The Study External Dynamic Multi-leaf Collimator Used For Intensity Modulated Radiation Therapy

With the technological development of radiotherapy,one of the vital treatments to malignant tumors,there has been outstanding advantages of intensity modulated radiotherapy(IMRT)in various aspects,like improving the gain ratio of radiotherapy,killing tumor cells for a less or none exposure of the surrounding normal tissues and organs in unnecessary irradiation.Getting more and more recognition from tumor radiation technicians and medical staff,IMRT has become the mainstream of radiotherapy,in which dynamic multi-leaf collimator(DMLC),a necessary device for IMRT,is playing a significant part.Due to the high price of the medical linear accelerator imported from abroad,we have purchased domestic medical linear accelerators and external DMLC for a lower cost.This paper mainly discusses the IMRT implementing study of installing external DMLC on the medical linear accelerators in the following four aspects.Firstly,study the physical properties,mechanical installation test and the calibration of external DMLC.Secondly,compare the different dosimetry characteristics of external DMLC and independent collimator in square fields,rectangle fields and irregular fields,including collimator scatter factor(SC),module scatter factor(SP),total scatter factor(SC,P),percent depth dose as well as profiles.Thirdly,verify the installation of external DMLC on KB1800 linear accelerator by means of 2D Chamber Array,followed by a dosimetry verification of both static and dynamic intensity modulation.Fourth,analyze factors that affect the pass rate of intensity modulation Gamma,including leaf transmission factor,leaf leakage factor,and leaf position precision and also the influence brought by DMLC deviations in gravity.As the result shows,external DMLC,the tertiary collimator fixed on the accelerator head by the adapter,should have all the performance indications that normal collimators have.Among all indications,the collimator-scatter factor and the percent depth dose have consistent properties with when independently using the collimator and external DMLC.As module scatter factor relies on the position setting of independent collimators,the planned verifications that make contrast of plane doses have passed with the pass rates of over 90.0%.The IMRT dosimetry verification of the medical linear accelerator KB1800 hang in with DMLC isstandard-compliant.With leaf transmission factor and leaf leakage factor of external DMLC both below 2%,the pass rate would decrease to a maximum of 2% if the leaf position expand 0.1mm,leading to a 2mm deviation of external DMLC in gravity.Lead the tested DMLC indications into TPS,and compare the analysis of inverse calculation result of IMRT with the measured result,we can get a conclusion that can fully meet the requirements of clinical applications.