Initial Staging of Locally Advanced Rectal Cancer and Regional Lymph Nodes: Comparison of Diffusion-Weighted MRI With 18F-FDG-PET/CT.

Purpose: The aim of the study was to compare diffusion-weighted MRI (DW-MRI) parameters with 18F-FDG PET/CT in primary locally advanced rectal cancer (LARC).

Methods: From October 2012 to September 2014, 24 patients with histologically confirmed and untreated LARC (T3-T4) prospectively underwent a pelvic 1.5-T DW-MRI (b = 0 s/mm, b = 600 s/mm2) and a whole-body 18F-FDG PET/CT, before neoadjuvant therapy. The 2 examinations were performed on the same day. Two readers measured 18F-FDG SUVmax and SUVmean of the rectal tumor and of the pathological regional lymph nodes on PET/CT and compared these with minimum and mean values of the ADC (ADCmin and ADCmean) on maps generated from DW-MRI. The diagnostic performance of ADC values in identifying pathological lymph nodes was also assessed.

Results: Regarding tumors (n = 24), we found a significant negative correlation between SUVmean and corresponding ADCmean values (ρ = -0.61, P = 0.0017) and between ADCmin and SUVmax (ρ = -0.66, P = 0.0005). Regarding the lymph nodes (n = 63), there was a significant negative correlation between ADCmean and SUVmean values (ρ = -0.38, P = 0.0021), but not between ADCmin and SUVmax values (ρ = -0.11, P = 0.41). Neither ADCmean nor ADCmin values helped distinguish pathological from benign lymph nodes (AUC of 0.24 [confidence interval, 0.10-0.38] and 0.41 [confidence interval, 0.22-0.60], respectively).

Conclusions: The correlations between ADCmean and SUVmean suggest an association between tumor cellularity and metabolic activity in untreated LARC and in regional lymph nodes. However, compared with 18F-FDG PET/CT, ADC values are not reliable for identifying pathological lymph nodes.