Lung cancer as a second primary malignancy (lung-2) is increasingly common, but its prognosis is poorly understood. This study aims to examine the overall and cancer-specific survival of patients diagnosed with lung-2 compared to lung-1. Primary lung cancer patients diagnosed from 1988 to 2014 in the Surveillance, Epidemiology, and End Results (SEER) program were included. Lung-2 was identified in patients with a previous diagnosis of nonlung primary malignancy in SEER. Hazard ratios (HRs) of overall and lung cancer-specific mortality were estimated among patients with lung-2 compared to lung-1, adjusting for age and calendar period at diagnosis, sex, race, socioeconomic status, tumor stage, histology, tumor grade, and treatment. A total of 679541 and 85758 patients were identified as lung-1 and lung-2, respectively. Compared to lung-1, patients with lung-2 were more likely to be diagnosed at localized stage, with smaller primary tumor, and treated with surgery. Lung-2 patients were at lower risk of lung cancer-specific mortality in the first 5years (HR, 0.77; 95% CI, 0.76-0.78 at <1year; HR, 0.87; 95% CI, 0.86-0.89 from 1 to <5years) but at higher risk thereafter (HR, 1.32; 95% CI, 1.27-1.37 from 5 to 10years), independent of tumor characteristics and cancer treatment. Similar pattern was found for overall mortality, although the survival benefit was restricted to the first year after diagnosis. Patients diagnosed with lung-2 face a favorable lung cancer-specific survival within the early period after diagnosis. A conservative approach to manage lung-2 solely based on malignancy history is not supported.
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