131I is a radiopharmaceutical used for the treatment of advanced lung cancer, resulting in high organ doses. The effects of the lung mass on the absorbed dose to the lung due to the administration of 131I were studied in this research. For this purpose, the lung was selected as the source of 131I. Furthermore, 98 similar mathematical phantoms, only different in their lung mass, were developed. The received dose per decay of 131I for each organ was calculated using MCNPX. The results indicate that for the electrons emitted from the decay of 131I, the dose changes proportionally to the inverse of the lung mass. Considering that the participation of the electrons resulting from the decay of 131I in the amount of the lung dose outweighs the photon participation by a great deal, changes in the dose for the sum of the electrons and photons per decay are proportional to the inverse of the lung mass, as for the electrons.

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