Key clinical point: Stray radiation of a primary breast tumor increased the risk for contralateral breast cancer among women aged younger than 40 years when treated.

Major finding: After a latency of 5 years or more among women with a high genetic risk aged younger than 40 years when they received the first diagnosis, 28% of the increased risk of contralateral breast cancer was attributable to stray radiation exposure.

Study details: A case-control study including 3,732 women who received a diagnosis for a first invasive local or regional breast cancer when they were aged younger than 55 years.

Disclosures: Dr. Watt had no financial disclosures. The research was funded by the National Cancer Institute.