Biomedical subjects

L Sakelliou

Publications and source records attributed to L Sakelliou.

At least 37 records · Page 2Linked to original sources

PubMed2002

Polymer gel dosimetry using a three-dimensional MRI acquisition technique.

In this work, three-dimensional (3-D) MRI techniques are employed in N-Vinylpyrrolidone-Argon-(VIPAR-) based polymer gel dosimetry. VIPAR gels were irradiated using a Nucletron microSelection 192Ir HDR brachytherapy remote afterloading system with single source dwell position and intravascular brachytherapy irradiation protocols. A single VIPAR gel and a single irradiation are adequate to obtain the full calibration curve needed. The 3-D dose distributions obtained with the 3-D MRI method were found to be in good agreement with the corresponding Monte Carlo calculations, for brachytherapy and intravascular irradiations. The method allows the reconstruction of isodose contours over any plane, with increased spatial resolution and accuracy following a single MR acquisition. VIPAR gel measurements using a 3-D MRI readout technique can be of particular use in the experimental dosimetry of brachytherapy sources, as well as for dose verification purposes when complex irradiation regimes and three-dimensional dose gradients are investigated.

Argon↗

PubMed2001

Narrow stereotactic beam profile measurements using N-vinylpyrrolidone based polymer gels and magnetic resonance imaging.

In this work, polymer gel-MRI dosimetry (using VIPAR gels), radiographic film and a PinPoint ion chamber were used for profile measurements of 6 MV x-ray stereotactic beams of 5 and 10 mm diameter. The VIPAR gel-MRI method exhibited a linear dose response up to 32 Gy. VIPAR gels were found to resolve the penumbra region quite accurately, provided that the in-plane image resolution of the related T2-map is adequate (< or = 0.53 mm). T2-map slice thickness had no significant effect on beam profile data. VIPAR measurements performed with a spatial resolution of 0.13 mm provided penumbra widths (80%-20% distance) of 1.34 and 1.70 mm for the 5 and 10 mm cones respectively. These widths were found to be significantly smaller than those obtained with the film (2.23 mm for the 5 mm cone, 2.45 mm for the 10 mm cone) and PinPoint (2.25 mm for the 5 mm cone, 2.52 mm for the 10 mm cone) methods. Regarding relative depth dose measurements, good correlation between VIPAR gel and PinPoint data was observed. In conclusion, polymer gel-MRI dosimetry can provide relatively accurate profile data for very small beams used in stereotactic radiosurgery since it can overcome, to some extent, the problems related to the finite size of conventional detectors.

Calibration↗

PubMed2001

Wide dynamic dose range of VIPAR polymer gel dosimetry.

In this work the extent of the linear dose response and the dynamic dose range of N-vinylpyrrolidone-argon based (VIPAR) polymer gels were investigated. VIPAR gels were irradiated using a 6 MV linear accelerator up to 60 Gy and a Nucletron microSelectron 192Ir HDR brachytherapy source to much higher doses to cover a dose range of two orders of magnitude. They were then MR scanned at 1.5 T to obtain T2-maps. VIPAR gel measurements obtained from the two irradiation regimes were calibrated against ion chamber measurements and dose calculations derived using the AAPM TG-43 protocol respectively. A satisfying agreement between the calibration results derived using the 6 MV x-rays and the 192Ir source was found for doses up to 60 Gy, implying that the response of the VIPAR gels is independent of photon energy and dose rate. A linear R2 dose response up to approximately 40 Gy and a dynamic dose range up to at least approximately 250 Gy were observed. VIPAR gel dose measurements derived using the monoexponentially fitted brachytherapy calibration data were found to be quite accurate.

Algorithms↗

PubMed2001

Bladder wall dosimetry for 131I administered activities.

