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T Kron

Publications and source records attributed to T Kron.

At least 73 records · Page 4Linked to original sources
PubMed1994

Thermoluminescence dosimetry and its applications in medicine--Part 1: Physics, materials and equipment.

Thermoluminescence dosimetry (TLD) is a versatile tool for the assessment of dose from ionising radiation. The wide variety of TLD materials and their different physical forms allow the determination of different radiation qualities at dose levels from microGy to kGy. Major advantages of TL dosimeters are their small physical size and that no cables or auxiliary equipment is required during the dose measurement. This makes them well suited for a wide range of applications in medicine. However, while larger institutions with extensive experience in TLD commonly achieve quite good results, thermoluminescence dosimetry appears a bit like a black art for physicists who attempt to set up a TLD system for the first time. Therefore, the present article aims at summarising the relevant literature describing problems and possible pitfalls in the evaluation of TLD readings. The second part of the review (to be published) will discuss typical applications of TLD in medicine. Most applications and the major part of the literature are based on lithium fluoride doped with magnesium and titanium (LiF:Mg,Ti). However, other materials such as CaSO4:Dy are widely used, and some more recent TL materials such as LiF:Mg,Cu,P show great potential for radiation dosimetry. Therefore, the present article while focusing on LiF:Mg,Ti also includes other TLD materials of interest for dosimetry in the medical field. Furthermore, TLD apparatus and a variety of TLD applications and techniques shall be discussed with two intentions: firstly to give an update of TLD in medicine, and secondly to enable readers to set up or optimise their own TLD system.

Equipment Design↗
PubMed1994

Portal films for relative exit dose determinations.

Portal films are radiographic images taken regularly in external photon beam radiotherapy in order to verify geometrical set-up. However, they also contain quantitative dose information. Absolute dosimetry with film is possible but the desired accuracy of +/- 5% is difficult to achieve. However, relative dosimetry with film would be useful in combination with a point determination of absolute dose, though a dose response linearisation is required for relative dosimetry over the range of doses given to portal films in routine clinical practice. For this purpose, a technique for the rapid dose response linearisation of portal films is investigated in simulated treatment situations and evaluated in terms of the ability to yield relative exit dose distributions from the portal films. A combination of such distributions with a single determination of radiation transmission with diodes should generate full two-dimensional transmission information. Distributions obtained from two anthropomorphic phantoms provided results consistent with distributions obtained from a treatment planning system to generally within +/- 5%, and most often within +/- 2%. It is concluded that the extraction of exit dose information from portal films would be useful for mantle, compensated, or other unusual treatment situations.

Calibration↗
PubMed1994

Surface doses from combined electron/photon fields in a radiotherapy.

Using mixed modality treatments of photon and electron beams, some skin sparing can be acquired whilst administering a safer dose to crucial structures such as the spine or lung. The combination of 6MV X-rays and 12MeV electron beams in the treatment of breast nodes is a clinical example where such treatments are beneficial. By weighting the photon and electron beams accordingly, the surface dose and dose at depth can be changed whilst not dramatically varying the depth at which the 90% dose level is maintained. In order to accurately predict near surface dose, build up results were obtained using TLD extrapolation, Markus parallel plate and Attix parallel plate ionisation chambers in a solid water phantom. This data was then used to predict surface dose due to different beam weights. Depending on the weightings given to the photon and electron beams, the surface dose and dose at depth varies. For example, when 6MV X-rays and 12MeV electrons are combined the percentage dose at surface and 20cm depth is 46%/23%, 54%/20%, 61%/15% for 60/40, 50/50 and 40/60 X-ray/electron weightings respectively. For these weightings, the depth of the 90% level remained at 30mm. From a clinical point of view this data is important, showing that the 90% level of radiation does not vary in depth significantly provided the ratio of photon/electron weights is kept within a range of 60/40 to 40/60. However by varying the weightings, the ability to control dose to skin in particular to produce the optimum level for both areas whilst still delivering the required tumour dose is obtained.

Electrons↗
PubMed1993

Clinical thermoluminescence dosimetry: how do expectations and results compare?

