Excess Body Weight and the Risk of Second Primary Cancers Among Cancer Survivors.

Importance: Little is known about the causes of second primary cancers among individuals with a history of cancer. Descriptive studies have suggested that lifestyle factors, including excess body weight, may be important. Objective: To investigate whether excess body weight is associated with the risk of a second primary malignant neoplasm among cancer survivors. Design, Setting, and Participants: This cohort study of adults in 21 states in the US used data from the Cancer Prevention Study II Nutrition cohort, a large prospective study that invited participants to respond to a survey in 1992 and biennial surveys starting in 1997, and who were followed-up through 2017. Eligible participants included those who received a diagnosis of a first primary nonmetastatic invasive cancer between 1992 and 2015. Data analysis occurred from September 2023 to March 2024. Exposure: Body mass index (BMI), computed from self-reported height and weight at the time of the first primary cancer diagnosis (mean [SD] years to diagnosis, 1.7 [1.5] years). Main Outcome and Measures: Main outcomes included a second primary cancer or an obesity-related second cancer. Cancer diagnoses were reported on biennial surveys and verified through medical record abstraction or linkage with state cancer registries. Results: This cohort included 26 894 participants who received a diagnosis of a first nonmetastatic primary cancer (mean [SD] age at first cancer diagnosis, 72.2 [6.5] years; 15 920 male [59.2%]). At the time of first diagnosis, 11 497 participants (42.8%) had overweight and 4684 (17.2%) had obesity. During a median (IQR) follow-up time of 7.9 (3.4-13.6) years, 3749 (13.9%) participants received a diagnosis of a second primary cancer, of which 1243 (33.2%) were obesity-related second primary cancers. Compared with cancer survivors whose BMI was in the normal range (18.5 to <25), there was 15% increased risk of any second primary cancer for those who had overweight (25 to <30; adjusted hazard ratio [aHR], 1.15; 95% CI, 1.07-1.25) and a 34% increased risk for those who had obesity (BMI ≥30; aHR, 1.34; 95% CI, 1.21-1.48), with greater risk for obesity-related second primary cancers, including a 40% increased risk for those with overweight (aHR, 1.40; 95% CI, 1.22,-1.61) and a 78% increased risk for those with obesity (aHR, 1.78; 95% CI, 1.51-2.11). Conclusions and Relevance: In this cohort study of older survivors of nonmetastatic cancer, those who had overweight or obesity at the time of their first cancer diagnosis were at higher risk of developing a second cancer, especially an obesity-related second cancer. Given the high prevalence of overweight and obesity among cancer survivors, it is important to promote survivorship care guidelines recommending weight management and increase awareness of second cancers among physicians and cancer survivors.