To provide a means for calculating doses resulting from radionuclide-contaminated wounds, the National Council on Radiation Protection and Measurements, in collaboration with the ICRP, has developed a biokinetic and dosimetric model for exposure to radionuclides from contaminated wounds (NCRP, ), The model (Fig. 14) was formulated and parameterized largely using experimental animal data due to the lack of adequate human information. The model can be used to calculate radiation doses to the wound site from deposited radionuclides, and, when coupled with an element-specific systemic biokinetic model, can also be used to calculate committed doses to organs and tissues and committed effective doses as well as to predict urinary and fecal excretion patterns for bioassay interpretation.
The NCRP Wound Model (Fig. 14) was designed to predict the biokinetic behaviour of both soluble and insoluble radioactive materials, regardless of initial physical and chemical state. To do so, five compartments were designated to describe certain physical or chemical states of the radionuclide within the wound site. These comprise Soluble (S); Colloidal and Intermediate State (CIS), Particles; Aggregates and Bound State (PABS); Trapped Particles and Aggregates (TPA); and Fragments. In some cases, the compartments contain the radionuclide in its original physico-chemical form. In others, the originally deposited material changes state and moves from one compartment to another with time. Although using five compartments to represent the wound site appears complex, in most cases the model simplifies to two or three compartments depending on the physical and chemical form of the radionuclide specified. This two- or three-compartment representation was shown to be widely consistent with the experimental data describing wound site retention (NCRP, ).
Four categories of retention were defined for radionuclides present in a wound initially in soluble form: Weak, Moderate, Strong and Avid, which refer generally to the magnitude of persistent retention in the wound. The criteria for categorization were: 1) the amount retained 1 d after deposition and 2) the rate of clearance of the remainder. In addition three other categories of deposit cover particles, colloids and fragments.
Release of radionuclide from the wound site occurs via the blood for soluble materials and lymph nodes (LN) for particulates. Further solubilisation of particles in LN also provides for radionuclide to the blood. The blood comprises the central compartment that links the wound model with the respective radioelement-specific systemic biokinetic model. Once the radionuclide reaches the blood, it behaves biokinetically as if it had been injected directly into blood, and in a soluble form. This is the same approach as is taken for the HRTM and HATM.
The presence of wounds, abrasions, burns or other pathological damage to the skin may greatly increase the ability of radioactive materials to reach subcutaneous tissues and then the blood and systemic circulation. Although much of the material deposited at a wound site may be retained at the site, and can be surgically excised, soluble (transportable) material can be transferred to the blood and hence to other parts of the body. These events occur only as a result of accidents, each event will, therefore, be unique and need to be assessed by occupational health physicists and medical staff.
To date, ICRP has not given advice on the interpretation of wound monitoring data following accidents involving radionuclides as each incident will be unique and general advice cannot be given. The biokinetic models that have been developed for various radionuclides are, however, applicable to the soluble component of any deposit in cuts or wounds that enters the blood circulation. The dose coefficients recommended by ICRP can be used in conjunction with the NCRP wound model parameter values to obtain organ and tissue doses and effective dose for radionuclides that have entered the blood from the wound site.
The application of the ICRP wound model for some real wound contamination cases has been discussed in the framework of the CONRAD project . Good fits to the urine data have been obtained for four out of six cases. However, it was not possible to obtain a good fit with default wound retention categories for the two other cases. So further investigations are required. One suggestion to solve the problem was to assume a mixture of two default retention categories, either inside the ‘soluble category’ (weak, moderate, strong and avid) or inside the ‘insoluble category’ (colloid, particles or fragment). Another option would be to vary model parameter values. However, which parameters should be varied and to what extent are questions that still need to be answered.