CT dosimetry considerations in non typical conditions: The effect of scan field of view and table height selection

PurposeTo experimentally investigate the effect of the scan field of view (SFOV) selection and table height settings on the Computed Tomography Dose Index (CTDI) and the implications concerning patient effective and skin dose.MethodsAir-kerma length product (AKLP) measurements were carried out in a helical CT scanner using a pencil type dosimeter positioned in air and inside the holes of a head and a body phantom, using all available SFOV selections and different table height settings. Furthermore, using radiotherapy verification films placed on the CT table surface, the entrance surface air kerma (ESAK) profiles were derived with different SFOV and table height selections, both with and without a phantom on top of the films.ResultsThe AKLP is strongly dependent on the SFOV selection and the table height settings. Different SFOV selections correspond to the selection of different bowtie filters that shape the X-ray beam intensity, resulting in different ESAK values at the isocenter and at the other points within the scanning plane. With the off-center positioning the calculated CTDI values within the center and the periphery of the phantom change also, as a result of the different intensity and width of the X-ray beam to which are exposed to.ConclusionsThe existing protocols for calculating effective dose are limited to only two patient anatomy-SFOV combinations and cannot account for off-center positioning. Therefore, more work will be required to estimate the effective and skin dose for non-standard SFOV-patient anatomy combinations and off-center patient positioning.     

A novel method for CT dosimetry with a suspended phantom setup

PurposeThis work presents a method for estimating CT dosimetric indices with a prototype designed for suspending the phantom/ion chamber system fixed at the CT isocenter. The purpose of this study was to validate the proposed methodology, which can be used to provide a direct assessment of dosimetric indices in helical scans.MethodsThe method is based on a reference setup in which the measuring system for CT dosimetry is in a stationary configuration, i.e. not bound to the CT table, and on a mathematical formalism developed for the proposed reference system. The reliability of the method was demonstrated through a set of experimental measurements. Firstly, dosimetric indices were measured with the new method and compared with the indices obtained with the procedure currently used for CT dosimetry (measuring system bound to the CT table). Secondly, dosimetric indices measured with the new method were compared with those displayed on the CT console.ResultsThere is good agreement between the dosimetric indices obtained with the standard setup and those obtained with the suspended phantom setup, within the expected range of errors. The difference between dosimetric indices estimated with the proposed method and those displayed on the CT console is below 2%.ConclusionsThe method enables CT dosimetry to be performed with the dose detector in a stationary longitudinal position thanks to the newly introduced suspended phantom setup. Using this approach, CT dose can be assessed for high pitch helical scans, acquisitions without complete tube rotation and for cases where dynamic collimation is used.     

Fetal radiation dose in three common CT examinations during pregnancy – Monte Carlo study

PurposeTo determine fetal doses in different stages of pregnancy in three common computed tomography (CT) examinations: pulmonary CT angiography, abdomino-pelvic and trauma scan with Monte Carlo (MC) simulations.MethodsAn adult female anthropomorphic phantom was scanned with a 64-slice CT using pulmonary angiography, abdomino-pelvic and trauma CT scan protocols. Three different sized gelatin boluses placed on the phantom’s abdomen simulated different stages of pregnancy. Intrauterine dose was used as a surrogate to a dose absorbed to the fetus. MC simulations were performed to estimate uterine doses. The simulation dose levels were calibrated with volumetric CT dose index (CTDI_vol) measurements and MC simulations in a cylindrical CTDI body phantom and compared with ten point doses measured with metal-oxide-semiconductor field-effect-transistor dosimeters. Intrauterine volumes and uterine walls were segmented and the respective dose volume histograms were calculated.ResultsThe mean intrauterine doses in different stages of pregnancy varied from 0.04 to 1.04 mGy, from 4.8 to 5.8 mGy, and from 9.8 to 12.6 mGy in the CT scans for pulmonary angiography, abdomino-pelvic and trauma CT scans, respectively. MC simulations showed good correlation with the MOSFET measurement at the measured locations.ConclusionsThe three studied examinations provided highly varying fetal doses increasing from sub-mGy level in pulmonary CT angiography to notably higher levels in abdomino-pelvic and trauma scans where the fetus is in the primary exposure range. Volumetric dose distribution offered by MC simulations in an appropriate anthropomorphic phantom provides a comprehensive dose assessment when applied in adjunct to point-dose measurements.     

