| Together with surgery and chemotherapy,radiotherapy plays an important role in oncology,both in the definitive and palliative aspects of treatment.It treats with about 60%-70%patients suffering from cancer and cures 40%of them.The clinical application of IMRT is a historic advancement for radiation therapy.The shape of high iso-dose surface is agree with that of target in three dimension,and the dose gradient is steeper.It has accurate target dose,small dose of the adjacent normal organ,protecting critical organ and homogeneous dose distribution.However,the complexity of the treatment may increase the probability of errors in the treatment process.And the error will seriously affect the treatment effect,that means it may reduce the tumor control rate and increase the normal tissue complication.Therefore, the QA and QC of IMRT become the urgent problems to be solved and should be pay great attention to.The dosimetric verification is extremely important.Absolute dose verification and relative dose verification are two aspects of dosimetric verification.Absolute dose verification is the way to check whether the dose at one point calculated by the quality assurance plan,which is produced by loading the IMRT treatment planning into the phantom,consists with the corresponding measurement.Relative dose verification is the way to check whether the referenced dose distribution on one plane calculated by the quality assurance plan consists with the corresponding measurements.For conventional therapy or conformal radiotherapy,we can check the dose of one or a few points to verify the plan and treatment delivery by manual.But for complex radiotherapy techniques,it is not sufficient because of the heterogeneous portal fluence and steep dose gradients.The film dosimeter,thermoluminescence gosimeter,gel dosimeter are effective tools for verification,but rather time-consuming and labor-consuming for use.As the advanced two-dimensional real-time dosimetric verification systems,MatriXX and EPID can obtain the two-dimensional data quickly.They have broad prospects in real-time dosimetric verification,which recently are hot issues in international radiotherapy.This paper mainly concerns the preliminary study of the two dimensional ionization chamber array(MatriXX) produced by IBA Company and Varian aS1000 a-Si EPID systems in dosimetric verification.The purpose of this study is to correct the lateral scatter contribution within the MatriXX system.Because of the lateral scatter within the MatriXX,there is considerable difference between MatriXX signal and the "real dose",which is defined as the dose measured with an ionization chamber in a mini-phantom.We introduced a scatter kernel to correct the dose values determined with the MatriXX.In order to determine the kernel parameters,MatriXX images of square fields of different sizes are recorded without a phantom in the beam.Then the dose is measured in air on the central axis with an ionization chamber in a mini-phantom for the same field sizes at the same SDD.Finally,there is an excellent agreement between the dose values measured with the ionization chamber in a mini-phantom and those obtained from the MatriXX after the scatter correction with the kernel.Another study of this paper is to investigate the dosimetric properties of a amorphous silicon electronic portal imaging device for dosimetric verification.All of the measurements were done using the Varian Clinac 23EX medical accelerator equipped with the Varian aS1000 a-Si EPID system.①EPID calibration.The EPID is calibrated by the acquisition of dark-field and uniform flood-field images;②Effect of buildup.To determine whether added buildup material was required for accurate dosimetry,the effect of added buildup placed on the detector surface on the EPID response for the 6 MV beam was investigated;③Monitor Unit.To verify linear response with dose,the EPID was positioned at a fixed detector distance and varying monitor unit settings;④Source to Detector Distance(SDD).The linearity of the EPID to variations in dose rate was investigated by comparison to ion-chamber measurements.To modify the dose rate the SDD was varied by varying the distance below iso-center;⑤Ghosting.The ghosting between images is being measured.The imager was first irradiated with a small field,after that with a large field and after that with another large field.Then a "comparison image" was calculated to determine the ghosting effects.Results:①After the calibration,the uniformity of dose distribution of Varian aS1000 a-Si EPID system is about 1.0%.②For dosimetric measurements without a patient or scattering material present,there is no necessity indicated here for extra buildup at this energy.③The relation between grey-scale value and the relative dose rate is perfectly linear as expected.④The relation between grey-scale value and the number of monitor units is perfectly linear as expected.⑤The maximum ghost signal is less than 0.3%.The end part of the paper gives an overview and puts forward the issues to do in the future.
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