Breast cancer survivors have a greater risk of developing a subsequent cancer but whether this is affected by obesity is unclear.
Cancer of the breast is the most common form of cancer in women although with an early diagnosis, the 5-year survival prognosis ranges from 86 to 99%. Nevertheless, women who survive breast cancer have a 17% increased risk for a second cancer compared to the general population. One factor known to be associated with cancer is obesity with one US study estimating that 40% of all cancer diagnoses occurred in people who were either overweight or obese. However, while much attention has been paid to the effect of obesity on the development of an initial cancer, far less is known about how obesity impacts on the development of a second cancer. As a result, a team from Kaiser Permanente, Denver, US, sought to examine the association between body mass index (BMI) and a second cancer among women who survived invasive breast cancer. Data were extracted from an electronic database and a surveillance tumour registry which provided information on the incidence and type of secondary cancers that occurred. Height and weight measurements within two years prior through one year after the date of the initial breast cancer diagnosis were used to calculate the BMI. All women included had surgery as part of their initial breast cancer and had no evidence of a second cancer one year later. The study outcomes included all second cancers, cancers for which there was a known association with obesity (e.g., oesophageal adenocarcinoma), and ER-positive second breast cancers.
A total of 6481 women were included in the analysis with a mean age of 60.2 years, of whom 33.4% were classed as overweight or obese (33.8%) at the time of their initial breast cancer diagnosis. During a median follow-up of 88 months, 822 (12.7%) women developed a second cancer, of which 508 (61.8%) were obesity-related and 333 (40.5%) were breast cancer, the majority of which (69.4%) were ER-positive. The authors calculated that every 5 unit increase in BMI was associated with a 7% increased risk of developing any second cancer (relative risk, RR = 1.07, 95% CL 1.01–1.14), a 13% increased for an obesity-related cancer and by 15% for a second ER-positive breast cancer.
The authors calculated that the risk of a second cancer was increased by 5% for every 5 unit increase in BMI. They concluded that these data had important public health implications given the prevalence of obesity and underscored the need for effective preventative strategies.
Feigelson HS et al. Body Mass Index and Risk of Second Cancer Among Women with Breast Cancer. J Natl Cancer Inst 2021