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F Verhaegen

Publications and source records attributed to F Verhaegen.

At least 37 records · Page 2Linked to original sources
PubMed2001

A systematic Monte Carlo study of secondary electron fluence perturbation in clinical proton beams (70-250 MeV) for cylindrical and spherical ion chambers.

Current dosimetry protocols for clinical protons do not take into account any secondary electron fluence perturbation in ion chambers. In this work, we performed a systematic study of secondary electron fluence perturbation factors for spherical and cylindrical ion chambers in proton beams (70-250 MeV). The electron fluence perturbation factor, pe, was calculated using Monte Carlo transport of protons and secondary electrons. The influence of proton energy, cavity wall material (graphite, water, A150, PMMA, polystyrene), cavity radius, cavity wall thickness and positioning depth in water is studied. The influence of inelastic nuclear proton interactions is briefly discussed. It was found that pe depends on wall material; the largest values for pe were obtained for ion chambers with A150 walls (pe=1.009), the smallest values for graphite walls. The perturbation factor was found to be largely independent of proton energy. A slight decrease of pe with cavity radius was obtained, especially for low energy protons. The wall thickness was found to have no effect on pe in the range studied (0.025-0.1 cm). The depth of the cavity in a water phantom was also found to have an insignificant effect on pe. Based on the results in the paper for spherical and cylindrical ion chambers, a method to calculate pe for a thimble ion chamber is presented. The results presented in this paper for cylindrical and spherical ion chambers are in contradiction to the calculated electron fluence perturbation factors for planar ion chambers in the paper by Casnati et al.

Electrons↗
PubMed2000

Backscatter towards the monitor ion chamber in high-energy photon and electron beams: charge integration versus Monte Carlo simulation.

In some linear accelerators, the charge collected by the monitor ion chamber is partly caused by backscattered particles from accelerator components downstream from the chamber. This influences the output of the accelerator and also has to be taken into account when output factors are derived from Monte Carlo simulations. In this work, the contribution of backscattered particles to the monitor ion chamber response of a Varian 2100C linac was determined for photon beams (6, 10 MV) and for electron beams (6, 12, 20 MeV). The experimental procedure consisted of charge integration from the target in a photon beam or from the monitor ion chamber in electron beams. The Monte Carlo code EGS4/BEAM was used to study the contribution of backscattered particles to the dose deposited in the monitor ion chamber. Both measurements and simulations showed a linear increase in backscatter fraction with decreasing field size for photon and electron beams. For 6 MV and 10 MV photon beams, a 2-3% increase in backscatter was obtained for a 0.5 x 0.5 cm2 field compared to a 40 x 40 cm2 field. The results for the 6 MV beam were slightly higher than for the 10 MV beam. For electron beams (6, 12, 20 MeV), an increase of similar magnitude was obtained from measurements and simulations for 6 MeV electrons. For higher energy electron beams a smaller increase in backscatter fraction was found. The problem is of less importance for electron beams since large variations of field size for a single electron energy usually do not occur.

Electrons↗
PubMed2000

Correlation of patient skin doses in cardiac interventional radiology with dose-area product.

The use of X-rays in cardiac interventional radiology has the potential to induce deterministic radiation effects on the patient's skin. Guidelines published by official organizations encourage the recording of information to evaluate this risk, and the use of reference values in terms of the dose-area product (DAP). Skin dose measurements were made with thermoluminescent dosemeters placed at eight different locations on the body. In addition, DAP was recorded in 100 patients for four types of interventional radiology procedures. Mean, median and third quartile for these results are presented. Maximum skin dose values found were 412 mGy, 725 mGy, 760 mGy and 1800 mGy for coronary catheterization, coronary catheterization with left ventricle investigation, and percutaneous transluminal angiography without and with stenting, respectively. Median DAPs for these same procedures were, respectively, 5682 cGy cm2, 10,632 cGy cm2, 10,880 cGy cm2 and 13,161 cGy cm2. The relationship between DAP and skin dose was investigated. We found a poor correlation of DAP with maximum skin dose (r = 0.77) and skin dose indicator (r = 0.78). Using conversion factors derived from Monte Carlo simulations, skin dose distributions were calculated based on the measured DAPs. Agreement between the calculated skin dose distribution, using DAP values averaged over a group of patients who underwent coronary catheterization and left ventricle investigation, and the measured skin dose averaged over the same group of patients was very good. However, there were large differences between the calculated skin doses using the individual DAP data per patient and measured skin doses for individual patients (r = 0.66). Hence, calculation of individual skin doses based on the specific DAP data per patient is not reliable and therefore measuring skin dose is preferable.

