The clinical application of 4D 18F-FDG PET/CT on gross tumor volume delineation for radiotherapy planning in esophageal squamous cell cancer

摘要

This study was a prospective analysis, approved by the local institutional review board (DMR98-IRB-171), of 4D-PET/CT in RT planning of EC. Patients with histologically approved EC who were scheduled to undergo definitive RT, concurrent chemoradiotherapy or radical surgery, were eligible for this study. Eighteen patients with esophageal squamous cell cancer were enrolled between December 2009 and January 2011. The image data from 12 patients with 13 GTV_CT were available for analysis in this study. The median age was 48.5 years (range 38–76 years). All patients were male. Table?1 presents the characteristics of these patients. All patients were asked to fast for at least 4 h before 18F-FDG PET/CT imaging. Each of them received 370 MBq (10 mCi) of 18F-FDG intravenously 40?min before scanning and rested in a supine position in a quiet and dimly lit room. All images were acquired with an integrated PET/CT scanner (Discovery STE, GE Medical Systems, Milwaukee, WI, USA). The patient's arms were elevated above their head. First, whole-body PET/CT images were taken according to the standardized protocol. The CT images were reconstructed onto a 512?×?512 matrix and converted to a 128?×?128 matrix, with 511-keV-equivalent attenuation factors for attenuation correction of the corresponding PET emission images. Immediately after finishing the whole-body PET/CT images, patients were repositioned and placed in a simulated RT planning position using the Real-time Position Management (RPM) system respiratory gating hardware (Varian Medical Systems Inc., Palo Alto, CA, USA). 4D-CT images with 2.50-mm slice thickness, and 4D-PET images with two table positions, 7?min per position, were acquired. The respiration cycle was divided into 10 phases. All CT images were automatically sorted using 4D software (Advantage 4D, GE Healthcare). The images were transferred from the PET/CT workstation via DICOM3 to the RTP (Eclipse version 8.6, Varian Medical Systems Inc.) for GTV delineation. All phases of CT images and PET images were automatically fused for this gating study. PET/CT-based GTV of the primary tumor (GTV_PET) was defined by the auto-contouring function at the AW workstation (Advantage SimTM 7.6.0, GE Healthcare), either by applying the isodensity volumes and adjusting the different percentages to the maximum threshold levels, or by simply using a fixed value of SUV. The threshold strategies for assessing the optimized SUV for GTV contouring were derived from the results of other studies [6, 7, 15, 21]. Eight different threshold methods were used in this study. They were SUV 15%, SUV 2, SUV 2.5, SUV 20%, SUV 25%, SUV30 %, SUV 40% and SUV 50%. The length of the GTV_PET provided by the auto-contouring function was not changed at all. All the artifacts within the GTV_PET, including the areas overlaid by the heart, bone and great vessels, were excluded manually in the RTP system (Fig.?1). The temporal resolution of PET is an average of several respiratory cycles. In contrast to helical CT, the temporal resolution of averaged CT (ACT) is comparable with that of PET. Furthermore, Chi et al. [22] demonstrated that respiration artifacts in PET from PET/CT can be minimized using ACT, and ACT is temporally and spatially consistent with PET. On the basis of axial ACT images, contouring of the tumor volume and critical structures was performed without knowing the PET results in an effort to decrease bias. Information about the tumor extent from the contrast CT scan, panendoscopy and endoscopic ultrasonography (EUS) was used when delineating the GTV_CT. Excluding the adjacent metastatic lymph nodes, the volume of primary tumors (GTV_CT) was contoured as a reference tumor volume. To reduce inter-observer variations, at least two different radiation oncologists carried out the contouring of the tumors for each patient. After completion of the GTV_CT contouring in the RTP system, the radiation oncologists reviewed the consistency of PET/CT images with nuclear medicine physicians. The volume of GTV_CT and GTV_PET was compared using the conformality index (CI) [23] and volume ratio (VR). The CI is the ratio of the volume of intersection of two volumes (A?∩?B) compared with the volume of union of the two volumes (A?∪?B) under comparison () [21, 24]. Volume ratio is the ratio of two volumes, and the denominator is the volume of GTV_CT. A suitable threshold level could be defined when GTV_PET was observed to be the best fit of the length, CI, or VR from the GTV_CT.All statistical tests were performed using SPSS 15 (SPSS, Chicago, IL, USA), and each GTV was analyzed by one-way ANOVA with Scheffe's post hoc test. P-values of 0.05 or less were considered statistically significant. Pearson's correlation was performed to assess the correlation between tumor length evaluated by GTV_CT wit