Papers by Evangelos Georgiou
South African Journal of Radiology, 2007
Objectives: This is a good example with interesting imaging of a condition which rarely presents ... more Objectives: This is a good example with interesting imaging of a condition which rarely presents in adulthood. Methodology: Case was described and a review and short summary of the literature was done. Conclusions: The complete resection of choledochal cysts is mandatory because of risk of malignant transformation.
Physics in Medicine and Biology, Sep 19, 2006
Seeking to assess the radiation risk associated with radiological examinations in neonatal intens... more 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.
Physica Medica, Sep 1, 2016
Introduction. Abdominal obesity is associated with increased risk for morbidity. The quantificati... more Introduction. Abdominal obesity is associated with increased risk for morbidity. The quantification of visceral and subcutaneous abdominal fat volume is an important task. Purpose. The present study aims to deploy and evaluate a method for the direct quantification of visceral (VAF) and subcutaneous abdominal fat (SAF) volume on CT data, based on a parallel trapezium geometrical model. Materials and methods. VAF and SAF volume estimations were performed on abdominal CT scans from 10 consecutive patients through systematic slice sampling using a sampling intensity of 1/10. Areas of VAF and SAF were defined on 4 or 5 slices of the complete abdominal imaging with a fixed HU threshold. Thereafter, the volumes were calculated assuming a linear variation of the fat area between consecutive slices and a parallel trapezium shape for the tissues of interest. The obtained estimations were compared with respective volumes measured on the entire abdominal image sets by the reference technique of manual planimetry. Results. VAF and SAF volumes measured by the proposed model did not statistically differ from the planimetric reference volumes (VAF: P = 0.543; SAF: P = 0.568). Correlations between the estimates of the two methods were found to be strong (VAF: r = 0.996; SAF: r = 0.988). The 95% limits of agreement between the two techniques, evaluated by Bland-Altman plots, were acceptable (VAF: À23.4%, +13.2%; SAF: À14.8%, +10.9%). The measuring process of the model-based method did not exceed 4 min per subject. Conclusion. The adoption of the parallel trapezium model may reduce the user effort, providing accurate quantification of abdominal fat volumes.
Osteoporosis International, 1996
Medical Physics, Dec 18, 2012
To compare TG43-based and Acuros deterministic radiation transport-based calculations of the Brac... more To compare TG43-based and Acuros deterministic radiation transport-based calculations of the BrachyVision treatment planning system (TPS) with corresponding Monte Carlo (MC) simulation results in heterogeneous patient geometries, in order to validate Acuros and quantify the accuracy improvement it marks relative to TG43. Methods: Dosimetric comparisons in the form of isodose lines, percentage dose difference maps, and dose volume histogram results were performed for two voxelized mathematical models resembling an esophageal and a breast brachytherapy patient, as well as an actual breast brachytherapy patient model. The mathematical models were converted to digital imaging and communications in medicine (DICOM) image series for input to the TPS. The MCNP5 v.1.40 general-purpose simulation code input files for each model were prepared using information derived from the corresponding DICOM RT exports from the TPS. Results: Comparisons of MC and TG43 results in all models showed significant differences, as reported previously in the literature and expected from the inability of the TG43 based algorithm to account for heterogeneities and model specific scatter conditions. A close agreement was observed between MC and Acuros results in all models except for a limited number of points that lay in the penumbra of perfectly shaped structures in the esophageal model, or at distances very close to the catheters in all models. Conclusions: Acuros marks a significant dosimetry improvement relative to TG43. The assessment of the clinical significance of this accuracy improvement requires further work. Mathematical patient equivalent models and models prepared from actual patient CT series are useful complementary tools in the methodology outlined in this series of works for the benchmarking of any advanced dose calculation algorithm beyond TG43.
