The continuous optimization of local-regional control under the guidance of molecular subtype allows clinicians to make reasonable adjustments based on the efficacy of NAT to achieve the maximum treatment benefits. For patients who plan to receive BCS after NAT, the 5-year LRR rate was 2~7% in patients with tumor-free margins, but the risk increased to as high as 22% if the margin was positive [7]. In EBCTCG meta-analysis involving 10 randomized controlled trials of NAT, more frequent LRR was associated with NAT compared with adjuvant chemotherapy (15-year risk of 21.4% for NAT vs. 15.9% for adjuvant therapy) after BCS [8]. For patients who received BCS after NAT, in cases of multifocal residual tumor and/or cases of scattered residual tumor, the 2017 St. Gallen consensus conference expressed an opinion to favor more “generous” margins [23]. However, the 2019 St. Gallen consensus conference recommended that the optimal resection remains removal of all known residual as opposed to original tumor lesions with a margin goal of “no ink on tumor” regardless of the presence of unifocal or multi-focal disease [24]. Three strategies to mitigate the increased LRR after BCS in tumors downsized by NAT should be considered: careful tumor localization (including place marker clip, tumor range, and shrinkage modes), detailed pathological assessment, and appropriate radiotherapy [8]. After NAT, tumor extent assessment can be difficult and shrinkage modes can be heterogeneous, making surgery technically more difficult than without use of NAT. So, for patients who plan to receive BCS after NAT, it is important to accurately assess residual tumor extent and shrinkage modes after NAT to ensure negative margins and reduce LRR as well as resection rate.
In this study, we constructed the 3D MRI and pathology model of residual tumor after NAT. Based on the gold standard of 3D pathology reconstruction model-measured tumor size, we found that the 3D MRI reconstruction after NAT could accurately predict the extent of residual tumor. At the same time, we explored and definite the clinical-pathological shrinkage modes which oriented by BCS purpose after NAT. In addition, a nomogram was developed based on the predictors of clinical-pathological shrinkage modes that might aid clinicians in surgical decisions. The nomogram indicated that patients with large primary tumor, mammographic malignant calcification, Luminal A/Luminal B HER2- subtype, and high nodal burden after NAT were more likely to present with CP-NCSM. With an AUC of 0.833 and internal validation using the bootstrap resampling method, the model exhibited sufficient ability to predict clinical-pathological shrinkage modes after NAT.
The main strength of the study was that we constructed the BCS-oriented clinical-pathological shrinkage modes which combined shrinkage modes with residual tumor extent. Compared with traditional shrinkage modes, clinical-pathological shrinkage modes were suitable to guide the individualized selection of BCS candidates and scope of resection. This mode could help to decrease the negative margins distance and simultaneously maintain the natural breast shape to facilitate better cosmetic outcomes. And it represents a transformation of treatment concept, which from maximum and tolerable treatment to the minimum and effective treatment. The traditional view believed that multinodular lesions and solitary lesion with adjacent spotty lesions were not suitable for BCS. However, in our study, for patients with a high probability of CP-CSM after NAT, even if they had multinodular lesion or solitary lesion with adjacent spotty lesion, BCS would also be safe if they had a negative margin. And for these patients, there would be no increase in LRR if they received BCS successfully. For patients with a high probability of CP-NCSM, the basic goal of NAT (tumor downstage) had not been achieved. If satellite lesions were missed during surgery, LRR would increase due to “false negative margins”. So, these patients need to be cautious when choosing BCS, at the same time, they also need a more “generous” resection extent. The 2019 St. Gallen consensus conference also recommended that patients with multi-focal disease could also accept BCS after NAT, but the scope of residual tumors need to be more accurately assessed. Therefore, our study might partly expand the indications of BCS: patients might also accept BCS safely even if they had multi-focal disease after NAT.
MRI has an increased sensitivity, accuracy and specificity in detecting residual disease in the breast compared to either mammogram or ultrasound, making it a potentially useful tool in neoadjuvant setting [25–26]. Over the past several years, the correlation between MRI and pathology in assessing residual tumors extent in breast cancer patients receiving NAT has been the topic of several publications [14, 25–32]. The meta-analysis of 35 clinical trials confirmed that the correlation of residual tumors size assessed by MRI and pathology varied from poor to excellent (range 0.210~0.982) [26]. The 3D MRI provides an intuitive image of tumor extent in the breast and is helpful for surgeons to plan surgery. Furthermore, it can display more precise information than routine bidimensional images, because 3D tumor images can be observed from various directions by rotation [9, 33–37]. Taking advantage of these characteristics, 3D MRI has a high degree of accuracy in assessing the residual tumor extent after NAT. Although several reports have demonstrated that 3D MRI significantly and strongly correlated with pathology examination [31–32, 38], most of these researches compared the tumor extent which was assessed by its largest diameter at 3D MRI model with the pathology examination of routine sliced images.
