Statistical analysis

Statistical analyses were performed using SPSS software (version 25.0, IBM Corporation, Armonk, New York, USA). The difference in response to NST of the axillary region between breast cancer subtypes was compared by use of a Pearsons's chi-squared test. Differences in PET-parameters between patients with and without axillary pCR were examined for statistical significance by the Mann–Whitney U test. Receiver-operating characteristics (ROC) analyses were performed to determine cut-off values of PET-parameters for the prediction of axillary response to NST for PET-parameters that differed significantly between response groups at baseline. Residual axillary lymph node disease was considered positive, and axillary pCR was considered negative. Sensitivity was defined as the proportion of patients with residual axillary disease that were correctly predicted. Specificity was defined as the proportion of patients with axillary pCR that were correctly predicted. Positive predictive value (PPV) was defined as the proportion of patients predicted to have residual axillary disease who had residual axillary disease following NST. Negative predictive value was defined as the proportion of patients predicted to achieve axillary pCR who had axillary pCR following NST. Due to the small sample size in combination with the low incidence of axillary pCR and low ¹⁸F-FDG uptake in ER-positive/HER2-negative patients, analyses for ER-positive/HER2-negative were performed separately from HER2-positive/TN breast cancer. Additionally, subgroup analysis of clinically node-positive breast cancer patients was performed. All statistical tests were two-sided, with the level of significance established at P < 0.05.