| The ideal state of radiotherapy is irradiating the tumor only without the normal tissues irradiated surrounding it.However,the modern radiotherapy effects haven’t ever reached this ideal state,the modern accurate radiotherapy benefit from the fast-track development of computer technology have made some tremendous progress heading for this ideal goals.Meanwhile,the core of the accurate radiotherapy is accurate positioning,precise plan designing and exact radiotherapy.In order to achieve the accurate radiotherapy,three questions need to be solved at the radiological physics level,i.e.,how to ensure X-rays deliver accurately,how to design the optimized radiotherapy plans based on choosing the optimized radiotherapy technique for each cancer patient,and how to ensure these plans implement accurately.Then the research of the above questions is performed by utilizing the complete radiotherapy system of radiotherapy department in Northern Jiangsu People’s Hospital,i.e.,Varian clinac-IX medical electron linear accelerator,Varian Eclipse treatment planning system(TPS),finger-type ionization chamber,PTW dosimeter,MatriXX two dimensional ionization chamber array,and COMPASS software.The research contents and conclusions are as follows:First,four aspects to ensure X-rays deliver more accurately were outlined:(1)the research was carried out for the measurement and computing methods of absorbed dose of both high energy X-rays and electron beams in water,which aimed at guaranteeing the accurate “quality” of X-rays.The results show that the errors between absorbed dose of the effective measuring point obtained by using the calculation methods in IAEA 277 report after the measurement in water phantom by using finger-type ionization chamber and the standard calculated values was between 0.17% and 0.54%,which met the requirements of the State(2.0%)and indicated the X-ray energies the accelerator delivered were correct.(2)An analysis of dose response characteristics of three kinds of frequently-used ionization chambers(i.e.,0.01 cm~3,0.125 cm~3,and 0.6 cm~3)was made in quality assurance(QA),which aimed at understanding these performance characteristics,to ensuring the accurate dose monitoring and thus to ensuring the accurate “quantity” of X-rays delivery.(3)The effects of MLC parameters(i.e.,transmission factor,interleaf leakage factor,leaf position correction factor,and penumbrae)on X-rays delivery were explored.These parameters were imported to TPS for calculation and comparison.The results indicate the contact ratio of 80% isodose line was greater than 95% and the curve degree of separation was less than 3 mm,which meant MLC parameters measurement is essential to ensure the X-rays transmission and the accuracy of dose calculation.(4)The impact of treatment couch on X-rays delivery demonstrate the attenuation factors of the Varian accelerator treatment couch for 6 MV and 15 MV X-rays were 0.981~0.997 and 0.984~0.999,respectively.Hence,it is necessary to design radiotherapy plans with a virtual couch,and the X-rays attenuation by the couch can achieve TPS compensation when the accelerator’s gantry rotates to oblique below or under the virtual couch and thus to ensure the delivery accuracy of X-rays.Second,in allusion to three kinds of currently mainstream radiotherapy(i.e.,3D-CRT,IMRT and VMAT),the comparative research of radiotherapy was presented for nasopharyngeal carcinoma of head and neck,breast cancer of chest,and cervical cancer of abdomen basin,wherein dosimetry parameters comparison was made between IMRT and VMAT plans for post-operative cervical cancer and nasopharynx cancer,and dosimetric comparison was conducted among three kinds of radiotherapy plans for breast cancer.The results demonstrate that although VMAT technique can obtain superior target dose distribution and maximum decrease the dose of organs at risk(OARs)received,the time-consuming optimization process decreases the radiotherapy efficiency as better stability of the accelerator itself is required in VMAT technique.Overall,IMRT technique is a superior choice for designing radiotherapy plans.Moreover,for cervical cancer in 5F-IMRT plans,the impact of four groups of conditional variables on dosimetry parameters was studied,wherein the variables were composed of X-ray energy(6 MV and 15 MV),treatment position(supine position and prone position),MLC motion mode(sliding window and multiple static segments),and dose calculation algorithms(anisotropic analytical algorithm and pencil beam convolution).The research indicates that the radiotherapy mode composed of prone position,AAA algorithm,SW and 15 MV X-rays is optimal.Finally,an analysis of the dosimetric verification and position verification was made.On the one hand,a dosmetric verification research was conducted for thoracic segment esophageal carcinoma patients by using MatriXX and COMPASS.The results show that the absolute dose calculated of the isocenter was consistent with the TPS results and γ passing rate can be achieved to over 97%,which met the clinical requirements.On the other hand,a position verification analysis was given for lung cancer patients by using KV-Cone-Beam CT(KV-CBCT).The results demonstrate that KV-CBCT can effectively reduce the setup errors and provide reference for obtaining CTV margins(planning target volume,PTV).In a word,in terms of the accuracy delivery of rays,the results in this dissertation explained how it was affected by the Varian clinac-IX accelerator system in several aspects,on the basis of which the accurate radiotherapy can be smoothly conducted;as for plan designing,the optimal design methods have been solved for a variety of typical cases through the research in this dissertation,i.e,achieving satisfactory target dose distribution and reducing the dose of organs at risk received;in respect to dose verification and location verification,the necessity of verification and the specific method for verification were analyzed and provided,respectively,which can provide reference for clinical application. |
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