Background: Todays, radiotherapy with high-energy photons is used as a common modality for cancer treatment. Unfortunately, high-energy photons can produce unwanted neutrons, and subsequently lead to increase the risk of secondary cancer in unshielded healthy organs. Methods: Using Monte Carlo (MC) code (MCNPX® ), the main parts of a typical Medical Linear Accelerator (LINAC) was modeled in 18 MV. Then, neutron source strength (Q) of the LINAC and neutron mean energy were evaluated for different treatment fields, and also different points at the treatment table. Additionally, to estimate the probability of secondary cancer risk, the neutron equivalent dose was calculated at some of the human’ s critical organs. For this purpose, the female medical internal radiation dosimetry (MIRD) phantom was included in the MC simulations. Findings: The Q was found as 1. 25-1. 38 (all in 1012 n/Gy) for different treatment fields meaning significant portion of neutrons with a mean energy of 0. 59-0. 90 MeV around the LINAC. Additionally, the total secondary cancer risk in unshielded healthy organs was calculated as 0. 0924% which maximum and minimum were related to stomach (0. 0216%) and thyroid (0. 0005%), respectively. Conclusion: Regarding the estimated values for the probability of secondary cancer risk, it is suggested that in conventional radiotherapy of deeply seated tumors, the 15-MV photons are preferred instead of using 18-MV photons due to their less biological side effects.