Patients with Lung Cancer Followed by Breast Cancer Have a Better Prognosis than Patients with these Cancers in the Opposite Order due to Differences in Pathological Components

Objectives: To explore the prognostic differences between lung cancer followed by breast cancer (LFB) and breast cancer followed by lung cancer (BFL) and the reasons for the differences. Methods: The database we chose was SEER 18 Regs, from which we retrieved data from patients diagnosed with multiple primary standardized incidence rate (MP-SIR) segments of cancer. Results: A total of 7169 patients were included, of whom 979 were patients with LFB and 6190 were BFL patients. The proportion of small cell lung carcinoma in LFB was 4%, which was significantly lower than that in BFL (p<0.001), while the proportion of carcinoid carcinoma in LFB was significantly higher than that in BFL (p<0.001). Survival analysis of LFB and BFL showed a slightly better prognosis for the former than the latter (HR=0.871 (0.804-0.944)), and the difference was statistically significant (p<0.001). The difference was not statistically significant after adjustment for the pathological type of tumor (HR=0.911 (0.827-1.003), p=0.057). Conclusion: LFB has a worse prognosis than BFL, and this difference is explained by the difference in the ratio of the two pathological components.