Dosimetric Comparison Betweeen Volumetric Modulated Arc Therapies Of Different Arc Radian And Dynamic Intensity-modulated Radiotherapy For Early-stage Peripheral Lung Adenocarcinoma

Objective To compare and analyze the dosimetric characteristics of two different types of intensity-modulated radiotherapy techniques—single-arc volumetric modulated arc therapie(VMAT)and 5-field dynamic intensity-modulated radiotherapy(d IMRT),for early-stage peripheral lung adenocarcinoma,and attempted to explore the appropriate curvature of single-arc VMAT for early-stage peripheral lung adenocarcinoma.Methods A total of 32 patients of early-stage peripheral lung adenocarcinoma with spherical or spheroidal tumor lesions from August 2016 to March 2018 in our hospital were collected,including 12 males and 20 females,aged 63 to 84 years.The median age is 75 years old.Tumors were pathologically confirmed as primary lung adenocarcinoma.According to the 2009 UICC,32 cases were all patients with stage Tl/T2N0M0.The tumor size was between 1.2-5.0cm,and the PTV was between 56.4cm3 and 198.3cm3.The MONACO 5.1 TPS was used to design the three treatment plans for selected cases:single-arc 180° and single-arc 360° VMAT,and 5-field dynamic intensity-modulated radiotherapy(d IMRT).Gross target volume(GTV): GTV is the size of the primary tumor in CT scans(lung window),the burr margin of the lesion is included in the GTV,and the clinical target volume(CTV)is GTV+ 8 mm extension.CTV should not exceed the boundary of anatomy and do not perform selective irradiation in the lymphatic drainage area unless there is an invasion.Plan target volume(PTV): CTV + ITV,and expansion with 5mm in anterior and posterior,up and down,left and right for the setup errort;95%PTV:6000c Gy / 30 f,5 times / week.Organs at risk(OARs): included the lungs,spinal cord,heart,and esophagus.The parameters to be compared include: 1.Target area:Compare DVH charts of the three plans,and analyze the maximum dose(Dmax),the minimum dose(Dmin),the average dose(Dmean),Homogeneity Index(HI),and Conformity Index(CI)of PTV.According to the ICRU83,defined as: HI =(D2%-D98%)/D50%.(D2%and D98% represent the exposure dose of 2% and 98% of the volume of the PTV,respectively.D50% is the exposure dose of the 50% volume of PTV,and is similar to theaverage exposure dose of the target.Conformity Index(CI)= VT.ref / VT × VT.ref / Vref(VT.ref is the target volume surrounded by the reference isodose line,VT is the target volume,Vref is all the volume surrounded by reference isodose line.The CI value is between 0 and 1.The value is as closer to 1,as higher the conformity of the plan.Organs at risk: Volumes percentage of both lungs and the affected lung received dose of 30 Gy,20 Gy,10 Gy,5Gy(V30,V20,V10,V5)and Mean Lung Dose(MLD)were measured,as well as the volume percentage of the affected lung received 50Gy(V50);The maximum dose(Dmax)and the average dose(Dmean)of spinal cord;the volume percentage of the heart received 30 Gy,and the received dose of 1/3 heart(D1/3);the average dose(Dmean)of the esophagus.Performance efficiency: Monitor Unit(MU): the sum of all the radiation fields.All statistical analyses were processed by SPSS 19.0 statistical software.The dosimetric differences of three kinds of planns were analyzed with one-way ANOVA of each index,and then paired t-test was used for comparison of each two groups.P<0.05 was considered statistically significant.Results The three types of plans can all meet the dosimetric requirements of the clinical target area.There was no significant statistical difference in Dmax,Dmin,Dmean,HI,and CI of PTV between the three groups(P>0.05).In protection of OARs,the MLD,V5,V10,and V20 of the bilateral-lungs and affected-lung in the d IMRT plan group were lower than that of the other two single-arc VMAT plans,with significant statistical difference(P<0.05).In the single-arc 360° VMAT plan,MLD,V5,V10,and V20 of the bilateral-lungs and affected-lung were slightly higher than hat of the single-arc180° VMAT plan,there was a significant statistical difference(P<0.05).For spinal cord,there was no significant difference in Dmax between the three groups(P>0.05),but there was a significant difference between the three groups in the Dmean,with d IMRT group <single-arc 180° VMAT group <single-arc 360° VMAT group(P<0.05).The three groups of radiotherapy plans had similar heart protection,and there was no significant difference between the V30 and D1/3 of the heart(P>0.05).For Dmean of esophagus,d IMRT group was less than the single-arc VMAT groups(P<0.05),there was no significant difference between single-arc 180° VMAT group and single-arc 360° VMAT group(P>0.05).Compared with d IMRT,MU of the single-arc VMAT plans was significantly reduced,and the 180° single-arc was less than the 360° single-arc VMAT plan.Conclusion In early-stage peripheral lung adenocarcinoma patients with spherical or spheroidal tumor lesions,the dose distribution of three treatment plans can all meet theneeds of clinical treatment.In the target conformity and dose uniformity,single-arc VMAT has no advantage,compared with d IMRT.And,d IMRT has better protection for the normal lung tissue,spinal cord,and esophagus.With respect to single-arc VMAT,the increasement in radian did not increase the dose conformity and uniformity of the target area,on the contrary,the average exposure of the normal lung tissue and the spinal cord increased,which was detrimental to normal organs protection.The MU of single-arc VMAT was significantly reduced relative to d IMRT,with the 180° single-arc VMAT being the lowest.Therefore,in early-stage peripheral lung adenocarcinoma patients with spherical or spheroidal tumor lesions,5-field d IMRT is preferred;for single-arc VMAT,180° single-arc is better.However,whether dosimetric advantages can ultimately be transfered into clinical benefits,still require further clinical studies to confirm.