Pretreatment MRI Versus Postoperative Pathology in Locoregional Staging of Rectal Cancer: A Germany-Wide Multicenter Re-Evaluation Study in Non-Pretreated Patients Based on the OCUM Study.

[BACKGROUND] Magnetic resonance imaging (MRI) is the standard diagnostic technique for the locoregional assessment of rectal cancer. The pertinent guidelines recommend neoadjuvant therapy depending on cT and cN categories. In this experimental study, we examined the accuracy of pretreatment MRI staging.

[METHODS] MRIs of 50 patients from the OCUM study (non-pretreated, 24 women, histologically confirmed rectal adenocarcinoma, located ≤12cm from the anal verge, stages I-III) were re-assessed by 74 radiologists affiliated with certified colorectal cancer centers. The radiologically determined Union for International Cancer Control (UICC) stages were compared with the histopathological findings, which were unknown to the radiologists. The degree of agreement between the radiologists' assessments and the histopathology was analyzed with κ, a weighted average of Brennan-Prediger agreement coefficients.

[RESULTS] 2915 complete assessments were performed for cTcN (UICC stages); 740 cases (25.4%) were not classifiable because of cTX (0.1%) or cNX (25.3%). The UICC stage accorded with the histopathology in 979 cases (33.6%); there was overstaging in 737 cases (25.3%) and understaging in 459 (15.7%). The κ for agreement was 0.114 (95% confidence interval, [0.032-0.20]). Agreement was also low for T- and N-categories (κ=0.354 and κ=0.235, respectively), but was better for distance to the mesorectal fascia (MRF) (κ=0.736) and extramural vascular invasion (EMVI) (κ=0.579).

[CONCLUSION] Although MRI remains the standard diagnostic technique for the locoregional assessment of rectal cancer, its accuracy for T- and N-staging in rectal cancer is low, particularly with regard to the distinction of T2 from T3a/b and the assessment of lymph nodes. The assessment of MRF and EMVI is much more reliable.