Patients
This retrospective study was approved by the ethics committee of the First Affiliated Hospital of Nanjing Medical University (Nanjing, China), and patient informed consent was waived. This research complied with the Declaration of Helsinki. Between April 2016 and January 2021, we identified pathology-proven fibroadenomas and pathology-proven TNBCs which underwent preoperative US examination. The inclusion criteria included the following: (1) verified lesions after inpatient surgery; (2) complete US and immunohistochemical data; (3) US images acquired from the same ultrasonic instrument; (4) US images stored by Digital Imaging and Communications in Medicine. The exclusion criteria included the following: (1) patients who had undergone neoadjuvant chemotherapy; (2) a diffuse extensive intraductal component on the US images; (3) patients receiving biopsy before US examination; (4) tumor size not fully included in the same plane. Finally, 360 patients involved in our study were randomly separated into training (n = 253) and testing (n = 107) groups. The recruitment process flowchart is shown in Figure 1A.
US Examination and Region-of-Interest (ROI) Segmentation
The flowchart of radiomics is summarized in Figure 1B and can be separated into three parts: imaging acquisition,ROIsegmentation, feature extraction. In addition, the subsequent study flowchart included feature selection, model analysis, and model evaluation. All the US images from the institution were collected using the same US instrument (MyLab Twice, Esaote, Italy) equipped with a 5–13 MHZ linear array transducer. All of the patients were examined in a supine position with full exposure of the breast, including the area closest to the chest wall for better detection of the breast lumps.
To test the inter-observer and intra-observer about the repeatability and reproducibility based on the features extracted from the ROIs. We randomly selected 60 patients for ROI segmentation, two radiologists with 3 years (Y.D.) and 15 years (H.W.) of experience in breast US scans who were unaware of the pathological results. The maximal-diameter plane was selected according to the US images of each breast lesion, and then the two radiologists drew an ROI along the mass boundary using an open source application called ITK-SNAP (http://www.itksnap.org). Then radiologist (H.W.) repeated the same workflow after one month. Features with intraclass correlation coefficients (ICCs) better than 0.80 in the inter-observer and intra-observer agreement extraction were considered for subsequent analysis. The remaining ROI segmentation of the images was accomplished by a radiologist (H.W.).
Clinical Information and US conventional features
Clinical characteristics such as age were acquired by reviewing the medical records. US features and the assessment of the BI-RADS category were retrospectively reviewed by two investigators (H.W and Y.D), who have 15 years’ and 3 years’ experience in breast US examination. Neither of the investigators was involved in the US examination, and both were unaware of the clinical information and the pathology data of the patients. They were asked to evaluate and record the imaging features of all the patients. The following image features of breast lesions were recorded and divided into various categories: 1) size (maximum tumor diameter: <3 or >3 cm); 2) shape (oval or round,irregular); 3) margin (well- or non-circumscribed); 4) orientation (parallel or anti-parallel); 5) echotexture (hypoechoic or heterogeneous); 6) posterior echo feature (none or enhancement); 7) BI-RADS category (3, 4A, 4B, 4C, or 5). In cases of discordance, a consensus reading was performed, and the consensus data were used for the following analysis.
Radiomics analysis and radiomics construction
Pyradiomics package 2.1.2 was used for the extraction of radiomics features from the ROIs [14]. We used the maximum relevance minimum redundancy (mRMR) algorithm to develop TNBC-related radiomics signatures. The most appropriate feature with nonzero coefficient in the training group among the 360 breast images features was selected using the least absolute shrinkage and selection operator (LASSO) logistic regression algorithm (Figure 2A and 2B). Then, penalty parameter tuning adjusted by 10-fold cross-validation was used to access the robust and nonredundant features from the initial cohort. Each patient with a Rad-score was created by a linear combination of selected features that were weighted by their respective coefficients [15]. The corresponding Rad-score was generated with the selected features and calculated with all the patients in the training and validation groups.
A total of 1283 features were computed and divided into three categories: (1) 9 shape-based features: used to evaluate typical morphological features, such as shape and size information about the mass; (2) 216 first-order statistics: first-order parameters that are used to calculate the distribution of individual voxel intensities through commonly used and basic metrics and ignore the spatial information within the tumors; among the first-order statistics, entropy is consistently considered one of the most stable features; (3) 1058 second-order features: generally known as textural features, encode valuable information about the scene, and have a relative spatial arrangement of intensity values in a medical image; in the second-order features we extracted, including Gray Level Co-occurrence Matrix (GLCM) features, Gray Level Dependence Matrix (GLDM) features,Gray Level Size Zone Matrix (GLSZM) features, Gray Level Run Length Matrix (GLRLM) features, Neighbouring Gray Tone Difference Matrix (NGTDM) features and wavelet-based features [16-19].
Establishment and validation of the radiomic nomogram
Among our study, we separated all the patients into training (n = 253) and validation (n = 107) datasets using a statistical software. One-way analysis of variance was used to compare demographic characteristics and continuous variables as appropriate. The χ2 test or Fisher’s exact test was used to establish significant differences in categorical variables as appropriate. To determine which clinical characteristics could serve as a candidate predictor for TNBC, we used univariate analysis to variable screen, those variables with p < 0.1 were subjected to the subsequent analysis. Therewith, a clinical predictive was developed through multivariate logistic regression analysis. Regarding the multivariate logistic regression analysis, we applied the variance inflation factor (VIF) to estimate the collinearity diagnostics. The characteristic features were integrated with radiomics signature, then a radiomics nomogram was developed with multivariate analysis and provided an individualized and visual model tool for distinguishing TNBC from fibroadenoma.
The potential predictive ability of the established models was assessed by the receiver operating characteristic (ROC) curve analysis with the area under the curve (AUC). The model performance with statistical difference of AUC was compared with the Delong algorithm (p < 0.05). The nomogram model was well-adjusted in accordance with a calibration curve evaluated by the Hosmer–Lemeshow test and used to assess the prediction capability of the nomogram. The discrimination diagnostic performance of the radiomics nomogram model was quantified with the access of concordance index (C-Index). The range of the C-index is defined 0.5 to 1.0, with 1.0 corresponds to the best model prediction, and 0.5 represents a random prediction [20]. Bootstraps with 1000 resamples were calculated for the prediction model relatively corrected C-index, and a corrected model was developed. The model internal performance was used on all patients in the validation cohort.
Clinical utility of the radiomics nomogram
The discrimination and predictive performance of the established models were assessed based on the decision curve analysis (DCA) of the training and validation datasets. DCA was developed to ascertain the clinical utility of the nomogram by quantifying the net benefits at different threshold probabilities in the whole cohort [21].
Statistical analysis
Statistical analyses were conducted with SPSS (version 26.0) and R statistical software (version 4.0.5). An independent sample T-test was used to compare continuous data as appropriate. Categorical variables were compared using Fisher’s exact test or Chi-square test. The LASSO logistic regression analysis was running by “glmnet” package. The “mRMRe” package was applied to implement mRMR algorithm. The VIF values were computed using the “car” package. The ROC curves were plotted using “pROC” package. The nomogram and calibration curve was constructed using the “rms” package. Decision curve was plotted using the “ dca.r” package. A two-tailed p-value of < 0.05 indicated statistical significance.