Understanding Tissue Sterilization, Part 2: Radiation Roundup
In Part 1, we reviewed the differences between aseptic processing and terminal sterilization. In this post, we explore two validated methods of tissue sterilization: gamma radiation and E-beam radiation.
Safety in tissue transplants is of critical importance. All potential donors are first subject to physical assessment, medical and social screening, and serological testing as the first line of defense against infectious disease transmission. To learn more about these standards of practice (SOPs), look at this post. Current Good Tissue Practice SOPs are established as recommendations by the FDA and are required as part of the accreditation process for tissue banks regardless of the processing and/or sterilization methods to be used.
Once donor tissue is accepted, it is aseptically processed and tested for intrinsic bioburden, which is the population of microorganisms existing on the tissue before sterilization. Unfortunately, there have been incidents of infections transmitted to patients by aseptically processed allografts in the past, including Clostridium species (an anaerobic and spore-forming toxin), viral hepatitis, and the HIV virus. Because of this, the Centers for Disease Control and Prevention requires the use of sterilization technologies proven to kill sporicidal organisms for a tissue to be declared sterile.2
Both gamma and E-beam radiation are delivered in quantifiable doses validated to kill or inactivate specific harmful microorganisms including bacteria, viruses, and fungi. However, there are some differences between the two methods.
Gamma radiation is the exposure of tissue to continuous gamma rays, electromagnetic radiation similar to X-rays that are extremely high frequency. The radiation is delivered to a large number of grafts at once. The dose applied to the tissue is measured in kilograys (kGy). When gamma radiation first began to be used in the late 1950s, radiation doses between 30 and 50 kGy were most often used. Clinical outcomes suffered, and early studies showed that these high doses of radiation had negative effects on tissues by altering biomechanical properties of the collagen through the breakdown of the protein.
Modernized methods are now able to validate lower dose ranges (15 to 20 kGy) to achieve terminal sterilization.1 Techniques to prevent collagen damage are typically used during radiation protocols today, including delivering radiation to grafts while they are in a lyophilized state, irradiating tissues while they are kept frozen at ultra-low temperature, or using radioprotectant anti-oxidants.
Electron beam radiation, also called E-beam radiation, is another form of radiation used to sterilize allografts. With E-beam, tissue is irradiated using a beam of accelerated (high energy) electrons. E-beam irradiation is delivered as tissues pass in front of the beam on a conveyor belt.
As with gamma radiation, early studies using high doses of radiation found unwanted negative side effects on the sterilized tissue. However, modern sterilization utilizes low doses of radiation; E-beam sterilization of allografts at doses of 9-21 kGy has been shown to achieve a sterility assurance level (SAL) of 10-6, the highest level of terminal sterilization.
E-beam is more powerful irradiation than gamma radiation, so tissues are exposed to E-beam radiation for a shorter time (on the scale of minutes instead of hours) to achieve sterilization. However, E-beam radiation does not penetrate as deeply and delivers a less uniform dose than gamma radiation. E-beam radiation is well-suited to smaller items that are less densely packed, and it allows faster turnaround of the final product. In addition, E-beam irradiation can also be used on plastic packaging, allowing for sterilization of a final manufactured product.
Both gamma and E-beam radiation are extremely safe sterilization methods; neither method results in any lingering radioactivity in the sterilized allograft. Sterilization through irradiation is considered to be a clean and efficient process. It prevents the need to use harsh chemicals that can damage the collagen in allografts, and it leaves no solvent residue on the tissue. Also, tissues sterilized by irradiation need no quarantine period after terminal sterilization.
Modern irradiation methods have minimal effects on the biomechanical properties of allografts. In fact, studies have shown that most tissues sterilized with low-dose gamma or E-beam radiation are biomechanically similar those that don’t undergo irradiation. Additionally, a substantial amount of research has helped the tissue industry determine how to mitigate negative effects of radiation.
In summary, both E-beam and gamma radiation are efficient, safe, and result in terminal sterilization of tissue with minimal negative effects on structure and function.
References
Singh, Rita; Singh, Durgeshwer; and Singh, Antaryami. (2016). Radiation sterilization of tissue allografts: A review. World Journal of Radiology 8(4): 355-369. 10.4329/wjr.v8.i4.355
