Measuring the Skyshine from Linear accelerator in Rajaee Oncology Hospital

Introduction: Radiation therapy is using the ionizing radiation to eliminate or minimize cancerous tissues. This method has been very much considered in recent years. Because of the high energy of the photons produced by the accelerator, it is necessary to protect the walls, roof and the floor of the treatment rooms using the concrete and leaden materials. One of these harmful radiations produced in radiation therapy centers is the Skyshine that should be considered in designing the shieldings. Skyshine rays are the primary beams that pass through the ceiling in the vertical direction and scatter in the upper atmosphere above the building and return to the ground; so they transfer radiation dose to a general area outside the radiotherapy room. This study aimed to evaluate and determine the levels of doses resulted from skyshine radiation in Shahid Rajaee Radiation Center of Babolsar in Northern Province of Iran and compare it with the values of international standards.  Materials and Methods: In this survey dose measurement has been done with LS01 ion chamber (1000 cc) and Unidose E electrometer produced by PTW company in 6mv Elekta compact accelerator (made in Germany) with exposing 400MUs and field size of 40*40. The distances of dose measurement were 300, 150, 100, 50 and 25 cm from the external wall of the treatment room and 2.10cm height from the ground. Temperature, pressure and correction factors of electrometer and dosimeter has been considered in results and total dose of skyshine rays based on reformed reading of electrometer acquired for 50 weeks,6 days and 8 working hours. Results: The dose rate at 25, 50, 100, 150 and 300 cm distances from the external wall of the treatment room were 1.017, 0.79, 0.595, 0.436, 0.275 µsv/h respectively. Also, the standard deviations were 0.012, 0.007, 0.015, 0.020, and 0.032. Conclusion: The obtained results were compared with other studies and the annual average worldwide rate; the result was compatible with most studies and was less than the average worldwide= 2.4 mSv/y.