Geometrical Comparison and Quantitative Evaluation of (18)F-FDG PET/CT- and DW-MRI-Based Target Delineation Before and During Radiotherapy for Esophageal Squamous Carcinoma

BACKGROUND AND PURPOSE: This study aimed to evaluate the geometrical differences in and metabolic parameters of (18)F-fluorodeoxyglucose positron emission tomography–computed tomography ((18)F-FDG PET-CT) and diffusion-weighted magnetic resonance imaging (DW-MRI) performed before and during radiotherapy (RT) for patients with esophageal cancer based on the three-dimensional CT (3DCT) medium and explore whether the high signal area derived from DW-MRI can be used as a tool for an individualized definition of the volume in need of dose escalation for esophageal squamous cancer. MATERIALS AND METHODS: Thirty-two patients with esophageal squamous cancer sequentially underwent repeated 3DCT, (18)F-FDG PET-CT, and enhanced MRI before the initiation of RT and after the 15th fraction. All images were fused with 3DCT images through deformable registration. The gross tumor volume (GTV) was delineated based on PET Edge on the first and second PET-CT images and defined as GTV(PETpre) and GTV(PETdur), respectively. GTV(DWIpre) and GTV(DWIdur) were delineated on the first and second DWI and corresponding T(2)-weighted MRI (T(2)W-MRI)-fused images. The maximum, mean, and peak standardized uptake values (SUVs; SUV(max), SUV(mean), and SUV(peak), respectively); metabolic tumor volume (MTV); and total lesion glycolysis(TLG) and its relative changes were calculated automatically on PET. Similarly, the minimum and mean apparent diffusion coefficient (ADC; ADC(min) and ADC(mean)) and its relative changes were measured manually using ADC maps. RESULTS: The volume of GTV(CT) exhibited a significant positive correlation with that of GTV(PET) and GTV(DWI) (both p < 0.001). Significant differences were observed in both ADCs and (18)F-FDG PET metabolic parameters before and during RT (both p < 0.001). No significant correlation was observed between SUVs and ADCs before and during RT (p = 0.072–0.944) and between ∆ADCs and ∆SUVs (p = 0.238–0.854). The conformity index and degree of inclusion of GTV(PETpre) to GTV(DWIpre) were significantly higher than those of GTV(PETdur) to GTV(DWIdur) (both p < 0.001). The maximum diameter shrinkage rate (∆LD(DWI)) (24%) and the tumor volume shrinkage rate (VRR(DWI)) (60%) based on DW-MRI during RT were significantly greater than the corresponding PET-based ∆LD(PET) (14%) and VRR(PET) (41%) rates (p = 0.017 and 0.000, respectively). CONCLUSION: Based on the medium of CT images, there are significant differences in spatial position, biometabolic characteristics, and the tumor shrinkage rate for GTVs derived from (18)F-FDG PET-CT and DW-MRI before and during RT for esophageal squamous cancer. Further studies are needed to determine if DW-MRI will be used as tool for an individualized definition of the volume in need of dose escalation.