18 F-FDG PET Definition of Gross Tumor Volume for Radiotherapy of Lung Cancer: Is a Single Standardized Uptake Value Threshold Approach Appropriate for Lymph Nodes and Tumor Delineation?

IntroductionPET with 18F-FDG has been used in radiation treatment planning for NSCLC. Thresholds of 15%–50% the maximum standardized uptake value (SUVmax) have been used for gross tumor volume (GTV) delineation by PET, with 40% being the most commonly used value. To date, lymph nodes have been contoured according to the threshold of % SUV max of the tumor.Purpose/Objective(s)Determine if the percentage threshold of maximum SUV (MBeq) of the tumor can provide a method that will adequately delineate volumes for all lymph nodes (PET-GTVn).Materials/MethodsThe study included 32 cases with non–small-cell lung cancer, who underwent a diagnostic 18F-FDG PET/CT scanner (GE Discovery ST). For CT planning, the primary tumor (CT-GTVt) was delineated using lung L/W settings (1600, -600) and the pathologic lymph nodes (CT-GTVn) using mediastinum L/W settings (400, 40). The PET-based GTV (PET-GTVt) was delineated for each tumor with 40% threshold isodensity contours. For estimation of the SUV, a square volume of interest was determined for each lesion at its maximal diameter in the axial, coronal, and sagittal planes on the fused PET and CT images. The method entailed locating the lesion within the slices and recording the maximum pixel of the PET image within the region of interest. This volume excluded any lymph nodes. The PET based GTV (PET-GTVn) for each lymph node/whole anatomical region of the mediastinum was contoured using the same method. First, we used 40% threshold isodensity contours based on SUV max of the tumor (PET-GTVnt40). A second GTV (PET-GTVnn40), based on 40% SUV max of each node was created. In order to avoid inter-observer variability, this was carried out by the same radiation oncologist.ResultsThe mean sizes of CT-GTVn, PET-GTVnt40 and PET-GTVnn40 were 15.80±15.0, 10.50±14.1 and15.20±16.2 cc, respectively. Compared with CT-GTVn, the intraclass correlation coefficient for two-way mixed effects model of PET-GTVnn40 (0.93; CI: 0.86–0.96) was higher than the coefficient of PET-GTVnt40 (0.49; CI: 0.18–0.71).Conclusions18F-FDG PET-GTV nn40 correlates more faithfully with CT-GTVn than PET-GTVnt40 and it is an effective tool to define the target of GTVn in radiotherapy planning. IntroductionPET with 18F-FDG has been used in radiation treatment planning for NSCLC. Thresholds of 15%–50% the maximum standardized uptake value (SUVmax) have been used for gross tumor volume (GTV) delineation by PET, with 40% being the most commonly used value. To date, lymph nodes have been contoured according to the threshold of % SUV max of the tumor.