X-ray dose reduction using additional copper filtration for abdominal digital radiography: Evaluation using signal difference-to-noise ratio

PurposeX-ray dose reduction using additional copper filters (Cu-filters) for abdominal general radiography was indicated in a report using a simulation study. We validated the dose reduction effects using a clinical digital radiography system equipped with an indirect-type CsI detector and an automatic Cu-filter insertion function.MethodsThe image qualities were evaluated using signal difference-to-noise ratio (SDNR) for different radiation qualities with and without Cu-filters for a 20-cm acrylic phantom. Acrylic and bone equivalent material plates were used for contrast measurements. The dose reduction using Cu-filters was estimated from the ratios of the SDNR² values.ResultsFor the same entrance surface dose (ESD), Cu-filters with 0.1- and 0.2-mm thicknesses increased the image quality as evaluated by SDNR² and the estimated dose reduction without degrading the image quality. For the acrylic contrast, the dose reductions with the 0.1- and 0.2-mm-thick Cu-filters were approximately 30% and 44% at 70 kV and 29% and 35% at 80 kV, respectively. For the bone contrast, the reduction rates were slightly reduced.ConclusionsWe validated the dose reduction capability of additional Cu-filters without degrading the image quality for abdominal radiography. The estimated entrance surface dose reductions of the Cu-filters were approximately 30–40% and 20–30% for the acrylic and bone contrasts, respectively, and effective dose reductions for acrylic were nearly half of those for ESD. At these reduced dose conditions, the current time product values needed to be increased by factors of 1.4 and 1.8 for the 0.1- and 0.2-mm-thick Cu-filters, respectively.     

Determination of radiation dose and low-dose protocol for digital chest tomosynthesis using radiophotoluminescent (RPL) glass dosimeters

PurposeThis study aimed to determine a low-dose protocol for digital chest tomosynthesis (DTS).MethodsFive simulated nodules with a CT number of approximately 100 HU with size diameter of 3, 5, 8, 10, and 12 mm were inserted into an anthropomorphic chest phantom (N1 Lungman model), and then scanned by DTS system (Definium 8000) with varying tube voltage, copper filter thickness, and dose ratio. Three radiophotoluminescent (RPL) glass dosimeters, type GD-352 M with a dimension of 1.5 × 12 mm, were used to measure the entrance surface air kerma (ESAK) in each protocol. The effective dose (ED) was calculated using the recorded total dose-area-product (DAP). The signal-to-noise ratio (SNR) was determined for qualitative image quality evaluation. The image criteria and nodule detection capability were scored by two experienced radiologists. The selected low-dose protocol was further applied in a clinical study with 30 pulmonary nodule follow-up patients.ResultsThe average ESAK obtained from the standard default protocol was 1.68 ± 0.15 mGy, while an ESAK of 0.47 ± 0.02 mGy was found for a low-dose protocol. The EDs for the default and low-dose protocols were 313.98 ± 0.72 µSv and 100.55 ± 0.28 µSv, respectively. There were small non-significant differences in the image criteria and nodule detection scoring between the low-dose and default protocols interpreted by two radiologists. The effective dose of 98.87 ± 0.08 µSv was obtained in clinical study after applying the low-dose protocol.ConclusionsThe low-dose protocol obtained in this study can substantially reduce radiation dose while preserving an acceptable image quality compared to the standard protocol.     

Impact of miscentering on patient dose and image noise in x-ray CT imaging: Phantom and clinical studies

The operation of the bowtie filter in x-ray CT is correct if the object being scanned is properly centered in the scanner’s field-of-view. Otherwise, the dose delivered to the patient and image noise will deviate from optimal setting. We investigate the effect of miscentering on image noise and surface dose on three commercial CT scanners. Six cylindrical phantoms with different size and material were scanned on each scanner. The phantoms were positioned at 0, 2, 4 and 6 cm below the isocenter of the scanner’s field-of-view. Regression models of surface dose and noise were produced as a function of miscentering magnitude and phantom’s size. 480 patients were assessed using the calculated regression models to estimate the influence of patient miscentering on image noise and patient surface dose in seven imaging centers. For the 64-slice CT scanner, the maximum increase of surface dose using the CTDI-32 phantom was 13.5%, 33.3% and 51.1% for miscenterings of 2, 4 and 6 cm, respectively. The analysis of patients’ scout scans showed miscentering of 2.2 cm in average below the isocenter. An average increase of 23% and 7% was observed for patient dose and image noise, respectively. The maximum variation in patient miscentering derived from the comparison of imaging centers using the same scanner was 1.6 cm. Patient miscentering may substantially increase surface dose and image noise. Therefore, technologists are strongly encouraged to pay greater attention to patient centering.     

