WHAT IS GAMMA IRRADIATION?
Gamma rays are a form of electromagnetic radiation—like x-rays, but with higher energy. The primary industrial sources of gamma rays are radionuclide elements such as Cobalt 60, which emit gamma rays during radioactive decay. Gamma rays pass readily through plastics and kill bacteria by breaking the covalent bonds of bacterial DNA. They are measured in units called kiloGrays (kGy).Gamma irradiation provides a number of benefits in cost and sterility assurance. It can be applied under safe, well-defined, and controlled operating parameters, and is not a heat- or moisturegenerating process. Consequently, there is no heat stress and condensate drainage or outgassing is not required. Most importantly, there is no residual radioactivity after irradiation.
Beyond having a different lethality mode, characterizing the radiation sensitivity of the product bioburden is another key difference from moist heat (i.e., steam) sterilization. Radiationresistant biological indicators are not used. After the mean bioburden is quantified and sensitivity to a low radiation dose (~8–10 kGy) is established, a statistically determined higher dose (typically >25 kGy) can be applied to provide the appropriate sterility assurance safety margin for every unit in the batch. This safety margin is similar to that of moist heat sterilization, where a target of <10–6 probability of a non-sterile unit (Sterility Assurance Level, SAL) is established.
A third difference is that the gamma dosage can be measured in each batch using detectors called dosimeters, which enable parametric release. Product batches subjected to gamma radiation do not need to be lotsample sterility tested for release.
STANDARDS FOR VALIDATION OF GAMMA STERILIZATION
Validation procedures for the sterilization of single-use systems using gamma irradiation are well established and based on widely used industry standards. These standards are recognized by regulatory agencies globally in lieu of any specific regulatory guidance.
The international standards are harmonized among three official standards bodies: the American National Standards Institute (ANSI), the American Association of Medical Instrumentation (AAMI), and the International Standards Organization (ISO). Their common document is ANSI/AAMI/ISO 11137, Sterilization of Health Care Products — Radiation (1).
ANSI/AAMI/ISO 11137 is comprised of three parts: Part 1 covers requirements for development, validation, and routine control of a sterilization process; Part 2 covers establishing the sterilization dose; and Part 3 provides guidance on dosimetric aspects, i.e., the measurement of the radiation dose. Part 2 describes three methods for establishing a sterilizing dose with SAL <10-6 . Methods 1 and 2 were designed with small medical devices in mind and involve determination of bioburden and multiple dose analyses that require more than 100 or 200 units respectively, both for initial validation and for quarterly dose-lethality audits. For large single-use systems, which are made in relatively small batches, both of these methods can be very costly and time consuming. However, the standard provides a third method called VDmax (VD stands for verification dose). Rather than determining the minimum dose to achieve a SAL of <10–6, the VDmax method substantiates the suitability of a predetermined dosage level, specifically 25 kGy or, for plastic devices with lower gamma tolerance, 15 kGy.
In conjunction with the publication of the VDmax method for doses of 25 or 15 kGy, additional doses were qualified and published by AAMI in their Technical Information Report 33:2005 (2). This is considered a supplement to ANSI/AAMI/ISO 11137 and they will likely be merged at the next scheduled revision. It expands the VDmax method to seven additional dosages; 17.5, 20, 22.5, 27.5, 30, 32.5 or 35 kGy, enabling flexibility of minimum sterilizing dosage based on mean bioburden levels for the product.
The VDmax method still requires at least 40 systems for sterilization validation; 30 for bioburden testing (10 from each of three lots) and 10 units for sterility confirmation after low-dose exposure. That's still a lot of systems and a primary reason to consider simply irradiating at >25 kGy and claiming microbial control wherever a validated sterile claim is not required.
Once the mean bioburden and minimum validated sterilizing dose is established, and the product goes into production with a sterile claim, quarterly dose audits are conducted to confirm that the levels of bioburden or their sensitivity to gamma irradiation have not changed over time. These quarterly dose audits require an additional 20 units from a current lot each time: 10 for mean bioburden analysis and 10 for irradiation at the low verification dose and sterility testing. If any of the irradiated units are found to be nonsterile, the test must be repeated at a higher verification dose, which will then qualify a new, higher production sterilization dose for subsequent batches to return the process to a <10–6 SAL.