Interaction of smoking and radon in rats: a biologically based mechanistic model.

Data on rats exposed to cigarette smoke before or after exposure to radon are used to estimate smoke-dependent parameters of the biologically based two-stage clonal expansion model. The baseline parameters and the action of radon acting on initiation and promotion were fixed based on earlier work. Cigarette smoke acting on transformation and inducing a reduction of the radon dose to the target cells after a smoking period gives an acceptable description of the data.

Age-adjustment in experimental animal data and its application to lung cancer in radon-exposed rats.

Procedures for age-adjustment of cancer fractions are proposed which do not require fixed age intervals. The full available information on survival times can then be used, which is especially important in small treatment groups. For incidental cancers a non-decreasing prevalence function and for fatal cancers the Kaplan-Meier estimator is used. In the latter case, the estimated competing risk of the control population is standardized, not its true survival. This makes the technique also applicable to treatment groups with high incidence, which otherwise may give adjusted rates above 100%. In the application part these age-adjustment techniques are used here to study lung cancer in radon-exposed Wistar and Sprague-Dawley rats. The data include a classification in fatal and incidental lung cancers. For fatal lung cancer, the lifetime excess absolute risk (LEAR) at 1 WLM averaged over all exposed groups is 0.67x10(-4) for the Wistar rats, while for the Sprague-Dawley rats it is 0.40x10(-4). For the Sprague-Dawley rats, there are several groups exposed later in life. When the averaging is restricted to animals with start of exposure prior to 150 days of age, the weighted average risk among the Sprague-Dawley rats is 0.79x10(-4). Compared to groups with similar exposures as young adults (up to about 150 days), animals exposed later in life have substantially lower lifetime risks. The Wistar rats include groups with roughly equal exposure rates and ages at start of exposure, but with increasing exposure duration. Within these groupings the LEAR at 1 WLM does not decrease with additional exposure at higher age, as would be expected if the risk from exposures at different ages would be additive.

Lung tumour risk in radon-exposed rats from different experiments: comparative analysis with biologically based models.

Data sets of radon-exposed male rats from Wistar and Sprague-Dawley strains have been investigated with two different versions of the two-step clonal expansion (TSCE) model of carcinogenesis. These so-called initiation-promotion (IP) and initiation-transformation (IT) models are named after the cell-based processes that are assumed to be induced by radiation. The analysis was done with all malignant lung tumours taken to be incidental and with fatal tumours alone. For all tumours treated as incidental, both models could explain the tumour incidence data equally well. Owing to its better fit, only the IP model was applied in the analysis of fatal tumours that carry additional information on the time when they cause death. A statistical test rejected the hypothesis that a joint cohort of Wistar and Sprague-Dawley rats can be described with the same set of model parameters. Thus, the risk analysis has been carried out for the Wistar rats and the Sprague-Dawley rats separately and has been restricted to fatal tumours alone because of their similar effect in humans. Using a refined technique of age-adjustment, the lifetime excess absolute risk has been standardised with the survival function from competing risks in the control population. The age-adjusted excess risks for both strains of rats were of similar size, for animals with first exposure later in life they decreased markedly. For high cumulative exposure the excess risk increased with longer exposure duration, for low cumulative exposure it showed the opposite trend. In addition, high cumulative exposure exerted lethal effects other than lung cancer on the rats.

Does mean lung dose calculated after inhalation of alpha emitters actually reflect the risk of induction of malignant lung tumours?

A comparison of incidence of lung tumours in rats after inhalation exposure to aerosols containing alpha emitters which have different physico-chemical properties has been performed. Aerosols of radon and progeny, uranium ore dust, NpO2, PuO2 or Cm2O3 were considered for intercomparison with similar or different particle sizes. Dose-effect relationships for the frequency of malignant lung tumours appear linear up to a few Gy and then become infralinear at higher doses delivered to the lungs. The initial slope of the curves reflects the risk of induction of a lung tumour. The highest slopes of incidence were observed for radon and uranium ore dust (about 70 and 20% Gy(-1) respectively) for which the most homogeneous alpha dose distribution to the lungs is expected. In a general trend, increasing the alpha-activity of deposited particles (higher specific activity of constituent radioisotopes or larger particle size) decreases the risk. The comparison of the reported data shows that the risk per Gy at 'low doses' could vary over more than one order of magnitude depending on the physico-chemical properties of the aerosols.

A new method specifically designed to expose cells isolated in vitro to radon and its decay products.

