A generic model for estimating the risk of deterministic effects of partial organ irradiation by hot particles

A generic (Weibull-type) model is presented for evaluating the risk of a specified deterministic effect of hot-particle (i.e., highly radioactive particle) exposure of the skin, eye, ear, respiratory tract, or gastrointestinal tract. The generic model is a composite of the two-parameter Weibull function used for evaluating the risk of deterministic effects of total-organ irradiation and an isoeffect equation for partial-volume (i.e., fractional-volume) irradiation. Isoeffect refers to a constant level of risk rather than a constant level of severity. The generic model provides a way to evaluate the uniform-exposure, isoeffect equivalent dose, H'T,V, to a partial volume V that incurs the same risk as is associated with a nonuniform exposure of volumes within V. When V = 1, the volume is called a reference volume. The reference volume can be associated with a specific organ or mass of tissue. The reference-volume-specific, uniform-exposure, isoeffect equivalent dose, H'T,1, can be used to limit the risk of deterministic effects of hot-particle exposure by requiring that H'T,1 remain below the equivalent dose limit for any reference volume of tissue at risk. Local doses H'T,V, can be converted to isoeffect doses H'T,1 by multiplying H'T,V by a volume weighting factor Wv, where Wv equals Vn. For deterministic effects, n < 1. For stochastic effects, n = 1. For stochastic effects, H'T,1 can be converted to an effective dose by multiplying H'T,1 by an appropriate tissue-weighting factor WT. Results presented apply to circumstances where, at most, only a few hot particles are involved. In such circumstances, it is highly unlikely that local tissue will be irradiated by more than one hot particle. Although the focus of this paper is on hot-particle effects, results presented also apply to partial-organ or partial-tissue irradiation by any radiation source including external beams (e.g., electrons, neutrons, protons, heavy ions, and X-ray and gamma-ray photons).