Iranian Journal of

In this study, the MCNPX code version 2.6 has been used to simulate a proton therapy in thyroid gland. The neck phantom is cylindrical including skin, adipose, skeletal muscle and thyroid gland. Radiation source is close to the skin surface and single energy protons are collided perpendicular to the skin surface. The results of the simulation shows that the best proton energy interval, to cover completely the thyroid tissue, is from 42 to 74 MeV, assuming that the thyroid gland has 2 cm thickness and is located 1.12 cm under the skin surface. Share of the energy distribution inside thyroid gland from protons and neutrons produced in (p,n) reaction for the best proton energy interval is 52-92 and 82-87 percent, respectively. Proton and neutron average energy deposition raises by increasing incident proton energy in thyroid gland and obey the quadratic polynomial equation