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Introduction In this study, we aimed to calculate dose enhancement factor (DEF) for gold (Au) and iron (Fe) nanoparticles (NPs) in brachytherapy and teletherapy, using Monte Carlo (MC) method. Materials and Methods In this study, a new algorithm was introduced to calculate dose enhancement by AuNPs and FeNPs for Iridium-192 (Ir-192) brachytherapy and Cobalt-60 (Co-60) teletherapy sources, using the MC method. In this algorithm, the semi-random distribution of NPs was used instead of the regular distribution. Diameters were assumed to be 15, 30, and 100 nm in brachytherapy and 15 and 30 nm in teletherapy. Monte Carlo MCNP4C code was used for simulations, and NP density values were 0.107 mg/ml and 0.112 mg/ml in brachytherapy and teletherapy, respectively. Results AuNPs significantly enhanced the radiation dose in brachytherapy (approximately 60%), and 100 nm diameter NPs showed the most uniform dose distribution. AuNPs had an insignificant effect on teletherapy radiation field, with a dose enhancement ratio of 3% (about the calculation uncertainty) or less. In addition, FeNPs had an insignificant effect on both brachytherapy and teletherapy radiation fields. FeNPs dose enhancement was 3% in brachytherapy and 6% (about the calculation uncertainty) or less in teletherapy. Conclusion It can be concluded that AuNPs can significantly increase the absorbed dose in brachytherapy; however, FeNPs do not have a noticeable effect on the absorbed dose. Overall, AuNPs and FeNPs cannot increase the absorbed dose for the Co-60 teletherapy source. [ABSTRACT FROM AUTHOR]