Monte Carlo determination of a nanoDot OSLD response using quality index for diagnostic kilovoltage X-ray beams

PurposeThis study aims to investigate the energy response of an optically stimulated luminescent dosimeter known as nanoDot for diagnostic kilovoltage X-ray beams via Monte Carlo calculations.MethodsThe nanoDot response is calculated as a function of X-ray beam quality in free air and on a water phantom surface using Monte Carlo simulations. The X-ray fluence spectra are classified using the quality index (QI), which is defined as the ratio of the effective energy to the maximum energy of the photons. The response is calculated for X-ray fluence spectra with QIs of 0.4, 0.5, and 0.6 with tube voltages of 50–137.6 kVp and monoenergetic photon beams. The surface dose estimated using the calculated response is verified by comparing it with that measured using an ionization chamber.ResultsThe nanoDot response in free air for monoenergetic photon beams (QI = 1.0) varies significantly at photon energies below 100 keV and reaches a factor of 3.6 at 25–30 keV. The response differs by up to approximately 6% between QIs of 0.4 and 0.6 for the same half-value layer (HVL). The response at the phantom surface decreases slightly owing to the backscatter effect, and it is almost independent of the field size. The agreement between the surface dose estimated using the nanoDot and that measured using the ionization chamber for assessing X-ray beam qualities is less than 2%.ConclusionsThe nanoDot response is indicated as a function of HVL for the specified QIs, and it enables the direct surface dose measurement.     

A Monte Carlo software bench for simulation of spectral k-edge CT imaging: Initial results

PurposeSpectral Computed Tomography (SCT) systems equipped with photon counting detectors (PCD) are clinically desired, since such systems provide not only additional diagnostic information but also radiation dose reductions by a factor of two or more. The current unavailability of clinical PCDs makes a simulation of such systems necessary.MethodsIn this paper, we present a Monte Carlo-based simulation of a SCT equipped with a PCD. The aim of this development is to facilitate research on potential clinical applications. Our MC simulator takes into account scattering interactions within the scanned object and has the ability to simulate scans with and without scatter and a wide variety of imaging parameters. To demonstrate the usefulness of such a MC simulator for development of SCT applications, a phantom with contrast targets covering a wide range of clinically significant iodine concentrations is simulated. With those simulations the impact of scatter and exposure on image quality and material decomposition results is investigated.ResultsOur results illustrate that scatter radiation plays a significant role in visual as well as quantitative results. Scatter radiation can reduce the accuracy of contrast agent concentration by up to 15%.ConclusionsWe present a reliable and robust software bench for simulation of SCTs equipped with PCDs.     

An X-ray spectrum estimation method from transmission measurement combined with scatter correction

PurposeConventional x-ray spectrum estimation methods from transmission measurement often lead to inaccurate results when extensive x-ray scatter is present in the measured projection. This study aims to apply the weighted L1-norm scatter correction algorithm in spectrum estimation for reducing residual differences between the estimated and true spectrum.MethodThe scatter correction algorithm is based on a simple radiographic scattering model where the intensity of scattered x-ray is directly estimated from a transmission measurement. Then, the scatter-corrected measurement is used for the spectrum estimation method that consists of deciding the weights of predefined spectra and representing the spectrum as a linear combination of the predefined spectra with the weights. The performances of the estimation method combined with scatter correction are evaluated on both simulated and experimental data.ResultsThe results show that the estimated spectra using the scatter-corrected projection nearly match the true spectra. The normalized-root-mean-square-error and the mean energy difference between the estimated spectra and corresponding true spectra are reduced from 5.8% and 1.33 keV without the scatter correction to 3.2% and 0.73 keV with the scatter correction for both simulation and experimental data, respectively.ConclusionsThe proposed method is more accurate for the acquisition of x-ray spectrum than the estimation method without scatter correction and the spectrum can be successfully estimated even the materials of the filters and their thicknesses are unknown. The proposed method has the potential to be used in several diagnostic x-ray imaging applications.     

