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Background and purpose: Electron beams are extensively used in superficial cancer therapy. Unlike photon therapy, percentage depth dose curves (PDD) do not specify a maximum point in high energy electron therapy and include a depth range. The aim of this study was to obtain PDD by an analytical formulation which can be helpful to acquire the size of treatment area and other treatment factors such as the depth of maximum dose, the most probable energy (Ep), practical range (Rp), and the depth which the dose reaches the peak of 50% (R50). Material and Methods: Measurements of PDD was done in radiotherapy department of Ahvaz Golestan Hospital for Siemens Primus Plus accelerator, using ionization chamber CC13 dosimeter and a water phantom for energies of 6, 9, 12, 15, 18, and 21 MeV and different field sizes at various depths. Data was analyzed using MATLAB 7.8 software Results: The function which had the best fit to the data was selected. This function was used to calculate the , Rp and R50 points for each field size and energy. The dependence of these points to field size and energy were also obtained. Conclusion: We found that 4 Gaussian function had the best fit to the data ( ). Rapid dose reduction was observed after a relatively flat area. The curve had a sequence that was due to photon contamination. R50and Rp points obtained for the various fields and energies by analytical function revealed that R50 depends on the field size and energy. But, Rp depends only on energy and does not depend on field size variation.

Keywords: Electron beams, 4 Gaussian function, percentage depth dose, the most probable energy, practical range

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