Radiation dose to patients and staff during angiography of the lower limbs. Derivation of local dose reference levels

BackgroundThe Euratom directive 97/43 recommends the use of patient dose surveys in diagnostic radiology and the establishment of reference dose levels (DRLs).PurposeTo perform measurements of the dose delivered during diagnostic angiography of the lower limbs using thermoluminescence dosimeters (TLDs), extraction of DRLs and estimation of the effective dose and radiation risk for this particular examination.MethodsDose measurement was performed on 30 patients by using TLD sachets attached in 5 different positions not only on the patient, but also to the radiologist. All the appropriate factors were recorded. Measurement of the ESD was performed after each examination.ResultsThe mean entrance skin dose (ESD) was calculated to be 70.8, 67.7, 24.3, 18.4, 9.7 mGy at the level of aorta bifurcation, pelvis, femur, knees, and at feet, respectively. The average effective dose is 9.8 mSv with the radiation risks for fatal cancer to be 5.4 × 10^?4. The effective dose of the radiologist was calculated to be 0.023 mSv per procedure.ConclusionRadiation dose variation depends on the physical characteristics of the patient, on the procedure preferences by radiologists and the difficulties in conducting procedures. The main reason for the increased patient dose, compared to other studies, is the number of frames rather than the duration of fluoroscopy. For DSA of the lower limbs, the DRL was chosen to be an entrance skin dose of 96.4 mGy in the pelvic region. The dose to the radiologist is negligible.     

The effect of heart rate,vessel angulation and acquisition protocol on the estimation accuracy of calcified artery stenosis in dual energy cardiac CT: A phantom study

PurposeTo investigate the effects of heart beat rate (bpm), vessel angulation and acquisition protocol on the estimation accuracy of calcified stenosis using a dual-energy CT scanner.MethodsA thorax semi-anthropomorphic phantom coupled with a motion simulator and a vessel phantom representing a 50% coronary artery calcified stenosis, were used. Electrocardiograph (ECG)-synchronized acquisitions were performed at different bpms. Acquisitions were performed using A, B, and C single-energy and D dual-energy protocols. Protocol A was prospective ECG-triggered axial and protocols B and C were retrospective single- and two-segment reconstruction ECG-gated helical acquisitions. Protocol D was prospective ECG-triggered axial acquisition. The vessel phantom was placed at two angulations relative to z-axis. Images were reconstructed using all available kernels with iterative reconstruction. Stenosis-percentage was estimated using the CT vendor’s vessel analysis tool. Effective dose (ED) was estimated using the dose-length product method.ResultsIn protocols A, B, and C, measured Stenosis-percentage increased with bpm. Stenosis-percentage estimate ranged from 56.8% at 40 bpm to 62.6% at 100 bpm. In protocol D, Stenosis-percentage ranged from 59.3% at 40 bpm to 54.8% at 80 bpm. Stenosis-percentage was overestimated on respect to the nominal value in most kernels. The detail kernel exhibited the highest accuracy. Stenosis-percentage was not affected by the vessel angulation. ED for protocols A, B, C, and D was 2.4 mSv, 5.1 mSv, 5.5 mSv, and 2.8 mSv, respectively.ConclusionsUse of the dual-energy cardiac CT examination protocol along with the detail kernel is recommended for a more accurate assessment of Stenosis-percentage.     

Low-tube-voltage selection for non-contrast-enhanced CT: Comparison of the radiation dose in pediatric and adult phantoms

PurposeWe used pediatric and adult anthropomorphic phantoms to compare the radiation dose of low- and standard tube voltage chest and abdominal non-contrast-enhanced computed tomography (CT) scans. We also discuss the optimal low tube voltage for non-contrast-enhanced CT.MethodsUsing a female adult- and three differently-sized pediatric anthropomorphic phantoms we acquired chest and abdominal non-contrast-enhanced scans on a 320-multidetector CT volume scanner. The tube voltage was set at 80-, 100-, and 120 kVp. The tube current was automatically assigned on the CT scanner in response to the set image noise level. On each phantom and at each tube voltage we measured the surface and center dose using high-sensitivity metal-oxide-semiconductor field-effect transistor detectors.ResultsThe mean surface dose of chest and abdominal CT scans in 5-year olds was 4.4 and 5.3 mGy at 80 kVp, 4.5 and 5.4 mGy at 100 kV, and 4.0 and 5.0 mGy at 120 kVp, respectively. These values were similar in our 3-pediatric phantoms (p > 0.05). The mean surface dose in the adult phantom increased from 14.7 to 19.4 mGy for chest- and from 18.7 to 24.8 mGy for abdominal CT as the tube voltage decreased from 120 to 80 kVp (p < 0.01).ConclusionCompared to adults, the surface and center dose for pediatric patients is almost the same despite a decrease in the tube voltage and the low tube voltage technique can be used for non-contrast-enhanced chest- and abdominal scanning.     

