MRI for nodal restaging after neoadjuvant therapy in rectal cancer with histopathologic comparison

Background After neoadjuvant therapy, most of the lymph nodes (LNs) will shrink and disappear in patients with rectal cancer. However, LNs that are still detectable on MRI carry a risk of metastasis. This study aimed to evaluate the performance of the European Society of Gastrointestinal and Abdominal Radiology (ESGAR) criterion (short-axis diameter ≥ 5 mm) in diagnosing malignant LNs in patients with rectal cancer after neoadjuvant therapy, and whether nodal morphological characteristics (including shape, border, signal homogeneity, and enhancement homogeneity) could improve the diagnostic efficiency for LNs ≥ 5 mm. Methods This retrospective study included 90 patients with locally advanced rectal cancer who underwent surgery after neoadjuvant therapy and performed preoperative MRI. Two radiologists independently measured the short-axis diameter of LNs and evaluated the morphological characteristics of LNs ≥ 5 mm in consensus. With a per node comparison with histopathology as the reference standard, a ROC curve was performed to evaluate the diagnostic performance of the size criterion. For categorical variables, either a χ2 test or Fisher’s exact test was used. Results A total of 298 LNs were evaluated. The AUC for nodal size in determining nodal status was 0.81. With a size cutoff value of 5 mm, the sensitivity, specificity, positive predictive value, negative predictive value and accuracy were 65.9%, 87.0%, 46.8%, 93.6% and 83.9%, respectively. No significant differences were observed in any of the morphological characteristics between benign and malignant LNs ≥ 5 mm (all P > 0.05). Conclusions The ESGAR criterion demonstrated moderate diagnostic performance in identifying malignant LNs in patients with rectal cancer after neoadjuvant therapy. It was effective in determining the status of LNs < 5 mm but not for LNs ≥ 5 mm, and the diagnostic efficiency could not be improved by considering nodal morphological characteristics.


Background
Patients with locally advanced rectal cancer (LARC) and those who are unable to undergo radical resection are recommend to receive neoadjuvant therapy, which aims to increase the possibility of complete resection and reduce the risk of local recurrence by downstaging and downsizing [1]. After neoadjuvant therapy, most of the lymph nodes (LNs) shrink and disappear, but the LNs that can still be detected on MRI carry a risk of metastasis, with LNs larger than 5 mm having a 38.6% possibility of being pathologically metastatic [2]. Even in patients with a pathological complete response, 3.2-14% of visible LNs are malignant [3,4]. Malignant LNs can lead to poor prognosis, including local recurrence and reduced long-term survival rate [1,5]. Therefore, for patients after neoadjuvant therapy, particularly whose who are nearly clinical complete responders, MRI restaging of LNs is valuable in evaluating the feasibility of conservative treatment, such as a local excision or a watch-and-wait strategy.
Restaging of benign and malignant LNs is more challenging than primary staging in rectal cancer. Most studies recognize that MRI criteria for LNs involvement in primary staging include size, shape, border, signal homogeneity, and enhancement homogeneity [6][7][8]. However, these criteria are not entirely suitable for nodal restating after neoadjuvant therapy [9,10]. Most research has primarily relied on the size criterion and has not considered whether morphological characteristics (including shape, border, signal homogeneity and enhancement homogeneity) can effectively distinguish benign and malignant LNs [4,9,11]. Only a few studies have shown that nodal morphological characteristics contribute to nodal restaging [12][13][14]. However, evaluating the details of morphological characteristics in small LNs (short-axis diameter < 5 mm) have been considered challenging [10].
In 2016, a new criterion was added to European Society of Gastrointestinal and Abdominal Radiology (ESGAR), considering LNs with a short-axis diameter < 5 mm as benign after neoadjuvant therapy [6]. To the best of our knowledge, no study has validated this criterion for a node-by-node evaluation. Thus, this study aimed to validate the ESGAR criterion and explore the diagnostic efficiency of nodal morphological characteristics for LNs ≥ 5 mm.

Patients
This retrospective study was approved by our Institutional Review Board, and written informed consent was obtained from all patients. A total of 262 consecutive patients with LARC treated with neoadjuvant therapy between March 2015 and June 2021 were considered for inclusion. Among these patients, 172 patients were excluded for the following reasons (a) no total mesorectal excision 6-8 weeks after neoadjuvant therapy (n = 108); (b) no post-treatment MRI within 2 weeks before surgery (n = 30); (c) poor image quality for evaluation (n = 12); (d) no LNs found on MRI (n = 20); and (e) the location of the LNs on MRI did not match the histopathologic results (n = 2). Finally, 90 patients (68 men, 22 women; median age: 55 years, range: 20-90 years) were included in this study (Fig. 1).

