Answer to Question #2790 Submitted to "Ask the Experts"

Before answering the specifics of your question, it is necessary to address the overall character of this question. Design of structural shielding for linear accelerators should be undertaken only by a qualified expert, such as a medical physicist certified by the American Board of Radiology (ABR) or the American Board of Medical Physics (ABMP), a health physicist certified by the American Board of Health Physics (ABHP) practicing in medical health physics, or a physicist certified by the Canadian College of Medical Physics. Within the United States, a number of states have lists of persons qualified to perform these calculations within that state and only plans reviewed by these individuals will be accepted. Several states also have licensing laws that prohibit anyone not licensed within that state from approving designs for structural shielding.

For persons residing outside of North America, the American Association of Physicists in Medicine (AAPM), maintains a Committee on International Affairs with liaison members assigned to various geographic regions all over the world. These people may be able to direct you to a person in your region who can provide assistance. The book, Shielding Techniques for Radiation Oncology Facilities, Second Edition, by Patton McGinley, published by Medical Physics Publishing, Madison, Wisconsin, in 2002 is available from the publisher.

Aside from the radiation protection issues involved, mistakes in structural shielding design are very costly and the overall cost of the facility will be driven both by the cost of the machine and the cost of the shielding and structure.

Part 1 - A methodology for performing structural shielding calculations is described in Reports 49 and 51 of the National Council on Radiation Protection and Measurements (NCRP). These reports are under revision, but those revisions are not yet available. The International Commission on Radiation Protection (ICRP) has also published some reports on this subject. These reports may also be under revision. A reference to Dr. McGinley's book on these methodologies is given above.

Part 2 - Data on the HVLs and TVLs for various energies can be found in Reports 49 and 51 of the National Council on Radiation Protection and Measurements (NCRP). Chapter 2 of Dr. McGinley's book, as noted above, also provides some of this information. In addition, most of the accelerator manufacturers provide this information for their machines and include the HVLs and TVLs for leakage radiation, data not yet available in the NCRP reports. This information may be contained in the installation data package from the accelerator manufacturer or you may request that the manufacturer supply this data.

Part 3 - Paraffin wax is not used as shielding in medical linear accelerators. Even if it is contained within a nonflammable casing, the wax is considered too flammable for use in medical applications. Concrete, due to its water content, provides shielding for both photons and neutrons. Where higher hydrogen content is required, as in doors for linear accelerator facilities, many facilities use polyethylene or boron-loaded polyethylene as a neutron shield. For some specifics on these materials, you may wish to access companies that manufacture these materials, for example, Nelco Corporation, Direct Scientific Radiation, and Thermo RMP Corporation. Some of these companies have PDF files available.

Jean St. Germain, CHP