A consistent two-mutation model of bone cancer for two data sets of radium-injected beagles

Mechanistic models of bone cancer induction by radium and plutonium in animals compared to humans

Two-mutation carcinogenesis models of mice and rats injected with (239)Pu and (226)Ra have been derived extending previous modellings of beagle dogs injected with (239)Pu and (226)Ra and radium dial painters. In all cases statistically significant parameters could be derived fitting data from several research groups jointly. This also lead to similarly parametrised models for (239)Pu and (226)Ra for all species. For each data set not more than five free model parameters were needed to fit the data adequately. From the toxicity ratios of the animal models for (239)Pu and (226)Ra, together with the human model for (226)Ra, an approximate model for the exposure of humans to (239)Pu has been derived. Relative risk calculations with this approximate model are in good agreement with epidemiological findings for the plutonium-exposed Mayak workers. This promising result may indicate new possibilities for estimating risks for humans from animal experiments.

Bone cancer risk in mice exposed to 224Ra: protraction effects from promotion

This paper analyzes data for the osteosarcoma incidence in life-time experiments of (224)Ra injected mice with respect to the importance of initiating and promoting action of ionizing high LET-radiation. This was done with the biologically motivated two step clonal expansion (TSCE) model of tumor induction. Experimentally derived osteosarcoma incidence in 1,194 mice following exposure to (224)Ra with different total radiation doses and different fractionation patterns were analyzed together with incidence data from 1,710 unirradiated control animals. Effects of radiation on the initiating event and on the clonal expansion rate, i.e. on promotion were found to be necessary to explain the observed patterns with this model. The data show a distinct inverse protraction effect at high doses, whereas at lower doses this effect becomes insignificant. Such a behavior is well reproduced in the proposed model: At dose rates above 6 mGy/day a longer exposure produces higher ERR per dose, while for lower rates the reverse is the case. The TSCE model permits the deduction of several kinetic parameters of a postulated two-step bone tumorigenesis process. Mean exposure rates of 0.13 mGy/day are found to double the baseline initiation rate. At rates above 100 mGy/day, the initiation rate decreases. The clonal expansion rate is doubled at 8 mGy/day, and it levels out at rates beyond 100 mGy/day.

Two-mutation models for bone cancer due to radium, strontium and plutonium

Data from beagle experiments and radium dial painters were used to derive two-mutation carcinogenesis models for bone cancer induced by the bone-seeking radionuclides radium, strontium and plutonium. For all data, the model fits indicate that at low doses both mutation rates depend linearly and equally strongly on dose rate. For the high-LET alpha-particle emitters, a cell killing term reduces the second mutation rate at high dose rates. In all cases, the combined effect of both mutation rates is a linear-quadratic dose-effect relationship for cancer at low doses. This behavior may lead to experimental data that could be mistaken as showing a threshold below which no cancers are induced. Derived parameters such as toxicity ratios and tumor growth times compare well with values reported in the literature. Furthermore, results for plutonium indicate that rapid burial of the nuclide in the growing bones of juvenile beagles leads to a significant reduction of its toxicity, as was suggested previously. The results for radium in beagles compare well with those for humans and suggest that the models derived for strontium and plutonium in beagles may be translated to humans. The significant model parameters for the accurate animal data could then also be used to fit human epidemiological data.