Results 1 - 10 of 3474
Results 1 - 10 of 3474. Search took: 0.022 seconds
|Sort by: date | relevance|
[en] According to experience and results of neutron radiotherapy in various radiotherapy centres, depth dose precision demands and neutron interactions with elemental tissue constituents, are the reasons presented in this paper, why neutron beams should not be used in radiotherapy. (author)
[en] Neutron KERMA factors calculated from the LLNL ENDL data file are tabulated for 15 composite materials and for the isotopes or elements in the ENDL file from Z = 1 to Z = 29. The incident neutron energies range from 1.882 x 10-5 to 20. MeV for the composite materials and from 1.30 x 10-9 to 20. MeV for the isotopes and elements
[en] We have tested value of the product of kerma in air and surface (kerma area product (KAP)) on 31 intra-oral X-ray units at different locations in the Republic of Serbia, in order to optimize the dose for patients. As image receptors are used films of class D, E and F, phosphor plate systems (storage phosphor plate (SPP)) and digital sensors (charged-couple-device (CCD)). The exposure time for each image receptors is given in accordance with the diagnostic requirements for incisors, canines and molars, upper and lower jaw. The measured values for the median molars of the upper jaw, the tested doses ranging from 22.4 to 116.2 mGy cm2. The tested values for film class D, are different compared to other receptors E, F, CCD and SPP, from 5 to 9 times, which is consistent with studies of similar character.(author).
[en] Experimental measurements of threshold contrast (Csub(T)) as a function of air kerma rate at the input plane of the image intensifier have been made for several diagnostic fluoroscopy units in clinical use. Threshold contrasts are determined by viewing a test object containing holes of fixed diameter and various depths under defined irradiation conditions. Kerma rate variations are effected by introducing aluminum sheets into the x-ray beam at fixed values of tube potential and current. At low kerma rates where quantum noise dominates, low tube potentials (60 kVsub(p)) usually yield lower values of Csub(T) than do higher potentials (100 kVsub(p)). At higher kerma rates the opposite is often true. A simple theoretical model for noise propagation in fluoroscopic imaging systems using models of diagnostic x-ray spectra lends qualitative support to the experimental findings. The often-quoted suggested upper limit of 100 μR s-1 (0.87 μGy s-1) at the input phosphor would seem to be justified under the test conditions since little improvement in Csub(T) is usually observed at higher kerma rates. However, application to clinical practice would ideally require the use of more realistic phantom studies. (author)
[en] A large air-filled parallel-plate extrapolation chamber with thin graphite front and back electrodes is used as a primary standard measuring device for low-energy interstitial brachytherapy sources from which the unit of air-kerma strength or reference air-kerma rate can be derived. The chamber is suitable for low-energy photons with energies up to 40 keV. The underlying principle is that the air-kerma rate at a given point is proportional to the increment of ionization per increment of chamber volume at chamber depths greater than the range of secondary electrons originating from the electrode. The fundamentals for evaluating the extrapolation curves are presented as well as a detailed description of the present set-up of the P.T.B. large-volume extrapolation chamber (G.R.O.V.E.X.). Comparisons between the G.R.O.V.E.X. and other primary standards for air-kerma and reference air-kerma rate are presented. (authors)
[en] The current value for weighted average primary plus scatter kerma at 1 m for intra-oral radiography shielding assessment is based on scatter radiation measurements made with the trunk of a RANDO phantom and an assessment of primary transmission that is unverified. Measurements of primary transmission and scatter radiation during intra-oral radiography were made at 30° intervals through a full 360° rotation using two anthropomorphic head phantoms and similar equipment at three different sites. The results suggest that a scatter factor of 5 µGy (Gy cm2)−1 and a primary transmission of 0.03% of the entrance surface dose are more appropriate and, therefore, we recommend that the weighted average primary plus scatter kerma used for shielding calculations can be reduced from 1 to 0.5 µGy per exposure at 1 m. This factor will adequately account for exposures made at 60 and 70 kV using a range of intra-oral units.
[en] Accuracy of dosimetric estimates can determine the value of the atomic bomb survivor experience in establishing radiation risks. The status of a major revision of this dosimetry, initiated in 1980, is assessed. 3 references, 6 figures
[en] In Iceland guidance levels for patient radiation doses are expressed in terms of kerma area product (KAP) for complete examinations. Radiological departments' compliance with these guidance levels is based on the average value from kerma area product measurements on a sample of patients. Along with the measurements, supplementary data linked to the performance of the examination are recorded, and analysed for influence on the measurement results. Key parameters recorded are the number of film-screen radiographs, fluoroscopy KAP rate and time, applied tube potential and speed of film-screen system. By comparing the average values of these parameters with other radiological departments or with guidelines for radiographic procedures, the reasons for a deviation from the guidance level can be identified. (author)
[en] The knowledge of the absorbed dose at the reference point is extremely important in all radiotherapich treatments. For the dosimetry of high-energy electron beams, the Medical Physics Department of the S. Chiara Hospital (Trento, Italy) uses the NACP-02 plane parallel chamber with the 2570A dosemeter. This chamber was calibrated in a beam of 60Co gamma rays according to the method suggested by the NCS Protocol. The air kerma calibration factor, Nk,pp, is 3.398±0.004 cGy/div. For all energy values, 11, 7 and 3 MeV nominal energy, the depth-ionisation curves were measured and from them the depth of ionisation maximum, R100, the half value depth, R50, and the practical range, Rp, were determined. After the values of E-bar0 =E-bar0(R50) and Cw,e=Cw,e(E-bar0, zmax) were assessed, the value of the absorbed dose to water at the reference point was also determined using both the standard method (source-skin-distance=SSD=1 m, square applicators, incidence angle 0 deg. /perpendicular) and the intraoperative radiation therapy method, IORT, (SSD variable, circular applicators, incidence angles: 0 deg. , 15 deg. and 30 deg. ). The absorbed dose was measured at SSD=1 m, at the depth of ionisation maximum, incidence angle 0 deg. , applicator 10*10 cm2, for the standard method, and circular applicator diam.=10 cm for the IORT method. The results obtained from the quality controls carried out during 1994 differ from the expected dose values for less than ±1.5%. The mean energies at the phantom surface, E-bar0, were also calculated according to the method suggested by Freim and Feldman. The values obtained from these measurements, carried out with the same set up used to determine the absorbed dose to water, can be compared with E-bar0 values determined following the NCS Protocol: 10.601±0.003 MeV (10.46 MeV, NCS), 6.543±0.003 MeV (6.73 MeV, NCS), 3.078±0.001 MeV (3.13 MeV, NCS)
[en] Neutron thermoluminescent sensitivities calculated via a kerma or combined kerma-linear energy transfer (LET) approach should be regarded as crude approximations. This due to the fact that (1) TL production efficiencies of heavy charged particles are not single valued functions of LET and (2) the TL yield dependence on ionization density is batch-dependent. (orig.)