Determination of absorbed ratio in I-131 dosimetry using MCNP-4A [Persian]

  Determination of the total body absorbed dose consists of two parts: Beta radiation absorbed dose and gamma radiation absorbed dose. The first part is generally determined by clinical data, while the second part has a parameter called absorbed ratio and is determined by computational methods. The Monte-Carlo computational methods have been shown to be the most suitable in determination of the absorbed dose ratio. We employed the Monte-Carlo code MCNP-4A and calculated the absorbed ratio for the whole body and thyroid gland. We used a distributed source of I-131 and calculated the absorbed ratio for emitted photons in different energies from this source. The phantom was an ellipsoid having the human body dimension. Water and material similar to body tissue were chosen as the phantom’s filling materials. Results of calculation were compared with Brownell works and the differences were acceptable. To check the accuracy of calculation, the phantom was divided into 5×5×5 cm cells and a complete symmetry in the absorbed dose within phantom was observed. The MCNP relative error was reduced to less than 1% by choosing more than 50,000 histories and applying variance reduction techniques.