Cardiac Catheterization↗
PubMed2000

[Stabilized epilepsy in three adult cases of pachygyria].

In complex malformations of the neocortex due to neuronal migration disorders, epilepsy is usually intractable and is observed in childhood. The study of such malformations in adults is rare. Three adult cases are described with easily treated epilepsy controlled by one or two anti-epilectic drugs. A brain CAT-scan of these three patients showed pachygyria (macrogyria) sometimes associated with other malformations and disorders of neurone migration. These three 28, 40 and 53 year-old-patients (one woman and two men) were mentally disturbed with complex neurological disturbances and confined to a wheelchair. Their epilepsy had began in childhood and stabilised as these patients became adults. We raised the question as to whether this is in fact the long-term outcome for epileptic patients with pachygyria. The outcome of such epilepsy due to pachygyria could be better than initially supposed.

Adult↗
PubMed1999

Secondary electron fluence perturbation by high-Z interfaces in clinical proton beams: a Monte Carlo study.

Fluence perturbation of secondary electrons from clinical proton beams (50-250 MeV) by thin high-Z planar interfaces was studied with Monte Carlo simulations. Starting from monoenergetic proton pencil beams, proton depth doses and proton fluence spectra were calculated, both in homogeneous water and near thin high-Z interfaces by using the proton transport Monte Carlo code PTRAN. This code was modified extensively to enable modelling of proton transport in non-homogeneous geometries. From the proton fluence spectra in water and in the interface materials, electron generation spectra were calculated analytically and were then used as input for an electron transport calculation with the Monte Carlo code EGS4/PRESTAII to obtain electron doses and electron fluence spectra. The interface materials used in the study were graphite, Al, Ti, Cu, Sn and Au. We found significant electron fluence perturbations on both sides of the planar interfaces, resulting in an electron dose increase upstream and a decrease downstream from the interfaces, with the magnitude of the effect depending strongly on the atomic number of the interface. For the most extreme case studied, 250 MeV protons and a gold interface, we obtained an electron dose increase of 41% upstream of the interface and a decrease of 15% downstream with both perturbations having a spatial extent of about 700 microm. The total dose perturbation due to this effect amounts to a 5% increase upstream and a 2% decrease downstream. A detailed analysis of dose and fluence perturbation is presented for a wide range of materials and proton energies.

Electrons↗
PubMed1999

Monte Carlo modelling of a virtual wedge.

Compared with a set of physical photon wedges, a non physical wedge (virtual or dynamic wedge), realized by a moving collimator jaw, offers an alternative that allows creation of a wedged field with any arbitrary wedge angle instead of the traditional four physical wedges (15 degrees, 30 degrees, 45 degrees and 60 degrees). It is commonly assumed that non-physical wedges do not alter the photon spectrum compared with physical wedges that introduce beam hardening and loss of dose uniformity in the unwedged direction. In this study, we investigated the influence of a virtual wedge on the photon spectra of a 6-10 MV Siemens MD2 accelerator with the Monte Carlo code EGS4/BEAM. Good agreement was obtained between calculated and measured lateral dose profiles at the depth of maximum dose and at 10 cm depth for 20 x 20 cm2 fields for 6 and 10 MV photon beams. By comparing Monte Carlo models of a physical wedge and the virtual wedge that was studied in this work, it is confirmed that the latter has an insignificant effect on the beam quality, whereas the former can introduce significant beam hardening.