Comparative performance evaluation of a flat detector and an image intensifier angiographic system both used for interventional cardiology procedures in adult and pediatric patients

PurposeTo compare two angiography systems of different image capture technology, one with flat detector (FD) and one with image intensifier (II), in terms of entrance surface air kerma (ESAK) rate, detector dose (DD) rate and image quality (IQ), in interventional cardiology procedures concerning both adult and pediatric patients.Materials and methodsIn order to determine ESAK and DD rates, a digital dosimeter and polymethylmethacrylate (PMMA) plates were used. For the evaluation of IQ, two contrast objects (the Leeds TOR 18FG and a 5 mm-thick Aluminum plate) were used and two figures of merit were defined in fluoroscopy and cine acquisition modes. Measurements of ESAK, DD rates and IQ were made for various fields of view, pulse and frame acquisition rates.ResultsFor the particular setup used in this study was noted that ESAK values in the II system were generally larger than the respective values in the FD system (on average 70% for fluoro mode, 5 times for cine mode). When halving the fluoroscopy pulse rate, reduction in ESAK was not proportional, in fluoroscopy mode. Image quality evaluations indicated that II performs better in terms of low contrast sensitivity (LCS) and signal-to-noise ratio (SNR) than the FD system which performs better regarding high contrast resolution (HCR). However, when considering image quality in relation to ESAK the FD system performs better than the II system (with the exception of low thicknesses and zooms for high pulse rates in the fluoroscopy mode).ConclusionsThe FD system, generally, provides a better image quality–dose relation than the II system although II unit provides better LCS and SNR. This means that with the right adjustments to both systems, FD unit is able to provide same image quality with lower dose. However, newer technology does not automatically imply better image quality and further investigation is necessary for deriving safe conclusions for units which utilize different capture technology.     

Dosimetric and localization accuracy of Elekta high definition dynamic radiosurgery

Background and PurposeWith the increasingly prominent role of stereotactic radiosurgery in radiation therapy, there is a clinical need for robust, efficient, and accurate solutions for targeting multiple sites with one patient setup. The end-to-end accuracy of high definition dynamic radiosurgery with Elekta treatment planning and delivery systems was investigated in this study.Materials and MethodsA patient-derived CT scan was used to create a radiosurgery plan to seven targets in the brain. Monaco was used for treatment planning using 5 VMAT non-coplanar arcs. Prior to delivery, 3D-printed phantoms from RTsafe were ordered including a gel phantom for 3D dosimetry, phantom with 2D film insert, and an ion chamber phantom for point dose measurement. Delivery was performed using the Elekta VersaHD, XVI cone-beam CT, and HexaPOD six degree of freedom tabletop.ResultsAbsolute dose accuracy was verified within 2%. 3D global gamma analysis in the film measurement revealed 3%/2 mm passing rates >95%. Gel dosimetry 3D global gamma analysis (3%/2 mm) were above 90% for all targets with the exception of one. Results were indicative of typical end-to-end accuracies (<1 mm spatial uncertainty, 2% dose accuracy) within 4 cm of isocenter. Beyond 4 cm, 2 mm accuracy was found.ConclusionsHigh definition dynamic radiosurgery expands clinically acceptable stereotactic accuracy to a sphere around isocenter allowing for radiosurgery of several targets with one setup with a high degree of dosimetric precision. Gel dosimetry proved to be an essential tool for the validation of the 3D dose distributions in this technique.     