A system was set up to provide direct exposure of cells cultured in vitro to radon and its decay products. Radon gas emanating from a uranium source was introduced at a measured concentration in a closed 10-m(3) exposure chamber. Cells were cultured on the microporous membrane of an insert that was floating over the culture medium in a six-well cluster plate. Plates with cells were placed in an open thermoregulated bath within the chamber. Under these conditions, cells were irradiated by direct deposition of radon and radon decay products. During exposure, all parameters, including radon gas concentrations, decay product activities, and potential alpha-particle energy concentrations, were determined by periodic air-grab samplings inside the chamber. The energy spectrum of deposited decay products was characterized. An estimation of alpha-particle flux density on the area containing cells was performed using CR-39 detector films that were exposed in cell-free wells during the cell exposure. The number of alpha-particle traversals per cell was deduced both from the mean number of CR-39 tracks per surface unit and from measurements of entire cells or nuclear surfaces. This paper describes the design of experiment, the dosimetry of radon and radon decay product, and the procedures for aerosol measurements. Our preliminary data show the usefulness of the in vitro cell culture approach to the study of the early cellular effects of radon and its decay products.

CGH analysis of radon-induced rat lung tumors indicates similarities with human lung cancers.

Epidemiological studies have shown that inhalation of radon, a radioactive gas, is associated with an increased risk for lung cancer. We have developed a model of radon-induced rat lung tumors to characterize cytogenetic and molecular events involved in radon-induced lung tumorigenesis. Using comparative genomic hybridization (CGH), gains and losses of genetic material were investigated in a series of 13 carcinomas and four adenomas of the lung. Frequent losses occurred at 4q12-21, 5q11-33, and 15q, which are homologous to human chromosome (HSA) bands 7q21-36, 1p31-36/9p21-31, and 13q14.1-14.3/3p14.2, respectively. These regions are frequently (30-80%) deleted in human lung cancer and contain tumor suppressor genes or proto-oncogenes such as MET, CDKN2A/p16/MTS1, CDKN2B/p15/MTS2, FHIT, and RB1 or yet to be identified genes. Frequent gains involved 6, 7q34-qter, and 19q; chromosomes 6 and 7 being homologous to human 2p21-25 and 8q21-24 where the MYCN and MYC oncogenes are located. The genetic similarities between rat and human lung cancer suggest common underlying mechanisms for tumor evolution in both species. Moreover, cytogenetic and molecular genetic analyses of radon-induced rat lung tumors could help to better understand the development and progression of radon-induced lung cancer in man.

Influence of exposure rate on lung cancer induction in rats exposed to radon progeny.

Animal studies were used in addition to epidemiological studies to investigate the effects of exposure, exposure rate and other factors in predicting risks resulting from exposures to radon progeny. A trend toward increasing tumor risk with decreasing exposure rate was observed in rats exposed at a cumulative exposure varying from about 0.72 J h m(-3) (200 WLM) up to 10.8 J h m(-3) (3,000 WLM) and high exposure rates varying from 25 WLM per week to 500 WLM per week. In contrast, at low cumulative exposure, comparable to lifetime domestic indoor exposures or lifetime occupational exposure in uranium mines, no evidence of an inverse exposure-rate effect was found. Chronic radon exposure at 0.09 J h m(-3) (25 WLM), protracted over 18 months, at a potential alpha-particle energy concentration (PAEC) of 0.042 mJ m(-3) (2 WL), resulted in fewer lung carcinomas in rats than a similar cumulative exposure protracted over 4 to 6 months at a PAEC of 2.1 mJ m(-3) (100 WL). The preliminary results of a new series of experiments carried out at relatively low cumulative exposures of 0.36 J h m(-3) (100 WLM) and PAEC varying from 0.21 mJ m(-3) (10 WL) to 3.15 mJ m(-3) (150 WL) indicate that at cumulative exposures comparable to lifetime indoor or occupational exposures, the risk of lung cancer in rats decreases with decreasing PAEC, i.e. exposure rate. These data suggest that the risk of radon-induced lung cancer results from a complex interplay between cumulative exposure and exposure rate at a given exposure level.

Intercomparison of measurement techniques used in radon exposure facilities for animals in Europe.