Radiotherapy and Oncology, May 1, 2017
Conclusion Our results suggested that Body6x1 might provide better SNR and image uniformity for T... more Conclusion Our results suggested that Body6x1 might provide better SNR and image uniformity for T2-weighted abdominal MRsimulation scan than other two settings. EP-1725 Predicting radiation-induced pneumonitis in NSCLC: a radiobiological and texture analysis study
Journal of physics, Nov 1, 2017
Stereotactic radiosurgery with volumetric modulated arc therapy (SRS-VMAT) has recently been intr... more Stereotactic radiosurgery with volumetric modulated arc therapy (SRS-VMAT) has recently been introduced for treatment of multiple brain metastases with a single isocenter. The technique's high efficiency is nevertheless dependent of metastatic tumors' characteristics such as size and number. In this work the impact of the metastases' size and number on the plan quality indices clinically used for plan evaluation and acceptance is investigated. Fifteen targets with a diameter of 1 cm and average volume of 0.7 cm 3 and ten targets with a diameter of 2 cm and average volume of 6.5 cm 3 were contoured on an anonymized patient CT dataset, in Monaco (Elekta) treatment planning system. VMAT plans for different target volumes (1 and 2 cm in diameter) and various target numbers (1-15) were generated using four noncoplanar arcs and the Agility (Elekta) linear accelerator (5 mm MLC width) using a Monte Carlo dose calculation algorithm and 1mm dose calculation grid resolution. Conformity index (CI), gradient index (GI) and heterogeneity index (HI) were determined for each target. High quality plans were created for both 1 cm and 2 cm in diameter targets for limited (<6) number of targets per plan. For increased number of irradiated targets (>6) both CI and GI, clinically used for plan evaluation and acceptance, were found to deteriorate.
Physics in Medicine and Biology, Jan 20, 2016
This work presents a comprehensive Monte Carlo (MC) simulation model for the Gamma Knife Perfexio... more This work presents a comprehensive Monte Carlo (MC) simulation model for the Gamma Knife Perfexion (PFX) radiosurgery unit. Model-based dosimetry calculations were benchmarked in terms of relative dose profiles (RDPs) and output factors (OFs), against corresponding EBT2 measurements. To reduce the rather prolonged computational time associated with the comprehensive PFX model MC simulations, two approximations were explored and evaluated on the grounds of dosimetric accuracy. The first consists in directional biasing of the (60)Co photon emission while the second refers to the implementation of simplified source geometric models. The effect of the dose scoring volume dimensions in OF calculations accuracy was also explored. RDP calculations for the comprehensive PFX model were found to be in agreement with corresponding EBT2 measurements. Output factors of 0.819 ± 0.004 and 0.8941 ± 0.0013 were calculated for the 4 mm and 8 mm collimator, respectively, which agree, within uncertainties, with corresponding EBT2 measurements and published experimental data. Volume averaging was found to affect OF results by more than 0.3% for scoring volume radii greater than 0.5 mm and 1.4 mm for the 4 mm and 8 mm collimators, respectively. Directional biasing of photon emission resulted in a time efficiency gain factor of up to 210 with respect to the isotropic photon emission. Although no considerable effect on relative dose profiles was detected, directional biasing led to OF overestimations which were more pronounced for the 4 mm collimator and increased with decreasing emission cone half-angle, reaching up to 6% for a 5° angle. Implementation of simplified source models revealed that omitting the sources' stainless steel capsule significantly affects both OF results and relative dose profiles, while the aluminum-based bushing did not exhibit considerable dosimetric effect. In conclusion, the results of this work suggest that any PFX simulation model should be benchmarked in terms of both RDP and OF results.
