Biomedical subjects

P Karaiskos

Publications and source records attributed to P Karaiskos.

At least 19 recordsLinked to original sources

PubMed2006

Dosimetric evaluation of a new collimator insert system for stereotactic radiotherapy.

The prototype of a stereotactic collimator set developed in our department is evaluated for clinical use. This set consists of three cylindrical blocks mounted on a tray which slides in the wedge insert of a Siemens Primus accelerator. Each block has a collimating hole along its long axis to produce radiation fields of circular cross-section at the isocentre plane with diameters of 15 mm, 20 mm and 25 mm. Different geometric and dosimetric quality assurance tests were performed and results are found within the limits set for stereotactic radiotherapy. Dosimetry results measured using Kodak EDR-2 radiographic film and a pinpoint ion chamber also show good agreement with corresponding results calculated by Monte Carlo simulation of the linear accelerator head and the collimators. Measured dosimetry data were used to adapt a conventional PLATO treatment planning system for stereotactic radiotherapy using the prototype collimator set. Treatment planning system calculations and film measurements for treatment of an intracranial lesion in an anthropomorphic head phantom using coplanar 180 degrees arcs are compared and found to agree within 2 mm. This supports the accuracy of dose delivery using the prototype stereotactic collimators. Despite their increased penumbra (2.5-3.5 mm relative to 2-2.5 mm for commercially available collimators) the ease of construction makes the proposed stereotactic collimators an interesting alternative for accomplishing cost effective stereotactic treatments.

Computer Simulation↗

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

Three-dimensional dose verification of the clinical application of gamma knife stereotactic radiosurgery using polymer gel and MRI.

This work seeks to verify multi-shot clinical applications of stereotactic radiosurgery with a Leksell Gamma Knife model C unit employing a polymer gel-MRI based experimental procedure, which has already been shown to be capable of verifying the precision and accuracy of dose delivery in single-shot gamma knife applications. The treatment plan studied in the present work resembles a clinical treatment case of pituitary adenoma using four 8 mm and one 14 mm collimator helmet shots to deliver a prescription dose of 15 Gy to the 50% isodose line (30 Gy maximum dose). For the experimental dose verification of the treatment plan, the same criteria as those used in the clinical treatment planning evaluation were employed. These included comparison of measured and GammaPlan calculated data, in terms of percentage isodose contours on axial, coronal and sagittal planes, as well as 3D plan evaluation criteria such as dose-volume histograms for the target volume, target coverage and conformity indices. Measured percentage isodose contours compared favourably with calculated ones despite individual point fluctuations at low dose contours (e.g., 20%) mainly due to the effect of T2 measurement uncertainty on dose resolution. Dose-volume histogram data were also found in a good agreement while the experimental results for the percentage target coverage and conformity index were 94% and 1.17 relative to corresponding GammaPlan calculations of 96% and 1.12, respectively. Overall, polymer gel results verified the planned dose distribution within experimental uncertainties and uncertainty related to the digitization process of selected GammaPlan output data.

Acrylic Resins↗

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

Monte Carlo and thermoluminescence dosimetry of the new IsoSeed model I25.S17 125I interstitial brachytherapy seed.

Monte Carlo simulation and experimental thermoluminescence dosimetry were utilized for the dosimetric characterization of the new IsoSeed model I25.S17 125I interstitial brachytherapy seed. The new seed design is similar to that of the selectSeed and 6711 seeds, with the exception of its molybdenum marker. Full dosimetric data are presented following the recommendations in the Update of the AAPM Task Group 43 report (TG-43U1). A difference of 3.3% was found between Monte Carlo dose rate constant results calculated by air kerma strengths from simulations using a point detector and a detector resembling the solid angle subtended to the seed by the Wide Angle Free Air Chamber (WAFAC) in the primary standard calibration geometry. Following the TG-43U1 recommendations, an average value of lambdaMC = (0.929 +/- 0.014) cGy h(-1) U(-1) was adopted for the new seed. This value was then averaged with the measured value of lambdaEXP = (0.951 +/- 0.044) cGy h(-1) U(-1) to yield the proposed dose rate constant for the new seed that is equal to lambda = (0.940 +/- 0.051) cGy h(-1) U(-1). The Monte Carlo calculated radial dose function and two-dimensional (2-D) anisotropy function results for the new seed were found in agreement with experimental results to within statistical uncertainty of repeated measurements. Monte Carlo simulations were also performed for 125I seeds of similar geometry and dimensions for the purpose of comparison. The new seed presents dosimetric characteristics that are very similar to that of the selectSeed. In comparison to the most extensively studied Amersham 6711 seed, the new one presents similar dosimetric characteristics with a slightly reduced dose rate constant (1.5%).

