Understanding Gamma Sterilisation

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 kiloGreys (kGy)

Jerold Martin

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 are not required. Most importantly, there is no residual radioactivity after irradiation.

Beyond having a different lethality mode, characterising the radiation sensitivity of the product bioburden is another key difference from moist heat (steam) sterilisation. 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 sterilisation, 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 sterilisation

Validation procedures for the sterilisation of single-use systems via gamma irradiation are well established and based on widely used industry standards. These standards are recognised by regulatory agencies globally in lieu of any specific regulatory guidance.

The international standards are harmonised 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 comprises 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, the measurement of the radiation dose. Part 2 describes 3 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 over 100 or 200 units respectively, both for initial validation and for quarterly dose lethality audits. When we consider 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 sterilising dosage based on mean bioburden levels for the product.

The VDmax method still requires at least 40 systems; 30 for bioburden testing (10 from each of three lots) and 10 units for sterility confirmation after low dose exposure. That's 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 sterilising 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 and/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 sterilisation dose for subsequent batches to return the process to a <10–6 SAL.

Addressing technical difficulties with testing of single-use systems

Because of their size and complexity, single-use systems can present some technical difficulties when assessing bioburden and sterility after irradiation. Fortunately, the standard provides some practical strategies by allowing for the grouping of similar products into families so that validation only needs to be performed with a "worst case" or representative unit. Product families are defined by common nature and source of raw materials, components and product design and size, along with assembly process, equipment and environment, etc. A single Master Product can then be identified or constructed to be the representative or "worstcase" version of all the products in the family. Sterilisation of comparable products in the family can then be rationalized as equivalent to the validated Master Product.

While this reduces the number of different systems that need to be qualified, bioburden recovery and sterility testing of large complex systems can still present formidable technical challenges. Therefore, it is common to only validate sterility of the internal product fluid contact pathway (with closures at any ports or openings) The system exterior and inner packaged space, which receives the same radiation dose, can be considered microbially controlled, but not validated as sterile.

To further ease the technical challenge of aseptic handling for bioburden and sterility testing, the Master Product can also be broken into smaller subunits, termed Sample Item Portions (SIP). These can be tested separately and have their mean bioburden and sterility results combined to establish the validated sterilising dose for the fully configured system.

Radiation steriliser — dose mapping and quarterly audit

Cobalt 60 can be stored safely in a pool of water, while the chamber above the pool is surrounded by a thick concrete barrier that prevents gamma rays from escaping when the gamma source is elevated into the irradiation chamber. Product intended for sterilisation is packaged, palletised and transported into the irradiation chamber using a conveyor.

Once a minimum sterilising dose is established for the Master Product, a pallet load configuration and density is established. Dose measuring devices called dosimeters are distributed throughout the packaged load to confirm that the minimum sterilising dose is reached throughout the batch. Because the received dose can vary based on density, materials are typically qualified to withstand up to 50 kGy. On a quarterly basis, the process is audited by, again, determining bioburden in 10 current product, Master Product or SIP samples. Verification dose sterility tests are conducted on 10 additional samples.

Sterile product documentation

The sterilisation validation and irradiator batch data support both the supplier's claim for sterility for the singleuse system(s) and the user's claim for cellculture process control and finished product sterility. Several documents serve to support both system and finished product sterile claims. First, the supplier should provide a letter explaining the rationale for the sterile claim of each specified system by part number, based on actual or Master Product validation. Suppliers can insert a Certificate of Quality within the unit packaging stating that the product is sterile after irradiation. External irradiation indicators that change colour upon exposure are not sufficiently quantitative to confirm sterility. A summary Sterilisation Validation Report supporting the minimum sterilising dose, as well as the Dose Mapping Study Report and most current Quarterly Dose Audit Report generated by the irradiator, should be provided by the system supplier. Original data and irradiator certificates, however, may only be available during supplier audit. Lastly, the supplier should provide a Certificate of Irradiation for the system lots in the batch, certifying the minimum dose recorded by the batch dosimeters.

This combination of sterilisation validation rationale, minimum dose sterilisation validation, batch load dose map, batch irradiation dose certificate and quarterly dose audit serves to support the ongoing sterility of the single use system and the sterile products produced with them. For more information on this subject, a good resource is available free of charge in the form of the BPSA's Guide to Gamma Irradiation and Sterilization (3).

References

1. AAMI/ANSI/ISO 11137:2006, "Sterilization of health care products — Radiation — Part 1: Requirements for the development, validation and routine control of a sterilization process for medical products; Part 2: Establishing the sterilization dose; Part 3: Guidance on dosimetric aspects"(2006).

2. American Association of Medical Instrumentation, "Sterilization of health care products—Radiation—Substantiation of a sterilization dose—Method VDmax", Technical Information Report TIR33:2005 (2005).

3. BPSA, "Guide to Irradiation and Sterilization of Single-use Systems" (2008).