Comparing Of Dosimetric Benefits Of VMAT And TOMO In Nasopharyngeal Carcinoma Patients Using A Priority-Classified Plan Optimization Model

Objective:The purpose of this study is to compare the dosimetric advantages and disadvantages of the planning target volume and the organs at risk in the treatment of nasopharyngeal carcinoma with volumetric modulated arc therapy（VMAT）and helical tomotherapy（TOMO）,and to provide a reference basis for selecting the best radiotherapy technique for patients with different T-stage nasopharyngeal carcinoma.Methods:Forty patients with nasopharyngeal carcinoma who were clearly diagnosed by pathology without distant metastases and received radiotherapy for the first time between January 2019 and March 2019 were randomly selected from the radiotherapy department of our hospital.Based on the international guidelines for dose prioritization and dose limitation of radiotherapy planning for nasopharyngeal cancer,we propose a priority-classified plan optimization model,which divides the planning target volume（PTVs）and organs at risk（OARs）into four priority classes,with brainstem,optic chiasma,optic nerves and spinal cord as priority class I;PTV and temporal lobes as priority class II;lenses,eyes and pituitary gland as priority class III;and parotid glands,mandible,temporomandibular joint,inner ears,oral cavity and thyroid gland being classified as priority class IV.This suggests that the dose coverage of PTV is placed after the dose limiting criteria of the priority class of the OARs before being considered.Based on this model,separate VMAT and TOMO plans are developed for each patient enrolled on the same set of CT images,with all plans optimized and evaluated by the same experienced and professionally trained of dosimetric parameters of PTVs,OARs,and normal tissue low-dose radiation areas for VMAT and TOMO plans for patients with T early and T advanced.The Wilicoxon paired signed-rank test in SPSS 26.0 software was used to assess dosimetric differences between VMAT and TOMO plans,and a two-tailed P<0.05 was considered to be a statistically significant difference in dosimetric parameters between the two radiotherapy plans.Results:1.All T-staged patients:VMAT had better protection for the optic nerves,eyes,pituitary gland and inner ears,while TOMO had better protection for the brainstem,spinal cord,temporal lobes,oral cavity and thyroid gland（P<0.05）,and both plans had similar protection for the remaining OARs（P>0.05）.Both VMAT and TOMO plans were within the dose limiting criteria for Class I priority OARs,and both plans achieved ideal prescription dose coverage for PGTVnx,PCTV-1,and PCTV-2（95.00%vs.95.24%,P=0.193;98.50%vs.98.96%,P=0.18;97.50%vs.98.09%,P=0.269）,but the conformity index（CI）of PCTV-1 and PCTV-2 was better in the TOMO plan（P=0.002;P<0.001）.The VMAT plan had a smaller volume of low-dose radiation,especially for V₁₀-V₂₅（P<0.05）.2.T early patients:Early T patients:VMAT plan had better protection for optic nerves,optic chiasma,eyes and pituitary gland;TOMO plan had better protection for brainstem,spinal cord,temporal lobes,mandible,temporomandibular joint and oral cavity（P<0.05）,and both plans had similar protection for lenses,parotid glands,inner ears and thyroid gland（P>0.05）.The priority class I OARs of both VMAT and TOMO plans were within the dose limiting criteria,and the ideal prescription dose coverage was achieved for PGTVnx,PCTV-1,and PCTV-2 in both plans,but the CI and HI were better for PCTV-2 in the VMAT plan（P<0.05）.The normal tissue low-dose radiation volume was smaller in the VMAT plan,especially for V₅-V₂₀（P<0.05）.3.T advanced patients:VMAT plan had better protection for optic nerves,eyes,mandible and inner ears,while TOMO plan had better protection for brainstem,spinal ideal prescription dose coverage for PCTV-1 and PCTV-2（98.00%vs.98.85%,P=0.013;97.00%vs.98.27%,P=0.01）,with better target area coverage for TOMO and better CI and HI for PCTV-1（P=0.024;P=0.027）.Both plans did not achieve the prescribed dose requirement for PGTVnx coverage（91.00%vs.93.61%,P=0.128）,but the TOMO plan achieved the maximum prescribed dose coverage for PGTVnx.The VMAT plan had a smaller volume of all low-dose radiation(V₅-V₃₀)（P<0.05）.Both VMAT and TOMO plans achieved cord,temporal lobes and thyroid gland（P<0.05）,and both plans had similar protection for the remaining OARs（P>0.05）.Both VMAT and TOMO plans achieved ideal prescription dose coverage for PCTV-1and PCTV-2（98.00%vs.98.85%,P=0.013;97.00%vs.98.27%,P=0.01）,with better target area coverage for TOMO and better CI and HI for PCTV-1（P=0.024;P=0.027）.Conclusion:In patients with T early stage（T1-2）nasopharyngeal carcinoma,both VMAT and TOMO plans achieved ideal target coverage（≥97%）,while the conformity index and homogeneity index of the VMAT plan planning target volume were superior.The TOMO plan had no significant advantage in the protection of the OARs,especially the protection of the optic nerves and optic chiasma was not as good as the VMAT plan,and the volume of low-dose radiation to normal tissues was larger.Therefore,patients with T early-stage nasopharyngeal carcinoma have greater dosimetric benefit of the VMAT plan.Patients with T advanced stage（T3-4）nasopharyngeal carcinoma,the TOMO plan achieved the greatest prescribed dose coverage of PGTVnx（93.61%）,and the TOMO plan had better protection of the brain stem,spinal cord,and temporal lobes.Therefore,patients with T advanced nasopharyngeal carcinoma had the greater dosimetric benefit of the TOMO plan.