Detailed dosimetric calculations of bladder dose from 131I accumulations are presented, using Monte Carlo simulations and the dynamic bladder model proposed in the recent revision of MIRD Pamphlet No 14. Penetrating radiation and electron dose to the bladder wall inner surface, as well as within the bladder wall, per unit of 131I activity accumulated in the bladder, are calculated for various bladder filling volumes. The effect of backscatter was taken into account for photon dose calculations while electron dose was determined for all the beta spectral components of 131I. Dosimetric results were used to derive simple parameterisation functions of photon and electron dose with respect to bladder filling volume in combination with any bladder physiology model. Calculations of bladder dose per unit of 131I-OIH and 131I-NaI administered activity, using the derived parametrisations, differ by less than 12% from corresponding results given in the recent revision of MIRD Pamphlet No 14.

Computer Simulation↗

PubMed2001

Dosimetry of 192Ir wires for LDR interstitial brachytherapy following the AAPM TG-43 dosimetric formalism.

Implementation of the AAPM Task Group 43 dosimetric formalism for 192Ir wires used as interstitial sources in low dose-rate (LDR) brachytherapy applications is investigated. Geometry factors, dose-rate constant values, radial dose functions, and anisotropy functions, utilized in this formalism, were calculated for various lengths of all commercially available wire source designs by means of a well-established Monte Carlo simulation code and an improved modification of the Sievert integral method. Results are presented in the form of look up tables that allow interpolation for dose-rate calculations around all practically used wire lengths, with accuracy acceptable for clinical applications.

Anisotropy↗

PubMed2001

Dosimetry close to an 192Ir HDR source using N-vinylpyrrolidone based polymer gels and magnetic resonance imaging.

In this work, the utilization of polymer gel-MRI dosimetry for measurements at distances relevant to clinical brachytherapy and intravascular applications [i.e., in the mm range, where steep three-dimensional (3-D) dose gradients exist] is investigated using N-vinylpyrrolidone-based gels. Transverse axis radial dose distributions, dose distributions parallel to the source axis, and 2-D dose distributions around the commonly used microSelectron 192Ir HDR source are measured for single source dwell position irradiations. Experimental results are found in good agreement with verified Monte Carlo calculations, even for distances less than 3 mm from the source. The effect of various MRI parameters, such as slice thickness, slice mispositioning, and in-plane resolution, on the accuracy of the method is also investigated. Possible limitations of the method are discussed, and its' overall potential in brachytherapy dosimetry is evaluated. Experimental 2-D dose distributions for an intravascular application following the Paris irradiation protocol are compared to corresponding commercial treatment planning system calculations. Results suggest that polymer gel-MRI dosimetry is capable of experimentally verifying dose distributions in relevant clinical intravascular applications.

Anisotropy↗

PubMed2001

Monte Carlo dosimetry of the selectSeed 125I interstitial brachytherapy seed.

This work provides full dosimetric data for the new selectSeed 125I prostate seed source to be distributed by Nucletron B.V. The AAPM TG-43 dosimetric formalism and the new 1999 NIST air kerma strength calibration standard have been followed. Air kerma strength, dose rate constant, radial dose functions, anisotropy functions, and anisotropy factors were calculated using Monte Carlo simulation. Corresponding calculations were also performed for the commercially available 6711 seed source, which is of similar design, for reasons of comparison. The calculated dose rate constant of the selectSeed was 0.954+/-0.005 cGy h(-1) U(-1) compared to 0.953+/-0.005 cGy h(-1) U(-1) for the 6711 source design. The latter value for the 6711 source suggests that the correction factor proposed by NIST for conversion of dose rate constants to the new 1999 NIST calibration standard may be overestimated by 2-3%. Radial dose functions of the two sources were found in good agreement for radial distances up to 4 cm, the selectSeed being less penetrating at greater radial distances (approximately 4% at 10 cm). The selectSeed source presents similar anisotropy characteristics with the 6711 source design. For both source designs, a distance and polar angle dependent discontinuity of anisotropy function values was observed owing to the dose contribution of radioactivity distributed on the ends of the cylindrical source cores. Variation of dosimetric parameters with possible variation in radioactive silver halide coating thickness of the silver source core of the new source was also investigated.

Anisotropy↗

PubMed2001

Beta versus gamma dosimetry close to Ir-192 brachytherapy sources.

