Associations between body mass index with cancer recurrence and all-cause mortality among survivors of breast cancer.
Authors: Chen Po-Hua , Gowda Karan , Leonard Myisha , Kim Rachel , Carnethon Mercedes , Hibler Elizabeth A. 
Published: June 2023

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Background: It is widely recognized that obesity is associated with increased breast cancer incidence. While body mass index (BMI) does not capture measures of body composition, it is routinely collected during clinical care. However, we do not know whether BMI at breast cancer diagnosis affects cancer recurrence or all-cause mortality. This study aims to investigate the associations between clinical categories of BMI, cancer recurrence, and all-cause mortality in this population. Methods: All breast cancer survivors treated at Northwestern Medicine on or after 1/1/2005 were included. We applied the WHO classification of BMI (kg/m2): underweight ( < 18.5), normal (18.5-24.9), overweight (25-29.9), class I obesity (30-34.9), class II obesity (35-39.9), and class III obesity (≥ 40). Cox proportional hazards models were used to determine the associations between BMI category at baseline and cancer recurrence plus all-cause mortality for breast cancer survivors. Models were adjusted for potential confounders including age, gender, race, ethnicity, histological grade, clinical stage, chemotherapy use, smoking, alcohol use, history of coronary artery disease, and history of Diabetes Mellitus. Results: Among all the 20,322 breast cancer survivors, 1,371 all-cause deaths and 1,183 cancer recurrences were observed. For patients aged 55 and above at diagnosis, there were 964 deaths and 632 cancer among 11,818 cases. When compared with a normal BMI (18.5-24.9), a BMI of 40 and above is associated with a higher risk of all-cause mortality among stage 1 breast cancer survivors (HR 1.90, 95 % CI 1.17-3.09), whereas a BMI of lower than 18.5 is associated with a higher risk of all-cause mortality among stage 4 breast cancer patients (HR 3.21, 95 % CI 1.24-8.30). For cancer recurrence, when compared with a normal BMI (18.5-24.9), a BMI of 40 and above is associated with a higher risk of cancer recurrence among stage 0 breast cancer survivors (HR 3.78, 95 % CI 1.21-11.81) and a BMI of 35 to 39.9 is associated with a higher risk of cancer recurrence among stage 3 breast cancer patients (HR 1.91, 95 % CI 1.01-3.61). We did not observe effect modification for either association by age at diagnosis using a cut-point of 55 years, which is considered as reaching postmenopausal status. Conclusions: Higher levels of obesity are associated with a higher mortality risk among early-stage breast cancer survivors whereas underweight is associated with a higher mortality risk for terminal-stage breast cancer. The associations between high-class obesity and increased risk of cancer recurrence were observed among stage 0 and stage 3 breast cancer survivors. These findings emphasize the importance of BMI as a tool for monitoring the health of breast cancer survivors. However, further study is needed to better understand the impact of BMI, and more importantly body composition, on the health and longevity of breast cancer survivors.