Mohammad javad Tahmasebi Birgani - Professor, Department of Medical Physics, School of Medicine; Department of Radiotherapy and Radiation Oncology, Golestan Hospital, Ahvaz Jundishapur University of Medical Sciences, Ahvaz,Iran,Tahmasebi_MJ@yahoo.com
Mansour Zabihzadeh - Assistant professor, Department of Medical Physics, School of Medicine, Ahvaz Jundishapur University of Medical Sciences, Ahvaz,Iran, zabihzadeh@ajums.ac.ir
Saeideh Aliakbari - Ph. D Candidate , Department of Medical Physics, School of Medicine, Ahvaz Jundishapur University of Medical Sciences, Ahvaz,Iran, aliakbarisaeedeh@gmail.com
Mohammad Ali Behrouz - Professor, Department of Medical Physics, School of Medicine; Ahvaz Jundishapur University of Medical Sciences, Ahvaz,Iran, mabehrooz@yahoo.com
Seyyed Mohammad Hosseini - Assistant Professor, Department of Radiation Therapy, University of JundiShapoor, Golestan Hospital, Ahvaz, Iran, kiahossainy@yahoo.com

Introduction: In this study, a specific combination (۷۰% W ۱۸.۶۱% Ni and ۱۱.۳۹% C) of lead free and flexible putty metal is introduced and validated as internal shielding by Mont Carlo study Materials and Methods: To evaluate putty metal as an internal shielding by Mont Carlo study Varian ۲۱۰۰ C/D was validated within measurement and then by given energy spectrum of Varian ۲۱۰۰ C/D shield thickness and backscatter factor were obtained by Mont Carlo study and compared to those of lead. Results: The results showed that this putty metal shield required comparable thickness for providing protection compared to lead. In addition, it is nontoxic and flexible in that it can be easily cut. Internal shielding with high atomic number causes dose enhancement that isn’t taken into account in treatment planning system. This study showed that this composition as internal shield causes ۵-۷% reduction of electron backscatter factor relative Lead. Conclusion: it can be concluded that lead can be replaced by putty metal with specific combination (۷۰% W, ۱۸.۶۱% Ni and ۱۱.۳۹% C). It is Lead free, flexible and thickness required for protecting tissue is acceptable under clinical condition and it causes ۵ -۷% reduction of electron backscattering

flexible internal shielding, Dose Enhancement, Electron- backscattering