Computer Simulation↗
PubMed1999

Monte Carlo modelling of radiotherapy kV x-ray units.

To obtain accurate information for absorbed dose calculations in water for kilovoltage x-rays, the photon spectrum, planar fluence and the angular distribution of the photons at the collimator exit of the x-ray unit have to be known. The only way to obtain this information is by Monte Carlo (MC) simulation. Compared with the situation for high-energy photons and electrons, where in recent years numerous papers have been devoted to MC modelling of complete clinical accelerator units, there is a lack of similar work for kV x-ray units. A reliable MC model for a kV x-ray unit would allow the output information to be used in a treatment planning system for regular and irregular treatment fields. Furthermore, with MC simulation, perturbation factors of dose-measuring devices, such as those specified in codes of practice, can be calculated. In this work, the MC code EGS4/BEAM was used to build realistic models of two complete x-ray units. The tungsten target, exit window, collimator, additional filtration and applicator were taken into account. For some aspects of the work, a comparison was made with the simulations from another MC code, MCNP4B. The contribution to the characteristic radiation from electron impact ionization and from the photoelectric effect of reabsorbed bremsstrahlung photons was studied. Calculated and measured photon fluence spectra in air and half-value layers for a Philips MCN410 tube were compared for several anode voltages and additional filtrations. Results from the two codes agreed well, and the agreement with measured spectra was found to be good for energies above 50 keV but rather less good below that energy. For a Siemens Stabilipan 2 Th300 x-ray tube, HVLs and dose distributions in water were compared with measurements for several clinical x-ray qualities. For most of the combinations of radiation qualities and applicators, good agreement was obtained, although there were also some cases where the agreement was not so good. Electron contamination and photon build-up at the water surface were studied using MC simulation. The influence of depth on the photon spectral distribution was investigated. Both EGS4/BEAM and MCNP4B, in their default versions, handle inadequately the production of characteristic x-rays. This was found to have only a minor influence on the calculated dosimetric quantities. Simulations with MCNP4B required the use of several variance reduction techniques in order to obtain results within reasonable calculation times.

Calibration↗
PubMed1999

Monte Carlo calculations of dose distributions around 32P and 198Au stents for intravascular brachytherapy.

3D dose distributions are calculated for a 32P impregnated stent and a 198Au stent for intravascular brachytherapy with the EGS4 Monte Carlo simulation code. The stents were modeled as a combination of eight helicoidal struts. This allowed investigation of the effect of the stent geometry and the electron absorption in the strut material on the dose distributions. Absorbed dose to water was calculated at radial distances ranging from 50 microm to 5 mm from the stent surface. The dose distributions around the stents are compared to the dose distribution around an intravascular brachy-therapy 192Ir source, also calculated with the EGS4 Monte Carlo code. The dose profiles near the struts show hot spots. At 50 microm distance a peak to valley ratio of 3 for 32P and 6 for 198Au in the dose distribution is obtained. For both the isotopes the inhomogeneities decrease with distance and at a radial depth of 350 microm the effect becomes negligible. The calculations showed the importance of the effect of the absorption in the stent material as this leads to a dose decrease to 67% for the 198Au stent and to 77% for 32P near the stent at a distance of 2 mm from the stent axis. It is concluded that from the dosimetric point of view, the 198Au stent is inferior to the 32P stent and the 192Ir source. Application of the 198Au stent in clinical practice requires further investigation of the importance of the adventitia in the restenosis process, and the tolerance dose of the intima.

Angioplasty, Balloon, Coronary↗
PubMed1999

Dose measurements compared with Monte Carlo simulations of narrow 6 MV multileaf collimator shaped photon beams.

