In the mid-1950s, the rising demand for sterile single-use medical devices paved the way for the expansion of radiation processing and specifically gamma sterilization using cobalt-60. Over the ensuing decades, the industry has not only grown in size, but also in its diversity and sophistication. Gamma irradiation is a very widespread application of nuclear technology for peaceful purposes, and positively impacts the lives of millions of people on a daily basis.
The United States currently produces 50% of the medical devices manufactured globally. These products, including syringes, catheters, surgical gloves, bandages, drapes, gowns and hundred of others must meet strict FDA regulations for sterility, to protect against patient infection. More than 40% of these products, equating to 200 million cubic feet per year in the U.S. alone, are sterilized by gamma irradiation.
Cobalt-60 is also used as a radiation source in external beam therapy equipment. An estimated 15 million cancer treatments are carried out each year in hospitals and clinics in over 80 countries using these machines, which enable reliable and affordable care for patients. As well the Leksell Gamma Knife, using tiny cobalt-60 sources, is the premier non-invasive, radiosurgery method for treating brain disorders.
In addition to these important healthcare applications, cobalt-60 is also used for materials modification processes such as the cross-linking of large plastic automotive parts that are strong and flame-retardant.
Cobalt-60 plays an important role in the scientific community as well, from promising new stem cell research to the design and testing of components for the aerospace and nuclear energy industries. Other beneficial uses include food preservation, decontamination of packaging materials, sanitization of cosmetics, microbial reduction in pharmaceuticals and quarantine application of consumer products.
The worldwide installed base of cobalt-60 is approximately 260 million curies, contained in more than 160 large-scale gamma facilities. The United States has just over 50% of the total cobalt-60 installed base contained in 51 commercial irradiation facilities.
Overall sterilization of single-use medical device sterilization accounts for approximately 80% of the volume of product commercially processed with gamma radiation. This underscores the important interdependent relationship between the gamma processing and medical device manufacturing industries.
Cobalt-60 is type of ionizing radiation. Gamma rays from a source penetrate the targeted material and knock electrons from orbit. For sterilization applications this has the effect of disrupting DNA, which renders any microorganisms present nonviable. For materials modification, the mechanism causes crosslinking of polymer chains, creating stronger molecular bonds or chain-scission for shorter polymer chains.
Gamma irradiation is known as a “cold process” as the temperature of the processed product does not significantly increase. Because of its excellent penetrating power, cobalt-60 enables treatment of products in their final hermetically sealed packaging, ensuring sterility until the product is removed from its package and put into use. It is ideal for complex devices with long lumens, sealed cavities and mated surfaces.
The most popular alternative technology, ethylene oxide (EO) gas is used for sterilizing and decontaminating many products. It is a very strong alkylating agent that stops cell reproduction and kills harmful microorganisms. Manufacturers of medical devices are using ethylene oxide (EO) gas to sterilize many products that can withstand heat, pressure and moisture conditions of the sterilization process and is permeable to the gas.
Accelerators are machines used to generate electrons (e-beam) and x-rays, which are also ionizing radiation, used to sterilize medical devices. E-beam is characterized by low penetration and high dose rates that performs best with light-density and uniformly simple, single packaged products. High energy x-rays, although expensive to produce, have similar penetrating power and slow dose rate comparable to gamma radiation.
Manufacturers of medical devices fully understand the different sterilization options and select the best method for their products and business. All of these technologies have been available and used for many years. Switching methods can be extremely costly; human resources intensive, wasteful and lead to delays in bring the product to market. In many cases because of product design and construction, regulatory approvals for the medical device, proximity to sterilization facilities and capital investment-- it is simply not feasible to change sterilization methods.
Placing non-radioactive, refined Cobalt-59 slugs or pellets into a nuclear reactor creates deliberately produced cobalt-60. Over time cobalt-59 absorbs a neutron to become cobalt-60. After removal from the reactor the cobalt-60 is double encapsulated in stainless steel sealed sources. The radioactivity or “energy level” is normally measured in curies per gram. Each source has a unique serial number and certified according to prescribed international standards.
Cobalt-60 is naturally unstable, therefore emits two gamma rays at energies of 1.17 MeV and 1.33 MeV, and decays at a rate of approximately 1% per month back to stable, non-radioactive nickel-60. It is these gamma rays (similar to UV but much more penetrating) that provide the energy to accomplish the desired objective, like sterilizing a medical device. It does not make the product radioactive.
Safety and security
Cobalt-60 usage has always been and continues to be highly regulated at the international, national, regional and local. The international irradiation industry prides itself on an impeccable safety and security record. The industry works closely with regulators to realize continuous improvement in all aspect of the safe manufacture, transportation, use and disposal of cobalt-60 sources. During the 50-year history of the industry, over 800 million curies of cobalt-60 has been safely and securely shipped to gamma processing sites more than 50 countries.
Cobalt-60 makes an important contribution to the health and well being of people, as well as providing significant economic benefit to many industries around the world. With concerns around the rising cost of everything from healthcare to industrial materials, safe, reliable and cost-effective technologies such as those employing cobalt-60 are critical to the success and social advancement of economies around the world.
2006, January 31
International Irradiation Association ( www.iiaglobal.org)