Abstarct: Nowdays, the use of radiotherapy has provided a great deal of applied research in technology and medicine. On the other hand, due to the harmful effects of gamma interaction with living cells (human), dealing with these sources has many problems. Hence, a solution is needed to avoid these dangers. The purpose of this thesis is to estimate the amount of reached doses in healthy tissues around the tumor to optimize the irradiation of cancer patients and reduce the side effects after radiation of the cancerous cell by the MCNPX computational code. In this project, according to radiation forming devices used in radiotherapy, the Linac 2100C / D is completely simulated with all its constituent parts by the MCNPX code and its radiation beam is used for radiation therapy. It should be noted that the exit beams of accelerators are different togather. Various models of these accelerators are being used in various hospitals. The used phantom in this study is the ORNL phantom in the range of the newborn baby's age, 1-, 5-, 10-, 15-, and 30- years old. As far as necessary, the noted phantoms is realistic in terms of size, volume and physical characteristics. The purpose of this work is to demonstrate that, despite the fact that radiotherapy is an important tool in the destruction of cancer cells, it may be self-induced by induction of secondary cancers in the patient. This content varies from tissue to othe tissues depending on the circumstances. The used tissue in this project is a liver tissue that is surrounded by tissues of adrenal, kidney the lumg, gallbladder, heart and Digestive system, and other tissues, which is described in detail in the text of the dissertation. The simulation will be carried out in three stages. The first phase, which is of great sensitivity, is to figue and design a radiation beam in radiotherapy, which will be simulated by the MCNPX simulator code and its output beam will be optimized to a standard level. In the second phase, the ORNL phantom, which is simulated by the code, is prepared by correction of its input data for use in later stages. This phase of work also plays an important role in the presented results. In most projects, the phantom is considered a water, which is a very misleading process. In many articles, geometric shape of the tissues is not considered, and the corresponding tissue is considers either as a cube or as any other form. This will also cause an error in the calculation of the dose. The provided phantoms by the respective companies are not void from error. Part of the errors are related to the implementation of the phantom program, which itself requires a specific time to correct the related errors. The main ingredient is the replacement of tissue-related materials and the change of geometry of the considered region. The reason for this is that most of the internal tissue in the phantom program is considered the same. With the advice of the doctors concerned, most materials are replaced to achieve more precise results with the real material of those tissues and their actual size, for the next possible comparison with the experimental results. In the final step, which is a combination of steps 1 and 2, the output of the code is acquired * F8 and F6. These data are converted to the reached doses to healthy tissues around cancerous tissues using coefficients. These doses will be compared with standard doses for each tissue if possible, and tissues that the radiation therapy of the canceroue portion caused to reach harmful doses to them will be identified and to inform the patient for possible future problems. Dose values will be calculated for 41 healthy tissues around the liver tumor with a volume of 30% liver. The highest dose is received by the liver's healthy tissue. For other organs, the received dose attribute to tumor (%) for phantoms of the newborn child, 1-, 5-, 10-, 15-, and 30-year-olds, respectively, was located at intervals of (0.015-19), (0.009-30.12), (0.0015-20.5), (5.16E-4-21.24), (2.76E-4-23.5) and (2.75E-4-21.43). According to the results, the patient's age affected the received dose. The received dose by each organ and the maximum tolerated dose by that organ are compared and the probability of the secondary cancers induction in healthy tissues around the tumor baxsed on the proposed formula was gain. For all ages, liver healthy tissues, adrenal, kidney, gallbladder, the lower portion of the spinal cord bones, pancreas, cage, the middle portion of the spinal cord bones and lung were 10 organs with highest possibility for secondary cancers, respectively. The received dose percentage in each tissue relative to the dose in the cancerous region is increased with age, and this indicates the sensitivity of radiotherapy at an early age. According to reaserch carried out in this study, such Monte Carlo calculations have not been conducted to estimate the likelihood of secondary cancers. Since time plays an important role in the treatment of cancer patients, it is important to anticipate early onset of these induction cancers and it is necessary to carry out such research in order to prevent further problems in radiotherapy for cancer patients and to prevent the development of cancer in healthy tissues around the tumor.