Dosimetric shield evaluation with tungsten sheet in 4, 6, and 9 MeV electron beams

In electron radiotherapy, shielding material is required to attenuate beam and scatter. A newly introduced shielding material, tungsten functional paper (TFP), has been anticipated to become a very useful device that is lead-free, light, flexible, and easily processed, containing very fine tungsten powder at as much as 80% by weight. The purpose of this study was to investigate the dosimetric changes due to TFP shielding for electron beams. TFP (thickness 0–15 mm) was placed on water or a water-equivalent phantom. Percentage depth ionization and transmission were measured for 4, 6, and 9 MeV electron beams. Off-center ratio was also measured using film dosimetry at depth of dose maximum under similar conditions. Then, beam profiles and transmission with two shielding materials, TFP and lead, were evaluated. Reductions of 95% by using TFP at 0.5 cm depth occurred at 4, 9, and 15 mm with 4, 6, and 9 MeV electron beams, respectively. It is found that the dose tend to increase at the field edge shaped with TFP, which might be influenced by the thickness. TFP has several unique features and is very promising as a useful tool for radiation protection for electron beams, among others.     

Tungsten-based material as promising new lead-free gamma radiation shielding material in nuclear medicine

PurposeThe main objective of this study was to evaluate the efficacy of tungsten carbide as new lead-free radiation shielding material in nuclear medicine by evaluating the attenuation properties.Materials and methodsThe elemental composition of tungsten carbide was analysed using Field-Emission Scanning Electron Microscopy (FESEM) with energy dispersive X-ray (EDX). The purity of tungsten carbide was 99.9%, APS: 40–50 µm. Three discs of tungsten carbide was fabricated with thickness of 0.1 cm, 0.5 cm and 1.0 cm. Three lead discs with similar thickness were used to compare the attenuation properties with tungsten carbide discs. Energy calibration of gamma spectroscopy was performed by using ¹²³I, ¹³³Ba, ¹⁵²Eu, and ¹³⁷Cs. Gamma radiation from these sources were irradiated on both materials at energies ranging from 0.160 MeV to 0.779 MeV. The experimental attenuation coefficients of lead and tungsten carbide were compared with theoretical attenuation coefficients of both materials from NIST database. The half value layer and mean free path of both materials were also evaluated in this study.ResultsThis study found that the peaks obtained from gamma spectroscopy have linear relationship with all energies used in this study. The relative differences between the measured and theoretical mass attenuation coefficients are within 0.19–5.11% for both materials. Tungsten carbide has low half value layer and mean free path compared to lead for all thickness at different energies.ConclusionThis study shows that tungsten carbide has high potential to replace lead as new lead-free radiation shielding material in nuclear medicine.     

High throughput film dosimetry in homogeneous and heterogeneous media for a small animal irradiator

PurposeWe have established a high-throughput Gafchromic film dosimetry protocol for narrow kilovoltage beams in homogeneous and heterogeneous media for small-animal radiotherapy applications. The kV beam characterization is based on extensive Gafchromic film dosimetry data acquired in homogeneous and heterogeneous media. An empirical model is used for parameterization of depth and off-axis dependence of measured data.MethodsWe have modified previously published methods of film dosimetry to suit the specific tasks of the study. Unlike film protocols used in previous studies, our protocol employs simultaneous multi-channel scanning and analysis of up to nine Gafchromic films per scan. A scanner and background correction were implemented to improve accuracy of the measurements. Measurements were taken in homogeneous and inhomogeneous phantoms at 220 kVp and a field size of 5 × 5 mm². The results were compared against Monte Carlo simulations.ResultsDose differences caused by variations in background signal were effectively removed by the corrections applied. Measurements in homogeneous phantoms were used to empirically characterize beam data in homogeneous and heterogeneous media. Film measurements in inhomogeneous phantoms and their empirical parameterization differed by about 2%–3%. The model differed from MC by about 1% (water, lung) to 7% (bone). Good agreement was found for measured and modelled off-axis ratios.ConclusionsEBT2 films are a valuable tool for characterization of narrow kV beams, though care must be taken to eliminate disturbances caused by varying background signals. The usefulness of the empirical beam model in interpretation and parameterization of film data was demonstrated.     

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.     

