Reproducibility of rectal tumor volume delineation using diffusion-weighted MRI: Agreement on volumes between observers

Purpose. - To retrospectively evaluate the inter -observer agreement between a radiologist and a radiation oncologist and volume differences, in T2 and diffusion -weighted (DWI) MRI of gross tumor volume (GTV) delineation, in rectal cancer patients. Materials and methods. - Two observers, a radiologist and a radiation oncologist, delineated GTVs of 50 patients on T2-weighted MRI (T2(GTV)) and echo planar DWI (DWIGTV). Observers agreement was assessed using DICE index, Bland -Altman analysis and intra-class correlation coefficient (ICC). Student's t-test was used for GTV comparison. Results. - Median T2(GTV) and DWIGTV were 17.09 +/- 14.12 cm(3) (1.92-62.03) and 12.79 +/- 12.31 cm(3) (1.23-62.25) for radiologist, and 16.82 +/- 13.66 cm(3) (1.78-65.9) and 13.72 +/- 12.77 cm(3) (1.29-69.75) for radiation oncologist. T2(GTV) were significantly larger compared to DWIGTV (P< 0.001 and P < 0.001, for both observers). Mean DICE index for T2(GTV) and DWIGTV were 0.80 +/- 0.07 and 0.77 +/- 0.06. The mean difference between the two observers were 0.26 cm(3) (95% CI: -5.36 to 5.88) and -1.13 cm(3) (95% CI: -5.70 to 3.44) for T2 and DWI volumes. The ICC for T2 volumes was 0.989 (95% CI: 0.981-0.994) (P < 0.001) and 0.992 (95% CI: 0.986-0.996) (P < 0.001) for DWI volumes. Conclusion. - DWI resulted in smaller volumes delineation compared to T2-weighted MRI. Substantial and almost perfect agreements were reported for DWIGTV and T2(GTV) between radiologist and radiation oncologist. Due to the fact that DWI could be considered a simple technique for volume delineation for radiation oncologist, DWI could be used to improve quality in radiation planning for an accurate boost volume delineation when a dose escalation is investigated. (C) 2019 Societe francaise de radiotherapie oncologique (SFRO). Published by Elsevier Masson SAS. All rights reserved.