Ionising Radiation Sterilization

Sterilisation by this method is achieved by exposure of the product to ionizing radiation in the form of gamma radiation from a suitable radioisotopic source (such as cobalt 60 or cesium 137) or of a beam of electrons energized by a suitable electron accelerator. The advantages of the method include low chemical reactivity, low measurable residues, and the fact that there are fewer variables to control. The unique feature of the method is that the basis of control is essentially that of the absorbed radiation dose, which can be precisely measured. Irradiation causes only a minimal rise in temperature but can affect certain grades of plastics and glass.

The two types of ionizing radiation in use are radioisotope decay (gamma radiation) and electron-beam radiation. In either case, the radiation dose to yield the required degree of sterility assurance should be established such that within the range of minimum and maximum doses set, the properties of the article being sterilized are acceptable.

For gamma radiation, the validation of a procedure should include the establishment of article materials compatibility, establishment of product loading pattern and completion of dose mapping in the sterilization container, establishment of timer setting, and demonstration of the delivery of the required sterilization dose. For electron-beam irradiation, in addition, the on-line control of voltage, current, conveyor speed, and electron beam scan dimension must be validated.

For gamma radiation, the reference absorbed dose is 25 kGy (2.5 megarads). Other doses may be used provided that it has been satisfactorily demonstrated that the dose chosen delivers an adequate and reproducible level of lethality when the process is operated routinely within the established tolerances. The procedures and precautions employed should be such as to give a SAL of 10^6 or better.

In order to validate the efficacy, particularly of the lower exposure levels, it is necessary to determine the magnitude of the natural radiation resistance of the microbial population of the product. Specific product loading patterns must be established and absorbed minimum and maximum dosage distribution must be determined by the use of chemical dosimeters. These dosimeters should be calibrated against a standard source at a reference radiation plant. The setting of the preferred absorbed dose should be carried out using a suitable biological indicator.