about in half overall.25PubMed. Tamoxifen and risk of contralateral breast cancer in BRCA1 and BRCA2 mutation carriers: a case-control study Taking it for 2 to 4 years lowered risk by about 75%.26PubMed. Tamoxifen and risk of contralateral breast cancer in BRCA1 and BRCA2 mutation carriers: a case-control study Radiation to the first breast slightly raised risk, but only in women treated before age 45.27PubMed. Cancer in the contralateral breast after radiotherapy for breast cancer Hormone therapy appeared to offset some of that radiation risk for hormone-sensitive cancers.28PubMed Central. Contralateral breast cancer after radiotherapy and hormone therapy in two cohorts of US breast cancer survivors
Side effects such as hot flashes, joint aches, and fatigue lead many women to stop their pills early. Protection for the other breast is one more reason to finish the planned course, and side effects can often be eased by changing the dose, the timing, or the drug. Talk with your doctor before stopping any prescribed drug.
Radiation treats the cancer you have now, and that benefit is far larger than the small added risk to the other breast. Modern planning aims to limit the dose reaching the other breast, so older studies may overstate today’s risk. If you are young, ask your radiation oncologist how your treatment is planned to protect it.
Among women with hormone-sensitive cancer, current smoking was linked to about twice the risk.29PubMed Central. Relationship between potentially modifiable lifestyle factors and risk of second primary contralateral breast cancer among women diagnosed with estrogen receptor-positive invasive breast cancer Seven or more drinks a week was linked to about 1.9 times the risk.30PubMed Central. Relationship between potentially modifiable lifestyle factors and risk of second primary contralateral breast cancer among women diagnosed with estrogen receptor-positive invasive breast cancer Obesity was linked to about 1.4 times the risk.31Journal of Clinical Oncology. Relationship Between Potentially Modifiable Lifestyle Factors and Risk of Second Primary Contralateral Breast Cancer Among Women Diagnosed With Estrogen Receptor-Positive Invasive Breast Cancer Quitting smoking, limiting alcohol, and managing weight are reasonable steps.
Removing the Healthy Breast
Removing the healthy breast is called contralateral prophylactic mastectomy. In BRCA carriers it lowered the risk of cancer in the other breast by about 90%.32PubMed Central. Risk reduction of contralateral breast cancer and survival after contralateral prophylactic mastectomy in BRCA1 or BRCA2 mutation carriers It did not clearly improve survival in those women, though removing the ovaries did.33PubMed Central. Risk reduction of contralateral breast cancer and survival after contralateral prophylactic mastectomy in BRCA1 or BRCA2 mutation carriers In another high-risk group, 0.5% developed a cancer after the surgery versus 8.1% without it, and 10-year survival was 83% versus 74%.34PubMed Central. Contralateral Prophylactic Mastectomy is Associated with a Survival Advantage in High-Risk Women with a Personal History of Breast Cancer
The surgery lowers the chance of a new cancer in that breast to very low, but it does not lower the chance that the first cancer returns or spreads. The stage and features of the first cancer often decide your outlook more than the other breast does. It is also a bigger operation than treating one breast, with a longer recovery and more risk of complications such as infection or problems with reconstruction.
Most women choosing it cite family history, fear of another cancer, or dense breasts that are hard to monitor.35PubMed Central. Factors impacting the decision of breast cancer patients to undergo contralateral prophylactic mastectomy Anxiety pushes some women toward it even when told it won’t extend life.36PubMed Central. Operating on Anxiety: Negative Affect toward Breast Cancer and Choosing Contralateral Prophylactic Mastectomy
Watching the Other Breast
Mammography is the main follow-up test after breast cancer.37PubMed Central. Surveillance for second breast cancer events in women with a personal history of breast cancer using breast MRI: a systematic review and meta-analysis In community care, mammograms caught about 70% of cancers and MRI added about 61%.38PubMed Central. Surveillance Breast MRI and Mammography: Comparison in Women with a Personal History of Breast Cancer But MRI led to biopsies in about 10% of exams versus 4% with mammograms.39PubMed Central. Surveillance Breast MRI and Mammography: Comparison in Women with a Personal History of Breast Cancer It found slightly more cancers yet did not reduce cancers showing up between scans.40PubMed Central. Surveillance Breast MRI and Mammography: Comparison in Women with a Personal History of Breast Cancer It remains unclear whether adding MRI improves health outcomes for most survivors.41PubMed Central. Surveillance for second breast cancer events in women with a personal history of breast cancer using breast MRI: a systematic review and meta-analysis
Women with BRCA mutations are often offered MRI along with mammograms because their risk is higher. Very dense breasts can hide tumors on a mammogram, so MRI may be offered to those women too. Many women at average risk are monitored with mammograms alone. Ask your care team whether your risk level makes MRI a good fit.
MRI picks up more suspicious-looking spots, so expect more callbacks and biopsies, and most turn out to be benign. A callback is not a diagnosis, though the wait can be stressful. MRI often needs an injected contrast dye and takes longer than a mammogram. Insurance coverage varies, so check before scheduling.
Is Cancer in Both Breasts More Serious
Women with cancer in both breasts at the same time had 10-year survival of about 57% versus 71% for one breast.42PubMed Central. Breast cancer mortality in synchronous bilateral breast cancer patients But after accounting for both tumors’ features, having two cancers added no extra risk.43PubMed Central. Breast cancer mortality in synchronous bilateral breast cancer patients With full guideline treatment for both tumors, survival matched one-breast cancer.44PubMed. Survival of patients with bilateral versus unilateral breast cancer and impact of guideline adherent adjuvant treatment: a multi-centre cohort study of 5292 patients Getting complete standard treatment for each tumor matters most. Survival figures describe groups and past years, and treatment has improved, so they aren’t a prediction for any one woman.
Each tumor is tested and staged separately, since the two can differ in type and receptor status. The plan is built to cover the more aggressive tumor, which often sets the choice of drugs. Ask how it addresses both cancers, including surgery, radiation, and drug therapy.