Anisotropy↗

PubMed2005

Technical note: evaluation of dosimetric performance in a commercial 3D treatment planning system.

The aim of this work was to evaluate the dosimetric performance of a commercial treatment planning system (TPS) which employs a three-dimensional calculation algorithm (Nucletron Plato version 2.2.3), following the guidelines of the AAPM Task Group 23 (TG23). Seven test cases were used to test the TPS dosimetric performance in homogeneous water. These cases involved absolute dose measurements on central as well as off-axis points situated at various depths, using simple field arrangements, and comparison with corresponding TPS calculations. This comparison yielded differences within +/-2% at all points, for all test cases. To test the ability of the TPS to account for tissue inhomogeneities, corresponding comparisons were performed with the presence of a low-density material in the beam to resemble an air inhomogeneity. Absolute dose measurements and corresponding TPS calculations showed a mean deviation of the order of +/-3.5%, reaching a maximum of 11.5% for small field sizes (5 cm x 5 cm). In summary, observed deviations are well within the set tolerance levels while comparison with previous TPS versions showed that Plato version 2.2.3 is significantly improved, especially in dose calculations in the presence of low density inhomogeneities.

Calibration↗

PubMed2004

Evaluation of a TG-43 compliant analytical dosimetry model in clinical 192Ir HDR brachytherapy treatment planning and assessment of the significance of source position and catheter reconstruction uncertainties.

A simple, time efficient, analytical model incorporating heterogeneities and body dimensions around a point 192Ir source is generalized for accurate dosimetry around commercially available 192Ir brachytherapy sources. The generalized model was verified in dosimetry of a clinical 192Ir high dose rate prostate monotherapy application, involving 16 catheters and 83 source dwell positions, through comparison with corresponding treatment planning system data. The computational time efficiency and accuracy of the proposed model allowed the assessment of the impact that uncertainties in source dwell positions and catheter reconstruction may have on dose distributions, and how these could potentially affect the clinical outcome. Results revealed that a 0.1 cm catheter reconstruction uncertainty and a 0.15 cm source position uncertainty along the catheter lead to a dose uncertainty of less than 2% for doses lower than 200% of the prescribed dose, reaching up to 5% for points lying in close proximity to the catheters. These uncertainties were found to have no impact (less than 1%) on dose volume histogram results of both the planning target volume and the urethra. A catheter reconstruction uncertainty as high as 0.2 cm results in a dose uncertainty greater than 2%, reaching up to 9%, only for points inside the 150% contour. However, even in this case, the impact on dose volume histogram calculations is less than 3%.

Brachytherapy↗

PubMed2004

A monte carlo dosimetry study of vaginal 192Ir brachytherapy applications with a shielded cylindrical applicator set.

A durable recommendation for brachytherapy treatment planning systems to account for the effect of tissue, applicator and shielding material heterogeneities exists. As different proposed approaches have not been integrated in clinical treatment planning routine yet, currently utilized systems disregard or, most commonly, do not fully account for the aforementioned effects. Therefore, it is of interest to evaluate the efficacy of current treatment planning in clinical applications susceptible to errors due to heterogeneities. In this work the effect of the internal structure as well as the shielding used with a commercially available cylindrical shielded applicator set (Nucletron part # 084.320) for vaginal and rectum treatments is studied using three-dimensional Monte Carlo simulation for a clinical treatment plan involving seven source dwell positions of the classic microSelectron HDR 192Ir source. Results are compared to calculations of a treatment planning system (Plato BPS v.14.2.7), which assumes homogeneous water medium and applies a constant, multiplicative transmission factor only at points lying in the shadow of the shield. It is found that the internal structure of the applicator (which includes stainless steel, air and plastic materials) with no shield loaded does not affect the dose distribution relative to homogeneous water. In the unshielded side of the applicator with a 90 degrees, 180 degrees, or 270 degrees tungsten alloy shield loaded, an overestimation of treatment planning system calculations relative to Monte Carlo results was observed which is both shield and position dependent. While significant (up to 15%) at increased distances, which are not of major clinical importance, this overestimation does not affect dose prescription distances by more than 3%. The inverse effect of approx. 3% dose increase at dose prescription distances is observed for stainless steel shields. Regarding the shielded side of the applicator, it is shown that the default treatment planning system transmission factors for tungsten alloy result in a consistent dose over-estimation thus constituting a safe approach given the nature of associated clinical applications. Stainless steel is shown to be an ineffective shielding material with transmission factors reaching up to 0.68 at increased distances irrespective of shield geometry.

