The new VARSKIN 4 photon skin dosimetry model.

A new photon skin dosimetry model, described here, was developed as the basis for the enhanced VARSKIN 4 thin tissue dosimetry code. The model employs a point-kernel method that accounts for charged particle build-up, photon attenuation and off-axis scatter. Early comparisons of the new model against Monte Carlo particle transport simulations show that VARSKIN 4 is highly accurate for very small sources on the skin surface, although accuracy at shallow depths is compromised for radiation sources that are on clothing or otherwise elevated from the skin surface. Comparison results are provided for a one-dimensional point source, a two-dimensional disc source and three-dimensional sphere, cylinder and slab sources. For very small source dimensions and sources in contact with the skin, comparisons reveal that the model is highly predictive. With larger source dimensions, air gaps or the addition of clothing between the source and skin; however, VARSKIN 4 yields over-predictions of dose by as much as a factor of 2 to 3. These cursory Monte Carlo comparisons confirm that significant accuracy improvements beyond the previous version were achieved for all geometries. Improvements were obtained while retaining the VARSKIN characteristic user convenience and rapid performance.

Assessment of plutonium exposures for an epidemiological study of US nuclear workers.

An ongoing case-control study evaluating the association between workplace external radiation exposures and leukaemia mortality required an assessment of internal plutonium exposures as a potential confounder. Of the study participants, 1,092 were employed at four Department of Energy sites where plutonium-bearing materials were processed or stored. Exposures were assessed by first categorising exposure potentials based on available bioassay data, then estimating doses for workers in the highest categories using recent recommendations of the International Commission on Radiological Protection. Given the aetiology of leukaemia, equivalent dose to active bone marrow was chosen as the exposure variable. There were 556 workers each with at least one plutonium bioassay result, assigned to one of three evaluation categories. Dose estimates were made for 115 workers resulting in a collective equivalent dose of 2.1 person-Sv for 2,822 exposure-years, compared with 29.8 person-Sv estimated from photon exposures. Modelling uncertainties were examined by comparison of results from independent analyses and by Monte Carlo simulation.