Lung tumors on multimodal radiographs derived from grating-based X-ray imaging – A feasibility study

PurposeThe purpose of this study was to assess whether grating-based X-ray imaging may have a role in imaging of pulmonary nodules on radiographs.Materials and methodsA mouse lung containing multiple lung tumors was imaged using a small-animal scanner with a conventional X-ray source and a grating interferometer for phase-contrast imaging. We qualitatively compared the signal characteristics of lung nodules on transmission, dark-field and phase-contrast images. Furthermore, we quantitatively compared signal characteristics of lung tumors and the adjacent lung tissue and calculated the corresponding contrast-to-noise ratios.ResultsOf the 5 tumors visualized on the transmission image, 3/5 tumors were clearly visualized and 1 tumor was faintly visualized in the dark-field image as areas of decreased small angle scattering. In the phase-contrast images, 3/5 tumors were clearly visualized, while the remaining 2 tumors were faintly visualized by the phase-shift occurring at their edges. No additional tumors were visualized in either the dark-field or phase-contrast images. Compared to the adjacent lung tissue, lung tumors were characterized by a significant decrease in transmission signal (median 0.86 vs. 0.91, p = 0.04) and increase in dark-field signal (median 0.71 vs. 0.65, p = 0.04). Median contrast-to-noise ratios for the visualization of lung nodules were 4.4 for transmission images and 1.7 for dark-field images (p = 0.04).ConclusionLung nodules can be visualized on all three radiograph modalities derived from grating-based X-ray imaging. However, our initial data suggest that grating-based multimodal X-ray imaging does not increase the sensitivity of chest radiographs for the detection of lung nodules.     

Application of the phase-space distribution approach of Monte Carlo for radiation contamination dose estimation from the (n,γ), (γ,n) nuclear reactions and linac leakage photons in the megavoltage radiotherapy facility

AimThe aim of this study was to characterize the radiation contamination inside and outside the megavoltage radiotherapy room.BackgroundRadiation contamination components in the 18 MV linac room are the secondary neutron, prompt gamma ray, electron and linac leakage radiation.Materials and MethodsAn 18 MV linac modeled in a typical bunker employing the MCNPX code of Monte Carlo. For fast calculation, phase-space distribution (PSD) file modeling was applied and the calculations were conducted for the radiation contamination components dose and spectra at 6 locations inside and outside the bunker.ResultsThe results showed that the difference of measured and calculated percent depth-dose (PDD) and photo beam-profile (PBP) datasets were lower than acceptable values. At isocenter, the obtained photon dose and neutron fluence were 2.4 × 10⁻¹⁴ Gy/initial e° and 2.22 × 10^-8 n°/cm², respectively. Then, neutron apparent source strength (Q_N) value was found as 1.34 × 10¹² n°/Gy X at isocenter and the model verified to photon and neutron calculations. A surface at 2 cm below the flattening filter was modeled as phase-space (PS) file for PDD and PBP calculations. Then by use of a spherical cell in the center of the linac target as a PS surface, contaminant radiations dose, fluence and spectra were estimated at 6 locations in a considerably short time, using the registered history of all particles and photons in the 13GB PSD file as primary source in the second step.ConclusionDesigning the PSD file in MC modeling helps user to solve the problems with complex geometry and physics precisely in a shorter run-time.     

Intercomparison of dose enhancement ratio and secondary electron spectra for gold nanoparticles irradiated by X-rays calculated using multiple Monte Carlo simulation codes

