Technical ReportPDF Available

Extrapolation ionisation chamber measurements on beta-emitting sources produced for the CEGB collaborative radiobiology programme. CEGB Report TPRD/B/0867/R88, 1988.

Technical Report

Extrapolation ionisation chamber measurements on beta-emitting sources produced for the CEGB collaborative radiobiology programme. CEGB Report TPRD/B/0867/R88, 1988.

Abstract

This report describes the structure and application of an extrapolation ionisation chamber used for measuring dose-rates from plane and point beta-emitting sources. These measurements form the basis of the dosimetry for a collaborative radiobiological study of skin to study both stochastic and non-stochastic effects. A small sample from the wide range of measurements undertaken in the programme has been selected to illustrate the procedures involved. The extrapolation chamber is currently being automated* and it is intended that this report should provide a source reference to the basis of the measurements made between 1977-86. (August 1988) * https://www.researchgate.net/publication/313290435
... Most of the measurements reported were made with a 1.2-mm diameter collecting electrode and 16 jum thick plastic chamber window. Several preliminary measurements made with this instrument using a variety of source sizes and collecting electrode diameters were reported by Wells(13). One measures charge collected (or current if dose rate is to be measured) in the ionization chamber at various spacings of the collecting electrodes in using this type of instrument. ...
Article
A critical review and a reassessment of data reviewed in NCRP Report 106 on effects of hot particles'' on the skin of pigs, monkeys, and humans were made. Our analysis of the data of Forbes and Mikhail on effects from activated UCâ 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 à 10⁹ beta particles from a point source on skin of mixed fission product beta particles, or about 10¹° beta particles from Sr--Y-90, since few of the Sr-90 beta particles reach this depth. The data of Hopewell et al. for their 1 mm Sr-Y-90 exposures were also analyzed with the above model and yielded a predicted threshold of 2 à 10¹° Sr-Y-90 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 alternate 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. 17 refs., 3 figs., 3 tabs.
Article
Background and objectives: Previous data, predominantly involving high dose-rate fractionated irradiation with incomplete repair intervals, had indicated that the kinetics of repair of sublethal damage for acute radiation reactions in pig skin could best be defined by a biphasic repair model with half-times for repair of 0.2 and 5.4 h, partition coefficient 0.5. To further test the validity of this finding and obtain a better estimate of the repair rate of the slow component of repair, the acute response of pig skin to very low dose-rates (VLDR), originally estimated to be 0.0067-0.0244 Gy/min, was investigated as part of a 4 fraction irradiation protocol involving an overall treatment time of <9 days to avoid confounding factors such as induced repopulation and enhanced radio-sensitivity in this animal tissue. Materials and methods: The flank skin of female Large White pigs, 3-4 months of age, was locally irradiated (8 sites/flank) with 22.5 mm diameter (90)Sr/(90)Y plaques. Irradiation with a 4 fraction protocol included 3 equal, high dose-rate, fractions with full repair, followed by a fourth VLDR fraction. The total doses administered were originally planned to represent the dose associated with the predicted ED(20), ED(50) and ED(80) (75% of total biological dose given at high dose-rate and 25% at VLDR) calculated on the basis of the repair kinetic parameters obtained from earlier studies. However, during the analysis a revision to the physical dosimetry was identified; this had been overlooked prior to the start of the study. Following completion of irradiation the irradiated sites were examined weekly and the presence or absence of moist desquamation recorded. Results: The incidence of moist desquamation was slightly higher than expected on the basis of the parameters used to calculate iso-effective doses, at least in part as a consequence of the change to the dosimetry. Using likelihood methods and the original dose estimates, the best model based estimate of the dose-rate correction factor for the LDR and VLDR plaques was 1.29. This was comparable with the physical calibration factor, median value 1.23. The VLDR fraction associated with a 50% incidence of moist desquamation, based on experimental observation, was 23.2+/-0.84, 27+/-2.6 and 30.1+/-3.2 Gy, for corrected VLDRs of 0.0247, 0.0093 and 0.0068 Gy/min, respectively. A biphasic model, which incorporated a dose-rate correction factor, provided a better fit than a monophasic repair model to the total data set, which now included the new VLDR data. Moreover, the monophasic repair model suggested a dose-rate correction factor of 1.63, well outside the range derived from the re-evaluation of the physical dosimetry. Conclusion: Using the total data (with model based corrected dose-rates), the analysis revealed two components of repair with half-times of 0.103 (0.0594-0.177) and 2.97 (1.96-4.50) h; partition coefficient 0.375 (0.225-0.526). These are comparable with the estimates for other tissues (the CNS in particular) and suggest that the kinetics of repair may be relatively species and tissue independent with variation observed being more related to experimental design rather than any true differences.
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