Evaluating the effect of reduced entrance surface dose on neonatal chest imaging using subjective image quality evaluation

IntroductionAs hospitalized neonates are radiosensitive and often require numerous X-rays, institutions should investigate the optimal beam parameter combinations to deliver diagnostically acceptable quality images at the lowest possible entrance surface dose (ESD). Using a subjective approach, this study evaluated the effect of different beam parameter combinations on image quality.Methods and materialsFive rabbits simulated the neonatal chest. The ESD was reduced using a variation of voltage, tube current and filtration. Eight radiology registrars, blinded to the dose parameter information, ranked the digital X-ray images of three anatomical regions from best to worst using a variation of the multiple rank order method. T-tests compared the average values, obtained from the scores assigned by each observer to images acquired at different ESDs, for each region. The calculated intraclass correlation coefficient (ICC) assessed observer agreement.ResultsResults showed that a 64% dose reduction was achievable by altering the beam parameters. Large variation among the observers was confirmed by an ICC value <0.5. A 95% confidence interval could not conclude that different ESD values, resulting in a 50–77% dose reduction compared to current practice, would result in different overall observed image quality. This was noted for all three regions indicating that no preference existed towards an image acquired with a specific beam parameter combination.ConclusionsThe large variation in observers’ opinion of acceptable image quality emphasizes the importance of subjective image quality evaluation in the clinical environment. The rabbit phantom and multiple rank order method are considered appropriate for these evaluations.     

Positioning accuracy of a single-isocenter multiple targets SRS treatment: A comparison between Varian TrueBeam CBCT and Brainlab ExacTrac

PurposeThis study compared the positioning accuracy between cone-beam CT (CBCT) and ExacTrac (ETX) for a single-isocenter multiple target stereotactic radiosurgery (SRS) on two TrueBeam STx systems.MethodsA single-isocenter treatment plan was simulated on an anthropomorphic head phantom with six spherical steel ball bearings (BBs). One of the BBs was chosen to be the isocenter. The five off-isocenter targets were located at various distances from the isocenter. MV portal images were generated to evaluate the deviations between the expected and the real center of the targets after CBCT and ETX positioning, respectively.ResultsThe evaluation of the positioning accuracy for the isocenter target showed that CBCT and ETX positioning provided comparable, sub-millimetric results. Deviations in positioning accuracy were also calculated for all other targets, also showing comparable results for CBCT and ETX. Moreover, our study showed that the deviation between CBCT and ETX positioning were in better agreement for TBSTx1 and deviated slightly higher on TBSTx2 (maximum: 1.23 mm at S/I direction), due to a less perfect alignment between the CBCT coordinate system and the ETX coordinate system on TBSTx2 compared to TBSTx1. This study also showed a correlation between the target positioning accuracy and the distance to the isocenter.ConclusionThe positioning accuracy of ETX and CBCT for targets located at isocenter and off-isocenter locations was compared on two treatment machines and found comparable. Our study highlights the importance of a proper calibration procedure, to ensure correct alignment between the CBCT, ETX and machine coordinate systems.     

Dose calculation validation of a convolution algorithm in a solid water phantom

PurposeThe dose calculated using a convolution algorithm should be validated in a simple homogeneous water-equivalent phantom before clinical use. The dose calculation accuracy within a solid water phantom was investigated.MethodsThe specific Gamma knife design requires a dose rate calibration within a spherical solid water phantom. The TMR10 algorithm, which approximates the phantom material as liquid water, correctly computes the absolute dose in water. The convolution algorithm, which considers electron density miscalculates the dose in water as the phantom Hounsfield units were converted into higher electron density when the original CT calibration curve was used. To address this issue, the electron density of liquid water was affected by modifying the CT calibration curve. The absolute dose calculated using the convolution algorithm was compared with that computed by the TMR10. The measured depth dose profiles were also compared to those computed by the convolution and TMR10 algorithms. A patient treatment was recalculated in the solid-water phantom and the delivery quality assurance was checked.ResultsThe convolution algorithm and the TMR10 calculate an absolute dose within 1% when using the modified CT calibration curve. The dose depth profile calculated using the convolution algorithms was superimposed on the TMR10 and measured dose profiles when the modified CT calibration curve was applied. The Gamma index was better than 93%.ConclusionsDose calculation algorithms, which consider electron density, require a CT calibration curve adapted to the phantom material to correctly compute the dose in water.     