Thermoluminescence dosimetry (TLD) for radiotherapy treatment verification is performed in the Prince of Wales Hospital in Sydney for a wide range of applications: (A) to determine the dose in difficult treatment geometries, (B) to record the dose to critical organs, and (C) to monitor special treatments such as total body irradiation (TBI). TLD measurements were performed with the aim to investigate cases where dose prediction is difficult and not as part of a routine verification procedure. We reviewed 1058 reports of TLD performed during the treatment of 502 patients between 1986 and 1991 to evaluate how the TLD results compare with the dose determined by the treatment plan. Reasons for possible discrepancies should be identified. In 19% of all investigated cases a discrepancy of more than 10% was found between expected and measured doses. The discrepancies could be divided into three groups: (1) errors made in the TLD determination or evaluation, such as placement errors of the TLD chips (21% of all discrepancies); (2) mistakes made during the patient set-up, such as insufficient shielding or inadequate patient immobilisation (30%); (3) inadequate treatment planning and dose calculation procedure, such as wrong inverse square law corrections or errors due to limitations of the two-dimensional treatment planning system used (41% of all). In 8% of all discrepancies the reason remained unclear. A number of changes to treatment plans and modalities (e.g. changed scrotal shield, modified bolus) were introduced due to TLD results. The increasing number of TLD requests per year attests to the value of TLD as a treatment verification method in clinical practice.

Electrons↗
PubMed1993

Investigation of the tissue equivalence of gels used for NMR dosimetry.

The transition of Fe2+ to Fe3+ in Fricke solution after irradiation results in a change of NMR proton relaxation times in agarose gels which can be used for the dosimetry of ionizing radiation. The main advantage of this system is the possibility of observing dose distributions in 3 dimensions in a medium which is supposed to be tissue equivalent. The aim of the present study was to quantify parameters which determine the tissue equivalence of NMR dosimetry gels. Electron densities and effective atomic numbers were calculated for gels with varying iron, sulphur and agarose concentration. The Hounsfield CT numbers so derived agree well with the CT numbers measured on a clinical CT scanner (effective photon energy 70.7 keV). The Hounsfield CT number of 7.2 +/- 1.5 (n = 9) measured for a 1.5% agarose gel doped with 0.5 mM ammonium ferrous sulphate and 125 mM sulphuric acid compares well with the calculated one of 8 +/- 5. Relaxation times were measured from a series of MR images obtained on a 1.5 T clinical MR scanner. The observed change in 1/T1 of the gel with dose was found to be linear up to 10 Gy (0.084 s-1 Gy-1). No difference in dose response for 10 Gy delivered by four different superficial radiation qualities (HVT = 1.4-7.5 mm Al) could be observed. These findings and the calculated effective atomic number of 7.46 demonstrate the close tissue equivalence of this agarose gel which makes it an ideal tool for the investigation of low energy therapeutic x-rays.

Ferrous Compounds↗
PubMed1993

A Monte Carlo technique to establish the water/tissue equivalence of phantom materials.

The quality of phantom materials is crucial for accurate dosimetry in radiotherapy. A wide range of factors such as density, electron density and elemental composition can influence the radiation properties, and hence the absorbed dose, of materials. New materials can be tested by direct measurements which requires considerable time and the availability of relatively large amounts of the material. Alternatively, the dosimetric properties of a proposed phantom material can be compared to those of water or tissue using Monte Carlo calculations. The aim of this study was to evaluate the use of a Monte Carlo technique for the investigation of the water/tissue equivalence of phantom materials. The material used for this investigation was Standard Dosimetry Agarose (SDA) gel, which is useful in MRI dosimetry. Depth doses in gel and water were calculated for mono-energetic electron beams of 6, 12 and 20 MeV, and photon beams of 60 keV and 6 MV. For each radiation quality the depth dose distributions are in close agreement.

Models, Structural↗
PubMed1993

Readout of thermoluminescence dosimetry chips using a contact planchet heater.