PurposeTargeted radiation therapy has seen an increased interest in the past decade. In vitro and in vivo experiments showed enhanced radiation doses due to gold nanoparticles (GNPs) to tumors in mice and demonstrated a high potential for clinical application. However, finding a functionalized molecular formulation for actively targeting GNPs in tumor cells is challenging. Furthermore, the enhanced energy deposition by secondary electrons around GNPs, particularly by short-ranged Auger electrons is difficult to measure. Computational models, such as Monte Carlo (MC) radiation transport codes, have been used to estimate the physical quantities and effects of GNPs. However, as these codes differ from one to another, the reliability of physical and dosimetric quantities needs to be established at cellular and molecular levels, so that the subsequent biological effects can be assessed quantitatively.MethodsIn this work, irradiation of single GNPs of 50 nm and 100 nm diameter by X-ray spectra generated by 50 and 100 peak kilovoltages was simulated for a defined geometry setup, by applying multiple MC codes in the EURADOS framework.ResultsThe mean dose enhancement ratio of the first 10 nm-thick water shell around a 100 nm GNP ranges from 400 for 100 kVp X-rays to 600 for 50 kVp X-rays with large uncertainty factors up to 2.3.ConclusionsIt is concluded that the absolute dose enhancement effects have large uncertainties and need an inter-code intercomparison for a high quality assurance; relative properties may be a better measure until more experimental data is available to constrain the models.     

A validation of SpekPy: A software toolkit for modelling X-ray tube spectra

PurposeTo validate the SpekPy software toolkit that has been developed to estimate the spectra emitted from tungsten anode X-ray tubes. The model underlying the toolkit introduces improvements upon a well-known semi-empirical model of X-ray emission.Materials and methodsUsing the same theoretical framework as the widely-used SpekCalc software, new electron penetration data was simulated using the Monte Carlo (MC) method, alternative bremsstrahlung cross-sections were applied, L-line characteristic emissions were included, and improvements to numerical methods implemented. The SpekPy toolkit was developed with the Python programming language. The toolkit was validated against other popular X-ray spectrum models (50 to 120 kVp), X-ray spectra estimated with MC (30 to 150 kVp) as well as reference half value layers (HVL) associated with numerous radiation qualities from standard laboratories (20 to 300 kVp).ResultsThe toolkit can be used to estimate X-ray spectra that agree with other popular X-ray spectrum models for typical configurations in diagnostic radiology as well as with MC spectra over a wider range of conditions. The improvements over SpekCalc are most evident at lower incident electron energies for lightly and moderately filtered radiation qualities. Using the toolkit, estimations of the HVL over a large range of standard radiation qualities closely match reference values.ConclusionsA toolkit to estimate X-ray spectra has been developed and extensively validated for central-axis spectra. This toolkit can provide those working in Medical Physics and beyond with a powerful and user-friendly way of estimating spectra from X-ray tubes.     

The TOPAS tool for particle simulation,a Monte Carlo simulation tool for physics,biology and clinical research

PurposeThis paper covers recent developments and applications of the TOPAS TOol for PArticle Simulation and presents the approaches used to disseminate TOPAS.Materials and methodsFundamental understanding of radiotherapy and imaging is greatly facilitated through accurate and detailed simulation of the passage of ionizing radiation through apparatus and into a patient using Monte Carlo (MC). TOPAS brings Geant4, a reliable, experimentally validated MC tool mainly developed for high energy physics, within easy reach of medical physicists, radiobiologists and clinicians. Requiring no programming knowledge, TOPAS provides all of the flexibility of Geant4.ResultsAfter 5 years of development followed by its initial release, TOPAS was subsequently expanded from its focus on proton therapy physics to incorporate radiobiology modeling. Next, in 2018, the developers expanded their user support and code maintenance as well as the scope of TOPAS towards supporting X-ray and electron therapy and medical imaging. Improvements have been achieved in user enhancement through software engineering and a graphical user interface, calculational efficiency, validation through experimental benchmarks and QA measurements, and either newly available or recently published applications. A large and rapidly increasing user base demonstrates success in our approach to dissemination of this uniquely accessible and flexible MC research tool.ConclusionsThe TOPAS developers continue to make strides in addressing the needs of the medical community in applications of ionizing radiation to medicine, creating the only fully integrated platform for four-dimensional simulation of all forms of radiotherapy and imaging with ionizing radiation, with a design that promotes inter-institutional collaboration.     

