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At least 523 records · Page 29Linked to original sources
PubMed1998

Tumour dose estimation using automated TLD techniques.

Lithium fluoride (TLD-700) dosimeters were used to measure exit surface absorbed doses in external beam radiotherapy using an automated TLD reader. Delivered tumour absorbed doses were derived from these measurements for head and neck, pelvis and breast treatments. For the head and neck treatments (first fraction only), the mean percentage difference between prescribed and delivered tumour absorbed doses was -0.15 +/- 3.0% (+/- 1 SD), for the pelvic treatments -0.83 +/- 2.8% and for the breast treatments +0.26 +/- 2.9%. The spread of results is approximately +/- 3% (+/- 1 SD). This is comparable with the estimated uncertainty in a single TLD absorbed dose measurement in phantom (+/- 2%; +/- 1 SD). Thus, ICRU recommended tolerances for absorbed dose delivery of +/- 5% may not be unequivocally detectable using this method. An action level of +/- 10% is suggested, allowing investigation of possible gross errors in treatment delivery at an early stage, before the course of treatment has progressed to a point at which absorbed dose compensation is impossible.

Breast Neoplasms↗
PubMedDate not supplied

Bipolar II disorder: current and future treatment options.

BACKGROUND: Bipolar II (BPII) disorder is a significant public health problem in the United States, and there is a dearth of studies of effective treatment modalities to deal with the recurrent major depressive episodes that accompany the disorder. This review attempts to summarize available data on agents useful in treating patients with the disease. METHODS: English language controlled clinical trials involving BPII patients obtained from an extensive Medline search were critically reviewed. RESULTS: Agents that have potential utility in the treatment of BPII are profiled, based on their efficacy in bipolar I (BPI) or unipolar depression. CONCLUSIONS: The most efficacious agents are likely those with bimodal stabilizing properties, such as lithium, carbamazepine, and quetiapine. In fact, on the strength of favorable efficacy data obtained in patients with major depressive symptoms accompanying bipolar disorder, quetiapine recently became the first agent to be indicated by the FDA for monotherapeutic use in the treatment of bipolar depression, including BPII depression. Aside from the aforementioned agents, lamotrigine also shows promise in the treatment of BPII.

Anticonvulsants↗
PubMed1980

Thermoluminescent response of LiF and Li2B4O7:Mn to pions.

LiF (TLD-100, TLD-600, and TLD-700) and Li2B4O7:Mn (TLD-800) have been exposed to pi+ and pi- beams over the clinically useful range of 0.05-4 Gy (5-400 rad). Thermoluminescence response curves were determined for the 200 degrees C and 260 degrees C and 260 degrees C glow peaks in LiF and for the 200 degrees C peak in Li2B4O7 when the dosemeters were exposed in the peak and plateau regions of the Bragg curve. TLD-700 appears to be suited for use in pion dosimetry since it is capable of measuring both total and high-LET dose by utilising the 200 and 260 degrees C peaks. TLD-100, TLD-600, and TLD-800 will not be useful because the neutrons emitted from pi- capture result in an enhanced dose to the thermoluminescence dosemeters via (n, alpha) reactions which cannot be extracted from the data. Linearity and fading of each glow peak was also investigated.

Elementary Particles↗
PubMed1980

Depth dose and off-axis characteristics of TLD in therapeutic pion beams.

The thermoluminescent (TL) response of LiF (TLD-100, TLD-600, TLD-700) and Li2B4O7 (TLD-800) has been measured as a function of depth and off-axis position in a therapeutic negative-pion beam in order to evaluate their usefulness in pion radiotherapy. TLD-100, TLD-600, and TLD-800 have been shown to be of little use as in vivo dosemeters because the neutron kerma relative to that in tissue changes grossly with depth. The neutron source comes primarily from pion absorption in the lead-alloy collimator. The 200 degrees C TLD-700 response agrees well with the depth dose spectra, except for small changes due to the varying linear energy transfer (LET) distributions. This variation can be partially accounted for by incorporating the known LET response of LiF. The 260 degrees C peak of TLD-700 has been found to be approximately four times more sensitive than the 200 degrees C peak to high LET dose. Using a simple model of the LET responses, the measured 200 degrees C and 260 degrees C peaks predict total dose within +/- 4% and high LET dose within +/- 50%, therefore indicating TLD-700 to be a good in vivo dosemeter for total dose but only an indicator of high LET dose.

Elementary Particles↗
PubMed1980

Pion dosimetry with thermoluminescent materials.

