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    ModeSAL

    The computer technology for simulation of practical tasks in radiation sterilization process in accordance with Method 1, Standard ISO 11137 and IAEA Code of Practice for the Radiation Sterilization of Tissue Allografts was developed. The technology was realized on the base of the RT-Office modules for modeling by Monte Carlo method of an absorbed dose in irradiated heterogeneous targets with electron beam (EB), X-ray and gamma-ray and specialized software ModeSAL. The software ModeSAL is intended for calculation and comparative analysis of a sterility assurance level (SAL) and a sterilizing dose for bioburden to achieve a required SAL in an irradiated product.
    The mathematical approach of the technology is based on the detailed and precise consideration of a self-consistent physical and geometrical models of calculation an EB, X-ray and gamma-ray dose maps in an irradiated product, a SAL, a sterilizing dose and the spatial and time uncertainties for dose provided to an irradiated product.

    The features of mathematical model of the software ModeSAL are the following:

    • generalized stochastic model of radiation sterilization process is developed.
    • basic stochastic parameters proper in the modern radiation technologies are taken into account.
    • stochastic parameters refer to: the bioburden in a product and in its separate parts; a spatial distribution of microbes in a product; a spatial dose distribution in an irradiated product; uncertainties because of nonstability of radiation facility parameters in time of irradiation; operational characteristics of radiation facility required to achieve a given SAL.

    The feature of the theoretical approach at development of the generalized stochastic model of radiation sterilization process is introduction of the scale factor (SF). SF is the basic characteristic of change of an irradiation process for the analysis of dose map uncertainties effect. Analysis and solution of problem tasks in radiation sterilization technology are performed as follows:

    • RT-Office modules for specific EB, X-ray and gamma-ray radiation facility and irradiated products optimize a thickness of irradiated products, a configuration of package for the product, the method of irradiation as one- or two-sided types, the choice of irradiation regimes.
    • RT-Office modules simulate a profile of an absorbed dose map in an irradiated product for optimal irradiation regimes.
    • simulation prediction for the dose map should be periodically verified in special dosimetric experiments.
    • output results for EB, X-ray and gamma-ray processing simulated with the RT-Office modules for an absorbed dose in forms of probability distribution and space distribution (dose map) are used directly as input data for the software ModeSAL;
    • software ModeSAL determines the sterilization dose and scale factor for parameters of radiation facility to achieve a required SAL for microbial population on/in an irradiated product in accordance with Method 1 ISO 11137 or IAEA Code. The software ModeSAL decides as well known and established tasks, so the specific practical tasks in radiation sterilization technology.
    • RT-Office modules optimize the operational parameters of radiation facility on the base of scale factor data.

    Such execution sequence of work provides a computer technology for sterilization process in accordance with Method 1 ISO 11137 and IAEA Code with regard to the spatial and time uncertainties for dose delivery to irradiated product. Some practical tasks which can be decided with help of the ModeSAL code are as follows:
    1. Calculation the sterilized dose required to achieve a given SAL for an arbitrary values of average bioburden having standard distribution of resistances (SDR) on the base of Dmin accordingly Method 1 ISO 11137 and IAEA Code. (Dmin is called as a sterilization dose). It means that:

    • for all values average bioburden from the Tables of ISO 11137 and IAEA Code having SDR, the data calculated by ModeSAL and the Tables data are in an absolute agreement;
    • for an arbitrary values of average bioburden having SDR and which are absent in the Table, the Software ModeSAL calculates exactly the value of sterilized dose required to achieve a given SAL. I.e. in this case, user can use these data without interpolation of the ISO 11137 Table 5 data.

    2. Calculation the sterilized dose required to achieve a given SAL for an arbitrary values of average bioburden having any distribution of resistances on the base of Dmin accordingly Method 1 ISO 11137 and IAEA Code. It is useful in case, when user works with known dangerous viruses or known microbial populations which are differed from SDR, for example, in case of bioterrorism (anthrax, birds influenza, etc.). User can take the value of microbial resistances for dangerous viruses, for example from Database on Microbial Resistance to Radiation, and enter as input data to the Software ModeSAL. With help of the Software ModeSAL user can made PC experiment to estimate the necessary absorbed dose and radiation facility parameters to eliminate the known dangerous viruses or known microbial populations.

    3. Calculation the sterilized dose required to achieve a given SAL for an arbitrary values of average bioburden having any distribution of resistances on the base of Dmin accordingly Method 1 ISO 11137 and IAEA Code with taking into account:

    • uncertainty effect of absorbed dose and of parameters of process irradiation;
    • uncertainty effect in a composition and in radiation resistances of microbial population.

    4. Calculation the sterilized dose required to achieve a given SAL for above points 1, 2, and 3 with regard to the dose distribution in an irradiated product.

    • comparison of calculation results on the base of Dmin accordingly Method 1 ISO 11137 and IAEA Code with taking into account the dose distribution in an irradiated product allows to estimate the safety factors for mathematical models which are used in the Standard ISO 11137 and in the Software ModeSAL.

    The ModeSAL code is a scientific tool which is accessible for broad audience of users, such as microbiologists, dosimetrists, physicists, and other specialists working with radiation sterilization tasks.

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