Extramural depth of rectal cancer tumor invasion at thin-section MRI: predicting treatment response to neoadjuvant chemoradiation

OBJECTIVES To assess whether the maximal extramural depth (EMD) of T3 tumor spread on magnetic resonance imaging(MRI) correlates with tumor response parameters and whether it can predict tumor response to neoadjuvant chemoradiation. METHODS 111 rectal cancer patients with American Joint Committee on Cancer (AJCC) T3 tumors underwent MRI staging before neoadjuvant chemoradiotherapy were included. Tumor EMD was measured as mm tumor beyond the muscular and compared between the following groups by Kruskal-Wallis test: pathological complete response(pCR) versus nonpCR, good regression versus poor regression, downstage versus nondownstage. RESULTS The final study population consisted of the 111 patients (79 male, 32 female). Median age was 56 years (range, 23-75 years). The EMD was significantly higher in nonpCR patients (7.8 ± 3.2 mm) than in pCR patients(6.1 ± 1.8 mm) (p = 0.033). According to good regression (tumor regression grade(TRG) 0-1 vs. TRG 2-3) and downstaging (ypStage 0-I vs. ypStage II-III), the difference was not significant. Receiver operating characteristic curve analysis revealed a good value for the area under the curve (0.775) and the cutoff value for EMD to predict pCR was 5.6 mm. Compared with patients with a EMD ≥ 5 mm, more patients with EMD <5 mm showed pCR (p = 0.019), while there was no correlation between EMD and good regression or downstaged. CONCLUSIONS EMD value obtained on initial staging MRI may serve as an imaging biomarker which predicts patients that have an incomplete response pathological response after standard neoadjuvant therapy.