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A Angelopoulos

Publications and source records attributed to A Angelopoulos.

At least 19 recordsLinked to original sources
PubMed2006

Polymer gel dosimetry for the TG-43 dosimetric characterization of a new 125I interstitial brachytherapy seed.

In this work, a polymer gel-magnetic resonance (MR) imaging method is employed for the dosimetric characterization of a new 125I low dose rate seed (IsoSeed model I25.S17). Two vials filled with PABIG gel were prepared in-house and one new seed as well as one commercially available 125I seed of similar dose rate and well-known dosimetric parameters (IsoSeed model I25.S06) were positioned in each vial. Both seeds in each vial were MR scanned simultaneously on days 11 and 26 after implantation. The data obtained from the known seed in each vial are used to calibrate the gel dose response which, for the prolonged irradiation duration necessitated by the investigated dose rates, depends on the overall irradiation time. Data for this study are presented according to the AAPM TG-43 dosimetric formalism. Polymer gel results concerning the new seed are compared to corresponding, published dosimetric results obtained, for the purpose of the new seed clinical implementation, by our group using the established methods of Monte Carlo (MC) simulation and thermo-luminescence dosimetry (TLD). Polymer gel dosimetry yields an average dose rate constant value of lambda = (0.921 +/- 0.031) cGy h(-1) U(-1) relative to (MC)lambda = (0.929 +/- 0.014) cGy h(-1) U(-1), (TLD)lambda = (0.951 +/- 0.044) cGy h(-1) U(-1) and the average value of Lambda = (0.940 +/- 0.051) cGy h(-1) U(-1) proposed for the clinical implementation of the new seed. Results for radial dose function, g(L)(r), and anisotropy function, F(r, theta), also agree with corresponding MC calculations within experimental uncertainties which are smaller for the polymer gel method compared to TLD. It is concluded that the proposed polymer gel-magnetic resonance imaging methodology could be used at least as a supplement to the established techniques for the dosimetric characterization of new low energy and low dose rate interstitial brachytherapy seeds.

Brachytherapy↗
PubMed2005

Dose verification of single shot gamma knife applications using VIPAR polymer gel and MRI.

This work describes an experimental procedure with potential to assess the overall accuracy associated with gamma knife clinical applications, from patient imaging and dosimetry planning to patient positioning and dose delivery using the automated positioning system of a Leksell Gamma Knife model C. The VIPAR polymer gel-MRI dosimetry method is employed due to its inherent three-dimensional feature and linear dose response over the range of gamma knife applications. Different polymer gel vials were irradiated with single shot gamma knife treatment plans using each of the four available collimator helmets to deliver a maximum dose of 30 Gy. Percentage relative dose results are presented not only in the form of one-dimensional profiles but also planar isocontours and isosurfaces in three dimensions. Experimental results are compared with corresponding Gammaplan treatment planning system calculations as well as acceptance test radiochromic film measurements. A good agreement, within the experimental uncertainty, is observed between measured and expected dose distributions. This experimental uncertainty is of the order of one imaging pixel in the MRI gel readout session (<1 mm) and allows for the verification of single shot gamma knife applications in terms of acceptance specifications for precision in beam alignment and accuracy. Averaging net R(2) results in the dose plateau of the 4 mm and 18 mm collimator irradiated gel vials, which were MR scanned in the same session, provides a crude estimate of the 4 mm output factor which agrees within errors with the default value of 0.870.

Argon↗
PubMed2005

Relative output factor measurements of a 5 mm diameter radiosurgical photon beam using polymer gel dosimetry.

