Hounsfield units variations: impact on CT-density based conversion tables and their effects on dose distribution.

PURPOSE: Determination of dose error margins in radiation therapy planning due to variations in Hounsfield Units (HU) values dependent on the use of different CT scanning protocols. PATIENTS AND METHODS: Based on a series of different CT scanning protocols used in clinical practice, conversion tables for radiation dose calculations were generated and subsequently tested on a phantom. These tables were then used to recalculate the radiation therapy plans of 28 real patients after an incorrect scanning protocol had inadvertently been used for these patients. RESULTS: Different CT parameter settings resulted in errors of HU values of up to 2.6% for densities of <1.1 g/cm(3), but up to 25.6% for densities of >1.1 g/cm(3). The largest errors were associated with changes in the tube voltage. Tests on a virtual water phantom with layers of variable thickness and density revealed a sawtooth-shaped curve for the increase of dose differences from 0.3 to 0.6% and 1.5% at layer thicknesses of 1, 3, and 7 cm, respectively. Use of a beam hardening filter resulted in a reference dose difference of 0.6% in response to a density change of 5%. The recalculation of data from 28 patients who received radiation therapy to the head revealed an overdose of 1.3 ± 0.4% to the bone and 0.7 ± 0.1% to brain tissue. On average, therefore, one monitor unit (range 0-3 MU) per 100 MU more than the correct dose had been given. CONCLUSION: Use of different CT scanning protocols leads to variations of up to 20% in the HU values. This can result in a mean systematic dose error of 1.5%. Specific conversion tables and automatic CT scanning protocol recognition could reduce dose errors of these types.

Polymorphisms in the methylenetetrahydrofolate reductase gene are determinant for vascular complications after liver transplantation.

OBJECTIVE: The aim of this study was to evaluate the role of the C677T-MTHFR (methylenetetrahydrofolate reductase)-polymorphism (CC, CT and TT) for vascular complications in liver transplant recipients. DESIGN: Retrospective study. SETTING: Hepatology-Transplantation-Unit, Johann Wolfgang Goethe-University, Frankfurt am Main. SUBJECTS: 48 liver transplant recipients were included, no dropouts. METHODS: MTHFR polymorphism was detected by PCR amplification and digestion with Hinfl restriction enzyme. Vascular complications after liver transplantation were detected from the patients' records. The total serum homocysteine (HCY) was analyzed with high-pressure liquid chromatography. RESULTS: In the wild-type group (CC), the HCY levels were slightly high (14.0+/-1 micro M). Among the patients with the CT polymorphism, the HCY values were elevated (22.5+/-3 micro M). In the homozygous TT group, there was a significant increase (31.2+/-6 micro M, P<0.01) of the HCY values. The percentage of vascular complications was higher in the heterozygous CT (47%) and homozygous TT (62.5%) group compared with wild-type CC (21%). Patients with a homozygous TT genotype of the MTHFR polymorphism with a vascular complication had a highly significant elevated HCY level compared to the other genotype groups, both with and without any vascular complications (P<0.001). Recipients with an elevated HCY and the TT polymorphism have a higher probability of developing a vascular complication after transplantation (odds ratio: 4.3 and 11.0; 95% confidence interval: 1.15, 12.25 and 1.41, 85.24). CONCLUSIONS: The C677T polymorphism in the MTHFR gene and subsequent elevation of the total serum HCY is significantly associated with an increased incidence of vascular complications in liver transplant recipients.

The right choice: extremity dosemeter for different radiation fields.

Measurements of weakly penetrating radiation in personal dosimetry present problems in the design of suitable detectors and in the interpretation of their readings. For the measurement of the individual beta radiation dose, personal dosemeters for the fingers/tips are required. LiF:Mg,Cu,P is a promising thermoluminescent (TL) material which allows the production of thin detectors with sufficient sensitivity. Dosimetric properties of two different types of extremity dosemeters, designed to measure the personal dose equivalent Hp(0.07), have been compared: LiF:Mg,Ti (TLD100) and LiF:Mg,Cu,P (TLD700H). A type test for energy response for photon and beta radiation according to ISO 4037-3 and ISO-6980 was carried out and the results for both dosemeters were compared. Simultaneous measurements with both types of dosemeters were performed at workplaces, where radiopharmaceuticals containing different radioisotopes are prepared and applied. Practices in these fields are characterized by handling of high activities at very small distances between source and skin. The results from the comparison of the two-dosemeter types are presented and analysed with respect to different radiation fields. Experiments showed a satisfactory sensitivity for the thinner dosemeter (TLD 700H) for detecting beta radiation at protection levels and a good energy response.

