Magnetic resonance imaging in locally advanced rectal cancer: Quantitative evaluation of the complete response to neoadjuvant therapy

Purpose: To assess the diagnostic performance of diffusion-weighted imaging (DWI) for the discrimination of complete responder (CR) from the non-complete responder (n-CR) in patients with locally advanced rectal cancer (LARC) undergoing chemotherapy and radiation (CRT). Material and methods: Between December 2009 and January 2014, 32 patients (33 lesions: one patient had two synchronous lesions) were enrolled in this retrospective study. All patients underwent a pre-and post-CRT conventional MRI study completed with DWI. For both data sets (T2-weighted and DWI), the pre-and post-CRT tumour volume (V T2 ; V DWI ) and the tumour volume reduction ratio (∆V%) were determined as well as pre-and post-CRT apparent diffusion coefficient (ADC) and ADC change (∆ADC%). Histopathological findings were the standard of reference. Receiver operating characteristic (ROC) curves were generated to compare performance of T2-weighted and DWI volumetry, as well as ADC. Results: The area under the ROC curve (AUC) revealed a good accuracy of pre-and post-CRT values of V T2 (0.86; 0.91) and V DWI (0.82; 1.00) as well as those of ΔV T2 % (0.84) and ΔV DWI % (1.00) for the CR assessment, with no statistical difference. The AUC of pre-and post-CRT ADC (0.53; 0.54) and that of ΔADC% (0.58) were significantly lower. Conclusions: Both post-CRT V DWI and ΔV DWI % (AUC = 1) are very accurate for the assessment of the CR, in spite of no significant differences in comparison to the conventional post-CRT V T2 (AUC = 0.91) and ΔV T2 % (AUC = 0.84). On the contrary, both ADC and ΔADC% values are not reliable.