Comparison of CT and MRI in the pre-operative staging of rectal adenocarcinoma and prediction of circumferential resection margin involvement by MRI

Objective It has been suggested that MRI may be used as the sole modality of choice in pre‐operative staging in rectal cancers. Knowledge of tumour stage and a threatened Circumferential Resection Margin (CRM) pre‐operatively are essential for planning neo‐adjuvant therapy and as predictors of local recurrence. At present most units utilize CT scanning to assess these parameters. The aim of our study was two fold: firstly to examine the accuracy of preop CT and MRI staging of rectal cancers compared with final histology and secondly to assess the accuracy of MRI in predicting penetration of the mesorectal envelope (ME). Patients and methods All patients with biopsy proven rectal adenocarcinoma underwent thin slice MRI and CT scan pre‐operatively. Forty‐seven patients have been prospectively entered into the study: 24 male (median age 68 years; range 38–91 years). Eleven patients were unsuitable for surgery leaving 36 patients available for study. Results CT correctly staged patients with T1/T2 rectal cancers more often than MRI (77%vs. 43%, P = 0.226). Patients with T1/T2 tumours were overstaged more often by MRI compared with CT (54%vs. 23%, P = 0.226). A greater proportion of patients with T3 tumours were correctly staged by MRI than CT (76%vs. 41%, P = 0.08); and more T3 disease was understaged by CT than MRI (54 vs. 18%, P = 0.032). CT and MRI staged T4 disease equally. In the assessment of mesorectal envelope integrity, MRI had a sensitivity of 80% and a specificity of 84%. The positive predictive value was 44% and the negative predictive value 96%. Conclusions These results suggest significant differences between accurate pre‐operative ‘T’ staging by CT and MRI for rectal cancer. MRI has the potential however, to accurately assess mesorectal envelope invasion. Further analysis is required to assess whether MRI can be used as the sole modality in pre‐operative staging of rectal cancers.