Uncertainty budget for a whole body counter in the scan geometry and computer simulation of the calibration phantoms.

At the Austrian Research Centers Seibersdorf (ARCS), a whole body counter (WBC) in the scan geometry is used to perform routine measurements for the determination of radioactive intake of workers. The calibration of the WBC is made using bottle phantoms with a homogeneous activity distribution. The same calibration procedures have been simulated using Monte Carlo N-Particle (MCNP) code and FLUKA and the results of the full energy peak efficiencies for eight energies and five phantoms have been compared with the experimental results. The deviation between experiment and simulation results is within 10%. Furthermore, uncertainty budget evaluations have been performed to find out which parameters make substantial contributions to these differences. Therefore, statistical errors of the Monte Carlo simulation, uncertainties in the cross section tables and differences due to geometrical considerations have been taken into account. Comparisons between these results and the one with inhomogeneous distribution, for which the activity is concentrated only in certain parts of the body (such as head, lung, arms and legs), have been performed. The maximum deviation of 43% from the homogeneous case has been found when the activity is concentrated on the arms.

Calibration and background measurements with a tissue equivalent proportional counter.

A tissue-equivalent-proportional counter (TEPC) instrument has been used as the reference instrument for cosmic radiation measurement at flight altitudes by several institutes. For purposes of characterisation the response of the instrument has been investigated under different standard radiation conditions, in terms of radiation particle, energy and angle of incidence. Photon sources and photon beams of energies up to 6.6 MeV and neutron beams up to 200 MeV were used. To have a better understanding of the shielding influence of the instrument assembly, the angle dependence of response was analysed for several radiation conditions. Specific measurement conditions were simulated with the Monte Carlo transport code, FLUKA. The measured instrument response was compared with simulation results. It was demonstrated, that simulations were very helpful to understand the instrument's response. The TEPC instrument used by the Austrian Research Centre Seibersdorf (ARCS) research simulates the energy deposition in a unit density tissue volume of 2 microm diameter, of similar size to a cell nucleus. Pure propane at low pressure is used as measurement gas. To characterise the instrument at low dose rates background measurements were done 800 m below ground and at the ultra low level laboratory in Gran Sasso, 1380 m below ground. These results were compared with measurements on the Earth's surface at different altitudes on mountains up to 3480 m above sea level. The significant increase of the expected dose rate is well reproduced by the experiments at mountain altitudes. As a result of this study a full characterisation and a thorough understanding of the performance and reliability of the detector was achieved.

Quality assurance for the Leksell gamma unit: considering magnetic resonance image-distortion and delineation failure in the targeting of the internal auditory canal.

Our aim in this study was to distinguish quantitatively between the localization accuracy of a commercially available stereotactic fixation device as claimed by the manufacturer and the target accuracy as measured by a user, applying neuroradiologic imaging in Gamma Knife planning and phantom irradiation. Missing the target is the most serious possible failure in Gamma Knife and Linac therapy. To reduce this risk, we developed a quality control algorithm and designed a phantom. To evaluate the accuracy of the targeting procedure with a Leksell Gamma unit, and to experience the possible errors in all procedural steps, irradiations of phantoms were performed, using the so-called "unknown" targeting method. Accuracy is defined by the extent of spatial deviation of the irradiated target from the calculated target. Digital imaging was used for therapy planning. GafChromic films, which had been irradiated while affixed to a specially developed phantom, were used for measuring the precision of the radiation unit. A series of MR images (in two plains: transverse and coronal) was acquired sequentially to image the three-dimensional (3-D) volume of the phantom. The results obtained for isocentric accuracy of the Leksell Gamma unit, model B, were in good agreement to the calculated position. The observed spatial deviations between calculated and irradiated targets is less than 1 mm. The newly designed phantom and quality control algorithm are useful in quality assurance measurements of stereotactic radiation therapy.

A novel 675.2 nm diode laser densitometer for use with GafChromic films.

In this article we compare the accuracy of a diode laser densitometer emitting 675.2 nm to that of a commercial He-Ne laser densitometer emitting 632.8 nm for GafChromic MD-55 film readout. A Leksell gamma unit (AB Elekta Stockholm, Sweden) Model B with a 14 and 8 mm collimator at the same isocenter (combined 11 mm collimator) was used to irradiate GafChromic MD-55 films. Dose response curves, dose cross profile and FWHM were measured with a custom-designed diode laser scanning device, emitting light at 675.2 nm. The same data were recorded with a commercial He-Ne laser densitometer (PTW FIPS Plus, Freiburg, Germany), emitting light at 632.8 nm. Both measurements were compared to dose cross profiles of a radiosurgery dose planning program (GammaPlan 5.12, Elekta, Sweden). Compared to the commercial He-Ne laser densitometer, the custom-designed diode laser scanning device showed better agreement with the calculated dose cross profile. For two axes, the full width half maxima (FWHM) of the diode laser scanning device was within 0.1 mm deviation compared to the data calculated by the dose planning program. The FWHM of the commercial He-Ne laser densitometer was less accurate (1.6 and 2.1 mm deviation). Our data show that a diode laser scanning device using a light source emitting 675.2 nm increases the accuracy of a GafChromic MD-55 film readout. This greater accuracy may be related to the diode laser measuring the optical density close to maximum absorption of the GafChromic film MD-55 (671-675 nm).

