Biomedical subjects

P Sandilos

Publications and source records attributed to P Sandilos.

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

Radiation risk assessment in neonatal radiographic examinations of the chest and abdomen: a clinical and Monte Carlo dosimetry study.

Seeking to assess the radiation risk associated with radiological examinations in neonatal intensive care units, thermo-luminescence dosimetry was used for the measurement of entrance surface dose (ESD) in 44 AP chest and 28 AP combined chest-abdominal exposures of a sample of 60 neonates. The mean values of ESD were found to be equal to 44 +/- 16 microGy and 43 +/- 19 microGy, respectively. The MCNP-4C2 code with a mathematical phantom simulating a neonate and appropriate x-ray energy spectra were employed for the simulation of the AP chest and AP combined chest-abdominal exposures. Equivalent organ dose per unit ESD and energy imparted per unit ESD calculations are presented in tabular form. Combined with ESD measurements, these calculations yield an effective dose of 10.2 +/- 3.7 microSv, regardless of sex, and an imparted energy of 18.5 +/- 6.7 microJ for the chest radiograph. The corresponding results for the combined chest-abdominal examination are 14.7 +/- 7.6 microSv (males)/17.2 +/- 7.6 microSv (females) and 29.7 +/- 13.2 microJ. The calculated total risk per radiograph was low, ranging between 1.7 and 2.9 per million neonates, per film, and being slightly higher for females. Results of this study are in good agreement with previous studies, especially in view of the diversity met in the calculation methods.

Abdominal Cavity↗

PubMed2006

Radiobiological modelling of dose-gradient effects in low dose rate, high dose rate and pulsed brachytherapy.

This paper presents a generalization of a previously published methodology which quantified the radiobiological consequences of dose-gradient effects in brachytherapy applications. The methodology uses the linear-quadratic (LQ) formulation to identify an equivalent biologically effective dose (BED(eq)) which, if applied uniformly to a specified tissue volume, would produce the same net cell survival as that achieved by a given non-uniform brachytherapy application. Multiplying factors (MFs), which enable the equivalent BED for an enclosed volume to be estimated from the BED calculated at the dose reference surface, have been calculated and tabulated for both spherical and cylindrical geometries. The main types of brachytherapy (high dose rate (HDR), low dose rate (LDR) and pulsed (PB)) have been examined for a range of radiobiological parameters/dimensions. Equivalent BEDs are consistently higher than the BEDs calculated at the reference surface by an amount which depends on the treatment prescription (magnitude of the prescribed dose) at the reference point. MFs are closely related to the numerical BED values, irrespective of how the original BED was attained (e.g., via HDR, LDR or PB). Thus, an average MF can be used for a given prescribed BED as it will be largely independent of the assumed radiobiological parameters (radiosensitivity and alpha/beta) and standardized look-up tables may be applicable to all types of brachytherapy treatment. This analysis opens the way to more systematic approaches for correlating physical and biological effects in several types of brachytherapy and for the improved quantitative assessment and ranking of clinical treatments which involve a brachytherapy component.

Algorithms↗

PubMed2006

Entrance radiation doses during paediatric cardiac catheterisations performed for diagnosis or the treatment of congenital heart disease.

The purpose of this study was to estimate the radiation exposure of children, during cardiac catheterisations for the diagnosis or treatment of congenital heart disease. Radiation doses were estimated for 45 children aged from 1 d to 13 y old. Thermoluminescent dosemeters (TLDs) were used to estimate the posterior entrance dose (DP), the lateral entrance dose (DLAT), the thyroid dose and the gonads dose. A dose-area product (DAP) meter was also attached externally to the tube of the angiographic system and gave a direct value in mGy cm2 for each procedure. Posterior and lateral entrance dose values during cardiac catheterisations ranged from 1 to 197 mGy and from 1.1 to 250.3 mGy, respectively. Radiation exposure to the thyroid and the gonads ranged from 0.3 to 8.4 mGy to 0.1 and 0.7 mGy, respectively. Finally, the DAP meter values ranged between 360 and 33,200 mGy cm2. Radiation doses measured in this study are comparable with those reported to previous studies. Moreover, strong correlation was found between the DAP values and the entrance radiation dose measured with TLDs.

Adolescent↗

PubMed2006

Technique charts for Kodak EC-L film screen system for portal localization in a 6MV X-ray beam.

Port films are used in radiotherapy for visual evaluation of the radiation fields and subsequent quantitative analysis. Common port films suffer from poor image quality compared to the simulator-diagnostic films and is desirable to determine the appropriate exposure required for the best image contrast. The aim of this work is to generate technique charts for the Kodak EC-L film screen system for use in a 6MV x-ray beam. Three homogeneous water phantoms were used to simulate head-neck, thorax and abdomen dimensions of adult human, correspondingly. The film screen system was calibrated in a 6MV x-ray beam and under various irradiation conditions. The film screen system behavior was studied as a function of phantom thickness, field size and air gap between the phantom and the film screen system. In each case the optimum film exposure which produces the maximum image contrast was determined. The generated technique charts for the EC-L film screen system and for a 6 MV x-ray beam are used in our radiotherapy department for daily quality assurance of the radiotherapy procedure.

Calibration↗

PubMed2005

Polymer gel dosimetry close to an 125I interstitial brachytherapy seed.

