Feature Article

By: P. Dethier, J.-L. Bol and B. Mullier
IBA Industrial, Louvain-la-Neuve, Belgium

Almost half of the medical devices that are sterilised today are treated with a traditional irradiation technology using gamma radiation produced by cobalt-60 radioactive sources. The gamma industry is confronted on a daily basis with issues such as cobalt-60 price fluctuations, availability, increasing regulations on transportation and difficulty related to the disposal of radioactive sources. These issues are causing medical device manufacturers to evaluate alternative sterilisation technologies such as X-ray.

 

X-ray sterilisation of medical products has been studied theoretically and experimentally, in detail, by the United States National Institute of Standards and Technology (www.nist.gov), universities and accelerator manufacturers since it was first proposed over 40 years ago. Commercial use began approximately 15 years ago, but full commercial adoption has been slow because of the low output power of early accelerators. Today’s availability of high-power accelerators is rapidly changing the landscape. There are sterilisation facilities with X-ray capacity in Europe, Japan and North America and high volume sterilisation of medical devices with X-rays will begin in Switzerland in 2010.

 

Most of the requirements of ISO 11137-1:2006, Sterilisation of Healthcare Products, Radiation¹ for gamma and X-ray irradiation are similar. Only the energy threshold may require more validation.

High-energy X-rays (bremsstrahlung) are high frequency, short-wave length electromagnetic photons. They are emitted when high-energy electrons are stopped by a material that has a high atomic number. The efficiency for X-ray emission increases with the electron energy and the atomic number of the target material. Thin sheets of tantalum are used for the long targets that are needed to irradiate tall pallet loads of product. The X-ray energy spectrum is broad; the maximum photon energy is the same as the kinetic energy of the incident electrons. With X-ray energies of 5 MeV and 7 MeV, product penetration is greater than that provided by gamma rays from an uncollimated cobalt-60 source.

 

High-energy X-rays are ideal for sterilising large packages and pallet loads of medical devices. In contrast to gamma rays, which are emitted in all directions from a cobalt-60 source, high-energy X-rays are concentrated in the direction of the incident electron beam, and their angular dispersion decreases as the electron energy increases. The high intensity in the forward direction enhances the efficiency of X-ray utilisation and allows a reduction in the size of the irradiation room. Only a few product carriers are in the treatment room at one time and treatment time per carrier is reduced.

The directional concentration and high penetration capability of X-rays also allows pallet loads of low-density medical products to be treated with excellent dose uniformity. To obtain good dose uniformity and efficient X-ray utilisation, product loads must be irradiated from both sides.

 

The optimum thickness for two-sided treatment increases as the package density decreases. With 5 MeV X-rays, the optimum thickness for a good dose uniformity ratio-throughput balance is approximately 2.6 m (for an average density of 0.1 g/cm³).

The directional concentration property of X-rays makes them ideal for medical product sterilisation on pallets (Figure 2). Sterilising on pallets eliminates the costly process of de-palletisation and re-palletisation. Less handling significantly reduces the opportunity for product damage and streamlines product tracking and quality assurance processes.

 

X-rays are equivalent to gamma rays; in both cases, the nature of radiation flux is photons. Therefore, there is no need to adapt the type of material or the packaging when migrating from gamma to X-ray sterilisation

Three major issues are driving industry to look for alternatives to gamma sterilisation: cobalt prices, its future availability and the increasing number of regulations concerning the transportation of cobalt-60, which are also increasing the cost of gamma sterilisation.

 

Cobalt producing reactors are becoming old and will require expensive upgrades in the coming years. This not only includes the cobalt producing reactors in Russia, but also the major cobalt reducing reactors in Canada. Many industry participants are forecasting significant increases in cobalt price in the next decade. Availability is also likely to be an increasing issue because reactor shutdowns for re-tooling are lengthy and little new cobalt production capacity is foreseen.

 

The global regulatory environment regarding the transportation of cobalt-60 is currently difficult and regulations are increasing every year. Moving industrial quantities of cobalt 60 by air is no longer allowed and sea freight is becoming increasingly difficult. Few carriers accept radioactive products and many ports globally refuse ships carrying cobalt. Cobalt sea freight shipments are commonly late and can be delayed for periods up to six months, because of the current regulatory environment. It is anticipated that the future will only become more challenging.

 

Electricity seems to be the answer to the issues that isotopes face. There is also the advantage that radiation can be turned off and on as required.

Financial models considering the direct and indirect costs of operating a sterilisation centre demonstrate that the cost to process a pallet of medical products with X-rays is more economical than gamma-equivalent capacities from 2 MCi. The higher the throughput, the more attractive X-ray sterilisation becomes. The accepted rule is that the electrical costs of an X-ray sterilisation facility are equivalent to the cobalt-60 refill costs in a gamma centre.

X-rays and gamma rays are both photons. They lose their energy in matter in the same manner and have a good penetration power. However, their different production processes lead to different emission characteristics:

Therefore, the X-ray and gamma ray sources are different and the dose rates in the product will also be different. The dose rate is the amount of radiation given per unit of time such as kGy/min. Simulations based on X-ray and gamma medical device sterilisation facility modelling show a dose rate two times higher for X-rays compared with gamma rays.

When deciding on a sterilisation technology one of the first considerations is product compatibility. X-ray and gamma are very similar in terms of their effects on medical plastics. Product re-design and/or material changes when switching from gamma to X-ray sterilisation will not be necessary.

 

X-ray sterilisation will improve treatment quality, the process will be faster, the long-term economical model is favourable, and there are no issues related to availability, transportation and disposal of radioactive sources.

Leoni-Studer (www.studerhard.ch) will be the first industrial player to provide large volume X-ray sterilisation services. Its new facility in Daniken, Switzerland will be operational in 2010. 

Benoit Mullier is Vice President Facility and Site Engineering
at IBA Industrial, Chemin du Cyclotron 3, B-1348 Louvain-la-Neuve, Belgium,
tel. +32 10 201 249,
e-mail: philippe.dethier@iba-group.com
www.xray-sterilization.com