| At the present time,Malignant tumor is the second one of the five reasons that induce people to die.The methods that treat malignant tumour are:Operation,radiation therapy and chemotherapy,radiation therapy is called radiotherapy for short.It is one of the effective methods for tumor therapy.Monte Carlo Method is a random sample method based on random numbers.It can vividly simulate particle transport,so the method has been widely used.MCNP is empoldered a large applicable code,which is made by Los Alamos Station Lab in America based on the Monte Carlo thoughts.Because MCNP can treats an arbitrary three-dimensional geometrical problem,it is widely used in radiotherapy.At present, treatment planning system which is used in clinic therapy is made by MCNP program.The treatment with 131I has a good effect on thyroid tumour,and it is a hotspot these years.When beta-ray emiiting,in order to reach the therapeutic purpose,it should be getted to the maximum possible safe dose,and we should also reduce the dose to avoid radiation damage.The dose calculation is very complex.Commonly-accepted international recommendations for dosimetry do not exist.Tissue-equivalent phantom are required to study the behavior of the mixed field.Water and PMMA is commonly used as thyroid tissue substitutes.Lately TH/L2 is made as thyroid tissue substitutes based on the chemical compositions of normal thyroid tissue.The aim of the present study was to define the most suitable normal thyroid tissue substitute for The treatment with 131I.With electron(0.19MeV-0.61MeV)incidence,the calculated distributions of the energy to normal thyroid tissue(defined by ICRU-44)along the central axis by MCNP-4C were compared to those in three thyroid tissue substitutes: water,PMMA and TH/L2.Through the comparison and analysis of the results,TH/L2 is the suitable thyroid tissue substitute for dose calibration and intercomparisions of different beams. |
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