Evaluation of Putty Metal for Internal Shielding for Patient Protection in Electron Therapy by Monte Carlo Study

background: lead has been widely used for internal shielding. it is recognized as a toxic material that pollutes the environment and does not fit well the patient's body because of its inflexibility. objectives: in this study, a specific combination (70% w, 18.61% ni, and 11.39% c) of lead-free and flexible putty metal was introduced and validated for internal shielding by monte carlo study. methods: to evaluate the putty metal for internal shielding by monte carlo study, varian 2100 c/d was validated within measurements. then, using the given energy spectrum of varian 2100 c/d, the shield thickness and backscatter factors were calculated by monte carlo study and compared with those of lead. results: the results showed that this putty metal shield required a comparable thickness compared to lead for providing protection. in addition, it is nontoxic and flexible and can be easily cut. internal shielding with high atomic number materials causes dose enhancement that is not taken into account in treatment planning systems. this study showed that this composition as an internal shield causes 5% - 7% reduction in electron backscatter factor compared to lead. conclusions: it can be concluded that lead can be replaced by putty metal with a specific combination (70% w, 18.61% ni, and 11.39% c). it is lead-free and flexible and its required thickness for protection is acceptable under clinical condition. it causes a 5% - 7% reduction in electron backscattering.