Cytogenetic effects induced by accelerated carbon ions with shielding

Our work aims to understand the effects of shielding on the induction of biological damage by heavy charged particles and to compare the shielding effects of different materials at the same LET from two aspects: the biological effectiveness including or not including secondary particles emitted at large angles and the biological effectiveness at different angles with respect to the beam direction. We designed and conducted biological experiments to determine the biological effectiveness of 200 MeV/u carbon ions after traversing different shielding materials (Lucite and aluminium). Whole blood samples, which were either attached to the shielding material (48 mm Lucite or 29 mm aluminium)or positioned at 300 cm away from it at different angles with respect to the beam axis, were exposed to carbon ion beams. For comparison, whole blood samples were exposed directly to 200 MeV/u carbon ions. Chromosomal aberrations in lymphocytes were scored. The results indicated that the biological effectiveness per unit dose was not significantly changed by 48 mm Lucite or 29 mm aluminium, and no significant differences were observed in lymphocytes attached to the target and in lymphocytes positioned at a distance of 300 cm away from the target, at 0o angle of the beam axis. However, when plotted as a function of the number of ions hitting the shielding target, the curves are separated and the shield increases the effectiveness per unit ion. The frequency of chromosomal aberrations at tilted angles behind 29 mm Al and 48 mm Lucite was almost the same. These lesions were considered to be caused by secondary particles due to the passage of particles through the shielding materials.     

Determination of the surface dose of a water phantom using a semiconductor detector for diagnostic kilovoltage x-ray beams

PurposeTo determine the surface dose of a water phantom using a semiconductor detector for diagnostic kilovoltage x-ray beams.MethodsAn AGMS-DM+ semiconductor detector was calibrated in terms of air kerma measured with an ionization chamber. Air kerma was measured for 20 x-ray beams with tube voltages of 50–140 kVp and a half-value layer (HVL) of 2.2–9.7 mm Al for given quality index (QI) values of 0.4, 0.5, and 0.6, and converted to the surface dose. Finally, the air kerma and HVL measured by the AGMS-DM+ detector were expressed as a ratio of the surface dose for 10 × 10 and 20 × 20 cm² fields. The ratio of both was represented as a function of HVL for the given QI values and verified by comparing it with that calculated using the Monte Carlo method.ResultsThe air kerma calibration factor, CF, for the AGMS-DM+ detector ranged from 0.986 to 1.016 (0.9% in k = 1). The CF values were almost independent of the x-ray fluence spectra for the given QI values. The ratio of the surface dose to the air kerma determined by the PTW 30,013 chamber and the AGMS-DM+ detector was less than 1.8% for the values calculated using the Monte Carlo method, and showed a good correlation with the HVL for the given QI values.ConclusionIt is possible to determine the surface dose of a water phantom from the air kerma and HVL measured by a semiconductor detector for given QI values.     

Signal-to-noise ratio improvements using anti-scatter grids with different object thicknesses and tube voltages

PurposeThe appropriate object thickness to start using anti-scatter grids (grids) has not sufficiently investigated in previous studies, and thus we rigorously investigated the effectiveness of two generally used grids with grid ratios of 6 and 10 (G6 and G10) for different 50–200 mm thicknesses at tube voltages of 60–100 kV.MethodsAcrylic phantoms with 30 × 30 cm² and different thicknesses were used to measure the signal-to-noise ratio improvement factors (SIFs) of grids. To evaluate the infants’ conditions, field sizes of 225, 400, and 625 cm² were also evaluated at 60–80 kV. In addition, the signal difference-to-noise ratio (SDNR) was used to evaluate tube voltage dependencies of grids for each thickness.ResultsSIF values exceeded 1.0 for ≥70 mm thicknesses and mostly exceeded 1.07 for the 100 mm thickness with 400 cm² field size corresponding to a 1-year-old infant abdomen. The estimated dose reduction capabilities for a 1-year-old infant were approximately 15% using G10 at 70 and 80 kV. The tube voltage dependencies for grid use was almost not prominent for all conditions tested, except for some conditions that are not clinically realistic.ConclusionsG6 and G10 can improve SNR for  ≥100 mm thickness. The results from this work demonstrate approximately 15% dose reduction or image quality improvements at the same dose level for the use of G6 and G10 grids for 100 mm thickness, traditionally excluded from the recommended grid use conditions.     

First proof of bismuth oxide nanoparticles as efficient radiosensitisers on highly radioresistant cancer cells

This study provides the first proof of the novel application of bismuth oxide as a radiosensitiser. It was shown that on the highly radioresistant 9L gliosarcoma cell line, bismuth oxide nanoparticles sensitise to both kilovoltage (kVp) or megavoltage (MV) X-rays radiation. 9L cells were exposed to a concentration of 50 μg.mL⁻¹ of nanoparticle before irradiation at 125 kVp and 10 MV. Sensitisation enhancement ratios of 1.48 and 1.25 for 125 kVp and 10 MV were obtained in vitro, respectively. The radiation enhancement of the nanoparticles is postulated to be a combination of the high Z nature of the bismuth (Z = 83), and the surface chemistry. Monte Carlo simulations were performed to elucidate the physical interactions between the incident radiation and the nanoparticle. The results of this work show that Bi₂O₃ nanoparticles increase the radiosensitivity of 9L gliosarcoma tumour cells for both kVp and MV energies. Monte Carlo simulations demonstrate the advantage of a platelet morphology.     

