Introduction: There are many ways to treat cancer and pain relief. One of the methods is radiopharmaceutical therapy. Indium 111-rituximab contains the radioisotope indium-111 and rituximab. The Indium-111, after decay, emits gamma rays with energies of 245 KeV and 171 KeV, and beta-radiation of 26 KeV. In this study, the absorbed dose of 111In-Rituximab in human body was calculated using mice data and MCNPX simulation code for the organs of heart, lung, liver, spleen, kidney, stomach, intestine and pancreas.
 
Materials and Methods: Male phantom was used for this simulation. The source and target organs were defined in code and absorbed dose obtained for per particle with error of 5 percent. The source organs were determined as volume source in SDEF card of MCNP code. Simulations were done in two steps for gamma and beta sources and the final absorbed dose of each organ were multiplied by the amount of cumulated activity of 1MBq.
 
Results: The results showed that the absorbed dose due to injection of 111In-Rituximab for organs of spleen and stomach are 2.2252E-02, 5.5979E-03 respectively. so these organs are at risk in treating cancer with this radionuclide. The errors for were less than 5 percent.
Conclusion: Monte Carlo method and MCNPX simulation code are important methods for calculation of the absorbed dose after the distribution of rradionuclides in the body. Also the results show that two organs spleen and stomach are at risk in Indium 111-rituximab therapy.