Journal of physics, Nov 1, 2017
In Stereotactic Radiosurgery (SRS), MR-images are widely used for target localization and delinea... more In Stereotactic Radiosurgery (SRS), MR-images are widely used for target localization and delineation in order to take advantage of the superior soft tissue contrast they exhibit. However, spatial dose delivery accuracy may be deteriorated due to geometric distortions which are partly attributed to static magnetic field inhomogeneity and patient/object-induced chemical shift and susceptibility related artifacts, known as sequencedependent distortions. Several post-imaging sequence-dependent distortion correction schemes have been proposed which mainly employ the reversal of read gradient polarity. The scope of this work is to review, evaluate and compare the efficacy of two proposed correction approaches. A specially designed phantom which incorporates 947 control points (CPs) for distortion detection was utilized. The phantom was MR scanned at 1.5T using the head coil and the clinically employed pulse sequence for SRS treatment planning. An additional scan was performed with identical imaging parameters except for reversal of read gradient polarity. Inhouse MATLAB routines were developed for implementation of the signal integration and average-image distortion correction techniques. The mean CP locations of the two MR scans were regarded as the reference CP distribution. Residual distortion was assessed by comparing the corrected CP locations with corresponding reference positions. Mean absolute distortion on frequency encoding direction was reduced from 0.34mm (original images) to 0.15mm and 0.14mm following application of signal integration and average-image methods, respectively. However, a maximum residual distortion of 0.7mm was still observed for both techniques. The signal integration method relies on the accuracy of edge detection and requires 3-4 hours of post-imaging computational time. The average-image technique is a more efficient (processing time of the order of seconds) and easier to implement method to improve geometric accuracy in such applications.
Physica Medica, Sep 1, 2016
associated with less complications and preservation of fertility is the prophylactic hypogastric ... more associated with less complications and preservation of fertility is the prophylactic hypogastric artery balloon occlusion (HABO) under fluoroscopic guidance. However, concerns regarding fetal radiation exposure and the associated radiogenic risks cannot be ignored. Purpose. To provide methods and data for fetal radiation dose estimation from HABO procedures. Material and methods. Mathematical phantoms that simulate a pregnant patient at the 9th month of gestation and Monte-Carlo-Nparticle (MCNP) transport code were employed. Projection-specific normalized fetal dose (NFD) data for various beam qualities were produced through simulations of the left and right internal iliac arteries. The effects of X-ray field location relative to the fetus, field size and maternal body size on NFD were investigated. To verify MCNP results, fetal dose measurements were carried out by using a physical anthropomorphic phantom simulating pregnancy at the 3rd trimester and thermoluminescence dosimeters (TLDs). Results. NFD was found to markedly depend on tube voltage, filtration, X-ray field location and size. Presented results have taken into account the effect of maternal body size on NFD. NFD derived from TLDs showed a difference of less than 13.5% compared to those estimated by MCNP simulations. Conclusion. Methods provided allow for reliable estimation of fetal radiation burden from HABO performed at any institution.
Physica Medica, Sep 1, 2016
differentiate between normal and pathologic cases and also related to type of scoliosis. Results.... more differentiate between normal and pathologic cases and also related to type of scoliosis. Results. Skin temperature distribution and homogeneity were quantitatively characterized. Temperature homogeneity is altered as a consequence of scoliosis, between the concave and convex sides of the spine curvature. Alterations tend to depend on the degree of scoliosis. The combination of parameters resulting from thermographic image analysis can effectively describe skin temperature pattern. Conclusion. Image analysis techniques can reveal alterations of skin temperature distribution due to scoliisis. Thermography has the potential to serve as a complementary tool on the evaluation of scoliosis development and treatment.
Physica Medica, Sep 1, 2016
differentiate between normal and pathologic cases and also related to type of scoliosis. Results.... more differentiate between normal and pathologic cases and also related to type of scoliosis. Results. Skin temperature distribution and homogeneity were quantitatively characterized. Temperature homogeneity is altered as a consequence of scoliosis, between the concave and convex sides of the spine curvature. Alterations tend to depend on the degree of scoliosis. The combination of parameters resulting from thermographic image analysis can effectively describe skin temperature pattern. Conclusion. Image analysis techniques can reveal alterations of skin temperature distribution due to scoliisis. Thermography has the potential to serve as a complementary tool on the evaluation of scoliosis development and treatment.
