Evaluation of Time-Phase Effect on 18F-FDG PET/CT Delineation Methods for Treatment Planning of Nasopharyngeal Carcinoma

Purpose Tumor boundary delineation using 18F-FDG PET/CT is a promising tool for radiotherapy applications, but no consensus has been established regarding the optimal delineation method. Time-phase variability of 18F-FDG PET/CT imaging frequently affects metabolically active volumes and treatment planning for nasopharyngeal carcinoma (NPC). This study aimed to evaluate the time-phase robustness of 8 methods commonly used for tumor volume delineation in NPC. Patients and Methods Twenty patients with biopsy-proven NPC were included and underwent multiple time-phase 18F-FDG PET/CT imaging. Gross tumor volumes (GTVs), absolute SUV, gradient-based watershed segmentation (GTV-GWT), and anatomic biologic contouring (GTV-ABC) values were determined. The volume of overlap between GTV-CT and the 8 PET-based GTVs was enclosed and the overlap fraction (OF-CT) calculated. Color matrix was used to semiquantify the time-phase differences. Gross tumor volume values obtained with different methods were recorded and compared using paired t test. Time-phase differences of GTVs and SUVmax were compared among groups by analysis of variance with Tukey honest significance tests. The coefficients of variation were computed to assess intrapatient time-phase variability. Similarity coefficient was calculated to evaluate similarity. Results Differences were observed between GTVs obtained at different time points using various delineation procedures. Nonsignificantly higher percentages were obtained for GTV-GWT (88.17%) and GTV-ABC (86.98%) compared with other methods, showing their robustness. GTV-40% (0.81–0.88) and GTV-ABC (0.82–0.88) indicated higher similarity with GTV-MRI than the other methods. Conclusions PET/CT-based GTV-ABC between 35 and 55 minutes should be the first choice for NPC treatment planning.