Proton therapy is one of the prostate cancer treatments which destroys tumor using a proton beam. Bragg peak in proton depth-dose curve is the most important characteristic of this method. Bragg peak causes the most amount deposition of energy in the tumor. Prostate is not a fixed organ and is moved in the range of 1-7mm in body, so this movement results in uncertainty in total absorbed dose. In this research total absorbed dose and equivalent dose of secondary particle are calculated regarding prostate movement using FLUKA code which works with Monte Carlo method. At first phantom used in this research including prostate healthy tissue, prostate tumor, bladder, rectum and testis was simulated. then tissue materials were added. Regarding depth of 15cm of prostate in body the needed energy to form Bragg peak in prostate tumor was calculated 148MeV with 0.02% precision. At first total absorbed dose for one proton particle for prostate and reside organs was calculated while prostate was considered fixed in (0,0,0), it was shown that 99.9% of total absorbed dose for one proton particle which is 7.2 nGy is deposited in prostate. Then prostate was moved 7mm perpendicular with proton beam, total absorbed dose for one proton particle in prostate reduces to 7.04 nGy. Due to this movement uncertainly of total absorbed dose was 2.2 %. Finally prostate was moved 3mm in the direction of proton beam and total absorbed dose for one proton particle in prostate was calculated 6.72 nGy. Due to this movement uncertainty of total absorbed dose was 6.66%
