Reassessment of Data Used in Setting Exposure Limits for Hot Particles

A critical review and a reassessment of data reviewed in NCRP Report 106 on the effects of 'hot particles' on the skin of pigs, monkeys, and humans were carried out. Our analysis of the data of Forbes and Mikhail on effects from activated UC2 particles, ranging in diameter from 144 µm to 328 µm, led to the formulation of a new model for prediction of both the threshold for acute ulceration and for ulcer diameter. A dose of 27 Gy at a depth of 1.33 mm in tissue in this model will result in an acute ulcer with a diameter determined by the radius over which this dose (at 1.33 mm depth) extends. Application of the model to the Forbes-Mikhail data yielded a 'threshold' (5% probability) of 6 x 109 beta particles from a point source on skin of mixed fission product beta particles, or about 1010 beta particles from 90Sr/90Y, since few of the 90Sr beta particles reach this depth. The data of Hopewell et al for their 1 mm 90Sr/90Y exposures were also analysed with the above model and yielded a predicted threshold of 3 x 1010 90Sr/90Y beta particles for a point source on skin. Dosimetry values were employed in this latter analysis that are 3.3 times higher than previously reported for this source. An alternative interpretation of the Forbes and Mikhail data, derived from linear plots of the data, is that the threshold depends strongly on particle size with the smaller particles yielding a much lower threshold and smaller minimum size ulcer. Additional animal exposures are planned to distinguish between the above explanations.