RESUMO
Objective To establish a new method to analyze the position accuracy of multileaf collimator (MLC) in the dynamic mode.Methods The MLC test sequence was created in a field,where intentional leaf positional errors ranging from 0.1 to 1 mm per centimeter were introduced.In order to establish the relationship between the ion chamber readings and leaf position,whose slope indicated the leaf position error per centimeter,a two-dimensional ion chamber array was used to measure absorbed dose while leaves were moving at dose rates of 100,300 and 600 MU/min,respectively.For routine test,leaf position error was easily found via dose profile in y direction of the field created by dynamic leaves,where the position error could be quantitatively calculated as the slope of absorbed dose line of x direction of the same field.Results The error of 0.2 mm or more per centimeter was obviously shown through y dose profile.The calibration curve was linear at different dose rates.At 600 MU/min,a 0.1 mm leaf position error corresponded to a slope variation of 0.74％,and the differences between the tested errors and the introduced errors were within 0.1 mm.Conclusions The simple and reliable method is helpful to establish the intensity modulated radiation therapy (IMRT) quality control (QC) system.
RESUMO
Objective To explore the relationship between DLG value and planning dose distribution and actual treatment dose distribution.Methods IMRT and VMAT plans were generated in Eclipse TPS for a typical whole pelvis case,separately.For IMRT plan,MLC position and actual fluence were calculated for each DLG value with the same optimal fluence.Plans with different DLGs of 0 cm and 0.3 cm were compared by several dosimetric indexes.For exploring the PTV mean dose difference between planning dose and actual IMRT or VMAT treatment,planning dose was recalculated with fixed MLC position and different DLGs.Results Dosimetric differences of PTV V50,rectum V40,bladder V40,small bowel V35,left and right femoral head Dmax were 1.49％,0.72％,0.82％,0.68％,0.02 and 0.14 Gy respectively,the average leaf pair width of MLC segments was correlated with DLG (R2 =0.996,P ＜0.05) and reduced with the increase of DLG.In actual treatment,3.95％ and 1.5％ mean dose reduction in PTV were observed while DLG increased per 0.1 cm in the typical pelvis case,in IMRT and VMAT treatment respectively.Conclusions DLG can result in the change of MLC position and the dose difference between planning and actual treatment.
RESUMO
In this study, the physical compensator made with the high density material, Cerrobend, and the electronic compensator realized by the movement of a dynamic multileaf collimator were analyzed in order to verify the properness of a design function in the commercial RTP (radiation treatment planning) system, Eclipse. The CT images of a phantom composed of the regions of five different thickness were acquired and the proper compensator which can make homogeneous dose distribution at the reference depth was designed in the RTP. The frame for the casting of Cerrobend compensator was made with a computerized automatic styrofoam cutting device and the Millennium MLC-120 was used for the electronic compensator. All the dose values and isodose distributions were measured with a radiographic EDR2 film. The deviation of a dose distribution was +/-0.99 cGy and +/-1.82 cGy in each case of a Cerrobend compensator and a electronic compensator compared with a +/-13.93 cGy deviation in an open beam condition. Which showed the proper function of the designed compensators in the view point of a homogeneous dose distribution. When the absolute dose value was analyzed, the Cerrobend compensator showed a +3.83% error and the electronic compensator showed a -4.37% error in comparison with a dose value which was calculated in the RTP. These errors can be admtted as an reasonable results that approve the accuracy of the compensator design in the RTP considering the error in the process of the manufacturing of the Cerrobend compensator and the limitation of a film in the absolute dosimetry.