The relative importance of the dose rate component owing to the beta spectrum emitted by 192Ir brachytherapy sources at the short radial distances of interest in intravascular and endobronchial applications is investigated. Separate dosimetric calculations, using Monte Carlo simulations, were performed for the gamma and beta dose rate components of an 192Ir ideal point source as well as real 192Ir source designs used in clinical practice including wire and seed sources and both Nucletron and Varian, old and new, high dose rate (HDR) source designs. A significant dose rate enhancement due to the beta spectrum emitted by 192Ir, greater than 50% for radial distances r<2 mm, was observed for an ideal point source. For real source designs, however, the magnitude of this enhancement was found to depend strongly on the sources' geometric as well as compositional details of the active core and encapsulation. A detectable effect was found for the majority of the investigated sources at radial distances less than 1 mm, but overall findings suggest that the contribution of beta particles is not significant in 192Ir clinical intravascular applications that are currently carried out. However, since treatment of vessels with smaller diameters, in the future, may lead to the development of 192Ir sources and catheters of reduced diameters, the potential effect of the beta spectrum in terms of dose enhancement to tissues in close proximity to 192Ir sources should not be ignored.

Beta Particles↗

PubMed2000

Dose rate calculations around 192Ir brachytherapy sources using a Sievert integration model.

The classical Sievert integral method is a valuable tool for dose rate calculations around brachytherapy sources, combining simplicity with reasonable computational times. However, its accuracy in predicting dose rate anisotropy around 192Ir brachytherapy sources has been repeatedly put into question. In this work, we used a primary and scatter separation technique to improve an existing modification of the Sievert integral (Williamson's isotropic scatter model) that determines dose rate anisotropy around commercially available 192Ir brachytherapy sources. The proposed Sievert formalism provides increased accuracy while maintaining the simplicity and computational time efficiency of the Sievert integral method. To describe transmission within the materials encountered, the formalism makes use of narrow beam attenuation coefficients which can be directly and easily calculated from the initially emitted 192Ir spectrum. The other numerical parameters required for its implementation, once calculated with the aid of our home-made Monte Carlo simulation code, can be used for any 192Ir source design. Calculations of dose rate and anisotropy functions with the proposed Sievert expression, around commonly used 192Ir high dose rate sources and other 192Ir elongated source designs, are in good agreement with corresponding accurate Monte Carlo results which have been reported by our group and other authors.

Anisotropy↗

PubMed2000

Monte Carlo dosimetry of a new 192Ir high dose rate brachytherapy source.

This work provides full dosimetric data for a new high-strength 192Ir source currently launched by Varian Oncology Systems for use in their high dose rate remote afterloading systems. The active core length of the new source is reduced to 5 mm compared to a value of 10 mm for the existing VariSource source design, with all other geometric source and encapsulation details being similar. Dose-rate constant, radial dose functions, geometry factors, and anisotropy functions, utilized in the AAPM Task Group 43 dose calculation formalism, were calculated using Monte Carlo simulation. Results are compared with corresponding data published for the existing VariSource and microSelectron high dose rate sources. The dose-rate constant for the new Varian source was found to be equal to 1.101 +/- 0.006cGyh(-1) U(-1), compared to values of 1.043 +/- 0.005 and 1.116 +/- 0.006 cGyh(-1) U(-1) calculated for the existing VariSource and microSelectron sources, respectively. The radial dose functions between the three sources are similar with the exception of their values at radial distances very close to the source (r approximately 2 mm) where differences of approximately 3% are observed. The new Varian source demonstrates a smaller anisotropy relative to the existing VariSource source design for polar angles close to the source longitudinal axis, due to its smaller active core length.

Anisotropy↗

PubMed2000

Limitations of the point and line source approximations for the determination of geometry factors around brachytherapy sources.