Neoadjuvant therapy
Neoadjuvant therapy included preoperative chemoradiotherapy and preoperative chemotherapy. Preoperative chemoradiotherapy regimen involved long-term radiotherapy using volumetric modulated arc therapy with a total dose of 50-60 Gy for primary and nodal gross tumor volumes administered with conventional segmentation, along with concurrent chemotherapy. Chemotherapy regimens included the mFOLFOX regimen (leucovorin + fluorouracil + oxaliplatin), CapeOX regimen (capecitabine + oxaliplatin), or capecitabine monotherapy.

MRI examination
Patients, except those with lower and large rectal tumors, were infused with an appropriate amount (20-80 mL) of ultrasonic gel into the rectum. To reduce bowel motility, 20 mg of raceanisodamine hydrochloride was injected intramuscularly 10 min before MRI examination unless contraindicated.
MRI was performed using a 3.0-T scanner (Magnetom Verio, Siemens Healthcare) with a 6-channel phasedarray body coil. Patients were positioned supine with feet first. The rectal MRI protocols included (a) sagittal, coronal and oblique axial (orthogonal to tumor base) non-fatsuppressed high-spatial-resolution T2-weighted imaging (HR-T2WI) using a turbo spin-echo sequence; and (b) coronal fat-suppressed isotropic contrast-enhanced three-dimensional high-spatial-resolution T1-weighted imaging (CE-3D-HR-T1WI) using a gradient-echo sequence. Axial, sagittal and coronal multiplanar reconstructions were performed on CE-3D-HR-T1WI with a slice thickness of 3 mm. An intravenous bolus of 0.2 mL/ kg gadopentetate dimeglumine was injected at a rate of 3.0 mL/s, followed by a 25-mL saline flush at the same rate. Detailed protocols are listed in Table 1.

Image interpretation
Two radiologists with 2 and 7 years of experience in rectal MRI, reviewed all MR images blinded to histopathologic findings. Firstly, all visible LNs on HR-T2WI were determined by the two radiologists in consensus. Secondly, the short-axis diameters of LNs were measured independently, and the average values were calculated for subsequent analyses. Thirdly, the two radiologists assessed the shape (oval/round), border (smooth/irregular), and signal homogeneity (homogeneous/heterogeneous) of LNs ≥ 5 mm on HR-T2WI and enhancement homogeneity (homogeneous/ heterogeneous) on CE-3D-HR-T1WI in consensus.

Radiologic-histopathologic comparison
All visible regional LNs on HR-T2WI were divided into three groups, including mesorectal, superior rectal and inferior mesenteric LNs. Based on the agreed-upon nodal position by the radiologist and surgeon, an experienced surgeon specializing in colorectal cancer successively localized and removed the regional LNs in different groups during surgery. In order to provide an accurate node-by-node comparison between MR images and histopathological findings, special attention was paid to nodal size and morphology, as well as nodal relative position to the tumor, rectal wall, mesorectal fascia, vessels and adjacent LNs [15]. The excised LNs were then sent to the pathology department and quickly placed in individual trays. All LNs were fixed in formalin and stained with hematoxylin and eosin. Thereafter, an experienced gastrointestinal pathologist analyzed and classified each LN as benign or malignant under light microscope. The LNs reported by histopathology were rematched with HR-T2WI in the corresponding groups and were excluded if they could not be matched.

Statistical analysis
Statistical analyses were performed using SPSS software (version 25.0, IBM). Figures were generated using Graph-Pad Prism (version 9.0, GraphPad Software) and Med-Calc statistical software (version 15.8, MedCalc Software bvba, Ostend, Belgium). The normality of quantitative data was test by using Kolmogorov-Smirnov test. Normally distributed data were compared with the independent samples t-test, while nonnormally distributed data were presented as medians with ranges and compared with the Mann-Whitney U test. Categorical data were expressed as numbers with percentages and compared with the χ 2 test or Fisher's exact test. ROC curve was constructed, and the AUC with a 95% confidence interval was calculated to evaluate the diagnostic efficacy of nodal size. With a short-axis diameter cutoff value of 5 mm, the sensitivity, specificity, positive predictive value (PPV), negative predictive value (NPV) and accuracy were calculated. The morphological characteristics were compared for LNs ≥ 5 mm using the χ 2 test or Fisher's exact test. A two-tailed P value < 0.05 indicated a statistically significant difference.  (Fig. 3).  (Fig. 4). However, none of these morphological characteristics were found to be statistically significant in differentiating benign and malignant LNs (P > 0.05) ( Table 4).

