Answer to Question #2247 Submitted to "Ask the Experts"

Why is lead used as protective shield in diagnostic radiology departments while concrete is used in radiotherapy departments?

Technically, any material can be used in shielding. The basic idea is that you want to have a sufficient quantity of atoms to intercept and absorb or scatter the radiation away from the area you are trying to protect. Obviously, the denser the material the greater the number of atoms that are available.

However, other factors besides density affect the choice of the shielding material. These include space, weight, cost of material and installation, and availability. As noted in the National Council on Radiation Protection and Measurements (NCRP) Report No. 49, "Structural Shielding Design and Evaluation for Medical Use of X Rays and Gamma Rays Up to 10 MeV," a concrete primary barrier for a 100 kVp x-ray unit would be 80 times as thick and 17 times heavier than a lead barrier that would offer the same protection. However, for a 1 MV x-ray therapy unit, the concrete would be six times as great and only 25 percent heavier than an equivalent lead barrier.

In the shielding for diagnostic x-ray units and x-ray therapy units below 300 kVp, lead is commercially available in sheets, lead-lined wallboard, lead-lined lath, and lead-lined blocks that are easy to install, particularly where space is limited. NCRP Report No. 35, "Dental X-ray Protection," provides a number of wall designs and their minimum concrete equivalences. Designs include those employing cinder blocks, gypsum plaster on metal lath, and fiberglass. In 2003, NCRP Report No. 35 was replaced by NCRP Report No. 145, "Radiation Protection in Dentistry."

Alternately, concrete is a very common building material that is used in floors, ceilings, and walls of many buildings. In addition, poured concrete can be loaded with materials such as barites, magnetite, steel, or lead to increase the density and attenuation characteristics. In addition, for linear accelerators operating above about 9 MeV, neutrons are produced from photoneutron activation, as noted in NCRP Report 79, "Neutron Contamination from Medical Electron Accelerators." While not as effective as other shielding materials such as polyethylene, concrete sufficient to shield for photons provides adequate neutron shielding. The shielding of neutrons by lead or other metals is very poor.

John P. Jacobus, MS
Certified Health Physicist