Small fields where electronic equilibrium is not achieved are becoming increasingly important in clinical practice. These complex situations give rise to problems and inaccuracies in both dosimetry and analytical/empirical dose calculation, and therefore require other than conventional methods. A natural diamond detector and a Markus parallel plate ionization chamber have been selected for clinical dosimetry in 6 MV photon beams. Results of simulations using the Monte Carlo system BEAM/EGS4 to model the beam geometry have been compared with dose measurements. A modification of the existing component module for multileaf collimators (MLCs) allowed the modeling of a linear accelerator SL 25 (Elekta Oncology Systems) equipped with a MLC with curved leaf-ends. A mechanical measurement method with spacer plates and a light-field edge detection technique are described as methods to obtain geometrical data of collimator openings for application in the Monte Carlo system. Generally a good agreement is found between measurements and calculations of depth dose distributions and deviations are typically less than 1%. Calculated lateral dose profiles slightly exceed measured dose distributions near the higher level of the penumbras for a 10x2 cm2 field, but agree well with the measurements for all other cases. The simulations are also able to predict variations of output factors and ratios of output factors as a function of field width and field-offset. The Monte Carlo results demonstrate that qualitative changes in energy spectra are too small to explain these variations and that especially geometrical factors affect the output factors and depth dose curves and profiles.

Biophysical Phenomena↗
PubMed1998

Heterogeneous photobleaching in confocal microscopy caused by differences in refractive index and excitation mode.

The photobleaching of fluorescence emission during confocal laser scanning was studied on well-defined, stained objects [microspheres of polystyrene or fluorescent gels of fluorescein isothiocyanate (FITC)-labeled dextran] and on biological samples. X,Y laser scanning with confocal microscopy induces fundamental differences in exposure rate and time in different z-planes orthogonal to the optical axis. A heterogeneous bleaching rate was observed at different focal levels in the polystyrene spheres and in the gels. This phenomenon can be caused by refractive index differences or is correlated with a photobleaching rate, which is dependent not only on the excitation light intensity but also on the photon flux (total intensity per unit of time). Heterogeneous excitation induced by refractive index differences results in photobleaching differences but will not necessarily cause heterogeneous emission intensity. Altered emission originating from altered excitation will be annihilated if the emitted light returns to the image plane along the same inverse path, compensating for the proportional increase or decrease in excitation intensity with an increased or decreased emission intensity. High numerical aperture or increased scanning speed increases the photobleaching rate. This leads to the conclusion that photobleaching in confocal scanning laser microscopy is dependent on photon energy flux density (joule/m2s).

Fluorescent Dyes↗
PubMed1998

Monte Carlo study of fluence perturbation effects on cavity dose response in clinical proton beams.

Current protocols for clinical proton beam dosimetry have not implemented any chamber-dependent correction factors for absorbed dose determination. The present work initiates a Monte Carlo study of these factors with emphasis on proton fluence perturbation effects and preliminary calculations of perturbation effects from secondary electrons. The proton Monte Carlo code PTRAN was modified to allow simulation of proton transport in non-homogeneous geometries of both unmodulated and modulated beams. The dose to water derived from the dose calculated in an air cavity agrees well with results from analytical calculations assuming a displacement of the point of measurement. For unmodulated beams small differences, limited to 0.8%, could be partially attributed to proton multiple scattering. Effects of replacing water around the cavity with wall material are explained by the introduction of a water-equivalent wall thickness. For modulated beams no significant perturbation effects arise. Secondary electron spectra are calculated analytically. Preliminary electron transport calculations with EGS4 show that wall perturbations of the order of 1% could result. Perturbation effects caused by the energy transport of secondary particles from inelastic nuclear interactions have not been studied here. Inclusion of inelastic nuclear energy transfers in the cavity dose, assuming total local absorption, indicate that separate scaling of this contribution with the ratio of total inelastic nuclear cross sections could be important.

Biophysical Phenomena↗
PubMed1998

Monte Carlo dosimetry study of a 6 MV stereotactic radiosurgery unit.