Thermoluminescence dosimetry (TLD) is performed in many radiotherapy departments using LiF chips which are evaluated by means of a contact heater. The aim of the present study was to investigate the influence of the TL chip position on the heating planchet and to study the self absorption of light in the chips during readout. Experiments were performed using standard LiF chips (3.1 x 3.1 x 0.89 mm3) on a circular heating planchet with a diameter of 13 mm. A two step readout cycle (10s 160 degrees C, 10s 300 degrees C) was used and the integral under the luminescence curve during the second step evaluated. Moving the chip away from the centre of the planchet without adjusting the temperature cycle leads to a peak broadening which can result in a signal loss of up to 40%. The use of a purpose build positioning device tended to improve the reproducibility of the readings. Self absorption of light in a normal chip on the heating planchet was studied by exposing chips in the strong dose gradient of the build up region of a 18 MV X-ray beam. The dose throughout the chip was investigated using a stack of 0.14 mm thick chips. In a 5 x 5 cm2 field the dose varied from 7% at the surface facing the beam to 23% at the opposite side of the chip. Reading the chips with the side facing the beam up in the reader resulted in a 1.9% smaller dose reading than with the chips upside down.(ABSTRACT TRUNCATED AT 250 WORDS)

Thermoluminescent Dosimetry↗
PubMed1992

Artifacts in chemical shift selective imaging.

The role of susceptibility effects in the production of artifacts in chemical shift images generated by the selective excitation technique is discussed. The effects are demonstrated in images of phantom samples of agarose gels containing small air bubbles. The artifacts can lead to erroneous interpretations, in which the resonances to be resolved exhibit a relatively small chemical shift separation, such as that between water and soluble carbohydrates (sugars).

Artifacts↗
PubMed1992

Radiotherapy x-ray dose distribution beyond air cavities.

Radiation oncologists are particularly concerned about tumours growing on the surface of air cavities in the head and neck regions, which involve treatment with small x-ray fields. An inhomogeneous dose distribution exists within and beyond the cavity. This is caused by the loss of electron equilibrium and the attenuation of both the primary and scattered photons is altered. The scatter function photon beam models for tissue inhomogeneity, such as the ETAR correction algorithm, currently implemented in commercial treatment planning systems do not predict the dose distribution accurately in many situations where lateral electron equilibrium does not exist. Using a Markus ionization chamber and different solid water slabs to simulate different air cavities, it is found that internal body cavities, depending upon their sizes, experience underdose or overdose on the distal surfaces of the cavities when compared with the results predicted by an ETAR correction method for 6 MV and 18 MV x-ray beams. For an infinitely long air passage of dimensions 2 cm x 2 cm, the error in the ETAR correction for a 6 MV x-ray beam is 4.8%, 0.5% and 1.1% for the field size of 5 cm x 5 cm, 7 cm x 7 cm and 10 cm x 10 cm respectively. The ETAR correction is accurate to within 1.6% for a 6 MV x-ray beam provided that the field size is 5 cm across the cavity and greater than 7 cm along it.

Absorption↗
PubMed1991

Stable isotopes for determining biokinetic parameters of tellurium in rabbits.

We have compared the use of stable and radioactive isotopes for determining the concentration of tellurium in body fluids of animals and man, specifically in the blood plasma of rabbits. Particular effort has been devoted to developing a sample-processing technique that allows the total amount of tellurium and isotope ratios to be measured by graphite furnace atomic absorption spectrometry (GFAAS) and secondary ion mass spectrometry (SIMS), respectively. The procedure employed in the SIMS analysis is discussed in detail. Investigations on the plasma clearance and the fractional intestinal absorption were carried out on four rabbits. Tracer solutions containing stable tellurium enriched in 124Te or 126Te and radioactive tellurium (121mTe or 123mTe) were administered by gavage and/or intravenously. Blood samples were drawn during the first 2 days after application. The activity of the separated plasma was measured by standard gamma ray spectrometry. After wet ashing and solvent extraction with MIBK the samples were analyzed for stable tellurium. A detection limit of 1 ng/mL of plasma could be achieved with GFAAS. For SIMS analysis the processed samples were deposited on high-purity graphite backings. Reliable isotope ratios could be determined with sample fractions containing 1 ng of tellurium or even less. The results obtained by applying stable isotopes were found to be in good agreement with the data achieved by using radioactive tracers. Studies on the intestinal absorption and the metabolic behavior of tellurium in human volunteers may thus be performed with stable isotopes.

