A buyer’s guide to radiation shielding

September 02, 2016
By: Jim Noller, Daniel Harrell and Manjit Chopra

Radiation has long been wielded as a vital tool by health care providers, but the shielding that is always necessary to protect patients and staff from unwanted exposure is, perhaps, not that well understood. Although many types of shielding materials exist, concrete in its various forms often provides the preferred solution.

Radiation shielding applications

Radiation and radiation shielding can be broken down into two primary categories: diagnostic imaging and therapeutic applications.

Diagnostic: Low-energy diagnostic procedures, such as chest X-rays, are typically shielded with lead-lined drywall. Higher energy diagnostic procedures, such as CT scans, require increased amounts of shielding. Concrete is often used for these applications.

Therapeutic: The second category is therapeutic radiation treatments for cancer, and this is where heavy-duty shielding comes into play. These internal and external beam therapies include high dose rate (HDR) brachytherapy, linear accelerator and particle therapy (including proton therapy). Because these treatments utilize more intense radiation than diagnostic procedures, they require substantial shielding.

For example, it is not unusual for a linear accelerator treatment room to require 1.5 million pounds of shielding. Not surprisingly, concrete and heavy concrete are the preferred shielding materials for radiation treatment applications. Radiation treatment room walls, ceilings and floors are carefully designed with materials that will provide the required level of protection, which is determined by applicable codes and regulations. The physicist of record will calculate the shielding barriers based on the quality and energy of the radiation, the treatment modalities and the expected number of patients per day.

Concrete, due to its relatively low cost, high structural properties and ease of use in construction, has long been the primary material for radiation shielding. As the cost of real estate and construction has increased, so has the use of heavy (or high-density) concretes in order to decrease shielding space requirements. Under certain conditions, lead (Pb) may be incorporated into the shielding design, but is often limited due to high cost, toxicity and lack of structural qualities, which require additional construction and cost for support.

How much is just right?
To provide some perspective, a typical PET/CT room might have walls that consist of approximately 10 to 14 inches of concrete shielding, while the walls of an HDR/brachytherapy room may be designed with 15 to 20 inches of concrete. Linear accelerators typically require two shielding barriers. The primary shielding barrier where the beam from the linear accelerator is pointed usually requires six to nine feet of concrete. The secondary barriers, which absorb leakage and scatter radiation, usually require three to four feet of concrete. Very high energy particle therapy applications like proton therapy utilize a cyclotron to produce the particles, a beam line to carry the particles and a gantry to deliver them to the patient, and can require eight to 12 feet of concrete shielding.

Types of concrete and methods of installation
Poured-in-place regular (conventional) weight concrete (147 pounds per cubic foot, pcf, density) is the most common type of concrete used for radiation shielding. It is relatively inexpensive and, like other forms of concrete, has high structural properties. It can, however, require a substantial amount of space, depending on the shielding requirements. Fortunately, there are other types of concrete and methods of installation that can offset this limitation. Heavy (high-density) concrete has superior shielding properties and requires significantly less space than regular weight concrete. Heavy concrete can achieve in-place densities ranging from 148 pcf to more than 330 pcf.

In addition to the traditional poured-in-place method, pre-manufactured modular concrete shield block units are available which can be stacked to create effective shielding systems. Modular concrete block is available as both conventional and heavy concrete formulations. With its wide array of product types and installation methods, it’s easy to understand why concrete is the most common and versatile radiation shielding material in use today.

Benefits of different concrete systems
There is no “one-type-fits-all” approach when it comes to selecting the best concrete shielding system for a project. Poured-in-place, regular-density concrete is a common approach, primarily due to its familiarity of use as a general building material. A treatment room (also referred to as a vault or bunker) is essentially a small building, and poured-in-place conventional concrete shielding is often selected due to availability and ease of construction. In many markets, regular density concrete offers a relatively low-cost shielding option. Finally, poured-in-place concrete is structural by design and can be readily adapted to a variety of seismic locations.

Heavy concrete has all the ease of construction and structural advantages of regular-density concrete and requires less space, which reduces excavation and foundation costs. The use of high-density concrete can effectively reduce the wall and/or ceiling thickness by up to 55 percent compared to regular concrete. This significantly reduces the footprint of the shielding barriers, thereby freeing up space for other profit-generating and/or patient enhancement uses.