Physica Medica, Sep 1, 2016
Introduction Geometric disposition in MR images is partly attributed to patient/object induced di... more Introduction Geometric disposition in MR images is partly attributed to patient/object induced distortions stemming from susceptibility differences and chemical shift artifacts. Purpose To develop and implement a procedure for patient-induced distortion characterization, as well as to evaluate and compare three distortion correction schemes in MR images used for target localization in stereotactic radiosurgery. Materials and methods A specially designed phantom was modified to accommodate two cylindrical-shape volumes lying in close proximity to control points (CPs) for distortion correction. These volumes, simulating brain metastases, were filled with Gd-solution, while the rest phantom was filled with standard copper sulfate solution. Following 1.5T MRI, Gd-induced distortion was estimated using CPs disposition analysis. Moreover, three distortion correction schemes were employed: the phase difference map (based on a dual-echo gradient echo sequence), as well as the average-image and the signal integration techniques (both based on frequency-encoding direction reversal). Residual distortion was assessed by comparing with corresponding CT scans. Results Gd-induced distortion results in a structure disposition of the of the order of 1 mm. The phase difference method proves excellent at high-signal regions but lack of phase information at low-signal areas results in unwrapping errors. The signal integration method relies on the accuracy of edge detection and noise removal at low-signal areas. The average-image technique is a simple and efficient method to improve geometric accuracy in such applications. Conclusion Patient-induced distortion in MR images should be considered and corrected for high fidelity target localization. One should be aware of the relative advantages/drawbacks of alternative correction schemes.
Physica Medica, Sep 1, 2016
Introduction The use of small fields in modern radiotherapy techniques has stretched the importan... more Introduction The use of small fields in modern radiotherapy techniques has stretched the importance of verifying the delivered 3D dose distributions characterized by steep dose gradients and complex shapes. In the absence of an ideal dosimeter a multi-detector approach is essential. Purpose To apply 3D PRESAGE dosimetry for the verification of helical TomoTherapy dose distributions and assess possible corrections of the A1SL ion-chamber response for a head-and-neck plan. Materials and methods A plan-class specific reference field (fpcsr), as recommended by TG-148, and a typical head-and-neck irradiation plan were delivered to a PMMA phantom incorporating appropriate inserts for A1SL ion-chamber, film and PRESAGE dosimeters. Irradiation of fpcsr permitted the evaluation of the corresponding k(Qpcsr, fpcsr; Qref, fref) correction factor for the A1SL chamber, through comparison to PRESAGE measurements. PRESAGE dosimetry results for the head-and-neck plan, were compared to TPS calculations and corresponding measurements from 1D/2D dosimeters commonly employed for TomoTherapy QA purposes. Results Although within experimental uncertainties A1SL chamber was found to overestimate the dose at fpcsr by almost 2%, in agreement with published results using alanine dosimeters. Agreement within uncertainties was also observed between planned and presage-measured dose distribution for the head-and-neck plan, exhibiting local gamma index passing rates >90%. When placed within a homogeneous dose distribution, the response of A1SL seemed to be correction free. Conclusion Besides water equivalence and exquisite spatial resolution, necessitated for small field dosimetry and determination of appropriate correction factors, PRESAGE dosimeters offer the advantage of 3D dose verification. Disclosure Research supported by an IKY fellowship of excellence for postdoctoral research in Greece–Siemens Program.
