DWI-MR and PET-CT Functional Imaging for Boost Tumor Volume Delineation in Neoadjuvant Rectal Cancer Treatment

T2 weighted magnetic resonance (MR) imaging is the gold standard for locally advanced rectal cancer (LARC) staging. The potential benefit of functional imaging, as diffusion-weighted MR (DWI) and positron emission tomography-computed tomography (PET-CT), could be considered for treatment intensification strategies. Dose intensification resulted in better pathological complete response (pCR) rates. This study evaluated the inter-observer agreement between two radiation oncologists, and the difference in gross tumor volume (GTV) delineation in simulation-CT, T2-MR, DWI-MR, and PET-CT in patients with LARC.

Two radiation oncologists prospectively delineated GTVs of 24 patients on simul-CT (CT_GTV), T2-weighted MR (T2_GTV), echo planar b1000 DWI (DWI_GTV) and PET-CT (PET_GTV). Observers' agreement was assessed using Dice index. Kruskal-Wallis test assessed differences between methods.

Mean CT_GTV, T2_GTV, DWI_GTV, and PET_GTV were 41.3±26.9 cc, 25.9±15.2 cc, 21±14.8 cc, and 37.7±27.7 cc for the first observer, and 42.2±27.9 cc, 27.6±16.9 cc, 19.9±14.9cc, and 34.8±24.3 cc for the second observer, respectively. Mean Dice index was 0.85 for CT_GTV, 0.84 for T2_GTV, 0.82 for DWI_GTV, and 0.89 for PET_GTV, representative of almost perfect agreement. Kruskal-Wallis test showed a statistically significant difference between methods (p=0.009). Dunn test showed there were differences between DWI_GTV vs. PET_GTV (p=0.040) and DWI_GTV vs. CT_GTV (p=0.008).

DWI resulted in smaller volume delineation compared to CT, T2-MR, and PET-CT functional images. Almost perfect agreements were reported for each imaging modality between two observers. DWI-MR seems to remain the optimal strategy for boost volume delineation for dose escalation in patients with LARC.