ABSTRACT
Objective To compare the sensitivity of Deha4 and ArcCHECK 3D detectors to detect the multi-leaf collimator (MLC) positioning errors in the dose verification of volumetric modulated arc therapy (VMAT) plans for nasopharyngeal carcinoma (NPC).Methods Ten NPC patients receiving VMAT plans were selected.The positioning error of 0.5-4.0 mm was introduced into the leaves of each MLC segment in the original file to expand,contract or shift the whole segments.The possible positioning errors of MLC in the treatment of VMAT were simulated.The Delta4 and ArcCHECK were utilized to verify the measurements.The absolute gamma passing rate was compared between the calculated dose and the measured dose of the VMAT plan by using the paired t-test.Results When the evaluation criterion was taken as 3 mm/3％,the absolute passing rate verified by the original plans of two detectors was greater than 95％.The positioning errors of MLC expansion,contraction and shifting detected by Delta4 and ArcCHECK were 1.5 mm,1.0 mm,2.0 mm and 3.0 mm,1.0 mm,and 3.0 mm,respectively.When taking 2 mm/2％ as the evaluation criterion,the absolute passing rate verified by the original plan was decreased significantly.Delta4 and ArcCHECK detected that the positioning errors of MLC expansion,contraction and shifting were 1.0 mm,1.0 mm,2.0 mm and 1.5 mm,0.5 mm,and 2.0 mm,respectively.Conclusions The dose verification of the VMAT plan for NPC by Delta4 and ArcCHECK can detect different types and sizes of MLC positioning errors,whereas the detection sensitivity slightly differs between Delta4 and ArcCHECK.Both of them are not sensitive to detect the MLC positioning errors less than 1.0 mm.It is fairly necessary to strengthen the quality assurance of MLC in the daily work.
ABSTRACT
Objective To compare the neck skin dose between fixed-field dynamic intensity-modulated radiation therapy (dlMRT),volumetric modulated arc therapy (VMAT),and helical tomotherapy (HT) in the treatment of early-stage nasopharyngeal carcinoma.Methods A total of 16 early-stage nasopharyngeal carcinoma patients undergoing radiotherapy were enrolled as subjects.The neck skin was delineated by contraction of the outer edge of neck by 3 mm.Dose planning was made by the traditional method (TP group)and a new method (NP group),in which the neck skin was considered as the organ at risk.Dmean and V5-V70 for the neck skin were recorded.The paired t-test was used to analyze the differences between two plans in each radiotherapy method.An analysis of variance was used to compare the same plan between the three radiotherapy methods.Results The HT group had significantly higher Dmean and V5-V70 for the neck skin than the dIMRT group and the VMAT group (P=0.00,0.00,0.00,0.00,0.00,0.00,0.00,0.02).Using dIMRT,the D and V10-V60 for the neck skin were reduced by 7％,8％,22％,25％,38％,59％,and 85％ in the NP group than in the TP group (P=0.00,0.00,0.00,0.00,0.00,0.00,0.00).Using VMAT,the D and V20-V40 for the neck skin were reduced by 4％,19％,29％,and 34％ in the NP group than in the TP group (P=0.02,0.01,0.02,0.01).Using HT,the V30-V60 for the neck skin were reduced by 20％,29％,50％,and 67％ in the NP group than in the TP group (P=0.00,0.00,0.00,0.00,0.03).Conclusions In the treatment of early-stage nasopharyngeal carcinoma,HT causes a higher radiation dose to the neck skin than dIMRT and VMAT,while dIMRT and VMAT have similar neck skin doses.The neck skin dose can be significantly reduced with the neck skin as the organ at risk.