Physica Medica, Sep 1, 2016
steep dose gradients can benefit from the high spatial resolution offered by radiochromic films. ... more steep dose gradients can benefit from the high spatial resolution offered by radiochromic films. Purpose. To evaluate single against triple channel film dosimetry techniques for patient specific dose verification procedures in SRS applications. Materials and methods. Dose verification of a gamma knife (GK) patient plan (target volume: 3.2 cm 3) was performed using Gafchromic EBT3 films placed in a solid water phantom. The phantom with the films in place followed all steps in the GK treatment chain and irradiated with 12 shots at a maximum dose of 10 Gy using the Perfexion unit. Scanned images were obtained using an Epson V750PRO flatbed optical scanner in 48-bit RGB transmission mode with a resolution of 150 dpi and converted to dose maps using both single-and triple-channel techniques. The 3D Gamma Index (GI) tool was used to quantitatively compare calculated (GammaPlan) against experimental dose distributions, using passing criteria of 1mm distanceto-agreement and 2% local dose-difference, while a threshold of 10% of maximum dose was applied. Results. Dosimetric results from both techniques revealed an overall excellent agreement between calculated and measured dose distributions. GI maps for the single and triple channel technique showed a passing rate of 95% and 99%, respectively. Moreover, implementation of triple channel technique is simpler, faster and characterized by reduced systematic uncertainties. Conclusion. Compared to the single channel, triple channel presents better passing rates, thus making it advantageous for dose verification in SRS applications.
Physics in Medicine & Biology, 2017
Detector-, field size- and machine-specific correction factors are required for precise dosimetry... more Detector-, field size- and machine-specific correction factors are required for precise dosimetry measurements in small and non-standard photon fields. In this work, Monte Carlo (MC) simulation techniques were used to calculate the [Formula: see text] and [Formula: see text] correction factors for a series of ionization chambers, a synthetic microDiamond and diode dosimeters, used for reference and/or output factor (OF) measurements in the Gamma Knife Perfexion photon fields. Calculations were performed for the solid water (SW) and ABS plastic phantoms, as well as for a water phantom of the same geometry. MC calculations for the [Formula: see text] correction factors in SW were compared against corresponding experimental results for a subset of ionization chambers and diode detectors. Reference experimental OF data were obtained through the weighted average of corresponding measurements using TLDs, EBT-2 films and alanine pellets. [Formula: see text] values close to unity (within 1%) were calculated for most of ionization chambers in water. Greater corrections of up to 6.0% were observed for chambers with relatively large air-cavity dimensions and steel central electrode. A phantom correction of 1.006 and 1.024 (breaking down to 1.014 from the ABS sphere and 1.010 from the accompanying ABS phantom adapter) were calculated for the SW and ABS phantoms, respectively, adding up to [Formula: see text] corrections in water. Both measurements and MC calculations for the diode and microDiamond detectors resulted in lower than unit [Formula: see text] correction factors, due to their denser sensitive volume and encapsulation materials. In comparison, higher than unit [Formula: see text] results for the ionization chambers suggested field size depended dose underestimations (being significant for the 4 mm field), with magnitude depending on the combination of contradicting phenomena associated with volume averaging and electron fluence perturbations. Finally, the presence of 0.5 mm air-gap between the diodes' frontal surface and their phantom-inserts may considerably influence OF measurements, reaching 4.6% for the Razor diode.
Journal of B.U.ON. : official journal of the Balkan Union of Oncology
The aim of this study was to evaluate the usefulness of scintigraphy with (99m)Tc-depreotide in t... more The aim of this study was to evaluate the usefulness of scintigraphy with (99m)Tc-depreotide in the staging of lung cancer, especially in cases where CT findings are doubtful. 53 patients with suspected lung cancer were investigated with whole body planar acquisition and single photon emission computed tomography (SPECT) of the thorax after i.v. injection of 740 MBq (99m)Tc-depreotide (Neospect). The results were compared with CT and correlated with histological findings. In 50 of 53 patients the biopsy was positive for bronchogenic carcinoma (31 patients with non small cell lung cancer [NSCLC], 18 with small cell lung cancer (SCLC), and 1 patient with neuroendocrine cancer). In the remaining 3, biopsy revealed no bronchogenic carcinoma (3 patients with inflammation). In 2/31 patients with NSCLC and doubtful CT findings, uptake of (99m)Tc-depreotide was the only non invasive reliable staging method. It seems that scintigraphy with (99m)Tc-depreotide in patients with SCLC does not ch...
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Papers by Evangelos Georgiou