In locations where space is limited or obstructions need to be worked around, high-density concrete can provide the appropriate amount of shielding in that limited space. Heavy concrete can be poured, pumped, conveyed or placed with concrete buckets for delivery where needed. While heavy concrete consists of select raw materials that are higher in cost compared to regular concrete, the batching and placement processes utilize the same methods and a lower overall volume of material is required. Therefore, the cost increase can be limited to the cost of the select materials.

Modular concrete shield block systems provide the greatest degree of flexibility of all the concrete systems and offer several advantages. Designed to come apart and be removed and relocated, modular block systems can easily be built in both green field and shell spaces. The use of modular systems in leased spaces, for example, allows the owner to accept the “temporary use” of the structure. The modular block system may be disassembled and removed to restore the landlord’s space when a lease expires. Modular block systems can be made with conventional and heavy concrete and offer the most flexibility to strategically place materials. Space savings and other benefits offered by heavy concrete can be applied to compact shield block systems, which can result in reduced leasing costs.

Another benefit of a modular system is the pre-testing individual units and product guarantee. The blocks are factory manufactured off-site and their density is measured prior to shipping. This ensures that all blocks will meet the project density requirements. Alternatively, poured-in-place concrete batches have to be tested in the field prior to placement, so there is a risk that some concrete batches may have to be discarded if the density is not achieved. Other remediation might also be required for a poured-in-place system if a lower-than-desired density batch is discovered after placement.

Additionally, modular shield blocks can be delivered and installed in most inclement weather conditions, thereby reducing the potential for weather delays. Modular systems can be assembled in a fraction of the time required for a poured-in-place structure. The ceiling for a modular block system does not require the expense or delay of shoring. The entire modular structure can be assembled and then immediately handed over to subsequent trades faster than a typical poured-in-place concrete process. A modular block system can reduce the overall construction timeline for a treatment room by weeks or months when compared to poured-in-place concrete.

Another benefit of modular blocks is that they are easily utilized for treatment room retrofits and upgrades. They can quickly be installed in existing treatment rooms to provide supplemental shielding in places where the accessibility for poured-in-place concrete options is limited or impossible. By varying the density of the modular shield blocks, the amount of shielding installed can be adjusted to fit the space available within the treatment room being upgraded.

The modular nature of shield block systems may qualify a structure to be eligible for accelerated depreciation because it can be dismantled and reassembled at another location. Each facility is unique and will require the appropriate review by a certified accountant, but if the system is determined to qualify, this may provide a significant financial benefit over a poured-in-place concrete system. Modular systems may have a slightly higher initial cost than poured concrete systems, but the multiple benefits achieved, such as recoverable materials, reduced installation time and reduced “time to first treatment” may offset and outweigh such costs.

Hybrid/specialty applications
Due to the versatility of concrete systems, especially in modular form, there are a number of hybrid or specialty concrete shielding applications available. Some facilities have decided to combine the space savings of heavy concrete with the cost savings of regular-density concrete. One example of this is the use of heavy concrete in the primary shielding areas (with the most intense radiation) while using regular-density concrete in the secondary shielding areas (lower radiation intensity) of a linear accelerator treatment room. By using heavy concrete to reduce the primary shielding thickness (from as much as 8 feet or more down to as little as 4 feet or less), the room can have consistent wall and ceiling thicknesses throughout.

The increase in concrete density in the primary locations has the benefit of reducing the footprint for the treatment room (area and volume) and relieves the architect or interior designer from having to figure out how to utilize the areas created on either side of the “bump-outs” created in a single-density concrete design. Another flexible use of concrete in treatment room design is the use of modular shield blocks to fill an opening in a treatment room wall after a machine has been installed. A facility may have limited space available to bring a treatment machine into a shielded room.

Whether the structure is built from poured-in-place concrete or modular blocks, the machine can be brought in through a hole in a wall that can be filled with modular shield blocks after installation. This modular shielded area can easily be removed and replaced any number of times to allow for servicing or replacement of a machine in the future.

Incorporating the extremely valuable benefits of high-density concrete and pre-cast modular shield block systems provides owners, architects and builders with multiple beneficial design solutions to consider. Whether shielding for a PET/CT, HDR/brachytherapy, linear accelerator or a higher energy particle therapy system, high-density concrete and modular shield block systems deserve due consideration.

About the authors: Jim Noller is vice president, sales and marketing at Shielding Construction Solutions, Inc. Daniel Harrell is vice president, operations at Shielding Construction Solutions, Inc. Manjit Chopra is the president of Nuclear Shielding Supplies & Service.