Effect of breast cancer radiotherapy and cigarette smoking on risk of second primary lung cancer

Purpose 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. Methods 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. We used conditional logistic regression to estimate odds ratios for the independent and joint effects of smoking and PMRT on risk of overall, ipsilateral, and contralateral LC. Results: 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 LC; adjusted odds ratios were 5.9 (95% CI, 2.7 to 12.8) for ever-smokers who did not receive PMRT and 18.9 ( 95% CI, 7.9 to 45.4) for ever-smokers who received PMRT. Adjusted odds ratios for the joint effects of smoking and PMRT were 10.5 ( 95% CI, 2.9 to 37.8) for the contralateral lung and 37.6 ( 95% CI, 10.2 to 139.0) for the ipsilateral lung. Smoking and PMRT were associated with increased risk for all histologic types of LC. Conclusion PMRT 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.