Monte Carlo simulation of gold nanoparticles for X-ray enhancement application

BackgroundGold nanoparticles (Au NPs) are regarded as potential agents that enhance the radiosensitivity of tumor cells for theranostic applications. To elucidate the biological mechanisms of radiation dose enhancement effects of Au NPs as well as DNA damage attributable to the inclusion of Au NPs, Monte Carlo (MC) simulations have been deployed in a number of studies.Scope of ReviewThis review paper concisely collates and reviews the information reported in the simulation research in terms of MC simulation of radiosensitization and dose enhancement effects caused by the inclusion of Au NPs in tumor cells, simulation mechanisms, benefits and limitations.Major conclusionsIn this review, we first explore the recent advances in MC simulation on Au NPs radiosensitization. The MC methods, physical dose enhancement and enhanced chemical and biological effects is discussed, followed by some results regarding the prediction of dose enhancement. We then review Multi-scale MC simulations of Au NP-induced DNA damages for X-ray irradiation. Moreover, we explain and look at Multi-scale MC simulations of Au NP-induced DNA damages for X-ray irradiation.General significanceUsing advanced chemical module-implemented MC simulations, there is a need to assess the radiation-induced chemical radicals that contribute to the dose-enhancing and biological effects of multiple Au NPs.     

Photoneutron-induced damage reduction for cardiac implantable electronic devices using neutron-shielding sheets in high-energy X-ray radiotherapy: A phantom study

PurposeTo evaluate damage reduction in cardiac implantable electronic devices (CIEDs) caused by photoneutrons in high-energy X-ray radiotherapy using a neutron-shielding sheet (NSS).MethodsThe NSS consists of a bolus with a thickness of 1 or 2 cm (Bls1 or Bls2) as a moderator and several absorbers (20%, 50%, or 80% B₄C silicone sheet [B₄C20, B₄C50, or B₄C80] or a 40% LiF silicone sheet [LiF40]). First, a linear accelerator (LINAC) with a water-equivalent phantom was modeled in the simulation and measured experimentally. Several NSSs were placed on the phantom, a Eu:LiCaAlF₆ scintillator was placed between the phantom and the NSS, and X-rays were irradiated. The relative counts (Cr = counts when placing the NSS or Bls2) were compared between the experiment and simulation. Second, CIED damage was evaluated in the simulation. The relative damage (Dr = damage when placing or not placing the NSS) was compared among all the NSSs. In addition, the γ-ray and leaking X-ray dose from B₄C was measured using a dosimetric film. After determining the optimal NSS combination, Dr value analysis was performed by changing the length of one side and the thickness.ResultsThe Cr values of the simulation and experiment agreed within a 30% percentage difference, except for Bare or LiF40-only. The Dr value was reduced by 43% when Bls2 + B₄C80 was applied. The photon dose was less than 5 cGy/1500 MU. The Dr values were smaller for the smaller lengths of one side of B₄C80 and decreased as the M-layer thickness increased.ConclusionsThe CIED damage induced by photoneutrons generated by a LINAC was effectively reduced by applying the optimal NSS.     

Evaluations of a flat-panel based compact daily quality assurance device for proton pencil beam scanning (PBS) system

PurposeTo evaluate the flat-panel detector quenching effect and clinical usability of a flat-panel based compact QA device for PBS daily constancy measurements.Materials & MethodThe QA device, named Sphinx Compact, is composed of a 20x20 cm² flat-panel imager mounted on a portable frame with removable plastic modules for constancy checks of proton energy (100 MeV, 150 MeV, 200 MeV), Spread-Out-Bragg-Peak (SOBP) profile, and machine output. The potential quenching effect of the flat-panel detector was evaluated. Daily PBS QA tests of X-ray/proton isocenter coincidence, the constancy of proton spot position and sigma as well as the energy of pristine proton beam, and the flatness of SOBP proton beam through the 'transformed' profile were performed and analyzed. Furthermore, the sensitivity of detecting energy changes of pristine proton beam was also evaluated.ResultsThe quenching effect was observed at depths near the pristine peak regions. The flat-panel measured range of the distal 80% is within 0.9 mm to the defined ranges of the delivered proton beams. X-ray/proton isocenter coincidence tests demonstrated maximum mismatch of 0.3 mm between the two isocenters. The device can detect 0.1 mm change of spot position and 0.1 MeV energy changes of pristine proton beams. The measured transformed SOBP beam profile through the wedge module rendered as flat.ConclusionsEven though the flat-panel detector exhibited quenching effect at the Bragg peak region, the proton range can still be accurately measured. The device can fulfill the requirements of the daily QA tests recommended by the AAPM TG224 Report.     

