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

Abstract

Background and purpose: This study aimed to evaluate the geometrical differences in and metabolic parameters of ¹⁸F-fluorodeoxyglucose positron emission tomography-computed tomography (¹⁸F-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, ¹⁸F-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 T₂-weighted MRI (T₂W-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 ¹⁸F-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 ¹⁸F-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.

Keywords: apparent diffusion coefficient (ADC); diffusion magnetic resonance imaging; esophageal squamous carcinoma; gross target volume; positron emission tomography; standard uptake value (SUV).

Figure 1

Scatter plots of correlation between the target volume delineated on¹⁸F-FDG PET–CT (GTV_PET) and DW-MRI (GTV_DWI) and on the corresponding CT (GTV_CT) before and during radiotherapy. The best-fit line is shown as the solid line for each scatterplot. (A) GTV_PETpre versus GTV_CTpre; (B) GTV_PETdur versus GTV_CTdur; (C) ΔGTV_PET versus ΔGTV_CT; (D) GTV_DWIpre versus GTV_CTpre; (E) GTV_DWIdur versus GTV_CTdur; (F) ΔGTV_DWI versus ΔGTV_CT.

Figure 2

A transversal diagram of gross target volumes on esophageal cancer with high uptake on ¹⁸F fluorodeoxyglucose positron emission tomography/computed tomography fused images. (A,a) Corresponding tumor on T₂-weighted imaging (C,c) with a high signal on diffusion weighted magnetic resonance imaging (b=600 s/mm²) (B,b) and corresponding apparent diffusion coefficient map (D,d) with restricted diffusion at the location of the tumor before radiotherapy (A–D) and during radiotherapy (a–d).