ESR1 and breast cancer: In detail, we demonstrated that the smoking [odds ratio (OR) = 9.73, P < 0.001] and radiotherapy [relative risk (RR) = 1.40, P < 0.001] were high-risk factors for developing second lung cancer in breast cancer patients and the chemotherapy (RR = 0.69, P = 0.002), positive estrogen receptor (ER) status (RR = 0.93, P = 0.014) and positive progesterone receptor (PR) status (RR = 0.86, P < 0.001) were protective factors for second primary lung cancer [5].