A new analytical formula for neutron capture gamma dose calculations in double-bend mazes in radiation therapy

BackgroundPhotoneutrons are produced in radiation therapy with high energy photons. Also, capture gamma rays are the byproduct of neutrons interactions with wall material of radiotherapy rooms.AimIn the current study an analytical formula was proposed for capture gamma dose calculations in double bend mazes in radiation therapy rooms.Materials and methodsA total of 40 different layouts with double-bend mazes and a 18 MeV photon beam of Varian 2100 Clinac were simulated using MCNPX Monte Carlo (MC) code. Neutron capture gamma ray dose equivalent was calculated by the MC method along the maze and at the maze entrance door of all the simulated rooms. Then, all MC resulted data were fitted to an empirical formula for capture gamma dose calculations. Wu–McGinley analytical formula for capture gamma dose equivalent at the maze entrance door in single-bend mazes was also used for comparison purposes.ResultsFor capture gamma dose equivalents at the maze entrance door, the difference of 2–11% was seen between MC and the derived equation, while the difference of 36–87% was found between MC and the Wu–McGinley methods.ConclusionOur results showed that the derived formula results were consistent with the MC results for all of 40 different geometries. However, as a new formula, further evaluations are required to validate its use in practical situations. Finally, its application is recommend for capture gamma dose calculations in double-bend mazes to improve shielding calculations.     

Towards breast cancer rotational radiotherapy with synchrotron radiation

PurposeWe performed the first investigations, via measurements and Monte Carlo simulations on phantoms, of the feasibility of a new technique for synchrotron radiation rotational radiotherapy for breast cancer (SR³T).MethodsA Monte Carlo (MC) code based on Geant4 toolkit was developed in order to simulate the irradiation with the SR³T technique and to evaluate the skin sparing effect in terms of centre-to-periphery dose ratio at different energies in the range 60–175 keV. Preliminary measurements were performed at the Australian Synchrotron facility. Radial dose profiles in a 14-cm diameter polyethylene phantom were measured with a 100-mm pencil ionization chamber for different beam sizes and compared with the results of MC simulations. Finally, the dose painting feasibility was demonstrated with measurements with EBT3 radiochromic films in a phantom and collimating the SR beam at 1.5 cm in the horizontal direction.ResultsMC simulations showed that the SR³T technique assures a tumour-to-skin absorbed dose ratio from about 7:1 (at 60 keV photon energy) to about 10:1 (at 175 keV), sufficient for skin sparing during radiotherapy. The comparison between the results of MC simulations and measurements showed an agreement within 5%. Two off-centre foci were irradiated shifting the rotation centre in the horizontal direction.ConclusionsThe SR³T technique permits to obtain different dose distributions in the target with multiple rotations and can be guided via synchrotron radiation breast computed tomography imaging, in propagation based phase-contrast conditions. Use of contrast agents like iodinated solutions or gold nanoparticles for dose enhancement (DE-SR³T) is foreseen and will be investigated in future work.     

Shielding effectiveness of X-ray protective garment

PurposeCertification of the X-ray shielding garment is based on attenuation testing on flat material samples. We investigated the difference of shielding effectiveness compared to realistic use when the garment is worn on the body of a staff person.MethodsAttenuation factors of X-ray protective aprons have been evaluated for several clinical scenarios with Monte Carlo (MC) calculations based on the ICRP female reference model and an experimental setup. The MC calculated attenuation factors refer to the effective dose E, whereas the measured attenuation factors refer to the personal dose equivalent Hp(10). The calculated/measured factors were compared to the attenuation factors of the identical materials measured under the conditions of the standard IEC 61331-1 that is currently in use for the type testing of X-ray protective aprons.ResultsAs a result, for example, at a common tube voltage of 80 kV, the real attenuation factors of a 0.35 mm Pb apron worn by a 3-dimensional body were 38% to 76% higher than when measured under IEC conditions on flat samples. The MC-calculated organ doses show the maximum contribution to E being within the operatoŕs abdomen/pelvis region.ConclusionsWith our findings, personal X-ray protective garments could be improved in effectiveness.     

