Tehran University of Medical Sciences Frontiers in Biomedical Technologies 2345-5837 8 3 2021 09 01 219 225 Mohammad Mohammadzadeh Medical Radiation Sciences Research Team, Tabriz University of Medical Sciences, Imam Hospital, Tabriz, Iran Hosein Ghiasi Medical Radiation Sciences Research Team, Tabriz University of Medical Sciences, Imam Hospital, Tabriz, Iran 2021 03 04 2021 05 17 Purpose: The current study aimed to estimate photon skyshine dose rate from a Varian linac equippedwith a Flattening Filter (FF) and its FF-Free (FFF) mode. The skyshine photons from a Linac bunker can influence the radiation dose received by personnel and the public in radiation therapy centers. Materials and Methods: In the current study skyshine dose from the conventional flattened beam and the flattening-free beam were compared. The MCNPX Monte Carlo code was used to model an18 MeV photon beam of Varian linac. The skyshine radiation was calculated for FF and FFF linac photon beams at the control room, parking, sidewalk, and corridor around the linac room. Results: For the conventional beam, the skyshine dose rates of 0.53, 0.42, 0.45, and 0.50 mSv/h were estimated for the control room, corridor, sidewalk, and parking, respectively. While for the FFF beam, dose rates of 0.21, 0.20, 0.20, and 0.23 mSv/h were estimated for the same positions, respectively. The results indicated that the empirical method of NCRP 151 can not distinguish between FF and FFF beams in skyshine dose calculations. Our results found a 50% lower level dose rate from the FFF beam at distant and nearby locations. Conclusion: The findings of current can be helpful in the radiation dose calculations and the radiation protection designation of radiation therapy bunkers.  https://fbt.tums.ac.ir/index.php/fbt/article/view/320 https://fbt.tums.ac.ir/index.php/fbt/article/download/320/213