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

Background/aim: 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. Patients and methods: Two radiation oncologists prospectively delineated GTVs of 24 patients on simul-CT (CTGTV), T2-weighted MR (T2GTV), echo planar b1000 DWI (DWIGTV) and PET-CT (PETGTV). Observers' agreement was assessed using Dice index. Kruskal-Wallis test assessed differences between methods. Results: Mean CTGTV, T2GTV, DWIGTV, and PETGTV 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 CTGTV, 0.84 for T2GTV, 0.82 for DWIGTV, and 0.89 for PETGTV, 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 DWIGTV vs. PETGTV (p=0.040) and DWIGTV vs. CTGTV (p=0.008). Conclusion: 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.