Monte Carlo dose in a prosthesis phantom based on exact geometry vs streak artefact contaminated CT data as benchmarked against Gafchromic film measurements

PurposeIn radiotherapy, accurate calculation of patient radiation dose is very important for good clinical outcome. In the presence of metallic implants, the dose calculation accuracy could be compromised by metal artefacts generated in computed tomography (CT) images of patients. This study investigates the influence of metal-induced CT artefacts on MC dose calculations in a pelvic prosthesis phantom.MethodsA pelvic phantom containing unilateral Ti prosthesis was CT-scanned and accurate Hounsfield unit (HU) values were assigned to known materials of the phantom as opposed to HU values produced through the artefact CT images of the phantom. Using the DOSXYZnrc MC code, dose calculations were computed in the phantom model constructed from the original CT images containing the artefacts and artefact-free images made from the exact geometry of the phantom with known materials. The dose calculations were benchmarked against Gafchromic EBT3 film measurements using 15 MeV electron and 10 MV photon beams.ResultsThe average deviations between film and MC dose data decreased from 3 ± 2% to 1 ± 1% and from about 6 ± 2% to 3 ± 1% for the artefact and artefact-free phantom models against film data for the electron and photon fields, respectively.ConclusionsFor the Ti prosthesis phantom, the presence of metal-induced CT artefacts could cause dose inaccuracies of about 3%. Construction of an artefact-free phantom model made from the exact geometry of the phantom with known materials to overcome the effect of artefacts is advantageous compared to using CT data directly of which the exact tissue composition is not well-known.     

Image quality for radiotherapy CT simulators with different scanner bore size

PurposeWe compare image quality parameters derived from phantom images taken on three commercially available radiotherapy CT simulators. To make an unbiased evaluation, we assured images were obtained with the same surface dose measured using XR-QA2 model GafChromic™ film placed at the imaging phantom surface for all three CT-simulators.MethodsRadiotherapy CT simulators GE LS 16, Philips Brilliance Big Bore, and Toshiba Aquilion LB were compared in terms of spatial resolution, low contrast detectability, image uniformity, and contrast to noise ratio using CATPHAN-504 phantom, scanned with Head and Pelvis protocols. Dose was measured at phantom surface, with CT scans repeated until doses on all scanners were within 2%.ResultsIn terms of spatial resolution, the GE simulator appears slightly better, while Philips CT images are superior in terms of SNR for both scanning protocols. The CNR results show that Philips CT images appear to be better, except for high Z material, while Toshiba appears to fit in between the two simulators.ConclusionsWhile the image quality parameters for three RT CT simulators show comparable results, the scanner bore size is of vital importance in various radiotherapy applications. Since the image quality is a function of a large number of confounding parameters, any loss in image quality due to scanner bore size could be compensated by the appropriate choice of scanning parameters, including the exposure and by balancing between the additional imaging dose to the patient and high image quality required in highly conformal RT techniques.     

Comparative dose levels between CT-scanner and slot-scanning device (EOS system) in pregnant women pelvimetry

PurposeTo estimate fetal absorbed doses for pregnant women pelvimetry, a comparative study between EOS imaging system and low-dose spiral CT-scanner was carried out. For this purpose three different studies were investigated: in vivo, in vitro and Monte Carlo calculations.MethodsIn vivo dosimetry was performed, using OSL NanoDot dosimeters, to determine the dose to the skin of twenty pregnant women. In vitro studies were established by using a cubic phantom of water, in order to estimate the out of field doses. In the latter study, OSLDs were placed at depths corresponding to the lowest, average and highest position of the uterus. Monte Carlo calculations of effective doses to high radio-sensitive organs were established, using PCXMC and CTExpo software suites for EOS imaging system and CT-scanner, respectively.ResultsThe EOS imaging system reduces radiation exposure 4 to 8 times compared to the CT-scanner. The entrance skin doses were 74% (p-values <0.01) higher with the CT-scanner than with the EOS system. In the out of field region, the measured doses of the EOS system were reduced by 80% (p-values <0.02).Monte Carlo calculations confirmed that effective doses to organs are less accentuated for EOS than for CT pelvimetry.ConclusionsThe EOS system is less irradiating than the CT exam. The out-of-field dose which is significant, is lower in the EOS than in the CT-scanner and could be reduced even further by optimizing the time used for image acquisition.     