Discussion
For the evaluation of LNs in rectal cancer, many studies have focused on assessing the agreement between MRI and histopathology for N-staging [16,17]. Although this method is easy to implement, it ignores the characteristics of each individual node. In our study, we conducted a node-by-node analysis, matching and verifying each LN found on MRI with the pathological results. This approach has been proven to be more specific and valuable in establishing diagnostic criteria for LNs [7,18]. Our study validated that the ESGAR criterion had a moderate diagnostic performance (AUC = 0.81) for nodal restaging in rectal cancer after neoadjuvant therapy. With a short-axis diameter cutoff value of 5 mm, the sensitivity, specificity and accuracy were 65.9%, 87.0% and 83.9%, respectively. The size criterion is the most widely used indicator for nodal staging and restaging in current studies on rectal cancer LNs [9,19]. According to the ESGAR consensus meeting in 2012, the size criterion after neoadjuvant therapy is more reliable than the baseline MRI assessment for diagnosing malignant LNs [19].
Many studies have shown that the number of LNs usually decreases, and the short-axis diameter becomes smaller or even disappears after neoadjuvant therapy. Downsized LNs have a very low chance of metastasis [2]. However, there is still no consensus on the optimal cutoff value. In the 2016 ESGAR, a new item was added for MRI nodal restaging: all LNs < 5 mm should be considered benign, but there are no reliable criteria for LNs ≥ 5 mm. Our study also showed that short-axis diameter cutoff value of 5 mm had a high NPV, as almost all (93.6%, 221/236) LNs < 5 mm were benign. Thus, we can confidently conclude that there are no malignant LNs if all LNs on MRI after neoadjuvant therapy are < 5 mm. However, among the 62 LNs ≥ 5 mm, malignant LNs accounted for only 46.8% (29/62), resulting a low PPV. Therefore, it is impossible to effectively distinguish benign and malignant LNs ≥ 5 mm.
Although we used HR-T2WI and CE-3D-HR-T1WI with thin thickness in our study, confidently observing the morphological features of small LNs remains challenging [9,10]. Consequently, we only evaluated the morphological features of LNs ≥ 5 mm. Although morphological characteristics were effective in predicting the status of LNs before treatment [18], we found no significant differences in shape, border, signal homogeneity, and enhancement homogeneity between benign and malignant LNs after neoadjuvant therapy. Nodal shape is likely influenced by the scanning planes. Even benign LNs showed irregular border and heterogeneous signal intensity. This could be attributed to fibrous thickening of the capsule and fibrotic or mucinous changes after  treatment, which make elevation more complex [20].
Regarding enhancement homogeneity evaluation, we applied fat-suppressed isotropic CE-3D-HR-T1WI, which has demonstrated good performance in nodal staging [7]. Compared with HR-T2WI, this sequence offers advantages such as higher spatial resolution, improved signalto-noise ratio, and multiplanar reconstruction, providing better-detailed of LNs. However, heterogenous enhancement could still be observed in benign LNs ≥ 5 mm, likely due to nodal fibrosis or the presence of acellular mucin lakes caused by neoadjuvant therapy [20,21]. Malignant LNs with smooth border, homogeneous signal intensity, and enhancement may contain micrometastases. There were some limitations in our study. Firstly, we only evaluated the morphological features of LNs ≥ 5 mm because accurately assessing the morphological features of small LNs is challenging. Secondly, the number of metastatic LNs was lower than that of benign because patients with obvious LN metastasis usually do not undergo surgery. Finally, we did not evaluate iliac LNs as extended pelvic lymphadenectomy is not usually performed in total mesorectal excision.

Conclusions
In summary, our study demonstrates that the ESGAR has moderate diagnostic performance for nodal restaging in rectal cancer after neoadjuvant therapy. LNs < 5 mm can be effectively identified as benign using the size criterion alone, in line with the ESGAR recommendation. However, morphological features do not aid in the diagnosis of LNs ≥ 5 mm on MRI restaging. Future research should focus on refining the criteria for distinguishing benign and malignant LNs ≥ 5 mm and exploring alternative imaging techniques to improve the accuracy of nodal restaging in rectal cancer after neoadjuvant therapy. high-spatial-resolution T2-weighted imaging CE-3D-HR-T1WI contrast-enhanced three-dimensional high-spatialresolution T1-weighted imaging PPV positive predictive value NPV negative predictive value  Fig. 4 Coronal non-fat-suppressed high-spatial-resolution T2-weighted imaging (a, c and e); coronal fat-suppressed isotropic contrast-enhanced threedimensional high-spatial-resolution T1-weighted imaging (b, d and f ). The lymph nodes were signed by white boxes. a-b, Benign node with 3.7 mm in short-axis diameter, showed oval, smooth border, homogeneous signal and homogeneous enhancement. c-d, Benign node with 5.6 mm in short-axis diameter, showed round, irregular border, heterogeneous signal and heterogeneous enhancement. e-f, Malignant node with 5.8 mm in short-axis diameter, showed round, irregular border, heterogeneous signal and heterogeneous enhancement