Study Of The Variances In Clinical Target Volume Delineation For Intensity-modulated Radiation Therapy Of Nasopharyngeal Carcinoma

Purpose: There are great inter-physician and inter-institute bias in delineation of clinical target volume(CTV)for nasopharyngeal cancer(NPC).The current study enrolled 14 radiation oncologists from five centers in China to analyze the variance of their contouring of prophylactic CTV for primary site and CTVln for lymph node.The comparison between these CTVs and suggestions made by the 2017 international guidelines for CTV delineation of NPC was also performed.The findings of the current study are expected to help design future studies to further optimize target volume of NPC.Methods: The simulative CT and MRI images for a histologically proven NPC patient with stage T3N1M0 were passed to all participated centers.A total of 14 senior radiation oncologists independently contoured the gross tumor volume(GTV),CTV,and nodal CTV(CTVln)based on fusion images of CT and MRI.Contouring documents were collected and the variances were analyzed.Results: For CTV of primary site,Cancer Hospital of Chinese Academy of Medical Science(CHCAM)and Fudan University Shanghai Cancer Center(FUSCC)use one-CTV design with 60 Gy dose prescription(CTV60).Sichuan Cancer Hospital(SCH)have two CTV as CTV66 and CTV60,among which only CTV60 can be recognized as prophylactic CTV.SCH thus could be grouped as one-CTV design.Fujian Cancer Hospital(FCH)and Sun Yat-Sen University Cancer Center(SYSUCC)both adopt two-CTV design as high risk CTV(CTV1,60Gy)and low risk CTV(CTV2,54Gy).Quantitative analysis showed that CTV60 s from two-CTV design groups were slightly but significantly smaller than CTV60 s from one-CTV design group(P=0.006,t-test),whereas CTV54 s from two-CTV design group is comparable with CTV60 s of one-CTV design group(P=0.131).Both CTV60 s from one-CTV design group and CTV54 s from two-CTV design group well cover the area with sub-clinical involvement,and are all consistent with what suggested by the 2017 international guidelines.However,some differences between Chinese physicians and the guidelines were seen.For instance,compared with guidelines,all participated physicians covered less normal tissue at the contralateral side of nasopharynx(this patient has no tumor at the left nasopharynx),and less frontal part of para-pharyngeal space at lower levels of nasopharynx was covered as well.In addition,variances among physicians at some detailed points were also observed,which include the extent to cover the pterygopalatine fossa and the cavernous sinus at the contralateral side.When tumor only situates at one lateral side of nasopharynx,a great divergence in risk assessment of the skull base and cavernous sinus at the contralateral side has been seen.Furthermore,disagreements on the caudal edge of CTV and inclusion of the air cavity of nasopharynx were also observed.For nodal CTV,there is also one-CTVln and two-CTVln disparity.SCH and FCH perform one-CTVln design.Their CTVln includes II-V levels from both sides and receive 54 Gy.Other three centers used two-CTVln design including high risk CTVln(60Gy)and low risk CTVln(54Gy).CHCAM and FUSCC define upper neck level,ipsilateral neck level,and level with positive node as high risk CTVln,whereas SYSUCC define high risk CTVln as 5mm expansion from positive node.Conclusion: The current study observed different CTV design strategies and dose prescriptions both for primary site and neck of nasopharyngeal cancer IMRT treatment among five centers.There are also variances in CTV contouring among participated physicians.For instance,significant differences were seen at the coverage of contralateral skull base structure when unilateral nasopharynx involved,the caudal edge of CTV for nasopharynx,and the switch of nodal CTV between high and low risk according to risk assessment.However,in general,CTVs from all physicians covered the high and medium risk site suggested by the 2017 international guidelines.The current study is the first study that investigated the consistence and variance of CTV and CTVln among different centers in China.The results provided basis for further optimization of CTV for NPC,and for establishing consensus of CTV delineation as well.