The biological effects of exposure to radon and its progeny are being studied in animals by three laboratories in Europe. The facilities used for such exposures are described, together with the methods used to estimate radon progeny concentrations and the activity deposited in the lungs of exposed animals. As the facilities and methods vary, a series of comparison exercises has been carried out at the three facilities; CEA/COGEMA, Razes, France, TNO, Rijswijk, The Netherlands and AEA Technology, Harwell, U.K. The results of the exercise are presented together with reasons for the discrepancies in results between the groups thus ensuring that estimates of exposure provided by the groups for their studies is directly comparable.

Carcinogenic and cocarcinogenic effects of radon and radon daughters in rats.

It has been previously established that lung cancer could be induced in rats by exposure to radon and radon daughters. Although the oat-cell carcinomas that are common in humans were not found in rats, other histological types of lung carcinomas, especially squamous cell carcinomas and primitive lung adenocarcinomas, were similar to those observed in humans. A dose-effect relationship was established for cumulative doses varying from 25 to 3000 working-level-months (WLM), which was similar for medium and high cumulative doses to that observed in uranium miners. This experimental protocol was also used to study the potential cocarcinogenic effects of other environmental or industrial airborne pollutants such as tobacco smoke, mineral fibers, diesel exhausts, or minerals from metallic mine ores that may act synergistically with radon exposure. In rats exposed to radon and tobacco smoke combined, the incidence of lung cancers was higher by a factor of 2-4 according to the cumulative radon exposure and the duration of tobacco smoke exposure. When mineral fibers were injected intrapleurally, an increased incidence of malignant thoracic tumors was observed in rats exposed to radon and fibers combined, but synergistic effects resulted in additivity. With diesel exhausts or minerals from metallic ores, a slight, nonsignificant increase in the incidence of lung carcinomas was observed compared with rats exposed to radon alone. These results demonstrated that it is possible to establish the potential cocarcinogenic action, showing either multiplicative, additive, or no effect of various environmental or industrial airborne pollutants combined with radon exposure. This radon model is valid for investigating possible interactions between two occupational exposures.

[Importance of the role of dose rate on tumor induction in rats after radon inhalation]. / Importance du rôle du débit de dose sur l'apparition des cancers chez le rat après inhalation de radon.

Lung cancer can be induced in rats, by radon daughter products, after exposure as low as 25 WLM (80 mJ.h.m-3) protracted over 4 to 6 months with a dose rate of 100 to 150 WL (2 to 3 mJ.m-3). The incidence of lung cancer is not increased and is equal to that of controls when the same cumulated dose is protracted over 18 months at 2 WL (0.042 mJ.m-3).

Increased biopterin production in rats with tumors induced by radon inhalation and benzonaphthoflavone administration.

Serial determinations of urinary biopterin were performed in rats during the development of lung tumors induced by radon inhalation and 5,6-benzonaphtoflavone administration. A striking increase in biopterin levels was observed in animals which developed single or multiple epidermoid carcinoma of the lung and this increase occurred several weeks before tumors could be detected radiographically.

Translocation of subcutaneously injected chrysotile fibres: potential cocarcinogenic effect on lung cancer induced in rats by inhalation of radon and its daughters.

Exposure to radon 222 and its daughters has been shown to induce lung cancer in rats. The cocarcinogenic effect of intrapleurally injected mineral fibres in rats which have previously inhaled radon has also been established. The aim of this work was to establish whether a similar process could be induced at a distance from the lungs by subcutaneous injection of chrysotile fibres. Three groups of animals were used: (1) 109 rats which inhaled radon only (dose: 1600 working-level months (WLM]; (2) 109 rats given a subcutaneous injection in the sacrococcygeal region of 20 mg of chrysotile fibres after inhalation of the same dose of radon; and (3) 105 rats injected with fibres only. No mesotheliomas occurred in any of the 3 groups. The incidence of lung cancer was 55% in group 2, 49% in group 1 and 1% in group 3. Statistical analysis using Pike's model showed that the carcinogenic insult was slightly higher in group 2 than in group 1. Electron microscopy analysis of fibre translocation from the injection site showed that less than 1% of injected fibres migrated to the regional lymph-nodes and only about 0.01% to the lungs. After injection, the mean length of the fibres recovered in lung parenchyma increased with time, suggesting that short fibres are cleared by pulmonary macrophages whereas long fibres are trapped in the alveolar walls. Although the high tumour incidence observed in group 1 might have masked the cocarcinogenic effect induced by the fibres, it is possible that this effect can occur only at short distances.(ABSTRACT TRUNCATED AT 250 WORDS)