Small-field and stereotactic radiosurgery (SRS) dosimetry with radiation detectors, used for clinical practice, have often been questioned due to the lack of lateral electron equilibrium and uncertainty in beam energy. A dosimetry study was performed for a dedicated 6 MV SRS unit, capable of generating circular radiation fields with diameters of 1.25-5 cm at isocentre using the BEAM/EGS4 Monte Carlo code. With this code the accelerator was modelled for radiation fields with a diameter as small as 0.5 cm. The radiation fields and dosimetric characteristics (photon spectra, depth doses, lateral dose profiles and cone factors) in a water phantom were evaluated. The cone factor (St) for a specific cone c at depth d is defined as St(d, c) = D(d, c)/D(d, c(ref)), where c(ref) is the reference cone. To verify the Monte Carlo calculations, measurements were performed with detectors commonly used in SRS such as small-volume ion chambers, a diamond detector, TLDs and films. Results show that beam energies vary with cone diameter. For a 6 MV beam, the mean energies in water at the point of maximum dose for a 0.5 cm cone and a 5 cm cone are 2.05 MeV and 1.65 MeV respectively. The values of St obtained by the simulations are in good agreement with the results of the measurements for most detectors. When the lateral resolution of the detectors is taken into account, the results agree within a few per cent for most fields and detectors. The calculations showed a variation of St with depth in the water. Based on calculated electron spectra in water, the validity of the assumption that measured dose ratios are equal to measured detector readings was verified.

Brain↗
PubMed1998

In-water calibration of PDR 192Ir brachytherapy sources with an NE2571 ionization chamber.

An ionometric calibration procedure for 192Ir PDR brachytherapy sources in terms of dose rate to water is presented. The calibration of the source is performed directly in a water phantom at short distances (1.0, 2.5 and 5.0 cm) using an NE2571 Farmer type ion chamber. To convert the measured air-kerma rate in water to dose rate to water a conversion factor (CF) was calculated by adapting the medium-energy x-ray dosimetry protocol for a point source geometry (diverging beam). The obtained CF was verified using two different methods. Firstly, the CF was calculated by Monte Carlo simulations, where the source-ionization chamber geometry was modelled accurately. In a second method, a combination of Monte Carlo simulations and measurements of the air-kerma rate in water (at 1.0, 2.5 and 5.0 cm distance) and in air (1 m distance) was used to determine the CF. The obtained CFs were also compared with conversion factors calculated with the adapted dosimetry protocol for high-energy photons introduced by Tölli. All calculations were done for a Gammamed PDR 192Ir source-NE2571 chamber geometry. The conversion factors obtained with the four different methods agree to within 1% at the three distances of interest. We obtained the following values (medium-energy x-ray protocol): CF(1 cm) = 1.458; CF(2.5 cm) = 1.162; CF(5.0 cm) = 1.112 (1 sigma = 0.7% for the three distances of interest). The obtained results were checked with TLD measurements. The values of the specific dose rate constant and the radial dose function calculated in this work are in accordance with the literature data.

Brachytherapy↗
PubMed1997

Calculated depth dose distributions for proton beams in some low-Z materials.

The extended use of proton beams in clinical radiotherapy has increased the need to investigate the accuracy of dosimetry for this type of beam. As for photon and electron beams, Monte Carlo simulations are a useful tool in the study of proton dosimetry. The existing proton Monte Carlo code PTRAN developed for dosimetry purposes is designed for transport of protons in homogeneous water only. In clinical proton dosimetry as well as in treatment conditions several other materials can be present, such as plastic phantoms, plastic modulator wheels and several materials in ionization chambers. To investigate the transport of protons in other media we started from the PTRAN code, and implemented proton transport in other materials including heterogeneous systems composed of different materials. With this extended code, calculations of depth dose distributions for some low-Z materials are performed and compared with those obtained for water. The results show that for plastics (PMMA, polystyrene and A150) the depth dose characteristics are comparable to those of water. For graphite, air and aluminium larger differences are observed. The differences between water and the low-Z materials studied here are small but can be important for accurate dosimetry.

Air↗
PubMed1996

[Water calorimetry measurements in a 85 MeV clinical proton beam].

