ABSTRACT
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.