Despite its advantages, the polymer gel-magnetic resonance imaging (MRI) method has not, as yet, been successfully employed in dosimetry of low energy/low dose rate photon-emitting brachytherapy sources such as 125I or 103Pd interstitial seeds. In the present work, two commercially available 125I seed sources, each of approximately 0.5 U, were positioned at two different locations of a polymer gel filled vial. The gel vial was MR scanned with the sources in place 19 and 36 days after seed implantation. Calibration curves were acquired from the coupling of MRI measurements with accurate Monte Carlo dose calculations obtained simulating the exact experimental setup geometry and materials. The obtained gel response data imply that while linearity of response is sustained, sensitivity (calibration curve slope) is significantly increased (approximately 60%) compared to its typical value for the 192Ir (or 60Co and 6 MV LINAC) photon energies. Water equivalence and relative energy response corrections of the gel cannot account for more than 3-4% of this increase, which, therefore, has to be mainly attributed to physicochemical processes related to the low dose rate of the sources and the associated prolonged irradiation time. The calibration data obtained from one 125I source were used to provide absolute dosimetry results for the other 125I source, which were found to agree with corresponding Monte Carlo calculations within experimental uncertainties. It is therefore suggested that, regardless of the underlying factors accounting for the gel dose response to 125I irradiations, polymer gel dosimetry of new 125I or 103Pd sources should be carried out as originally proposed by Heard and Ibbot (2004 J. Phys.: Conf. Ser. 3 221-3), i.e., by irradiating the same gel sample with the new low dose rate source, as well as with a well-characterized low dose rate source which will provide the dose calibration curve for the same irradiation conditions.

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

An evaluation of the TSE MR sequence for time efficient data acquisition in polymer gel dosimetry of applications involving high doses and steep dose gradients.

The use of magnetic resonance imaging as a readout method for polymer gel dosimetry commonly involves long imaging sessions, particularly when high spatial resolution is required in all three dimensions, for the investigation of dose distributions with steep dose gradients and stringent dose delivery specifications. In this work, a volume selective turbo spin echo (TSE) pulse sequence is compared to the established Carr-Purcell-Meiboom-Gill (CPMG) multiecho acquisition with regard to providing accurate dosimetric results in significantly reduced imaging times. Polyethylene glycol diacrylate based (PABIG) gels were irradiated and subsequently scanned to obtain R2 relaxation rate measurements, using a CPMG multiecho sequence and a dual echo TSE utilizing an acceleration (turbo) factor of 64. R2 values, plotted against corresponding Monte Carlo dose calculations, provided calibration data of PABIG gels dose response over a wide dose range. A linear R2 versus dose relationship was demonstrated for both sequences with TSE results presenting reduced dose sensitivity. Although TSE data were found to deviate from linearity at lower doses compared to CPMG data, a relatively wide dynamic dose range of response extending up to approximately 100 Gy was observed for both sequences. The TSE and CPMG sequences were evaluated with a brachytherapy irradiation using a high dose rate 192Ir source and a gamma knife stereotactic radiosurgery irradiation with a single 4 mm collimator helmet shot. Dosimetric results obtained with the TSE and CPMG are shown to compare equally well with the expected dose distributions for these irradiations. The 60-fold scan time reduction achieved with TSE implies that this sequence could prove to be a useful tool for the introduction of polymer gel dosimetry in clinical radiation therapy applications involving high doses and steep dose gradients.

Brachytherapy↗

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

Limitations of quantitative bone-mass measurements using assessments of first-order statistics of grey-level histograms in plain radiographs.

PURPOSE: To evaluate the quality assurance of image-processing techniques in plain radiographs of skeletal structures. MATERIAL AND METHODS: Twenty-two patients were studied, each with one osteolytic metastasis. Accuracy and precision of tube voltage and timer were confirmed. The mean value of grey-level histograms in plain radiographs (MVGLHs) was assessed. The deviation was monitored after five sets of sequential X-rays retaining the same settings for each radiograph. RESULTS: Deviation was significantly higher in anatomical areas of thorax (21.2%) and abdomen (42.4%), while the consistency of MVGLH for weight-bearing bones was satisfactory with a maximum deviation of 2.9% (P<0.001, Kruskal-Wallis test). CONCLUSION: Assessment of MVGLH in plain radiographs is a reliable method for the extremities and generally for regions without superimposed movable tissues.

Adult↗

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↗

PubMed2003

A model for calculating shielding requirements in diagnostic X-ray facilities.

In this study a new model for calculating shielding requirements in diagnostic X-ray facilities is presented. It is based on the combination and modification of models and concepts originally proposed by other authors in order to calculate barrier requirements in diagnostic X-ray facilities accurately and realistically without unjustified exaggerations. With this model, multiple sources of radiation operating at different potentials, leakage radiation reduction when operating at potentials less than the maximum rated value, secondary radiation use factors reduction for primary barriers, attenuation by image receptor hardware and existing building materials are all taken into account. Examples of shielding calculations for typical cases are given illustrating the differences between the various models and concepts proposed, as well as the potential reduction in shielding requirements without compromising the radiation protection of public and staff.

Humans↗

PubMed2003

Field flatness and symmetry of photon beams: review of the current recommendations.

Flatness and symmetry are main parameters determining the quality of a photon beam produced by linear accelerators. The quality assurance in routine clinical practice of radiotherapy and consequently the treatment-outcome depend definitely on the physical parameters of treatment-delivery. Several recommendations from national and international associations are reported. By reviewing the current literature and mainly according to the World Health Organization (WHO) report of quality assurance in radiotherapy, we may suggest that for flatness and symmetry, the optimal level of deviation should be within +/- 3%. Flatness and symmetry should be checked monthly or once a year in accordance to the guidelines of national societies.

Calibration↗

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↗