Survival of Patients with First and Metachronous Second Primary Breast Cancer or Lung Cancer Malignancy: Comparisons Using the SEER Database

Introduction Breast cancer (BC) and lung cancer (LuC) are common malignancies. The survival of patients with metachronous second primary malignancy (MSPM) of BC and LuC after a first primary of BC and LuC remains unclear. Methods Data of patients with BC and LuC, with or without MSPM of BC and LuC, who were diagnosed from 2000 to 2014, were extracted from the Surveillance, Epidemiology, and End Results (SEER) database. The baseline characteristics of the different groups were compared using chi-square tests. The survival curves were compared using the log-rank test. Survival analysis was also performed in other malignancies with data extracted from the SEER database using the same method. Results Significant differences in most of the demographics and clinicopathological factors were found between the groups. Patients with LuC with an MSPM of BC (LuC2BC) exhibited better survival than those with LuC (P < 0.001), while patients with BC with an MSPM of LuC (BC2LuC) exhibited worse survival than those with BC (P < 0.001). A similar trend was found in other malignancies. Prostate cancer (PC) followed by thyroid cancer (TC) (PC2TC) and TC followed by PC (TC2PC) did not show an obvious survival trend against their index malignancy (IM). Conclusions MSPM benefits the IM prognosis if the survival rate of MSPM is better than that of the IM and vice versa. However, the situation is somewhat confusing when the survival differences between MSPM and IM are minimal.