PubMed · 4758206
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J M Gamble, J C Peake. 1973. Letter: Errors in cobalt teletherapy timer units.. https://doi.org/10.1088/0031-9155%2F18%2F5%2F118
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The aim of the TLD pilot study was to investigate and to reduce the uncertainties involved in the measurements of absorbed dose and to improve the consistency in dose determination in the regional radiotherapy centres in Poland. The intercomparison was organized by the SSDL. It covered absorbed dose measurements under reference conditions for Co-60, high energy X-rays and electron beams. LiF powder type MT-N was used for the irradiations and read with the Harshaw TLD reader model 2000B/2000C. The TLD system was set up and an analysis of the factors influencing the accuracy of absorbed dose measurements with TL-detectors was performed to evaluate and minimize the measurement uncertainty. A fading not exceeding 2% in 12 weeks was found. The relative energy correction factor did not exceed 3% for X-rays in the range 4-15 MV, and 4% for electron beams between 6 and 20 MeV. A total of 34 beams was checked. Deviation of +/- 3.5% stated and evaluated dose was considered acceptable for photons and +/- 5% for electron beams. The results for Co-60, high energy X-rays and electron beams showed that there were two, three and no centres, respectively, beyond acceptance levels. The sources of errors for all deviations out of this range were thoroughly investigated, discussed and corrected, however two deviations remained unexplained. The pilot study resulted in an improvement of the accuracy and consistency of dosimetry in Poland.
The AAPM TG 39 protocol has proposed three different methods of calibrating plane-parallel ionization chambers, i.e., in-phantom irradiation with a high-energy electron beam and in-phantom and in-air 60Co irradiation. To verify the consistency of the three methods, we have measured Ngaspp values using each of these techniques for the five most commonly used plane-parallel chambers considered by the protocol. Our results demonstrate that the measured Ngaspp values for the three different methods for any of the chambers agree to within +/- 0.6%. Once Ngaspp was measured, the determination of absorbed dose for electron beams with different energies for an AECL Therac 20 and Philips SL25 was carried out according to the AAPM TG 39 protocol. The results show that the determination of the absorbed dose outputs for any of the five chambers agree to within +/- 0.7% for electron-beam energies of 4-20 MeV if all five chambers had Ngaspp values determined by the electron-beam method. The uncertainties are well within the expected error for these approaches.
Several families of negatively supercoiled topoisomers of plasmid pIBI30 were prepared by a modification of the procedure of Singleton and Wells (Anal. Biochem. 122, 253-257, 1982). The average superhelical density (sigma) was determined by two-dimensional agarose gel electrophoresis and varied from -0.010 to -0.067, corresponding to a change in the number of supercoils from 3 to 19 and an effective volume change from 1.6 x 10(8) to 4 x 10(8) A3. Samples were exposed to either fission-neutron or 60Co gamma radiation and assayed for single-strand breaks by agarose gel electrophoresis. Form I DNA for all topoisomers decreased exponentially with increasing dose. The D37 values for both neutron and gamma radiation increased monotonically with increasing magnitude of sigma. Using a branched plectonemic (interwound) form for DNA over the range of sigma studied and standard (single-hit) target theory, a quantitative linear fit to (D37)-1 as a function of the effective DNA radius, S(A), was obtained. The model predicts that both the slope (a) and the intercept (b) of (D37)-1 as a function of S(A) are directly proportional to the length of DNA and the radiation fluence. Furthermore, the ratio b/a (= ro) at sigma = 0 depends only on the ionic strength of the medium and is independent of the radiation source parameters. Our results support the model and we calculate ro = 13.4 +/- 1.4 nm, a value consistent with other investigations. Our results are consistent with studies using 137Cs (Milligan et al., Radiat.(ABSTRACT TRUNCATED AT 250 WORDS)