The 3D pathology reconstruction has been previously used in researches, and it could also provide more precise information about tumor extent than routine sliced images [16–17, 11, 19, 39]. Wang S et al. [11] reconstructed 3D pathology models and analyzed the correlation with clinical pathological factors. Kazuaki et al. [39] just reconstructed 3D models of whole breast. Zheng et al. [19] analyzed the association between 3D pathology reconstruction and mutant-allele tumor heterogeneity value. However, as far as we know, most of these studies did not compare the association and correlation between 3D MRI reconstruction and 3D pathology reconstruction in evaluation of residual tumor extent. In this study, taking 3D pathology reconstruction-measured tumor size as the gold standard, we further confirmed the accuracy of 3D MRI reconstruction in assessing residual tumor extent after NAT. At the same time, the MRI images were easy to obtain, and the 3D reconstruction technology was relatively mature. So, we recommend applying 3D MRI reconstruction techniques to evaluate residual tumor extent after NAT in clinical practice.
In this study, the correlation value was 0.942 among the longest diameter measured by 3D MRI and pathology reconstruction. In addition, the correlation about maximum cross-section and volume of residual tumors after NAT were also highly correlated. However, MRI may underestimate or overestimate residual disease compared with pathology examination [40]. In this study, 3D MRI reconstruction had a slight underestimation of the maximum diameter and cross-section compared with 3D pathology reconstruction. Reasons might be that the anti-vascular effects of chemotherapy resulted in lack of inflammatory reactions surrounding the tumor [41]. On the other hand, 3D MRI reconstruction overestimated the maximum volume of residual tumor, reasons might be that the changes in cellularity or vascularity of tumors after NAT did not reflect in the change of overall tumor volume. Although tumor cells were destroyed, the tumor fibrosis remained. Some drastic pathologic changes induced by chemotherapy, such as tumor degeneration, severe fibrosis, inflammatory reactions and surrounding necrosis [37], could result in non-specific contrast enhancement in the tumor bed, which might be mistaken as residual tumors by MRI.
The study of Ippei et al. [12] showed that traditional shrinkage modes were significant associated with tumor size and number of metastatic lymph nodes (all p<0.05). Our research group performed a series of studies, the results showed that patients with lower mutant-allele tumor heterogeneity value and lower primary tumor/nodal burden were more likely to present with CP-CSM after NAT (all p<0.05) [16, 19]. Xu et al. [42] confirmed that TN and HER2+ subtypes had more chance to achieve CP-CSM compared with Luminal A and Luminal B HER2- subtypes, (p=0.042). Katsuhiro et al. [43] reported that the concentric shrinkage pattern may be more commonly found in TN subtype than in other molecular subtypes. Our results also showed that molecular subtype was an independent predictor of the clinical-pathological shrinkage modes. The correlation between molecular subtype and clinical-pathological shrinkage modes might reflect tumor biologic characteristics. One possible reason might be the growth characteristic of Luminal A and Luminal B HER2- subtypes, tumor cells tend to grow slowly with low apoptosis rate and genetic instability [12]. Simultaneously, tumor cells in these subtypes may be more resistant to preoperative therapy. However, tumor cells in TN and HER2+ subtypes had poor differentiation and strong proliferation ability, the aggressive tumor cells were more sensitive to therapy [44]. After NAT, the tumor boundary of patients with CP-CSM was easy to judge, and the margins of these tumors were often negative after finishing tumor resection. But the tumor boundary of patients with CP-NCSM is difficult to determine accurately. For those patients, LRR might be increased due to “false negative margin” when performing BCS. Therefore, Luminal A and Luminal B HER2- patients with large primary tumor and/or high nodal burden after NAT should be cautious to receive BCS after NAT, and the negative margin distance might also need to be appropriately increased. Although some patients with TN and HER2+ subtypes had the poor prognosis, patients with these subtypes were more likely to present with CP-CSM after NAT, suggesting that BCS after NAT was also feasible for TN and HER2+ patients.
Shrinkage modes were reported to be associated with prognosis. Ippei et al. [12] found patients with concentric shrinkage pattern has an excellent DFS (p=0.007) and OS (p=0.037). Our study also found that the clinical-pathological shrinkage modes were related to the survival. Patients with CP-CSM also had a better DFS and OS. The reasons might be that the predictors associated with CP-CSM indicated lower tumor burden, and these predictors were associated with a better prognosis.
This study has certain limitations, and the most important of which is the small sample size. Additionally, in this study, the follow-up time is relatively short. So, long-term follow-up is still needed to verify our study. Thirdly, lacking multi-center external data to verify the accuracy of the nomogram is another limitation in our study. since our study has small sample size, so we performed internal cross-validation with a bootstrap resampling frequency of 1000. We will further increase the number of cases and divide them into training and validation set in the future. Therefore, further prospective multi-center studies are required to confirm and assess the results of this study.