Background: Radiation therapy  uses high-energy radiation to kill cancer cells. Photoneutron contamination and induced radioactivity of high energy therapeutic photon beams are considered as the main source of occupational exposure to radiation therapists who works with linear accelerators operating above 10 MV. Materials and Methods: The gamma dose rates were measured after termination of different treatment approaches using 15 MV photons and room entry times for each approach was determined. Based on the results of this study, the annual dose of radiation therapists was estimated. Results: The highest dose rate that measured in the treatment room was belonged to 3-field pelvic approach which was equal to 5 ± 1.1 µSv/h. The radiation therapist's room entry times were determined between 3 to 10 minutes and the annual dose was estimated up to 5.9 mSv/y. Conclusion: Although the estimated annual dose is less than the internationally permitted value, the undesirable dose to radiation workers could be reduced through considering recommended room entry times.