Dosimetry studies with TLDs for stereotactic radiation techniques for intraocular tumours.

Between March 1993 and January 1997, stereotactic radiation techniques were used to irradiate 66 intraocular tumour patients with the Gamma Knife (Leksell Gamma Knife, model B unit) at the University of Vienna, Austria. This study investigates the dosimetry for stereotactic irradiation of ocular structures. For the dosimetry program KULA 4.4, Gamma Knife stereotactic irradiation of the eye represents an extreme frontal skull position. In addition, irradiation of the eye may be performed in the usual supine position in exceptional cases only. With the patient in the prone position, the dose planning program has to calculate with a significantly large number of single-beam extrapolations. In our first experiment we measured the isocentre dose for eight different gamma-angle positions, both in prone and supine positions, using TLD measurements in an Alderson head phantom. We found a maximum deviation of +/- 1.6% using these individually calibrated TLDs. In the second experiment we examined the dose cross profiles for the two most frequently used treatment positions (supine position, gamma = 65 degrees, and prone position, gamma = 140 degrees). For this purpose we implanted a specially designed TLD array into the orbit of a human cadaver head. We found excellent agreement of the dose values measured for the isocentre as well as the posterior part of the eye with orbit with deviations of less than -2.7%. However, for the anterior part of the eye, deviations between computer-generated calculations and the TLD measurements were found to range up to -30%. These differences were noticed both for supine and prone positions. For the Gamma Knife stereotactic irradiation of ocular tumours or pathologies, precautions should be taken to avoid significant underdosage in the anterior part of the radiation field.

Shuttle dose at the Vienna Leksell Gamma Knife.

The aim of this study was to determine the shuttle dose for all collimator helmets (4, 8, 14 and 18 mm) of the Gamma Knife, model B, in Vienna, Austria. The additional dose accumulated during the transport of the patient in and out of the treatment position should be considered in the dose planning procedure of multicentre treatment regimens and in fractionated stereotactic Gamma Knife radiotherapy. The GafChromic film study was basically used to determine the shuttle dose of all four collimator helmets. In addition, measurements with an ionization chamber (18 and 14 mm collimator--and, for the 18 mm collimator helmet, TLD dosimetry--were performed in order to confirm the GafChromic film data. The shuttle dose ranged between 99.6 and 183.5 mGy, depending mainly on the size of the collimator and the irradiated isocentres at the half-life activity of Co-60 in a brand new Gamma unit. Our film-generated data were in good correlation with the dose levels obtained with the ionization chamber and the TLD dosimetry, showing a dose difference of less than 0.8%. Since it was possible to verify the shuttle dose even for the 4 and 8 mm collimator helmets, we consider it a non-negligible factor and would advocate the inclusion of the shuttle dose in radiosurgical dose planning.

[New aspects in evaluation of the average-absorbed dose of the parenchyma in mammography (author's transl)]. / Neue Aspekte für die Beurteilung der Parenchymbelastung bei der Mammographie.

Over 2000 measurements have been made on a mammaphantom to ascertain radiation exposure curves for skin and parenchyma. The exposure of the parenchyma is definitely depending on the thickness of the fatlayer above the parenchyma. Until now radiation exposure was calculated under the assumption that the different breast tissues are equally distributed. In doing so exposure was estimated too high (up to a factor 3-especially in fatter breasts). The individual absorbed dose in the parenchyma can be reckoned out by the various figures and tables, which consider technical criteria, different sizes of the breast and two film systems.

Results and conclusions of the Austrian radon mitigation project 'SARAH'.

The Austrian radon mitigation joint research project SARAH (supported by the Austrian Ministry of Economy and the Government of Upper Austria), a 2-year follow-up study of the Austrian National Radon Project (ONRAP), was started in 1996. The objectives of the research project were to find simple, cost-effective experimental methods for the characterisation of the radon situation in dwellings and to evaluate technically and economically the implementation of state of the art remedial actions for Austrian house types. After an intercomparison exercise of the assigned radon measuring instruments and detectors five houses were closely examined in regions with elevated radon levels in the federal state of Upper Austria. In this research work for the first time an extended Blower-Door method (which is conventionally used for determining the tightness of buildings) was successfully applied to radon diagnosis of buildings. In this paper the methods used for the radon diagnosis, the applied mitigation measures and the related technical and economical aspects are discussed. In conclusion of the results of this project a common strategy for solving the radon problem in Austria in the future is presented briefly.

