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Radiation shielding for megavoltage therapy in the post-NCRP 49 era.pdf
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AAPM Refresher Course: Monday 8:30-9:25 am, Ballroom B Radiation Shielding for Megavoltage Therapy Machines in the Post- NCRP 49 Era Peter J. Biggs Ph.D. Department of Radiation Oncology, Massachusetts General Hospital, Harvard Medical School Boston, MA 02114 Radiation shielding for megavoltage therapy in the post-NCRP 49 era 8:09 AM 07/19/01 Page 2 Table of Contents Page 1. Calculations 4 a. Primary 4 b. Primary barrier width 5 c. Scatter 6 d. Leakage 6 e. Obliquity factor 7 2. Workload, use and occupancy factors 7 a. Workload 7 (i) Effect of TBI, IMRT and IORT on workload 7 b. Use factor 8 c. Occupancy Factor 9 3. Shielding materials 9 a. Concrete 9 b. Lead 9 c. Steel 10 d. Earth 10 e. Polyethylene 10 4. Tenth value layers (TVL) 10 5. Room layout and features 11 a. Overall size 11 b. Machine orientation 11 c. Maze vs. direct door 12 d. Physics’ conduit 12 e. Safety features 13 f. Upgrading rooms to higher energy 13 6. Neutrons 14 7. Laminated shielding 14 8. Doors and mazes 15 a. Maze doors 15 (i) Low energy 15 (ii) High energy 17 (1) Neutrons 17 (2) Capture γ rays at the end of a maze 17 b. Direct shielded doors 18 (i) Low energy 18 (ii) High energy 18 c. Edge problem for high-energy direct-shielded doors 19 d. Other considerations 19 9. Ducts 19 a. HVAC ducts 19 (i) General 19 (ii) Rooms with mazes 20 Radiation shielding for megavoltage therapy in the post-NCRP 49 era 8:09 AM 07/19/01 Page 3 (iii) Rooms without m azes 20 (iv) Ducts passing through the ceiling 21 b. Machine cables 21 c. Water and narrow electrical conduits 21 10. Skyshine 21 Acknowledgements 22 Radiation shielding for megavoltage therapy in the post-NCRP 49 era 8:09 AM 07/19/01 Page 4 Introduction Designers of shielded rooms for radiation therapy with megavoltage electron linear accelerators rely principally upon the use of three documents: NCRP #49 (which superceded NCRP #34), NCRP #51 and NCRP #79. However, it is well recognized that these documents, published in 1976, 1977 and 1984, respectively, though they provide a conservative framework for calculating shielding barrier thicknesses, are nevertheless plete in terms of data or methodology for solving certain shielding problems, particularly those associated with high energy machines. Since the last report appeared, a substantial amount of literature has been published on many aspects of shielding design, particularly on high-energy machines and neutron production. Preparation of an update of the three existing reports to cover these new situations is currently being addressed by AAPM TG-57 that also doubles as an NCRP committee, SC46-13. 内容来自淘豆网www.taodocs.com转载请标明出处.