Besides the fine spatial resolution inherent in polymer gel-magnetic resonance imaging (MRI) dosimetry, the method also features the potential for multiple measurements of varying sensitive volume in a single experiment by integrating results in MRI voxels of finite dimensions (i.e., in plane resolution by slice thickness). This work exploits this feature of polymer gel dosimetry to propose an experimental technique for relative output factor (OF) measurements of small radiosurgical beams. Two gel vials were irradiated with a 5 and 30 mm diameter 6 MV radiosurgery beam and MR scanned with the same slice thickness and three different in plane resolutions. Using this experimental data set, 5 mm OF measurements with the PinPoint ion chamber are simulated by integrating results over a sensitive volume equal to that of the chamber. Results are found in agreement within experimental uncertainties with actual PinPoint measurements verifying the validity of the proposed experimental procedure. The polymer gel data set is subsequently utilized for OF measurements of the 5 mm beam with varying sensitive volume to discuss the magnitude of detector volume averaging effects. Seeking to correct for volume averaging, results are extrapolated to zero sensitive volume yielding a 5 mm OF measurement of (0.66+/-5%). This result compares reasonably with corresponding ionometric and radiographic film measurements of this work and corresponding, limited, data in the literature. Overall, results suggest that polymer gel dosimetry coupled with the proposed experimental procedure helps overcome not only tissue-equivalence and beam perturbation implications but also volume averaging and positioning uncertainties which constitute the main drawback in small radiosurgical beam dosimetry.

Calibration↗
PubMed2003

Evaluation of the performance of VIPAR polymer gels using a variety of x-ray and electron beams.

The aim of this investigation was the evaluation of the usefulness of N-vinyl pyrrolidone argon (VIPAR) polymer gel dosimetry for relative dose measurements using the majority of types and energies of radiation beams used in clinical practice. For this reason, VIPAR polymer gels were irradiated with the following beams: 6 and 23 MV photons (maximum dose: 15 Gy) and 6, 9, 12, 15, 18 and 21 MeV electrons (90% dose: 15 Gy). Using 6 MV x-rays, a linear gel dose response was verified for doses up to 20 Gy. Assuming linearity of response for the rest of the photon and electron beams used in this study, percentage depth dose measurements were derived. For all beams used and the range of relative doses studied, a satisfying agreement was observed between percentage depth dose measurements performed using the VIPAR gel-MRI method and an ion chamber, validating the assumption that a linear gel dose response holds for all photon and electron beams studied. VIPAR gels, therefore, can be used for relative dose distribution measurements using photons or electrons of any typical energy used in external radiotherapy applications. It is also demonstrated that two-dimensional dose distribution measurements through an irradiated (9 MeV electrons, 3 cm x 3 cm cone) VIPAR gel volume can be easily obtained.

Argon↗
PubMed2003

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

A new microSelectron pulsed dose rate source has been designed, containing two active pellets instead of one inactive and one active pellet contained in the old design, to facilitate the incorporation of higher activity up to 74 GBq (2 Ci). In this work, Monte Carlo simulation is used to derive full dosimetric data following the AAPM TG-43 formalism, as well as the dose rate per unit air kerma strength data in Cartesian, "away and along" coordinates for both source designs. The calculated dose rate constant of the new PDR source design was found equal to lambda=(1.121 +/- 0.006) cGy h(-1) U(-1) compared to lambda = (1.124 +/- 0.006) cGy h(-1) U(-1) for the old design. Radial dose functions of the two sources calculated using the point source approximated geometry factors were found in close agreement (within 1%) except for radial distances under 2 mm. At polar angles close to the longitudinal source axis at the sources' distal end, the new design presents increased anisotropy (up to 10%) compared to the old one due to its longer active core. At polar angles close to the longitudinal source axis at the sources' drive wire end however, the old design presents increased anisotropy (up to 18%) due to attenuation of emitted photons through the inactive Ir pellet. These differences, also present in "away and along" dose rate results, necessitate the replacement of treatment planning input data for the new microSelectron pulsed dose rate source.

Anisotropy↗
PubMed2003

Monte Carlo dosimetry of 60Co HDR brachytherapy sources.

