In a new study, researchers found risk of second cancers increases in women with breast cancer as body mass index (BMI) increases. They reported their results in the Journal of the National Cancer Institute.

“Cancer survivors are at increased risk of developing a second cancer, but relatively little is known about factors (other than treatment of the first cancer) that contribute to this increased risk,” explained the researchers.

The study was a retrospective cohort analysis of 6481 patients from within the Kaiser Permanente Colorado and Washington systems. Study participants were women who had invasive, nonmetastatic primary breast cancer. Patients were evaluated for outcomes related to any second cancer, in addition to particular types of second cancers. Height and weight data obtained in the 2 years before initial diagnosis and up to 1 year after diagnosis were used to calculate BMI, and incidence of second cancer was analyzed with respect to BMI.

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At the time of initial diagnosis, 33.4% of patients were overweight and 33.8% were obese. Patients were mean age 61.2 years at initial diagnosis. At a mean of 88.0 months (range, 12 to 322) of follow-up, 12.7% of patients had received a second cancer diagnosis. Most (61.8%) of these were cancers for which there is evidence of an association with obesity. Slightly fewer than half (40.5%) of second cancers diagnosed were breast cancer.

Multivariable analyses showed the risk of any second cancer diagnosis increased by 7% for every 5 kg/m2 increase in BMI (relative risk [RR], 1.07; 95% CI, 1.01-1.14; P =.01); risk of an obesity-related cancer increased by 13% (RR, 1.13; 95% CI, 1.05-1.21; P <.001); risk for a second breast cancer was elevated by 11% (RR, 1.11; 95% CI, 1.02-1.21; P =.01), and risk for a second estrogen receptor-positive breast cancer was elevated by 15% (RR, 1.15; 95% CI, 1.04-1.27; P =.008).

“BMI is a modifiable risk factor, and weight loss to reach a healthy body weight among breast cancer survivors may result in significant health and quality of life benefits,” the researchers concluded. They suggested that weight-loss strategies be considered for survivors of breast cancer, and also noted that weight loss can have benefits beyond cancer and including cardiovascular disease, which is another condition for which survivors of breast cancer may be at increased risk.


Feigelson HS, Bodelon C, Powers JD, et al. Body mass index and risk of second cancer among women with breast cancer. J Natl Cancer Inst. 2021;113(9):1156-1160. doi:10.1093/jnci/djab053