An optimal imaging modality based on different predictive risk factors for evaluating local staging of rectal cancer preoperatively: a comparative study of MRI and endorectal ultrasound

Accurate preoperative local staging of rectal cancer is essential for individualized treatment planning; however, the optimal imaging modality and personalized assessment approach remain controversial. This study aimed to compare the performance of different imaging modalities in the evaluation of local staging for rectal cancer and to identify factors related to their inaccurate predictions.

From January 2018 to December 2021, tumors were staged by endorectal ultrasound (ERUS) (277 patients) and magnetic resonance imaging (MRI) (243 patients). The predictive ability of the different imaging modalities was evaluated and compared, including pathologic T-staging and mesorectal fascia (MRF) status. The potential risk factors for accurate staging evaluation were analyzed, including body mass index (BMI) (≥23 or <23 kg/m²), tumor location (on longitudinal and transverse sections), and neoadjuvant chemoradiotherapy (NCRT) history (with or without).

Compared with postoperative pathological results, MRI showed a better accuracy in both T-staging (83.53%) and MRF status (84.24%), and three-dimensional endorectal ultrasound (3D-ERUS) also demonstrated relatively high accuracy (83.33% in both T-staging and MRF status). The results demonstrated that NCRT was a risk factor for predictions of T-staging (78.05% vs. 91.14% by MRI, 65.38% vs. 100% by 3D-ERUS) and MRF status (79.82% vs. 94.12% by MRI, 73.08% vs. 92.86% by 3D-ERUS). For MRI, low BMI and location were risk factors for predictions of T-staging (75.93% vs. 89.63% for BMI; 78.15% vs. 88.71% for location) and MRF status (77.03% vs.90.11% for BMI; 78.12% vs. 89.66% for location) (P<0.05). Compared with the non-anterior group, the predictive accuracy of MRI decreased in the anterior group, whereas 3D-ERUS showed the opposite trend, with no statistical significance. The accuracy of 3D-ERUS was unaffected by the patient's BMI and tumor location (P>0.05).

For patients with a low BMI (BMI <23 kg/m²) and rectal tumors located at a low position (≤5 cm from the anal margin), 3D-ERUS may have a better performance for local staging assessment of rectal cancers. Thus, it can be an important complement to conventional imaging modality on specific occasions.