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X-Ray Sterilisation


Test results from a new X-ray sterilisation facility in Europe confirm that X-ray is a good, practical alternative to traditional irradiation methods for medical device sterilisation, reports Jean-Louis Bol of IBA Industrial.

The latest Rhodotron TT1000 with power up to  700 kW, when previous accelerators were limited to 150 kW makes X-ray sterilisation available as an industrial processing technology

X-ray sterilisation of medical products has been studied theoretically and experimentally by the US National Bureau of Standards, universities and accelerator manufacturers since it was first introduced more than 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. The recent availability of high-power, high-energy accelerators is quickly changing the landscape.

X-ray is an available industrial solution
Modern industrial accelerators have increased the throughput rates in X-ray processing facilities so that this irradiation method is now competitive and more economical than medium and large cobalt-60 facilities. Today, there are sterilisation facilities with X-ray capability in Europe, Japan and North America.

High-energy X-rays are suitable for sterilisation processes in which the materials and products are too thick to be penetrated by electron beams (e-beams). The radiation processing of materials and commercial products with high-energy X-rays can produce beneficial changes that are similar to those obtained by irradiation with gamma rays emitted by cobalt-60 sources. The chemical and biological effects of X-ray and gamma-ray processing are similar because both types of energy can produce ions and free radicals in irradiated materials. The practical differences are attributable to physical characteristics such as angular distribution, penetration, dose uniformity and dose rate.

Effect of angle on penetration
A significant difference between X-ray and gamma rays is the angular distribution of the radiation. Nuclear gamma rays are emitted in all directions. High-energy X-ray photons are concentrated in the direction of the product to be irradiated. The narrow angular distribution of X-rays increases penetration in materials because the most intense zone of the emitted radiation is perpendicular to the surface of the irradiated products. In contrast, the nearly isotropic radiation in an industrial gamma facility has a wide angular distribution. Consequently, much of the gamma-ray emission is more divergent than the emission from a high-energy X-ray and enters the products at larger angles from the perpendicular direction.

These X-ray penetration properties explain partially why products sterilised with X-ray have a better dose uniformity ratio (DUR) compared with gamma processing. The other reason is the wider energy spectrum of X-ray when generated by accelerated electrons at energies higher than 5 MeV. X-ray sterilisation systems irradiate full pallet loads by moving them continuously through the X-ray beam. The loads are irradiated from the side as they pass in front of a long, vertically oriented target. The loads are irradiated on opposite sides at high and low elevations to obtain a nearly uniform dose in the vertical direction. A virtual animation of an X-ray irradiation process is available at

Putting X-ray to the test
Irradiation tests performed in a new X-ray facility in Europe used a full pallet load measuring 100 x 120 x 180 cm with a homogeneous density of 0.15 g/cm3. Alanine dosimeters were used to obtain precise dose measurements. Dosimeters were placed on a horizontal grid at various heights inside the load to identify the maximal and minimal dose locations. Then the load was irradiated by multiple pass cycles, each cycle consisting of four passes in front of the X-ray target (both pallet sides, top and bottom).

Several irradiation tests were performed to identify the best irradiation parameters. After optimisation, DUR values between 1.2 and 1.3 were measured when processing full pallets of homogeneous products with densities similar to medical devices. This is better than the typical DUR value of 1.45 obtained for the same pallet irradiated with a cobalt-60 irradiator, and it represents an excellent result for X-ray technology.

The superior DUR with X-ray allows medical devices to be treated on pallets, when previously they could only be treated with gamma tote sterilisation. The gamma tote process is labour intensive and not efficient because of the difficulty of achieving fully loaded totes.

eXelis sterilisation, a typical X-ray pallet processing facility allows mixing of different products with different sterilisation requirements during the same irradiation cycle

X-ray finds its role
The irradiation tests performed in the latest industrial X-ray facility confirm the performance of X-ray sterilisation when processing pallet loads of low-density materials. X-ray sterilisation can treat the same product range as gamma and is the only irradiation technology able to reach DUR values of between 1.2 and 1.3 when sterilising pallets. We believe that irradiation sterilisation will slowly migrate towards X-ray. Drivers for this migration will be market demand for better sterilisation quality and stronger hurdles related to cobalt-60 such as continual price increases, more regulations linked to the transport of radioisotopes and limited availability of cobalt-60 in the future.

Jean-Louis Bol, Vice President Sales ‚€®IBA Industrial,
tel. +32 10 475 825, ‚€®
e-mail: jean-louis.bol@iba-group.com ‚€®

Irradiation Technologies

  • E-beam sterilisationis a highly effective and low cost treatment.When products can accept low penetration, e-beam is the preferred technology.
  • Gamma treatment offers a better DUR than e-beam, but optimum DUR is reached with tote processing; tote processing introduces production inefficiencies estimated at 10% due to partial tote loading.
  • X-ray offers the best DUR even when treating full pallets. Today, products can be sterilised on their original pallet with excellent treatment quality making X-ray sterilisation the preferred technology for sterilising products on pallets.

Editors Note:
These results of the first commercial operation of X-ray sterilisation in Europe present a challenge to gamma and e-beam operators and suppliers. We welcome their views on the opinions expressed in this article,
e-mail: editorial@med-techinnovation.com


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