The importance of water calorimetry in clinical proton beam dosimetry has increased for various reasons: there has been an increasing interest in proton therapy from the radiotherapy world; existing dosimetry protocols for clinical proton beams recommend calorimetry as the primary dosimetry method (ie, AAPM and ECHED); water has recently become the reference material for dose specification. For the water calorimetric dose evaluation, the water calorimeter developed in Ghent was used. Ionometry was performed following the ECHED protocol. The study resulted in a calorimetric to ionometric dose ratio of 0.974 +/- 0.009. The discrepancy should in our opinion be attributed to the (Wle)p,air value of 35.2 J/C implemented in the protocol, possibly next to ion chamber dependent effects for which indications are found in the ionometry measurements.

Calorimetry↗
PubMed1996

Water calorimetry and ionization chamber dosimetry in an 85-MeV clinical proton beam.

In recent years, the increased use of proton beams for clinical purposes has enhanced the demand for accurate absolute dosimetry for protons. As calorimetry is the most direct way to establish the absorbed dose and because water has recently been accepted as standard material for this type of beam, the importance of water calorimetry is obvious. In this work we report water calorimeter operation in an 85-MeV proton beam and a comparison of the absorbed dose to water measured by ionometry with the dose resulting from water calorimetric measurements. To ensure a proper understanding of the heat defect for defined impurities in water for this type of radiation, a relative response study was first done in comparison with theoretical calculations of the heat defect. The results showed that pure hypoxic water and hydrogen-saturated water yielded the same response with practically zero heat defect, in agreement with the model calculations. The absorbed dose inferred from these measurements was then compared with the dose derived from ionometry by applying the European Charged Heavy Particle Dosimetry (ECHED) protocol. Restricting the comparison to chambers recommended in the protocol, the calorimeter dose was found to be 2.6% +/- 0.9% lower than the average ionometry dose. In order to estimate the significance of chamber-dependent effects in this deviation, measurements were performed using a set of ten ionization chambers of five different types. The maximum internal deviation in the ionometry results amounted to 1.1%. We detected no systematic chamber volume dependence, but observed a small but systematic effect of the chamber wall thickness. The observed deviation between calorimetry and ionometry can be attributed to a combination of the value of (Wair/e)p for protons, adopted in the ECHED protocol, the mass stopping power ratios of water to air for protons, and possibly small ionization chamber wall effects.

Calorimetry↗
PubMed1996

Dependence of overall correction factor of a cylindrical ionization chamber on field size and depth in medium-energy x-ray beams.

In this paper we examine the depth and field size dependence of the overall correction factor kch for in-phantom dose determinations in orthovoltage x-ray beams. The overall correction factor is considered to be composed of three contributions, i.e., (1) a contribution from the angular dependence of the chamber response free-in-air, derived based on the measured directional response of the NE2571 for different energies combined with Monte Carlo calculations; (2) a displacement effect and (3) a stem effect, both calculated using the Monte Carlo method for different field sizes and depths. The results show a variation of, at most, 2.2% at the lowest photon energies (29.8-keV average photon energy) when going from 2 cm to 5 cm for a small circular 20-cm2 field. In the medium-energy range (> or = 100 kV), variations are limited to, at most, 1.5% for 120 kV-150 kV when comparing the most extreme variations in field size and depth (i.e., 2-cm depth; 20-cm2 area compared to 5 cm depth; 200-cm2 area). Depth variations most importantly affect the overall correction factor by hardening of the photon fluence spectrum, whereas field diameter variations affect the factor by increase or decrease of contributions of photon scattering. The work shows that taking into account the uncertainties adopted in the recent review of data and methods recommended in the IAEA code of practice, the dependence of the overall correction factor on depth and field size is insignificant for the radiation qualities between 100 kV (HVL 0.17-mm Cu, average energy: 52 keV) and 280 kV (HVL 3.41-mm Cu, average energy: 144 keV).

Calorimetry↗