In the present study, MRI parameters of TNT patients in three different neoadjuvant treatment phases were used to predict response. It is the first study that using MRI parameters to predict response of TNT. Most studies have used good responder and poor responder according to TRG as an evaluation method(9, 20, 21). Actually, TRG is subjective(22). The sensitivity to TNT was grouped according to tumor volume reduction rate on MRI and TRG of LARC following treatment with TNT. In this study, there was no L group but only H group and M group, which may be because TNT is the strongest neoadjuvant therapy. More studies(3, 23–25) have reported that patients could achieve a high rate of CR. We used two modes to group the response to TNT. Compared with grouping scheme of H group vs M group vs L group, CR vs non-CR has more significant results in predicting response by MRI parameters, and this grouping mode is more suitable for it.
We found post-ICT ∆TL offered the good results for the detection of patients with a CR after TNT. Currently, there are very few researches that have described MRI parameters of post-ICT ∆TL for predicting a CR. The smaller post-nCRT tumor length predicted an increased pCR rate in the previous studies(20, 26, 27). FOWARC analyzed MRI images of 403 patients and found that baseline TL was a significant factor for predicting pCR probability and patients with TL (> 3 cm) may have a lower pCR probability(28). In our study, there was significant difference of post-ICT ∆TL between CR and non-CR. During induction chemotherapy, the tumor length of CR and non-CR decreased by 33% (range: 17% − 39%) and 4% (range: -5% − 27%). Although our sample size is only 15 cases, integrating the result and practice experience we may conclude that the larger the post-ICT ∆TL predicted the more the tumor regression and the higher the probability of CR. Post-ICT ∆TL might be an early prediction parameter of CR. It has important reference value to help predict the sensitivity of neoadjuvant chemoradiotherapy and adjust the treatment plan as soon as possible.
Unlike other studies, we explored that post-CRT EMVI had a great predicting performance in identifying H group. When the post-CRT EMVI status was negative, sensitivity of patients to TNT was higher than positive. In previous literature by Lee et al(12), post-CRT EMVI was the only significant MRI factor in DFS. Long-term results from the GEMCAD 0801 trial(29) and Meng et al(30) considered baseline mrEMVI positivity was an independent prognostic indicator for DFS. Most previous studies have explored the relationship between EMVI and prognosis, yet we explored the correlation between EMVI and response and obtained good positive results. This maybe because radiation was effective in wiping out pathways of vascular spread in the pelvis(29).
Regarding the definition of lymph node, Brown et al(14) found that if a node was defined as suspicious because of an irregular border or mixed signal intensity. Koh et al(16) recommend the use of the short-axis diameter of 8 mm for positive pelvic nodes. So we combined the above two to define the positive lymph node. In the study, the more post-CRT ∆LNN and post-CCT ∆LNV provoked more chance of CR and high sensitivity to TNT in our study. At this point, patients who had post-CRT ∆LNN ≥ 70% would be more easier to achieve CR. Bustamante-Lopez et al(31) found only pCR showed a significant association with < 12 baseline LN. However, none of the previous studies had indicated correlation between post-CRT ∆LNN or post-CCT ∆LNV and response so far. It may be associated with the definition of positive lymph node is different and subjective.
DWI is increasingly incorporated in clinical rectal MRI exams worldwide(32). Moreover, the DWI-derived ADC values can be used for quantitative analysis of tumorous cellular density and extracellular space(17). ADC values are mainly negatively related to cell density and positively related to extracellular space(17, 20, 21). After consolidation chemotherapy, some tumor tissue was replaced by fibrous tissue. Post-CCT ADCT reflects the tissue density after TNT rather than only the tumor cell. Fibrotic tissue generally has low ADC(21). Consequently, Post-CCT ADCT may be a useful parameter for discriminating between CR and non-CR. Several articles focused on post-CRT ADC and post-CRT ∆ADC(33–35). However, we found ADCT of post-CCT MRI was associated with response. This differs from above studies.
There were a few limitations in the study that must be considered. Firstly, our study was a retrospective analysis with a small sample size, thus above conclusions need further validation and support. In addition, we did not compare the same parameter of post-ICT MRI, post-CRT MRI and post-CCT MRI to confirm that it can better reflect the response at a certain phase. Because many patients may have only two phases of treatments. Thirdly, we assessed SDWI and ST2 by measuring manually its long diameter and short diameter, and the area measured may be not accurate. Use of the image processing software may increase the accuracy.