Program Information
Shielding Design and Affirmation for a Single Room Proton Radiotherapy System
E Klein*, Northwell Health, Lake Success, NY
Presentations
TU-H-CAMPUS-TT-3 (Tuesday, August 1, 2017) 4:30 PM - 5:30 PM Room: Therapy ePoster Theater
Purpose: This was the first installation of the Mevion single room proton therapy system, the Mevion s250. The manufacturer couldn’t supply entities such as extraction efficiency or proprietary machine design and components. Our challenge was to design shielding based on no history and limited information.
Methods: We used classical analytic methods to calculate the necessary barriers to absorb a wide variety of neutrons energy spectra. A very conservative extraction efficiency of 25% was used. Exposure from gamma rays and in room x-ray systems were considered negligible. The goal was to design shielding that maintains annual exposure for occupational regions below 5 mSv, but yet not over-shielded to keep costs contained. The goal for non-occupational regions was to limit exposure to 1 mSv. The materials used for shielding were regular density concrete, high-density concrete with a hematite mix to provide extra shielding where needed, and steel. Once the construction was completed, a cursory survey found the environment was safe for testing. A formal survey was performed using the WENDI-II detector which inherently uses a quality factor of 10 for neutrons. The detector also yields gamma values.
Results: The shielding strategy led to a lowered ceiling for the maze path to reduce the area of neutron flux, and an extra leg for the door entrance to increase the attenuation lengths, allowing a non-shielded door. We found measured exposure values below the limit of 5 mSv per year for occupational and below 1 mSv per year for non occupational areas. We also demonstrated the facility is not over-shielded when comparing measured vs. expected exposure levels. For the first year, personnel radiation monitors exhibited minimal exposure levels.
Conclusion: Overall, our mechanism to design shielding to provide low neutron exposure rates, and created a safe environment for use of the single room proton therapy system.
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