Animals↗
PubMed1991

Renal excretion of tellurium after peroral administration of tellurium in different forms to healthy human volunteers.

As tellurium ranks among the rare non-essential trace elements there is only little known of its intestinal absorption and its metabolic behaviour in humans. Data for risk evaluations needed for occupational medicine are based on animal experiments only. In order to investigate the metabolic behaviour of tellurium in man, tellurium in different forms was administered perorally to healthy male human volunteers. It was given as sodium tellurate, sodium tellurite, metallic colloid and intrinsically bound in cress. For the latter, cress was cultivated with tellurium-containing water in order to provide tellurium for ingestion in a form which is more equivalent to foodstuffs. After the administration the urinary excretion of tellurium was determined. Tellurium concentrations were measured in urine samples by means of graphite furnace atomic absorption spectroscopy (GFAAS) after wet ashing and a preconcentration of tellurium by solvent extraction with isobutyl methyl ketone (IBMK). From the cumulative tellurium excretion in the first four days after the administration, a percentage intestinal absorption of 25% +/- 10% for soluble tellurium salts can be calculated. The renal tellurium excretion is faster after administration of hexavalent tellurium than after ingestion of the tetravalent form. This can explain the higher toxicity of the tetravalent tellurium compounds found in animal experiments. The introduction of tellurium to cress lowered the intestinal absorption to approximately 15%. For metallic tellurium the fractional intestinal absorption was found to be about 10%.

Adult↗
PubMed1991

The dialogical dimension in therapists' dreams about their patients.

The author develops a conception of the dialogical dimension in therapists' dreams about their patients by using elements of Martin Buber's dialogue philosophy, particularly "Inclusion." By way of illustration, the author discusses one of her cases, and a dream she had about her patient, "The Dream of the Meeting." The dream is interpreted along the lines of the classical countertransference interpretation, the projective identification and the uncovering of the dialogical dimension. The use of dreams as part of "Inclusion" therapy is then discussed.

Adult↗
PubMed1990

[Not Available].

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History, Modern 1601-↗
PubMed1990

Thermoluminescence dosimetry in the Prince of Wales Hospital, Sydney.

Thermoluminescence dosimeters (TLDs) were introduced in the Radiotherapy Department of the Prince of Wales Hospital in 1975. They are used in three different types of applications: 1. to confirm doses in complicated geometries (e.g. axilla, ear, nose), 2. to measure the dose delivered to critical organs (e.g. lens, scrotum) and 3. to monitor certain treatments like total body irradiations (TBI). Using four LiF chips (TLD 100; size: 3.1mm x 3.1mm x 0.9mm) per measuring point the accuracy of each measurement is +/- 5%. Approximately 200 TLD measurements are requested from the planning radiographers per year. Lens dose determination (32% of all cases) and total body irradiations are the main applications. Since 1988 the planning radiographers have been asked to state their expectation of the dose where it could be estimated. In 14% of these cases (7% of all requests) the expected and the measured dose differed by more than 20%. This was usually followed by revising the treatment set-up and a repeated TLD measurement. For about one third of all patients more than one dose measurement is required either to monitor the treatment or to investigate changes in the delivered dose after changes in the treatment set-up. Several changes to treatment modalities (e.g. boli in various treatments, a changed scrotal shield) were made due to TLD results. This is reflected in the still increasing number of TLD requests which demonstrates the benefit of TLDs in clinical practice.

Humans↗
PubMed1985

The precise midline forehead flap in reconstruction of the nose.

The midline forehead flap has been a preferred choice for nasal repair in our practice for over 20 years, with no significant complication ever having developed (Figs. 9 to 11). The abundance and length of tissue it provides is remarkable, given the inconspicuous defect remaining in the forehead area. Its use is economical of tissue, time, and expense to the patient. Most importantly, the midline flap provides superior aesthetic and reconstructive repair in a relatively short period of time without resorting to more complicated, and therefore more risky, flap designs.

Adult↗