Fig. 3

The relationship between tumor characteristics and recurrence rates in breast cancer patients with healthy weight vs with obesity defined by BMI. Forest plot shows adjusted HR (95% CI) of breast cancer recurrence for patients with obesity (BMI ≥ 30.0) compared with patients with healthy weight (BMI 18.5–24.9), according to stratified tumor characteristics. *BCR = breast cancer recurrence; ER = estrogen receptor. ** The stratified analyses were adjusted for the following co-variables excluding the stratified variable of interest: age, histological grade, tumor size, nodal status, estrogen-receptor status, surgery, planned adjuvant radio-, chemo-, and endocrine therapy. *** The relative excess risk due to interaction (RERI) with 95% CI for tumor size > 20 mm was 1.78 (− 0.16–3.72)”. The relative excess risk due to interaction with 95% CI for ER-negative disease was 0.79 (− 0.84–2.46). In this case, the RERI value is 1.78 and 0.79, which are greater than zero, indicating the presence of positive interaction between the risk factors (tumor size > 20 mm or ER-negative disease and obesity) on the outcome. However, the 95% CI for RERI includes zero. This suggests some uncertainty, and the possibility that there is actually no interaction between the two risk factors cannot be ruled out. A larger sample size may be needed