Various thermoluminescent materials (6LiF, 7LiF, Li2B4O7:Mn, CaSO4:Dy, CaF2:Mn, CaF2-Dy, CaF2:Tm) were irradiated with pi- mesons throughout the depth dose distribution of a beam with central momentum of 176 MeV/c. Doses from 0.05 to 2 Gy were used and linearity and reproducibility were investigated. For LiF, CaF2:Dy and CaF2:Tm, low- and high-temperature peaks were analysed separately. CaF2:Tm was found to be the most suitable for pi- dosimetry since the total dose and the high LET component could be determined using the integral TL signal and the peak height ratio of the 150 degrees C and 240 degrees C peaks. In the dose range used, both peak responses are linear with dose and the sensitivity of the materials is 10 times that of LiF. The reproducibility was +/- 3%. The high temperature peak of 7LiF is too small to be determined accurately and the high temperature peak of CaF2:Dy decreases with increasing LET in the same way was the low temperature peak does, so that no LET information can be extracted.

Calcium Fluoride↗
PubMed1982

The suitability of different preparations of thermoluminescent lithium borate for medical dosimetry.

The thermoluminescence properties of three different preparations of lithium borate have been studied with specific reference to their use in medical dosimetry. The properties of lithium borate powder doped with copper make it more attractive for low dose measurements than the more conventional phosphor doped with manganese. However, the energy response of the copper-doped material was not quite so suitable as that of the manganese-doped material for measuring doses to tissue at photon energies below 100 keV. It also exhibited appreciable light induced fading.

Borates↗
PubMed1983

Modified general cavity theory applied to the calculation of gamma dose in 60Co thermoluminescence dosimetry.

Burlin's general cavity theory for intermediate sized cavities incorporated two weighting factors: d, the average reduction in the electron fluence from the medium inside the cavity, and 1 - d, the average fractional build-up to equilibrium of the cavity-generated electron spectrum--both calculated for an average path length g of electrons crossing the cavity. A new weighting factor d' has been introduced expressing the fact that average path length for medium- and cavity-generated electrons are not equal and this leads to a modified general cavity expression. The new expression is applied to 60Co thermoluminescence dosimetry experimental results of other workers. For LiF TLD in all the media agreement with experiment is improved. There is a significant dependence of the theoretical results on the effective mass attenuation coefficient, indicating the need for further studies.

Cobalt Radioisotopes↗
PubMed1986

Thermoluminescence of 7LiF in therapeutic high linear energy transfer (LET) charged-particle beams.

The thermoluminescence of the 200 and 260 degrees C peaks of 7LiF has been measured and compared with off-axis dose and depth-dose distributions for three therapeutic high-LET beams: negative pions, helium ions and neon ions. The methods of analysis consisted of both a single-peak analysis and the dual-peak analysis methods of Hogstrom and Irifune and Hoffmann et al. The results indicate that 7LiF, analysed by the dual-peak analysis methods, is potentially useful for extracting total dose, high-LET dose, and beam quality of helium-ion and negative-pion beams. For the higher-LET neon beam, differences in sensitivity between the 200 and 260 degrees C peaks of 7LiF were found to be independent of variations in LET within the beam; consequently, only the single-peak analysis method is applicable in which case the dosemeter at best can only estimate total dose.

Fluorides↗
PubMed1988

Influence of the variation of energy spectra with depth in the dosimetry of 192Ir using LiF TLD.

Absolute dose measurements around low activity seeds used in brachytherapy are difficult to perform with ionisation chambers. The physical size of the chamber does not allow good resolution close to the seeds and the ionisation current generated is very low. Small thermoluminescent dosimeters (TLDs) overcome these problems but the energy dependence of their response should be considered. In this work, dose in polystyrene was measured at several distances from the high activity 192Ir source (370 GBq) of a remote afterloading device using an ionisation chamber and LiF TLD chips. These data show that over a range of 1-10 cm from the source the sensitivity of LiF varies by up to 8.5%. This is attributed to the higher response of LiF to the lower photon energies, and to the shift of the photon spectrum to lower energies with increasing depth. The sensitivity of LiF to 192Ir was also calculated by weighting the energy-dependent response of LiF by the Monte Carlo calculated photon spectra. The calculations give a similar change in sensitivity with distance from the source.

Brachytherapy↗
PubMed1989

Sensitivity of LiF thermoluminescent dosemeters to 6-18 keV photons.

The sensitivity of thermoluminescent dosemeters (TLD) to 6-18 keV photons at six different effective energies was determined using an extrapolation chamber as a dosimetric standard. Photons having effective energies of 8.1, 9.2, 13.5, 13.8 and 18.0 keV were generated from an x-ray machine using appropriate kilovoltages and aluminium filters. Photons from an 55Fe source (6 keV) were also used to broaden the range of energies investigated. Observed sensitivities relative to 137Cs were found not to differ from unity with the possible exception of 6.0 keV photons. At 6.0 keV the sensitivity was found to be 0.83 +/- 0.5.

Fluorides↗
PubMed1989

Measurement and analysis of supralinearity in LiF TLD-100 irradiated by 1.5 keV x-rays.