Body Burden↗

PubMed2003

An analytical dosimetry model as a step towards accounting for inhomogeneities and bounded geometries in 192Ir brachytherapy treatment planning.

A simple analytical dose rate calculation model based on primary and scatter separation that treats 192Ir as a monoenergetic source by use of appropriate attenuation and mass energy absorption coefficients is documented for accurate dosimetry in water. This model is then generalized and tested for use in any homogeneous tissue material of radiobiological interest using scatter to primary ratios calculated in water with a material density scaling to account for the difference in the scattering properties of these materials and water. The potential of the analytical model for predicting the effect of the interference of an inhomogeneity is then evaluated by comparison with corresponding Monte Carlo calculations. It is found that regardless of the inhomogeneity dimensions and position relative to the source, the model is capable of increased accuracy (better than 2%) in calculating the primary dose rate at any point not only for low-Z tissue materials but also for high-Z shielding materials where a severe hardening of the primary photons occurs. Overall, for low-Z tissue inhomogeneities the proposed model succeeds in correcting dosimetry results towards the right direction compared to commercial treatment planning systems that currently ignore the effect of phantom dimensions and inhomogeneity interference. Regarding high-Z shielding materials the proposed model accurately predicts the dose reduction just beyond the inhomogeneity (for example it predicts a dose reduction of 47% just behind a tungsten alloy cylinder of 1 cm diameter and 2 mm thickness placed at 1.4 cm away from an 192Ir source, in agreement with corresponding results in the literature) but does not account for the increasing contribution of the laterally scattered photons with increasing distance from the bounded inhomogeneity.

Algorithms↗

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↗

PubMed2003

3D dose verification in 192Ir HDR prostate monotherapy using polymer gels and MRI.

VIPAR polymer gels and 3D MRI techniques were evaluated for their ability to provide experimental verification of 3D dose distributions in a simulation of a 192Ir prostate monotherapy clinical application. A real clinical treatment plan was utilized, generated by post-irradiation, CT based calculations derived from Plato BPS and Swift treatment planning systems. The simulated treatment plan involved the use of 10 catheters and 39 source positions within a glass vessel of appropriate dimensions, homogeneously filled with the VIPAR gel. 3D high resolution MR scanning of the gel produced T2 relaxation time maps, from which 3D dose distributions were derived via an appropriate calibration procedure. Results were compared to corresponding dose distributions obtained from the Plato and Swift treatment planning systems. Quantitative comparison, on a point by point basis, was based on user adopted acceptance criteria of 5% dose-difference and 3 mm distance-to-agreement. Significant deviations between experimental and calculated dose distributions were found for doses lower than 50% due to the reduced dose resolution of the method in the low dose, low dose gradient region. Measurement errors were observed at 1.0-1.5 mm around each catheter due to MR imaging susceptibility artifacts. For most remaining points the acceptance criteria were fulfilled. Systematic offsets of the order of 1-2 mm, observed between measured and corresponding calculated isocontours at specific segments, are attributed to the 1 mm uncertainty in catheter reconstruction and 1 mm uncertainty in the alignment of the MR and CT imaging planes.

Algorithms↗

PubMed2002

Thermoluminescent dosimetry of the selectseed 125I interstitial brachytherapy seed.

This work presents experimental dosimetry results for the new selectSeed 125I prostate seed design for use with the seedSelectron afterloading device, in accordance with the AAPM advisory that all new low energy interstitial brachytherapy seeds should undergo one Monte Carlo (MC) and at least one experimental dosimetry characterization. TLD dosimetry was performed using 120 cylindrical LiF TLD type-100 rods calibrated using a 6 MV photon beam. They were irradiated in solid water phantoms for the experimental determination of the seed dose rate constant, radial dose functions and anisotropy functions. MC simulations were performed for the determination of the TLDs relative energy response that was found position independent and equal to 1.40+/-0.03, and for the calculation of the ratio of dose in liquid water to dose in solid water that was found to be well described by Dliquidwater/Dsolidwater= 1.013*r+0.030 presenting only a minor dependence on polar angle. The selectSeed dose rate constant in liquid water was found equal to 0.938+/-0.065 cGy h(-1) U(-1), which agrees within experimental uncertainties with corresponding MC results of lambdaselect Seed=0.954+/-0.005 cGy h(-1) U(-1). The experimental radial dose and anisotropy function results were also found in good agreement with corresponding MC calculations.

Anisotropy↗

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

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↗