Dosimetric impacts of beam-hardening filter removal for the CyberKnife system

PurposeEquipment refurbishment was performed to remove the beam-hardening filter (BHF) from the CyberKnife system (CK). This study aimed to confirm the change in the beam characteristics between the conventional CK (present-BHF CK) and CK after the BHF was removed (absent-BHF CK) and evaluate the impact of BHF removal on the beam quality correction factors k_Q.MethodsThe experimental measurements of the beam characteristics of the present- and absent-BHF CKs were compared. The CKs were modeled using Monte Carlo simulations (MCs). The energy fluence spectra were calculated using MCs. Finally, k_Q were estimated by combining the MC results and analytic calculations based on the TRS-398 and TRS-483 approaches.ResultsAll gamma values for percent depth doses and beam profiles between each CK were less than 0.5 following the 3%/1 mm criteria. The percentage differences for tissue-phantom ratios at depths of 20 and 10 cm and percentage depth doses at 10 cm between each CK were −1.20% and −0.97%, respectively. The MC results demonstrated that the photon energy fluence spectrum of the absent-BHF CK was softer than that of the present-BHF CK. The k_Q values for the absent-BHF CK were in agreement within 0.02% with those for the present-BHF CK.ConclusionsThe photon energy fluence spectrum was softened by the removal of BHF. However, no remarkable impact was observed for the measured beam characteristics and k_Q. Therefore, the previous findings of the k_Q values for the present-BHF CK can be directly used for the absent-BHF CK.     

Impact of transverse magnetic fields on dose response of a nanoDot OSLD in megavoltage photon beams

PurposeWe investigated the impact of transverse magnetic fields on the dose response of a nanoDot optically stimulated luminescence dosimetry (OSLD) in megavoltage photon beams.MethodsThe nanoDot OSLD response was calculated via Monte Carlo (MC) simulations. The responses R_Q and R_Q,B without and with the transverse magnetic fields of 0.35–3 T were analyzed as a function of depth at a 10 cm × 10 cm field for 4–18 MV photons in a solid water phantom. All responses were determined based on comparisons with the response under the reference conditions (depth of 10 cm and a 10 cm × 10 cm field) for 6 MV without the magnetic field. In addition, the influence of air-gaps on the nanoDot response in the magnetic field was estimated according to Burlin’s general cavity theory.ResultsThe R_Q as a function of depth for 4–18 MV ranged from 1.013 to 0.993, excepting the buildup region. The R_Q,B increased from 2.8% to 1.5% at 1.5 T and decreased from 3.0% to 1.1% at 3 T in comparison with R_Q as the photon energy increased. The depth dependence of R_Q,B was less than 1%, excepting the buildup region. The top air-gap and the bottom air- gap were responsible for the response reduction and the response increase, respectively.ConclusionsThe response R_Q,B varied depending on the magnetic field intensity, and the variation of R_Q,B reduced as the photon beam energy increased. The air-gaps affected the dose deposition in the magnetic fields.     

Photon-beam radiotherapy in pregnant patients: Can the fetal dose be limited to 10 cGy or less?