Geometry factors were calculated around commercially available pellets and elongated brachytherapy sources taking into account their actual active core geometries. These calculations were compared with corresponding ones derived using the point and line source approximations commonly used for the determination of geometry factors, as proposed by AAPM Task Group 43. The point source approximation was found to be efficient for the determination of geometry factors around single active pellets, even at radial distances very close to the source. It is also valid for the determination of geometry factors at radial distances r>2L from elongated brachytherapy sources of length L. For smaller radial distances, however, this approximation introduces significant errors, >50%, around elongated source designs, thus being unacceptable for the determination of geometry factors in this case. The line source approximation was found to accurately reproduce geometry factors around elongated brachytherapy source designs. Errors greater than 3%, due to the fact that the line source approximation ignores the radial dimension d of the source, are observed only at radial distances very close to the source (r< or =L/2), at polar angles far away from their transverse bisectors. These errors depend on the ratio d/L and increase as this ratio increases.

Biophysical Phenomena↗

PubMed2000

Alpha-particle-induced changes in the stability and size of DNA.

The effect of alpha-particle radiation on the thermal stability and size of calf thymus DNA molecules in deoxygenated aqueous solutions was investigated by thermal transition spectrophotometry, pulsed-field gel electrophoresis, and standard agarose gel electrophoresis. The thermal transition of DNA from helix to coil was studied through analysis of the UV A(260) absorbance. The results obtained for alpha particles of mean LET of 128 keV microm(-1) reveal a dual dose response: a tendency for thermal stability of the DNA helix at "low" doses, followed by an increasing instability at higher doses. The same phenomenon was observed for the mean molecular weight of DNA molecules exposed to alpha particles. The results reported here for alpha particles in the low-dose region of 0-16 Gy are consistent with our previous hypothesis of inter- and intramolecular interactions of a covalent character in gamma-irradiated DNA molecules in the dose region of 0-4 Gy.

Alpha Particles↗

PubMed1999

Low doses of alpha- and gamma-radiation enhance DNA thermal stability.

Isolated calf thymus DNA in buffered solutions has been exposed to 0-150 Gy of alpha- and gamma-radiation. The effects of alpha- and gamma-radiation on the thermal stability and electrophoretic mobility of the DNA molecules have been studied by UV spectroscopic 'melting' and Pulsed Field Gel Electrophoresis (PFGE), respectively. The observed thermal denaturation parameters were fitted to the energy propagation descriptive model. The experimental results for the samples exposed to relatively low (low) doses indicate an increased thermal stability and a reduced mobility over that of the controls. The expected overall degradation of the DNA molecules was confirmed for the samples exposed to high doses. Our results are in good agreement with the predictions of the energy propagation model, which now is also tested in the low dose region and for an additional type of ionising radiation (alpha-particles). Our findings are consistent with conformational changes at low doses resulting in a DNA form characterised by localised alterations, which affect the energy flow along the DNA molecule.

Alpha Particles↗

PubMed1999

A new polymer gel for magnetic resonance imaging (MRI) radiation dosimetry.

New composition polymer gels, the N-vinylpyrrolidone argon (VIPAR) gels, were developed and investigated as MRI dosimeters. VIPAR gels were irradiated in the dose range of 0-12 Gy by a 6 MV x-ray linear accelerator and MR-scanned in a 1.5 T magnetic resonance imager. A linear relationship was found between absorbed dose and spin spin relaxation rate R2. The dose sensitivity was found to be approximately 0.1 s(-1) Gy(-1) for a gel composition of 4% w/w in N-vinylpyrrolidone, 4% w/w in N,N'-methylene-bisacrylamide, 5% w/w in gelatine type A and 87% w/w in water. This dose sensitivity was stable with time and did not deteriorate even when a boost radiation dose of 2.5 Gy was applied 15 days after the first irradiation. Good reproducibility of these results was observed when a new batch of gels was produced and used for corresponding measurements and analysis.

Gels↗

PubMed1999

A Monte Carlo investigation of the dosimetric characteristics of the VariSource 192Ir high dose rate brachytherapy source.

