The present study was a prospective case-control study. The study's design and protocol were approved by the ethics committee of the institutional review board at the Cancer Center of Sun Yat-Sen University (approval GZR2017-047). Written informed consent for study participation and data collection were obtained from all patients.
Patients and Study Procedure
Patients were consecutively recruited at the Cancer Center of Sun Yat-Sen University (Guangdong, China) between October 2016 and August 2020. The inclusion criteria were: (i) the patients had invasive breast cancer confirmed with US-guided core needle biopsy, without distant metastasis; (ii) the patients received eight cycles of a standardized NAC regimen; (iii) the patients had at least one targeted measurable lesion according to RECIST 1.1,23 and (iv) surgery was conducted after completion of NAC. The exclusion criteria were: (i) patients who did not complete full course of NAC as intolerance to chemotherapy, (ii) tumor progression, leading to palliative chemoradiotherapy instead of surgery, (iii) lesion size exceeding the scope of a color-coded map in the assessment view of SWE, and (iv) low-quality data in quantitative SWE with a large area of elasticity defects on the SWE velocity image.
Each included patient was continuously followed up with US and MRI before the surgery. US examinations, including conventional US and SWE, were performed one day before the biopsy (time point t0) and one day before the second (t1), third (t2), fourth (t3), and fifth cycles (t4) of NAC (time point t1-t4). MRI examinations were performed before the biopsy and within 3 days before the fifth cycle of NAC (time point t4). Surgical excision was performed approximately 2 weeks after eight cycles of NAC. The flow chart depicting the study design is shown in Fig. 1.
NAC strategies were performed according to the standard protocol at our institution. All patients received eight cycles of chemotherapy based on anthracycline/taxane. Moreover, patients with positive HER-2 expression received trastuzumab, starting from the fifth cycle of NAC.
B-mode US and SWE images were obtained with the Siemens S2000 ultrasound system (Siemens Medical Solutions, Mountain View, CA, USA) equipped with a 9L4 linear transducer. Conventional US and SWE images of breast lesions were acquired with patients in the supine position according to the breast US examination guideline from the American Institute of Ultrasound in Medicine.24 Firstly, conventional US scans were performed to locate the breast lesions and obtain greyscale and color Doppler flow images. The longest diameter (DUS) and area (AB) of the breast lesion on the B-mode image were recorded. Secondly, SWE was performed at the same position, depth, focus position, and gain setting used for conventional US scanning. Furthermore, a probe was held still and applied perpendicular to the skin with the minimum amount of pressure possible. SWE was performed by setting the region of interest (ROI) for stiffness assessment to include the breast tumors and surrounding normal tissue. SWE was performed with the patients holding their breath for approximately 5 seconds. A quality map displayed in green-yellow-red indicating high-intermediate-low quality was obtained first to evaluate the Shear Wave Velocity (SWV). Next, the image was switched to the SWV map, which was displayed in color mode to indicate the stiffness ranging from soft (blue) to intermediate (green or yellow) and hard (red). The part corresponding to high SW quality (green areas on the quality map) was selected for the measurement of the SWV to ensure SWE reliability. Within the ROI, SWV values ranging from 0.5 to 10 m/s on the velocity map were obtained by placing three SWV-ROIs (2×2 mm) over the lesion's stiffest and softest parts, respectively. Finally, the image was switched to a Shear Wave Time (SWT) map, and the profile of the breast lesion was delineated to measure the area of the lesion (AE) according to the color-coded differences between the tumor and the surrounding tissues.
The US examinations were conducted by two board-certified radiologists (Jia-Xin Huang and Shi-Yang Lin) with at least 2 years of experience in the performance and interpretation of breast US, as well as at least 6 months of experience in performing elastography before the onset of this study. SWE images were reviewed and confirmed to be eligible by a third radiologist (Xiao-Qing Pei) who had 20 years of experience in ultrasonography and 5 years of experience in breast elastography imaging. The US examination and measurement procedures are shown in Fig. 2.
All breast MRI examinations were performed on an eight-channel 3.0-T system (Discovery MR750, GE Medical Systems, Milwaukee, WI, USA). Patients were imaged in the head-first prone position, and images were obtained with bilateral axial views. Images were interpreted by radiologists with more than 5 years of experience with MRI of the breast. Measurements of the maximal diameter (DMRI) of breast cancer were obtained on post-contrast subtracted T1-weighted images.
Imaging Parameters Calculation
The maximal and minimal values of SWV (SWVmax and SWVmin) were recorded by averaging three measured values, respectively. The mean SWV values (SWVmean) for the six SWV-ROIs were calculated.25 Additionally, the area ratio (AE/B) was calculated as the ratio of the area of the breast tumor in the SWE time image to that in the B-mode screen. The relative changes of the breast lesion [Δ values (%)] in the B-mode US, SWE, and MRI images were calculated at each time point. The equations used to calculate these parameters can be found in Appendix A.
All pathological results were determined by two board-certified pathologists with consensus who were blinded to the imaging information for the patients.
Before the onset of NAC, the diagnosis was determined with a US-guided core needle biopsy. Samples from the core needle biopsy were examined to obtain the biological characteristics of the breast tumors. The expression levels of estrogen receptor (ER), progesterone receptor (PR), human epidermal growth factor receptor 2 (HER2), and Ki-67 in core needle biopsy specimens were evaluated with immunohistochemistry (IHC) and fluorescence in situ hybridisation (FISH). The criteria for the IHC, FISH and molecular subtype results were determined according to the St. Gallen consensus.26–30
After eight cycles of NAC, all patients received mastectomy along with ALND or sentinel lymph node biopsy (SLNB). Assessment of the NAC response was performed using the Miller-Payne system as defined in Appendix B. A grade of 4 or 5 was categorized as a Major Histological Response (MHR) and a grade of 1, 2, or 3 was defined as a Non-major Histological Response (NMHR).
Data on the demographics, IHC characteristics of breast cancer, breast tumor size/stage, lymph node stage, clinical stage, and pathological response grade of NAC were collected. During the chemotherapy follow-up, the maximum diameter of the breast tumor on the B-mode US, the maximum and average values of SWV, the ratio of tumor area on the elastic image to that on the B-mode US, and the maximum diameter of the breast tumor on the enhanced T1 MRI image were recorded.
Continuous data were presented as the mean and standard deviation (SD), and categorical variables were presented as counts. Univariate analysis for the pathological response was performed by using a t-test or Mann-Whitney U test to compare continuous quantitative variables between the responsive and non-responsive groups. The Kruskal-Wallis test, X2 test, or Fisher exact test were used to compare the categorical variables in the two pathological response groups.
When both the imaging parameters and clinicopathological characteristics showed favorable efficiency in univariate analysis, we generated a multivariate regression model by using the factors with statistical significance to create a new predictive modality. A p-value of < 0.05 was entry probability for stepwise of multivariate logistic regression while a p-value of more than 0.10 was defined as removal probability. The receiver operating characteristic (ROC) curve was drawn to determine the performance of the imaging findings and combined parameters (CP) in predicting the pathological response.32 An area under the ROC (AUC) value of > 0.9 indicated a great diagnostic value, 0.9 > AUC > 0.7 suggested a moderate diagnostic value, and AUC < 0.7 was considered to indicate a poor diagnostic value.32 The cut-off point for these features was determined by maximizing the Youden coefficient.33 A p-value of < 0.05 was considered to indicate statistical significance. Data analysis was performed with the Statistical Package for the Social Sciences (SPSS) version 22.0 and Medical Statistical Software version 16.2.