Measurements of the response of lithium fluoride TLD-100 to 1.5 keV ultrasoft x-rays, 240 keV x-rays and 137Cs gamma-rays at doses below 70 Gy are presented and the implications for the track interaction model are discussed. This model proposes that thermoluminescence is due to the combination electrons and holes formed along secondary charged particle tracks. As the dose is increased, the average distance between particle tracks is reduced and there is a greater probability of combination between neighbouring tracks; this mechanism is used to explain the supralinear response of lithium fluoride to certain radiations. Measurements using ultrasoft x-rays are interesting in this context because the energy from each photon is deposited in a small volume, therefore the probability of combination between neighbouring events should only be significant at high doses. The results, however, show that lithium fluoride exhibits a significant supralinear response to 1.5 keV x-rays after only a few tens of grays and suggests that the track interaction model alone cannot explain the phenomenon of supralinearity.

Cesium Radioisotopes↗
PubMed1994

Response of thermoluminescent lithium fluoride (TLD-100) to photon beams of 275, 400, 500, 600, 730, 900, 1200, 1500, and 2500 eV.

LiF:Mg,Ti (TLD-100) extruded ribbons and cleaved crystals were exposed to monoenergetic photons of 275-2550 eV energy to determine their potential usefulness as radiation dosimeters for radiobiology experiments at these energies. The radiation source was synchrotron radiation from the 1 GeV electron storage ring, Aladdin. The authors report TLD response and glow curves for He- and air-annealed dosimeters. The undesirable effects of air annealing increase with decreasing photon penetration in the dosimeter. Under certain experimental conditions, UV radiation produced anomalous bleaching of high-temperature traps. The crystals and the chips presented a supralinear response, Supralinearity factors were determined to be of the order of 1.5 for crystals, and 1.7 for the chips. The authors' results indicate that TLDs are a reliable means to monitor the total energy deposited in irradiated cells and are now used routinely for radiobiology cell irradiations.

Biophysical Phenomena↗
PubMed1996

Transmission ionization chambers for measurements of air collision kerma integrated over beam area. Factors limiting the accuracy of calibration.

Kerma-area product meters (KAP meters) are frequently used in diagnostic radiology to measure the integral of air-collision kerma over an area A (integral of A Kc,air dA) perpendicular to the x-ray beam. In this work, a precise method for calibrating a KAP meter to measure integral of A Kc,air dA is described and calibration factors determined for a broad range of tube potentials (40-200 kV). The integral is determined using a large number of TL dosimeters spread over and outside the nominal field area defined as the area within 50% of maximum Kc,air. The method is compared to a simplified calibration method which approximates the integral by multiplying the kerma in the centre of the field by the nominal field area Anom. While the calibration factor using the precise method is independent of field area and distance from the source, that using the simplified method depends on both. This can be accounted for by field inhomogeneities caused by the heel effect, extrafocal radiation and scattered radiation from the KAP meter. The deviations between the calibration factors were as large as +/- 15% for collimator apertures of 5-100 cm2 and distances from the source of 50-160 cm. The uncertainty in the calibration factor using the precise method was carefully evaluated and the expanded relative uncertainty estimated to be +/- 3% with a confidence level of 95%.

Equipment Design↗
PubMed1996

The quality dependence of LiF TLD in megavoltage photon beams: Monte Carlo simulation and experiments.

The quality dependence of LiF TLD in megavoltage photon beams with qualities from 60Co gamma-rays to 25 MV x-rays has been studied experimentally against ion chamber measurements and theoretically by Monte Carlo simulation using the EGS4 Monte Carlo code system. The experimental findings are that the energy dependence of 1 mm thick TLD-100 (micro-rods and chips) on average decreases slowly from 1.0 for 60Co gamma-rays to 0.989 +/- 1.3% for 6 MV x-rays (TPR20/10 = 0.685) and to 0.974 +/- 1.3% for 25 MV x-rays (TPR20/10 = 0.800) relative to 60Co gamma-rays. The Monte Carlo results vary from 0.991 +/- 0.9% for 6 MV x-rays to 0.978 +/- 0.8% for 25 MV x-rays. Differences between chips and micro-rods were negligible and there was no difference in the energy dependence between TLDs irradiated in water or Perspex (PMMA). The Monte Carlo simulation shows that the contribution to the total absorbed dose from photon interactions in the 1 mm diameter and 6 mm long TLD material varies from 50% for 60Co gamma-rays to 10% for 25 MV x-rays. When the diameter of the TLD micro-rod was increased from 1 mm to 5 mm there was no significant change in response computed by Monte Carlo even though the dose contribution to the total dose scored in the TLD material from photon interactions in the cavity increased to 85% for 60Co gamma-rays and 30% for 25 MV x-rays.

Biophysical Phenomena↗