Optimization of an active anti cosmic veto shielding.

The active veto shielding of a low-level gamma spectrometer has been optimized to reduce the background in the interval 20-2700 keV by a factor of nine. The signal to noise ratio was increased, due to the reduction of electromagnetic interference coming from the power line, by using an uninterruptible power supply and specially designed line filters. The overall performance of the veto shielding was improved by using time spectroscopy to find the optimum time duration for the coincidence window.

Characterisation of selected cadmium--zinc--telluride detectors.

The performance of new detection technologies like the ambient temperature cadmium--zinc--telluride (CdZnTe) detectors is of interest for search and identification of radioactive material in a range of applications. Various types of CdZnTe detectors that are currently available on the market as well as some specially designed ones were tested and characterised. The examination included peak shape, energy-resolution, energy-range and also the usability in hand-held applications. Additionally, the effects of the use of a cooling system were evaluated.

The use of the Monte Carlo simulation technique for the design of an H*(10) dosemeter based on TLD-100.

The thermoluminescence (TL) detector material LiF:Mg,Ti (TLD-100) and appropriate filter materials were combined in order to design a passive dosemeter measuring the operational quantity ambient dose equivalent, H*(10), for monitoring low-dose external photon radiation fields. Using the Monte Carlo simulation technique, optimisations of energy dependent conversion coefficients from air kerma free-in-air compared to ICRU and ISO proposed values. h*K(10), were performed by varying dosemeter detector positioning. geometrical arrangements, and filter materials. Deviations smaller than 5% compared to h*K(10) between 30 keV and 2.5 MeV of primary photon energies were achieved by a dosemeter design consisting of a 15 microm Sn metal layer and a 5 mm PMMA layer surrounding the LiF detector. Subsequently performed free-air verification experiments carried out in well defined standard photon radiation fields showed an obviously TL-specific effect. An underestimation up to -15% of the modelled data at low photon energies was observed.

Long term comparison of methods to sustain energy calibration in low level gamma-ray spectroscopy and investigation of possible sources for drift.

Two methods to compensate the gain drift of low level gamma-ray spectrometers are compared. For a period of 4 years, source injection and pulser method have been applied in parallel. The long term stability of a reference peak, injected by a precision pulser, has been validated by periodical source injection to sustain the energy calibration. The pros and cons of both methods and the advantage of active vs. passive energy calibration are discussed. The main cause for gain drift and the critical components have been identified.

Enhancing sensitivity of portal monitoring at varying transit speed.

The sensitivity of a portal monitoring system has been improved by introducing speed dependent integration time in the data processing of the count rate signal. Characterization and first tests have been performed using a pneumatically driven (137)Cs point source, before deploying the system to a test point at a railway junction near Vienna. At 14m/s the MDA could be reduced by a factor of two and at 28m/s by a factor of three compared to a fixed integration time of 1s, whereas high sensitivity at low speed remained unaffected.

IASON-FREE: theory and experimental comparisons.

A sophisticated flight code named FREE (acronym for Flight Route Effective Dose Estimation) was built for professional commercial usage. During its creation special precautions have been taken to take correctly into account all existing dependencies and details, so that the best possible result is achieved. Some of these factors are presented in detail and their effect on doses or dose rates is estimated. Detailed comparisons to more recent measurements for both quiescent as well as disturbed conditions are presented. The agreement at undisturbed conditions turns out to be excellent and also the comparisons to measured transient effects are very satisfactory.

Comparison of codes assessing galactic cosmic radiation exposure of aircraft crew.

The assessment of the exposure to cosmic radiation onboard aircraft is one of the preoccupations of bodies responsible for radiation protection. Cosmic particle flux is significantly higher onboard aircraft than at ground level and its intensity depends on the solar activity. The dose is usually estimated using codes validated by the experimental data. In this paper, a comparison of various codes is presented, some of them are used routinely, to assess the dose received by the aircraft crew caused by the galactic cosmic radiation. Results are provided for periods close to solar maximum and minimum and for selected flights covering major commercial routes in the world. The overall agreement between the codes, particularly for those routinely used for aircraft crew dosimetry, was better than +/-20 % from the median in all but two cases. The agreement within the codes is considered to be fully satisfactory for radiation protection purposes.