Prior studies have found that postmastectomy radiotherapy (PMRT) for breast cancer (BC) increases the risk of lung cancer (LC). We explored the joint effects of cigarette smoking and PMRT on LC risk.

We conducted a population-based nested case-control study among women registered in the Connecticut Tumor Registry diagnosed with nonmetastatic BC between January 1, 1965 and December 31, 1989. Patient cases developed a LC 10 years after BC diagnosis. Controls were matched to patient cases on age, year of BC diagnosis, and length of survival. Medical records were reviewed for pathology, BC therapy, and smoking history.

Among 113 second primary LC patient cases and 364 controls, compared with nonsmoking women who did not receive PMRT, nonsmoking women who received PMRT had no higher risk of LCPMRT after a diagnosis of BC sharply increased the risk of second primary LC, especially in the ipsilateral lung, among ever-smokers. Clinicians should consider including smoking history in their discussions with patients about the risks and benefits of PMRT.