PurposeTo estimate fetal dose and its components from three-dimensional conformal radiotherapy for several malignancies presented during pregnancy.Materials and methodsFetal dose was measured from radiotherapy for Hodgkin's lymphoma and for tumors in the region of nasopharynx, breast and lung. Anthropomorphic phantoms were used to simulate an average pregnant patient at the first, second and third trimesters of gestation. Thermoluminescent dosemeters (TLD) were employed for fetal dose measurements. Phantom exposures were also performed to estimate fetal dose due to head leakage, scatter from collimators and beam modifiers and scatter generated inside the phantom (D_in). All treatments were delivered for 6 MV photon beams.ResultsRadiotherapy of Hodgkin's lymphoma resulted in a fetal dose of 5.6–57.9 cGy depending upon the gestational age and the distance between the fetal level and the field edge. The corresponding dose ranges for treatment of nasopharyngeal, breast and lung cancer was 4.0–17.1 cGy, 3.9–24.8 cGy and 5.7–74.3 cGy, respectively. The D_in at the first trimester of gestation was always smaller than 10 cGy for all examined malignancies. Pregnancy progression resulted in D_in values above or below 10 cGy depending upon the treatment site and gestational age.ConclusionThis study provides data about the fetal exposure and the contribution of D_in to the total fetal dose from conformal radiation therapy. The D_in knowledge prior to patient's irradiation enables radiation oncologists and medical physicists to decide whether fetal dose may be limited to 10 cGy or less with or without the introduction of special shielding materials.     

The effect of tumour geometry on the quantification accuracy of planar 123I phantom images

IntroductionAccurate activity quantification is applied in radiation dosimetry. Planar images are important for quantification of whole-body images, enabling assessment of biodistribution from radionuclide administrations. We evaluated the effect of tumour geometry on quantification accuracy of ¹²³I planar phantom studies, including various tumour sizes, tumour-liver distances and two tumour-background ratios.Methods and materialsAn in-house manufactured abdominal phantom was equipped with a liver, different size cylindrical tumours, and a rod for tumour-liver distance variation. The geometric mean method with scatter and attenuation corrections was used for image processing. Scatter and attenuation corrections were made using the triple energy window scatter correction technique and a printed transmission sheet source, respectively. Region definitions for tumour activity distribution compensated for the partial volume effect (PVE). Activity measured in the dose calibrator served as reference for determining quantification accuracy.ResultsThe smallest tumour had the largest percentage deviation with an average activity underestimation of 34.6 ± 1.2%. Activity values for the largest tumour were overestimated by 3.1 ± 3.0%. PVE compensation improved quantification accuracy for all tumour sizes yielding accuracies of <12.4%. Scatter contribution to the tumours from the liver had minimal effect on quantification accuracy at tumour-liver distances >3 cm. With PVE compensation, increased tumour-background ratio resulted in a percentage increase of up to 26.3%.ConclusionWhen applying relevant corrections for scatter, attenuation and PVE without background activity, quantification accuracy of <13% was obtained. We demonstrated the successful implementation of a practical technique to obtain quantitative information from ¹²³I planar images.     

Gold coated contact lens-type ocular in vivo dosimeter (CLOD) for monitoring of low dose in computed tomography: A Monte Carlo study

PurposeThis study reports a sensitivity enhancement of gold-coated contact lens-type ocular in vivo dosimeters (CLODs) for low-dose measurements in computed tomography (CT).MethodsMonte Carlo (MC) simulations were conducted to evaluate the dose enhancement from the gold (Au) layers on the CLODs. The human eye and CLODs were modeled, and the X-ray tube voltages were defined as 80, 120, and 140 kVp. The thickness of the Au layer attached to a CLOD ranged from 100 nm to 10 μm. The thickness of the active layer ranged from 20 to 140 μm. The dose ratio between the active layer of the Au-coated CLOD and a CLOD without a layer, i.e., the dose enhancement factor (DEF), was calculated.ResultsThe DEFs of the first 20-μm thick active layer of the 5-μm thick Au-coated CLOD were 18.4, 19.7, 20.2 at 80, 120, and 140 kVp, respectively. The DEFs decreased as the thickness of the active layer increased. The DEFs of 100-nm to 5-μm thick Au layers increased from 1.7 to 5.4 for 120-kVp X-ray tube voltage when the thickness of the active layer was 140 μm.ConclusionsThe MC results presented a higher sensitivity of Au-coated CLODs (∼20-times higher than that of CLODs without a gold layer). Au-coated CLODs can be applied to an evaluation of very low doses (a few cGy) delivered to patients during CT imaging.