ABSTRACT
Objective To simulate the possible systematic delivery errors introduced by monitor units ( MU) and multi-leaf collimator ( MLC) in radiotherapy plans for nasopharyngeal carcinoma ( NPC) , and to analyze the dosimetric sensitivity of static intensity-modulated radiotherapy ( IMRT ) and volumetric modulated arc therapy ( VMAT) with these errors. Methods Five IMRT plans were replanned using VMAT modality with the same physical parameters, and then MU errors of 125%, 250%, and 5. 00% were introduced into IMRT and VMAT plans. Meanwhile, to simulate leaf position errors during delivery, MLC position errors (025 mm, 050 mm, 100 mm, 150 mm, and 200 mm) were introduced by modifying the original plan documents. The types of MLC errors were as follows:( 1) the MLC banks moved in the same direction;( 2) the MLC banks moved in opposing directions ( expand or contract the MLC gaps ) . The differences in dosimetric sensitivity introduced by MU and MLC errors between IMRT and VMAT plans for NPC were calculated by linear regression analysis. Results With the increase in MU errors, the doses to target and organs at risk ( OARs) of IMRT and VMAT plans increased in a linear way, and met R2=0992-1000( P<005) . For MLC errors, the average dosimetric sensitivity for target and OARs of IMRT and VMAT were-026%/mm and-065%/mm in case of offset errors, 487%/mm and 868%/mm in case of expansion errors, and -604%/mm and -988%/mm in case of indentation errors. In addition, the dosimetric sensitivity with the three types of MLC errors was greater for VMAT plan than for IMRT plan. ConclusionsMU and MLC errors have a significant effect on the dose distribution of IMRT, and particularly VMAT, for NPC. It is important to execute routine quality assurance of MLC to ensure accurate radiotherapy.
ABSTRACT
Objective To simulate the possible systematic delivery errors introduced by monitor units ( MU) and multi-leaf collimator ( MLC) in radiotherapy plans for nasopharyngeal carcinoma ( NPC) , and to analyze the dosimetric sensitivity of static intensity-modulated radiotherapy ( IMRT ) and volumetric modulated arc therapy ( VMAT) with these errors. Methods Five IMRT plans were replanned using VMAT modality with the same physical parameters, and then MU errors of 125%, 250%, and 5. 00% were introduced into IMRT and VMAT plans. Meanwhile, to simulate leaf position errors during delivery, MLC position errors (025 mm, 050 mm, 100 mm, 150 mm, and 200 mm) were introduced by modifying the original plan documents. The types of MLC errors were as follows:( 1) the MLC banks moved in the same direction;( 2) the MLC banks moved in opposing directions ( expand or contract the MLC gaps ) . The differences in dosimetric sensitivity introduced by MU and MLC errors between IMRT and VMAT plans for NPC were calculated by linear regression analysis. Results With the increase in MU errors, the doses to target and organs at risk ( OARs) of IMRT and VMAT plans increased in a linear way, and met R2=0992-1000( P<005) . For MLC errors, the average dosimetric sensitivity for target and OARs of IMRT and VMAT were-026%/mm and-065%/mm in case of offset errors, 487%/mm and 868%/mm in case of expansion errors, and -604%/mm and -988%/mm in case of indentation errors. In addition, the dosimetric sensitivity with the three types of MLC errors was greater for VMAT plan than for IMRT plan. ConclusionsMU and MLC errors have a significant effect on the dose distribution of IMRT, and particularly VMAT, for NPC. It is important to execute routine quality assurance of MLC to ensure accurate radiotherapy.
ABSTRACT
Objective To investigate the impact of planning parameter settings on plan quality and delivery efficiency of VMAT for nasopharyngeal carcinoma with two treatment planning systems (TPS),as references for clinic plan optimization. Methods 25 patients with nasopharyngeal carcinoma were selected and planned for SIB?VMAT treatment. The same planning aims were used in the two kinds of TPS ( TPS?1 and TPS?2). Multiple planning parameters were set for plan optimization. Dose distribution to the target volumes and organs at risk,monitor unit ( MU) and delivery time were compared. Paired t?test or one?way ANOVA was used for the data which was in accordance to normal distribution;otherwise, nonparametric Wilcoxon signed rank test or nonparametric Friedman test was used. Results More segments lead to better plan quality and less MU but longer delivery time ( Minor impact was observed when segment number was larger than 120) in TPS?1,while it had little impact on both plan quality and delivery efficiency in TPS?2. Comparing to single?arc plans,dual?arc VMAT achieved no significant benefit in plan quality but had more MU and longer delivery time in TPS?1 ( P= 0?000 ) . However, dual?arc VMAT plans had better dose distribution in TPS?2, decreased the maximum and mean dose for spinal cord in 3?9% and 13?7%respectively (P=0?000,0?000).Changing the settings of maximum or minimum dose rate did not affect the plan quality in both of the tested TPSs. Increasing the maximum or minimum dose rate reduced the delivery time but the latter increased the number of MU ( P=0?000,0?000) . Conclusions VMAT plan quality and delivery efficiency is affected significantly and differently by planning parameter settings for two TPSs. Trial test should be conducted for different TPS to determine the optimal parameter settings.