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

Purpose: Tumor boundary delineation using F-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 F-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 F-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.