Objective To define a suitable threshold setting for gross tumor volume (GTV)when using 18F-fluoro-deoxyglucose positron emission tomography and computed tomogram (PET/CT) for radiotherapy planning in Nasopharyngeal carcinoma(NPC).Methods Sixteen NPC patients respectively received PET/CT and MRI scan before their radiation treatment.All of the images were transferred to the radiotherapy planning system (TPS).MRI/CT-based primary GTV was defined as GTVf.Biological target volumes (BTVs) were derived from PET/CT-based GTVs of primary tumors.The BTVs were defined as the volumes when adjusting different percentage of the maximal standardized uptake value (SUVmax).GTVfs were compared with BTVs.The suitable threshold level (sTL) could be determined when BTV value and its morphology using a certain threshold level were observed to be the fittest GTVf.The suitable standardized uptake value (sSUV) was calculated as the sTL multiplied by the SUVmax.Results Our result demonstrated no single sTL or sSUV method could achieve an optimized volumetric match with the GTVf.The sTL was [20.93％±6.51％(15％-40％)],whereas the sSUV was [2.27±0.48(1.56-3.25)].The sTL was inversely correlated with the SUVmax sTL =-0.144ln(SUVmax) + 0.5548 (R2 =0.85,F =78.57,P＜0.01.The sSUV showed a linear correlation with the SUVmax sSUV =0.104(SUVmax) + 1.0398,(R2=0.75,F=41.88,P＜0.01).The sTL was not associated with the value of GTVf.Conclusions In PET/CT-based BTV for NPC,SUVmax threshold method is feasible.sTL is not a fixed value,which is correlated with the SUVmax instead of the value of tumor.

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-PETvis,GTV-PET30,GTV-PET40,GTV-PET50 (GTV was delineated according to the PET-based GTVs obtained by visual interpretationor,by percentage of the SUVmax (30％,40％,50％) thresholds,respectively).The differences were compared among the GTV-MRI,GTV-PETvis,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-PETvis 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-PETvis and GTV-PET30 (Z=-0.05,P=0.958),as well as GTV-MRI and GTV-PETvis or GTV-PET30 in 25 patients who were T1-2 stage (Z =-0.93,-0.93,P=0.353,O.353),the other GTVs 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-PETvis or GTV-PET30 delineated by FDG PET/CT are reduction of biological metabolic tumor volume in GTV delineation and reduction of the size of the GTV in NPC patients.