VALIDATION OF RADIATION STERILIZATION PROCESS

The major objective in validating a radiation sterilization process, regardless of
whether the mode of radiation is cobalt-60, cesium-137, or electron beam, is to
determine the D value of the indicator micro-organism used to monitor the process.
With radiation sterilization, the D value is defined as the dose of radiation in
Mrads or kilograys* necessary to produce a 90% reduction in the number of indicator
microbial cells. The D value depends on such factors as temperature, moisture,
organism species, oxygen tension, and the chemical environment and/or phys-
ical surface on which the indicator microorganism is present.If a probability of nonsterility of 10−6 is specified for a system sterilized by radiation and the D value of B. pumilus in that system is
0.20 Mrad, a radiation dose of 1.2 Mrads would produce a 6-log reduction in
the concentration of B. pumilus spores. Greater probability allowances (e.g.,
10−3) would permit lower radiation doses.*
The development of radiation sterilization cycles follows requirements of
the Association for the Advancement of Medical Instrumentation (AAMI) [38].
1. Determine microbial load on preirradiated products.
2. Determine the D value for natural flora on the product.
3. Determine the D value using biological indicators on the product to
make certain that the natural flora are not more radioresistant than the
biological indicator.
4. Determine the D value of biological indicator spore strips placed
within the product. Determine the location of the lowest radiation
dose point within the product. Then determine the dosage required for
a 10−6 probability of nonsterility for the product.
5. Determine whether or not the D value for the biological indicator
varies as a function of the dose rate. With cobalt-60, dose rate differences
are not of much concern (variance of 0.1–0.5 Mrad/hr), whereas
electron beam sterilization might produce dose rate variances of several
Mrads per min!
The microbiological studies above are conducted to establish the appropriate
dose level to be used to sterilize each specific product or commodity to an acceptable
level of statistical nonsterility. These studies should be conducted following
qualification of the irradiation facility. The Health Industry Manufacturers Associaton
(HIMA) [39] has suggested major items to be included in the qualification
phase of the validation scheme for radiation sterilization installation.
1. Specifications of the irradiator equipment—description, materials used,
instrumentation, etc.
2. Drawings of the equipment and the entire facility
3. Licensing agreement and supporting documentation from both the
Atomic Energy Commission and the appropriate state
4. Reliability and calibration of the dosimeter system
5. Radiation source strength when the sterilization cycle is validated
through D value determination
6. Speed of conveyor belt
7. Dose rate If it is assumed that the radiation sterilizer equipment and facilities have
been qualified and microbiological studies have been conducted as previously
outlined, the next step in the validation process is the complete evaluation of
the radiation sterilization cycle. Tests are conducted to determine the effect of
minimum and maximum product density on the ability of the minimum or nominal
radiation dose—determined during the microbiological studies to produce a
given log reduction in the biological indicator population—to sterilize the load.
For example, it was found that a 0.2-Mrad dose of cobalt-60 will produce a 1-log
reduction in the population of B. pumilus. The microbial load of a one-package
polyvinyl chloride (PVC) device (intravenous administration site) was estimated
to be approximately 1000. A probability of a nonsterility level of 10−6 is desired,
therefore theoretically, the minimum dose necessary to produce a 9-log reduction
in the microbial population is 1.8 Mrad.
Validation tests must be conducted in such a manner that the following
questions are answered:
1. Is the nominal radiation dose sufficient to destroy B. pumilus spore
samples at a relatively high concentration (e.g., 108 spores per ml or
per strip) using a minimum load of product (minimum density)?
2. Is the nominal radiation dose sufficient to destroy B. pumilus spore
samples at a relatively high concentration (e.g., 108 spores per ml or
per strip) using a maximum load of product (maximum density)?
3. What is the radiation sterilization efficiency; that is, how much of the
applied dose is actually absorbed by the product?
4. What is the isodose profile for each irradiated item; that is, what is
the dose of radiation absorbed as a function of the location within the
product being irradiated? What is the ratio between the highest and
lowest doses absorbed within the product?
5. What is the effect of conveyor loading conditions and line speeds on
the amount of radiation absorbed?
As these questions are answered, adjustments probably will be made in
the process. For example, it might be concluded that a higher radiation dose is
required for adequate exposure to all points of a particularly large and/or dense
container system. The loading size or pattern may have to be reduced to permit
adequate sterilization at a given dose level. Once all process parameters have
been defined through preliminary testing, the tedious but essential task of proving
consistency, repeatability, and reliability of the radiation sterilization cycle
must be established. Test records, data work sheets, and monitoring systems
schedules must be kept and organized for easy retrieval and analysis.
While radiation sterilization cycles are validated based upon the achievement
of sterility, many other factors must be considered in the utilization and
approval of the radiation sterilization process. Such factors include the effect of irradiation on (1) the physical appearance of the container system and its contents,
(2) stability of the active ingredient, if present, and (3) safety of the irradiated
material.