In phantom evaluation of targeting accuracy in MRI-based brain radiosurgery

PurposeTo determine the targeting accuracy of brain radiosurgery when planning procedures employing different MRI and MRI + CT combinations are adopted.Materials and methodA new phantom, the BrainTool, has been designed and realized to test image co-registration and targeting accuracy in a realistic anatomical situation. The phantom was created with a 3D printer and materials that mimic realistic brain MRI and CT contrast using a model extracted from a synthetic MRI study of a human brain. Eight markers distributed within the BrainTool provide for assessment of the accuracy of image registrations while two cavities that host an ionization chamber are used to perform targeting accuracy measurements with an iterative cross-scan method. Two procedures employing 1.5 T MRI-only or a combination of MRI (taken with 1.5 T or 3 T scanners) and CT to carry out Gamma Knife treatments were investigated. As distortions can impact targeting accuracy, MR images were preliminary evaluated to assess image deformation extent using GammaTool phantom.ResultsMR images taken with both scanners showed average and maximum distortion of 0.3 mm and 1 mm respectively. The marker distances in co-registered images resulted below 0.5 mm for both MRI scans. The targeting mismatches obtained were 0.8 mm, 1.0 mm and 1.2 mm for MRI-only and MRI + CT (1,5T and 3 T), respectively.ConclusionsProcedures using a combination of MR and CT images provide targeting accuracies comparable to those of MRI-only procedures. The BrainTool proved to be a suitable tool for carrying out co-registration and targeting accuracy of Gamma Knife brain radiosurgery treatments.     

Calculation and measurement of doses in the surface layers of a phantom when using Tomotherapy

BackgroundThe calculation and measurement on the surface of the skin presents a significant dosimetric problem because of numerous factors which have an influence on the dose distribution in this region.AimThe overall aim of this study was to check the agreement between doses measured with thermoluminescent detectors (TLD) during tomotherapy photon beam irradiation of the skin area of a solid water cylindrical phantom with doses calculated with Hi-Art treatment planning system (TPS).Material and MethodThe measurements of the dose were made with the use of a solid water cylindrical phantom - Cheese Phantom. Two bolus phantoms were used: 5 mm and 10 mm Six different planning treatments were generated. The doses were measured using TL detectors.ResultsIn the case of a tumor located near the surface of the skin, the mean dose for 0.5 cm bolus was - 1.94 Gy, and for 1 cm bolus - 2.03 Gy. For the tumor located inside the phantom and organ at risk on the same side that TL detectors, for a 0.5 cm bolus, mean dose was 0.658 Gy, and for a 1 cm bolus, 0.62 Gy.ConclusionThe analysis of results showed that the relative percentage difference between measured and planned dose in the field of irradiation was less than 10%, while the largest differences were on the board of the field of radiation and outside of the field of irradiation, where the dose was 0.08 Gy to 1 Gy.     

Dosimetric effects of brass mesh bolus on skin dose and dose at depth for postmastectomy chest wall irradiation

PurposeTo investigate the feasibility of using the brass mesh bolus as an alternative to tissue- equivalent bolus for post mastectomy chest wall cancer by characterizing the dosimetric effects of the 2-mm fine brass bolus on both the skin dose, the dose at depth and spatial distribution.Materials and methodsSurface dose and percent depth dose data were acquired for a 6 MV photon beam in a solid water phantom using MOSkin™, Gafchromic EBT3 film and an Advanced Markus ionization chamber. Data were acquired for the case of: no bolus, Face-up bass bolus, Face-down brass bolus, double brass bolus, 0.5 cm and 1.0 cm of Superflab TE bolus. The exit doses were also measured via MOSkin™ dosimeter and Markus ionization chamber. Gafchromic EBT3 film strips were used to plot dose profile at surface and 10 cm depth for Face-up brass, Face-down brass, double brass, 0.5 cm and 1.0 cm of Superflab TE bolus.ResultsThe surface dose measured via MOSkin™ dosimeter increased from 19.2 ± 1.0% to 63.1 ± 2.1% under Face-up brass discs, 51.2 ± 1.2% under Face-up brass spaces, 61.5 ± 0.5% under Face-down brass discs, and 41.3 ± 2.1% under Face-down brass spaces. The percentage difference in the dose measured under brass discs between Face-up versus Face-down was less than 2% for entrance dose and 10% for exit dose, whereas the percentage difference under brass spaces was approximately 3% for entrance dose and about 5% for the exit dose. Gafchromic EBT3 film strip measurements show that the mesh bolus produced ripple beam profiles due to the mesh brass construction.ConclusionsBrass bolus does not significantly change dose at depth (less than 0.5%), and the surface dose is increased similar to TE bolus. Considering this, brass mesh may be used as a substitute for TE bolus to increase superficial dose for chest wall tangent plans.