Bilateral breast cancer refers to cancer present in both breasts. While breast cancer commonly affects one breast, understanding the likelihood and contributing factors for developing it in both breasts is important. This article explores the chances of such a diagnosis and the various elements that can influence this occurrence.
Understanding Bilateral Breast Cancer
Bilateral breast cancer is categorized by the timing of diagnosis in each breast. Synchronous bilateral breast cancer (SBBC) occurs when tumors are found in both breasts at or around the same time, typically within three to twelve months. Metachronous bilateral breast cancer (MBBC), also known as contralateral breast cancer, involves a diagnosis in the second breast after the initial cancer in the first breast has been diagnosed and treated. The interval for metachronous diagnosis can vary, often occurring more than six months after the first diagnosis. Distinguishing between these two types is important because they can differ in their underlying causes, risk factors, and implications for management.
Statistical Likelihood
Bilateral breast cancer is uncommon compared to cancer in a single breast. It occurs in approximately 1% to 11% of all individuals diagnosed with breast cancer, with some sources narrowing this to 1% to 3%. This variation often depends on how cases are defined and the surveillance period of studies.
Synchronous bilateral breast cancer is rarer, with an incidence of 1% to 2% of all breast cancer cases. Metachronous bilateral breast cancer is more frequently observed, with incidence rates ranging from 5% to 8%. For individuals who have already had breast cancer in one breast, the lifetime risk of developing cancer in the opposite breast is notable. The 25-year cumulative incidence of contralateral invasive breast cancer can be around 9.9%. The annual risk of developing cancer in the opposite breast is approximately 0.37% per year, and this risk can persist over many years after the initial diagnosis.
Factors Influencing Risk
Several factors influence the chance of developing breast cancer in both breasts. Genetic predispositions are important, particularly inherited mutations in genes such as BRCA1 and BRCA2. Individuals with these mutations have a higher risk of developing bilateral breast cancer. Other genes, including CHEK2, ATM, PALB2, and TP53, are also associated with an elevated risk. For example, the 10-year cumulative risk of contralateral breast cancer for BRCA1 mutation carriers is 21.1% to 32%, and for BRCA2 carriers, it is 10.8% to 21%.
A strong family history of breast or ovarian cancer, even without a known genetic mutation, also increases risk. It is higher when multiple close relatives have had breast cancer, especially if they were diagnosed at a younger age or had bilateral disease. Younger age at the time of the first breast cancer diagnosis is another factor; individuals diagnosed at a younger age, particularly those under 40 or 50, face a higher risk of developing cancer in the contralateral breast. For BRCA1/2 mutation carriers, being diagnosed before age 41 leads to a higher 10-year cumulative contralateral breast cancer risk compared to those diagnosed later.
Certain characteristics of the initial tumor also impact risk. Invasive lobular carcinoma (ILC), a type of breast cancer, is associated with a greater rate of contralateral breast cancer compared to other forms, with a 5-year bilateral cancer rate of about 8%. This increased risk for ILC patients is often evident in the first year after diagnosis. Prior radiation therapy to the chest can increase the risk of developing breast cancer in the irradiated area. Breast density is another factor; women with extremely dense or heterogeneously dense breast tissue have a higher risk of developing contralateral breast cancer.
Screening and Monitoring
For individuals at increased risk of developing bilateral breast cancer, particularly those with a personal history of breast cancer or identified genetic predispositions, regular surveillance is important. Screening strategies are tailored to individual risk factors to facilitate early detection. Common imaging techniques include mammography and magnetic resonance imaging (MRI). MRI is often recommended for high-risk individuals due to its sensitivity in detecting new cancers, particularly in dense breast tissue where mammography might have limitations. These screening methods aim to identify new cancer development at an early, treatable stage.