Determination of the equivalent field size of irregular rays?fields in radiotherapy with different range of energies for X and

Introduction : The equivalent field in radiotherapy is a standard field with the shape of circular or square that have the same central axis depth dose properties (PDD,…) with irregular field . Objective : In this field different experimental and semi experimental methods have been presented so far (but to some extend successful). In this study our purpose is to obtain relatively perfect and theoretical methods based on physical rules in mater-radiation interaction which can be sensitively rapidly and easily used in different radiotherapy centers . Subjects and Methods : In this method for calculation of equivalent square of a rectangle fields with shild,decreased from length of collimator rectangle field an increased the width of it to achieved a square : Then to equate the square and irregular fields (central axis depth dose data) do in this manner : with applying the inverse square distance rule and exponential attenuation rule of radiations in crossing the mater for all of the scatter radiations that comes from the all regions that are shown in picture and take the subtracting of scatter radiation of the collimator rectangular field and square field equal to scatter radiations that comes from the shields regions to earn an integral equation For calculation of this equation has been used from numerical calculations such as composite Simpson(for integrations) and tasnif algorithm(for calculation of the equation). The calculations was done with computer and with the program that was made with visual C++ language . This method was applied to calculation the equivalent circular fields of square fields and equivalent square fields of several custom therapeutic fields with different triangle and rectangle shields in varied position . Computer calculations were done in two energy of 6MV and 18MV of 2100C Varian accelerator X-ray and 1.25MeV energy of Cobalt-60 ?-ray in Ahvaz Golestan Hospital . And in continue the equivalent square side of rectangular fields, has been compared with the acceptable data of ICRU. In addition the depth dose data of several conventional irregular fields in radiotherapy( )has been compared with depth dose data of equivalent field that extract from method in two x-ray energy of 6MV and 18MV of 2100.C Varian linear accelerator . Results and Conclusion : In this method, since the calculation was done with account the situation of field, energy and the position of the shield, show a very good accommodation with dose measurements (error, less then 2%). These intentions don’t be account in references. In addition, the calculations can be done with computer for each therapeutic irregular fields and can be added to the treatment planning programs .

Keywords:

Equivalent field , Exponential attenuation , Inverse square distance