Comparative study of gross tumor volume shown on MRI and FDG PET/CT of nasopharyngeal carcinoma

Objective: To compare gross tumor volume (GTV) of nasopharyngeal carcinoma (NPC) according to MRI and FDG PET/CT and to investigated four fixed threshold methods to delineate the GTV using FDG PET/CT. Methods: Fifty patients with primary biopsy-proven NPC were prospectively were enrolled into the study, FDG PET/CT scans and MRI were carried out within one week prior to pretreatment, respectively. The GTV was named GTV-MRI (GTV were delineated according to MRI), GTV-PET_vis, GTV-PET₃₀, GTV-PET₄₀, GTV-PET₅₀ (GTV was delineated according to the PET-based GTVs obtained by visual interpretation or, by percentage of the SUV_max (30%, 40%, 50%) thresholds, respectively). The differences were compared among the GTV-MRI, GTV-PET_vis, GTV-PET₃₀, GTV-PET₄₀ and GTV-PET₅₀ in different by Wilcoxon test. Results: Of 50 patients, the median of volume descending order were: GTV-MRI 27.8 cm³, GTV-PET_vis 22.2 cm³, GTV-PET₃₀ 22.7 cm³, GTV-PET₄₀ 14.4 cm³ and GTV-PET₅₀ 9.0 cm³. However, there was no significant difference between GTV-PET_vis and GTY-PET₃₀ (Z = -0.05, P = 0.958), as well as GTV-MRI and GTV-PET_vis or GTV-PET₃₀ in 25 patients who were T_1-2 stage (Z = -0.93, -0.93, P = 0.353, 0.353), the other GTYs were all different in 50 patients, (Z = -5.74-2.09, P =0.000-0.037). Conclusions: All the GTVs delineated by the different methods of using FDG PET/CT were less than the GTV delineated by MRI. The potential advantages with the GTV-PET_vis or GTV-PET₃₀ delineated by FDG PET/CT are reduction of biological metabolic tumor volume in GTY delineation and reduction of the size of the GTV in NPC patients. (authors)