In recent years, women facing breast cancer decisions have grappled with choices regarding their treatment. People who have certain genetic mutations (BRCA1/2) and breast cancer in one breast have usually thought about either having a bilateral mastectomy to remove both breasts or keeping the healthy breast and using radiation therapy (RT) on the cancerous breast.

Research has suggested that opting for breast-conserving therapy (RT for the affected breast and no intervention for the healthy one) does not significantly differ in terms of survival rates or recurrence compared to undergoing mastectomy. However, individuals with these mutations still face a heightened risk of developing cancer in the opposite breast (known as contralateral breast cancer, or CBC). Factors like younger age at diagnosis and family history of breast cancer also contribute to this risk. To better understand if radiation therapy affects the risk of CBC, researchers analysed data from thousands of individuals with BRCA1/2 mutations and primary breast cancer.

Among over 3,600 patients, about two-thirds received RT. Over almost a decade of follow-up, about 252 patients who underwent RT experienced CBC, with around 180 cases being invasive. Meanwhile, about 98 patients who did not receive RT encountered CBC, with roughly 70 being invasive cases.

The analysis highlighted a noteworthy association between RT and an increased risk of CBC in women with a BRCA2 mutation, showing a 44% higher risk. Although the trend was less clear for those with BRCA1 mutations, there was still an indication of elevated risk.

Additionally, the highest likelihood of a second cancer occurrence was observed around 5 to 6 years after the initial breast cancer diagnosis. These findings underline the complexity of treatment decisions for individuals with these mutations and emphasise the importance of informed discussions with healthcare providers to tailor the most appropriate approach for each person's unique situation.