Answer to Question #4131 Submitted to "Ask the Experts"

I am doing research for my supervisor on IMRT (intensity modulated radiation therapy) and was wondering if you knew of any shielding requirements for facilities that have it. If so what are the requirements, and who performs the shielding calculations?

Intensity modulated radiation therapy (IMRT) uses computer-generated images to plan and deliver more tightly focused radiation beams to tumors. During the therapy treatment a precise radiation dose that conforms to the shape of the tumor is delivered with the use of computer-controlled multileaf collimators (MLCs). Thus there is a significant dose reduction to surrounding healthy tissues when compared to conventional radiation therapy.

For shielding calculations the net effect of IMRT is that the workload for the primary barrier remains the same, but the workload for the secondary barrier increases. The workload is usually stated in terms of the weekly dose at 1 meter from the source. Most facilities use IMRT for about 20 to 30 percent of the patient cases.

The IMRT Factor is defined as the ratio of monitor units (MU) for IMRT to the monitor units for conventional therapy. The monitor unit is just a surrogate for the dose delivered at the isocenter of a linear accelerator and is normally measured with transmission ion chambers. Depending upon the particular procedure, the IMRT Factor can vary from 2 to 10. For example, with 20 percent IMRT cases and an IMRT Factor of 6, the workload for the secondary barrier increases by a factor of 2. Thus, the secondary barrier will require an additional half-value layer of shielding. The half-value layer (HVL) is the thickness of the shielding material that reduces the radiation intensity by a factor of 2. For some examples of shielding calculations for IMRT, you may refer to the following tutorial.

Shielding calculations are usually performed by a shielding consultant. Most states have very specific requirements for shielding consultants, frequently referred to as "qualified experts." Some states require that these qualified experts are registered or licensed with the state. There are also requirements for submission of shielding reports and integrity radiation surveys to the state. Please refer to your state regulations to determine the applicable requirements.  A shielding consultant should have a basic understanding of photon and neutron shielding methodology and should be familiar with the following National Council on Radiation Protection and Measurements (NCRP) reports: NCRP Report 49, "Structural Shielding Design and Evaluation for Medical Use of X Rays and Gamma Rays of Energies up to 10 MeV" (1976); NCRP Report 51, "Radiation Protection Design Guidelines for 0.1-100 MeV Particle Accelerator Facilities" (out of print); NCRP Report 79, "Neutron Contamination from Medical Electron Accelerators" (1984); and NCRP Report 144, "Radiation Protection for Particle Accelerator Facilities" (2003). The person should be well experienced in the shielding design of therapy accelerators in addition to being familiar with all applicable state regulations. The consultant should be registered or licensed in the applicable state. The person should also be familiar with architectural drawings and terminology. Ideally, the shielding consultant should be involved in all stages from design to construction to ensure that the facility is constructed per design. The person should also have experience working with architects and contractors.

Nisy E. Ipe, PhD, CHP