We retrospectively reviewed the medical records of patients from Fujian Medical University Union Hospital (n = 386) as the training cohort and those from Fujian Cancer Hospital, Zhangzhou Affiliated Hospital of Fujian Medical University, and No. 900 Hospital of The Joint Logistic Support Force (n = 211) as an external validation cohort from April 1, 2012, to March 31, 2022. Additionally, 119 patients were enrolled as the test cohort to assess the predictive power of the model from May 1, 2022, to May 31, 2023.The inclusion criteria included: 1) histologically proven primary breast cancer without distant metastatic lesions, 2) HER2-positive status (defined as immunohistochemistry (IHC) of 3+ overexpression or 2+ expression, and a ratio of ≥ 2.0 by fluorescence in situ hybridization), 3) initial axillary lymph node-positive status confirmed using core- or fine-needle biopsy before NST, 4) a full course of standard neoadjuvant therapy before surgery, and 5) complete clinicopathological characteristics and treatment information. The study was approved by ethics committee of Fujian Medical University Union Hospital. All patients in the training and validation cohorts underwent mastectomy, breast-conserving surgery, radiation, and ALND surgery after completing NST. All patients in the additional independent cohort underwent SLNB followed by ALND, which detected at least 2 SLNs using methylene blue dye alone. SLNs were defined as blue-stained lymph nodes guided directly by blue-stained lymphatic vessels. Methylene blue dye was injected alone at peritumoral or subareolar sites 5-15 min before SLNB. After constructing the nomogram for predicting axillary pCR with the independent predictive factors, the investigators quantified the predictive performance of the model using the AUC of the receiver operating characteristic curve. Calibration plots with bootstrapping and the Hosmer-Lemeshow test were used to illustrate the calibration power of the model, with p \> 0.05 indicating a good fit.20 The clinical utility of the model in guiding surgical options was evaluated using decision curve analysis by plotting net benefits.21 Internal validation was estimated using the bootstrap method. The investigators validated the nomogram using an external validation cohort from the other institutions. Following the Youden index, the investigators selected an optimal cutoff probability of predicting axillary pCR as a stratification criterion for identifying patients who underwent SLNB surgery after NST. The investigators additionally examined 119 patients who underwent SLNB with ≥ 2 SLNs removed, followed by ALND, as an independent cohort to evaluate the effect of the nomogram on identifying patients accurately. FNRs of SLNB were compared using two different strategies: performing SLNB in all patients without any selection criteria and performing SLNB in patients selected by the nomogram with the cutoff probability of axillary pCR. These results were compared with those of the two previous tests.
