The effect is true for all types of first cancer, but is strongest for second obesity-related cancers.
Excess body weight before a cancer diagnosis makes survivors more likely to develop a further primary cancer, especially an obesity-related cancer, a large US cohort study has found.
Previous research has established an increased risk of second cancers among breast and colorectal cancer survivors, but this study extends that finding to survivors of all cancer types.
Published in JAMA Network Open, the study used the Cancer Prevention Study II (CPS-II) Nutrition Cohort of more than 184,000 participants enrolled in 1982, with regular follow-up surveys conducted since 1997.
Participants with first primary invasive cancers diagnosed between 1992 and 2015 were followed up to 2017 for a second primary cancer diagnosis. Pre-diagnosis BMI was recorded.
After excluding nonmelanoma skin cancers, metastatic disease, people over 85, those with a BMI under 18.5, those with a second primary diagnosed within 60 days of their first, those who died within 60 days and those lost to follow-up, about 27,000 participants remained.
Of these, about 11,500 were overweight and 4700 were obese. The mean age was 72.
The most common first cancers were prostate, breast and colorectal. Almost 40% were obesity-related cancers, which include breast and colorectal.
During follow-up of a median eight years, 3750 were diagnosed with a second primary cancer, 33% of which were obesity-related.
Every 5kg/m2 increase in BMI was associated with a 13% increased risk of any second cancer and a 28% increased risk of a second obesity-related cancer.
Overweight survivors had a 15% increased risk of any second cancer while those with obesity had a 34% increased risk.
Every added 5kg/m2 was associated with a 42% increased risk of a second colorectal cancer, but there was no increased risk of a second breast cancer.
There was a 70% increased risk of a second kidney cancer for every 5kg/m2 increase in BMI among all survivors, but the association was attenuated for five-year survivors.
When the team removed breast and colorectal cancer survivors from the analysis – given their already known higher incidence of second cancers – the significant increase in second cancer risk remained.
The authors conclude that excess weight increases the risk for second primary cancers, especially obesity-related ones, and that “weight loss to reach a healthy body weight among cancer survivors may result in substantial health and quality of life benefits”, though this may be especially challenging for older cancer survivors.