Monte Carlo based dosimetry is presented for three 60Co HDR sources with a unique configuration of two active pellets in contact or spaced 9 and 11 mm apart. Results are presented in Cartesian "away and along" as well as polar coordinates following the AAPM TG-43 dosimetric formalism. Iso-dose rate contours around the sources in Cartesian coordinates reveal that significant differences between the three source designs exist only close to the source centers where dose rate distributions bear the effect of the unique source configurations. Dose rate constants of all three sources are accurately described by an equation of the form: lambda(cGyh(-1)U(-1))=lambda*pointG(1 cm,90 degrees) = 1.094*G(1 cm,90 degrees) where lambdapoint is the dose rate constant of a bare 60Co point source and G(1 cm,90 degrees) is the "exact" source geometry factor as defined by the TG-43. Radial dose and anisotropy function data extracted using the point source approximated geometry factors are tabulated for all three source designs. Finally, the dependence and variation of the above TG-43 parameters are discussed and it is demonstrated that the dosimetric properties of high-energy photon emitters are largely dependent on radionuclide source distribution.

Brachytherapy↗
PubMed2002

Dosimetry comparison of 192Ir sources.

192Ir sources besides being widely utilized in the field of conventional brachytherapy also find use in contemporary peripheral and coronal intravascular applications. In this study, the same Monte Carlo simulation code and input data were used to investigate differences between the dose rate distributions of the most commonly used 192Ir sources in the cm and mm distance range. Findings are discussed in view of differences in source and encapsulation dimensions as well as structural details. Results are presented in the AAPM TG-43 formalism, as generalized by AAPM TG-60, for five 192Ir HDR source designs as well as an LDR seed and an LDR wire source. Dose rate constants of the sources at r0 = 1 cm and r0 = 2 mm were found proportional to the corresponding geometry factors along the transverse source bisectors and an equation of the form lambda r0(cGyh(-1) U(-1)) = 1.12 x G(r0,90 degrees) provides results within clinical accuracy (less than 2%) for any 192Ir source. Radial dose functions do not depend significantly on source and encapsulation geometry and agree within 2% with that of a point 192Ir source. Anisotropy is of importance for accurate dosimetry at the cm distance range but it does not affect dose rate in the mm distance range significantly. At such short radial distances the source geometry factor defines the shape of isodose lines. Dose uniformity at given distances from the sources is strongly dependent on source dimensions as indicated by dose rate profiles in polar and Cartesian coordinates.

Anisotropy↗
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

Use of membrane and bone grafts in the reconstruction of orbital fractures.

OBJECTIVE: To present and analyze the clinical results derived from the use of different grafts for the reconstruction of orbital defects during a 10-year period. STUDY DESIGN: Fifty-five fracture cases with orbital bony defect, requiring a graft, are presented. The surgical treatment includes the reconstruction of the fracture (osteosynthesis) and the repair of the remaining bone defect by graft, with the type of graft dependent on the size of the defect. For minor defects membranes were used (lyophilized dura or alloplastic dura mater), whereas major defects were repaired with bone grafts (autografts, heterografts, or bone substitute material). All patients have been regularly evaluated for at least one year postoperatively. RESULTS: All grafts were well tolerated by the patients. Diplopia subsided in all but 5 cases, motility disturbance was fully repaired in all but 3 cases. Esthetics were improved in cases with severe bone defect. CONCLUSION: The wide variety of grafts available allows successful reconstruction of all types of orbital bony defects. The clinician should be able to use different types of grafts depending on the type and size of the defect.

Adolescent↗
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

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↗
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↗
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

Epidemiological aspects of tuberculosis in the Padua Health District 1985-1996.

The present study was carried out in order to monitor the epidemiology of Mycobacterium tuberculosis disease in the Padua Health District over the period 1985-1996 with a view to the situation in some population subgroups. The average annual rate per 100,000 persons per year was 7.9 with a peak in 1994 (15.6 per 100,000 persons). The most significant variations are not per annum but between the periods 1985-1990 and 1991-1996 (average rates per 100,000 persons: 4.8 and 10.9, respectively). The trend shows an increasing frequency in all age groups and especially among the elderly. Immigration from endemic areas and human immunodeficiency virus (HIV) infection appear to contribute to the increase the number of notified cases among adults. In the last four years an increase of tuberculosis cases was also observed among subjects with professional exposure risk. Improvements of programs both at national and regional level are believed to be essential to the tuberculosis control.

Adolescent↗