Dosimetric Comparison Between Volumetric Modulated Arc Therapy And Intensity Modulated Radiation Therapy Of Extended-field Radiation For Locally Advanced Cervical Cancer

Objective:To compare dosimetric difference between volumetric modulated arc therapy（VMAT）and intensity modulated radiation therapy（IMRT）of extended-field radiation therapy to seek for Optimal radiotherapy plan for locally advanced cervical cancer.Methods:From July 2016 to July 2017,24 cervical cancer patients were included who were diagnosed as squamous-cell carcinoma by pathological examination in our institution.Abdominal aortic lymph node metastasis was evaluated by computer tomography（CT）,and no other distant metastasis was observed.The average age was53.25 years old and median age was 54 years old（36⁷4 years old）.According to federation of gynecology and obstetrics clinical staging criteria（FIGO,2009）,9 cases were diagnosed withⅡB,2 cases were diagnosed withⅡA,and 13 cases were diagnosed withⅢB.Two sets of radiotherapy plans（7-field IMRT plan and VMAT plan）were designed on the same set of CT images to implement extended-field radiation therapy after completing CT-based simulation for all the patients.And patients underwent concurrent cisplatin chemotherapy and brachytherapy.According to ICRU83,delineation of target volume was completed by clinician,and prescription dose of external beam radiotherapy should reach planning target volume（PTV）50.4Gy/28f,primary gross tumor volume of lymph nodes（PGTVnd）59.92Gy/28f.95%target volume should reach prescribed dose for the two plans.The dose limit of organ at risk（OAR）was shown below:rectum V₅₀≤50%;bladder V₅₀≤50%;head of femur V₅₀≤5%;spinal cord D_max≤45Gy;small intestine V₃₀≤50%,V₅₄≤2cc;kidney V₂₀≤30%;liver V₂₀≤30%.Comprehensive evaluation of the two plans referring to isodose curve distribution and dose-volume histogram.The evaluation included the homogeneity index（HI）and conformity index（CI）of target volume,the dosimetry difference of OAR,the monitor units（MU）and the time of therapy（min）.SPSS 21.0 software was used for statistical analysis,and the results were expressed by （?）±s.Paired sample t test was used for comparison.P<0.05 was considered statistically significant.Results:Both VMAT and IMRT plans met the dosimetry requirements.Compared with the plan IMRT,plan VMAT had higher CI in PTV（P﹤0.001）and lower HI in PGTVnd（P﹤0.05）.For OAR,the dose value of plan VMAT was lower than plan IMRT in the following respects:V₂₀ of bladder;V₂₀0 of rectum;V₁₀0 and D_maxax of liver;V₂₀、V₃₀、V₄₀ and D_meanean of small intestine;V₂₀、V₃₀ and D_max of spinal cord;V₂₀0 and D_mean of double kidneys;V₁₀、V₂₀、V₃₀、V₄₀ and D_mean of left head of femur;V₄₀ and D_max right head of femur.The differences were all statistically significant with P<0.05.The average MU of VMAT and IMRT was 714.33 and 1230.86 respectively,down41.96%（P﹤0.001）,and the average treatment time of VMAT and IMRT was 3.55min and 6.99min respectively,down 49.28%（P﹤0.001）.Conclusions:For locally advanced cervical cancer extended-field radiation therapy,both IMRT and VMAT plan can achieve dose distribution of the target volume.VMAT has better CI and HI than that of IMRT.Accelerator average monitor units and treatment time of VMAT reduced significantly.In terms of specific OAR,VMAT has dosimetric advantages for locally advanced cervical cancer extended-field radiation therapy.