An analytical Monte Carlo simulation code, incorporating in detail source construction and dimensions, was used to investigate the dosimetric characteristics of the VariSource 192Ir high dose rate brachytherapy source. Dose-rate constant, radial dose functions, and anisotropy functions, utilized in the AAPM Task Group 43 dose estimation formalism, were calculated with the source centered in a spherical water phantom of 30 cm in diameter. The results, which are in agreement with the corresponding data available in the literature, are compared to those obtained in our previous study for the widely used microSelectron 192Ir high dose-rate brachytherapy source. The dose-rate constant was found to be equal to 1.043 +/- 0.005 cGy h(-1) U(-1) for the VariSource, compared to a value of 1.116 +/- 0.006 cGy h(-1) U(-1) calculated for the microSelectron. Significant differences in the anisotropy of the two sources are observed only for polar angles close to their long axis and are due to their different dimensions.

Anisotropy↗

PubMed1998

Monte Carlo and TLD dosimetry of an 192Ir high dose-rate brachytherapy source.

An analytical Monte Carlo simulation code has been used to perform dosimetry calculations around an 192Ir high dose-rate brachytherapy source utilized in the widely used microSelectron afterloaded system. Radial dose functions, dose rate constant and anisotropy functions, utilized in the AAPM Task Group 43 dose estimation formalism, have been calculated. In addition, measurements of anisotropy functions using LiF TLD-100 rods have been performed in a polystyrene phantom to support our Monte Carlo calculations. The energy dependence of LiF TLD response was investigated over the whole range of measurement distances and angles. TLD measurements and Monte Carlo calculations are in agreement to each other and agree with published data. The influence of phantom dimensions on calculations was also investigated. Radial dose functions were found to depend significantly on phantom dimensions at radial distances near phantom edges. Deviations of up to 25% are observed at these distances due to the lack of full scattering conditions, indicating that body dimensions should be taken into account in treatment planning when the absorbed dose is calculated near body edges. On the other hand, anisotropy functions do not demonstrate a strong dependence on phantom dimensions. However, these functions depend on radial distance at angles close to the longitudinal axis of the source, where deviations of up to 20% are observed.

Anisotropy↗

PubMed1998

Effects of gamma rays on the stability and size of DNA.

The effects of gamma radiation on the stability and size of mammalian DNA were studied by using thermal transition spectrophotometry and pulsed-field and standard agarose gel electrophoresis. The experiments were performed using deproteinized calf thymus DNA in buffered deaerated aqueous solutions. A dual dose response was observed: a tendency for increased helix stability at "low" doses (0-4 Gy) accompanied by a high tendency of the DNA molecules to interact, forming larger molecules, followed by a gradual increase of degradation and helix instability at higher doses. The results reported here for the low-dose region are consistent with the hypothesis of inter- and intramolecular interactions of covalent character (crosslinking) in irradiated DNA molecules.

Animals↗

PubMed1992

Dose rate distributions around 60Co, 137Cs, 198Au, 192Ir, 241Am, 125I (models 6702 and 6711) brachytherapy sources and the nuclide 99Tcm.

Simple analytical functions derived from our point source Monte Carlo calculations on the combined attenuation and scatter factor, B exp(-mu r), for 60Co, 137Cs, 198Au, 192Ir, 241Am, 125I (models 6702 and 6711) brachytherapy sources and the nuclide 99Tcm, for water spherical geometries of radii R = 15 and 20 cm, are presented. Our results for the broadly used 60Co, 137Cs, 198Au and 192Ir brachytherapy sources can be compared directly and found in excellent agreement with the widely accepted data of Meisberger et al in the limited distance range for which the latter are valid. Our data, however, can be used with high accuracy outside this distance range. Many discrepancies observed among different data sets available in recent literature are attributable to differences in geometries used. The results for the recently introduced 241Am source are very dissimilar to those produced by any other currently used brachytherapy source. Dose rate distributions, based on the above simple functions, are proposed in accordance with the recommendations for calibration of the brachytherapy sources in terms of reference air kerma rate and were found to be in good agreement with data available in the literature. Our calculations for 125I sources (models 6702 and 6711), provided that the characteristic x-rays from titanium encapsulation are taken into account, support recent experimental and theoretical dose rate distributions indicating that currently accepted values for 125I may be overestimated.

Americium↗