Prediagnosis obesity and secondary primary cancer risk in female cancer survivors: A national cohort study

BACKGROUND: This study evaluated the effects of body mass index (BMI) before the diagnosis of the first primary cancer on the development of secondary primary cancers (SPCs) in female cancer survivors. METHODS: This study population included 146 377 Korean female cancer survivors whose first primary cancer was diagnosed between 2002 and 2010. The incidence of SPCs was evaluated throughout follow‐up until December 2011. We used Cox proportional hazards models to calculate the hazard ratios of SPCs with prediagnosis BMI and compared it to those of first cancers in the general population. RESULTS: After 565 877 person‐years of follow‐up, 2222 patients with SPC were observed. The higher BMI was more in female cancer survivors than in general population. The age‐standardized incidence rate of cancer in cancer survivors was 2.02 times higher than that of the general population. There were positive linear trends between prediagnosis BMI and risk of overall, colorectal, ovary, thyroid, and obesity‐related SPCs. In addition, the BMI‐SPC risk association was statistically significant in female cancer survivors without smoking history (P (trend )= 0.001) and with a localized first primary cancer (P (trend )= 0.014). However, the magnitude of the BMI‐SPC risk association was similar to that for first cancers in the general population (P (heterogeneity )= 0.403 in BMI ≥ 30.0 kg/m(2)). CONCLUSIONS: In female cancer survivors, prediagnosis obesity was a risk factor for overall, individual, and obesity‐related SPCs. However, the magnitude of the BMI‐SPC risk association was similar to that for first cancers in the general population.