The contribution of second primary cancers to the mortality of patients with a gastric first primary cancer

Objectives Gastric first primary cancers (FPCs) have an increased risk of developing second primary cancers (SPCs). However, their effect on mortality remains poorly understood. Thus, we aimed to estimate the contribution of SPCs to the mortality of gastric FPCs. Patients and methods A population-based cohort of gastric FPCs diagnosed in 2000-2006 was followed for SPCs (31 December 2012) and vital status (31 December 2017). SPCs (138 synchronous and 205 metachronous, ≤1 and >1 year after the FPC, respectively) were matched (1 : 3, by sex, 5-year age group, and year of FPC diagnosis) with those without an SPC and alive when the corresponding SPC was diagnosed. Results In synchronous SPCs, the hazard ratio (95% confidence interval) for death was 1.07 (0.81-1.40) in males and 0.86 (0.58-1.28) in females; the corresponding estimates for metachronous SPCs were 1.89 (1.49-2.41) and 2.08 (1.42-3.05). In synchronous SPCs, the 10-year cumulative mortality was similar to that of FPCs only; the estimates were higher in lung SPCs. The 10-year cumulative mortality was 75.5% in males and 65.5% in females with metachronous SPCs, which is more than 20% higher than for FPCs only. Conclusion Among patients with gastric FPCs who survive long enough for metachronous SPCs, the latter substantially increases mortality over 10 years, whereas the effect of synchronous SPCs is generally smaller.