3D printed copper-plastic composite material for use as a radiotherapy bolus

The purpose of this work is to evaluate a commercially available copper-plastic composite material for use as a custom fit 3D printed bolus. Superficial dose under copper-plastic composite bolus was assessed for 0.4 mm, 0.6 mm, and 0.8 mm thicknesses. Superficial dose measurements were performed with an Attix parallel plate ionization chamber and radiochromic film. Additionally, a custom-fit bolus was designed for the temporal-frontal cranial region of an anthropomorphic phantom. A treatment plan with a tangential field arrangement was designed, and radiochromic film was used to measure the dose enhancement to the surface of the phantom from the bolus and compared to the calculated dose. It was shown that 3D printed copper-plastic composite bolus can provide the equivalent dose enhancement of thicker conventional bolus. Due to the limited thickness of the copper-plastic composite the bolus can remain flexible, which can aid in the placement of the bolus and improve patient comfort.     

X-ray dose reduction using additional copper filtration for dental cone beam CT

This study aims to quantitatively evaluate the effect of additional copper-filters (Cu-filters) on the radiation dose and contrast-to-noise ratio (CNR) in a dental cone beam computed tomography (CBCT). The Cu-filter thickness and tube voltage of the CBCT unit were varied in the range of 0.00–0.20 mm and 70–90 kV, respectively. The CBCT images of a phantom with homogeneous materials of aluminum, air, and bone equivalent material (BEM) were acquired. The CNRs were calculated from the voxel values of each homogeneous material. The CTDI_vol was measured using standard polymethyl methacrylate CTDI test objects. We evaluated and analyzed the effects of tube current and various radiation qualities on the CNRs and CTDI_vol. We observed a tendency for higher CNR at increasing tube voltage and tube current in all the homogeneous materials. On the other hand, the CNR reduced at increasing Cu-filter thickness. The tube voltage of 90 kV showed a clear advantage in the tube current–CNR curves in all the homogeneous materials. The CTDI_vol increased as the tube voltage and tube current increased and decreased with the increase in the Cu-filter thickness. When the CNR was fixed at 9.23 of BEM at an exposure setting of 90 kV/5 mA without a Cu-filter, the CTDI_vol at 90 kV with Cu-filters was 8.7% lower compared with that at 90 kV without a Cu-filter. The results from this study demonstrate the potential of adding a Cu-filter for patient dose reduction while ensuring the image quality.     

Energy response of radiophotoluminescent glass dosimeter for diagnostic kilovoltage x-ray beams

PurposeThis study aimed to investigate the energy response of a radiophotoluminescent glass dosimeter (RGD) for diagnostic kilovoltage x-ray beams by Monte Carlo (MC) calculations and measurements.MethodsThe uniformity and reproducibility of GD-352M (with Sn filter) and GD-302M (no filter) were tested with 45 RGDs in free air. Subsequently, the RGD response was obtained as a function of an Al-HVL using the parameter, quality index (QI), which is defined as the ratio of the effective energy (keV) to the maximum energy (keV) of the photons. The x-ray fluence spectra with QI of 0.4, 0.5, and 0.6 were set for tube voltages of 50 ~ 137.6 kVp. The RGD response was calculated in free air using the MC method and verified by the air kerma, K_air, measured using an ionization chamber.ResultsThe uniformity and reproducibility of the 45 RGDs were ± 2.3% and ± 2.7% for GD-352M and ± 0.7% and ± 1.6% for GD-302M at the one standard deviation level, respectively. The calculated RGD response was 0.965 to 1.062 at Al-HVL 2.73 mm or more for GD-352M and varied from 3.9 to 2.8 for GD-302M. Both RGD responses exhibited a good correlation with the Al-HVL for the given QI. K_air measured by RGDs for each beam quality with a QI of 0.5 was in the range of −5%~0.8% for GD-352M and −1.8%~3% for GD-302M, relative to the chamber measurements.ConclusionsThe RGD response was indicated as a function of the Al-HVL for the given QI, and it presented a good correlation with the Al-HVL.     

A review on photoneutrons characteristics in radiation therapy with high-energy photon beams

In radiation therapy with high-energy photon beams (E > 10 MeV) neutrons are generated mainly in linacs head thorough (γ,n) interactions of photons with nuclei of high atomic number materials that constitute the linac head and the beam collimation system. These neutrons affect the shielding requirements in radiation therapy rooms and also increase the out-of-field radiation dose of patients undergoing radiation therapy with high-energy photon beams. In the current review, the authors describe the factors influencing the neutron production for different medical linacs based on the performed measurements and Monte Carlo studies in the literature.     

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.     

Radiological properties of 3D printed materials in kilovoltage and megavoltage photon beams

PurposeThis study evaluates the radiological properties of different 3D printing materials for a range of photon energies, including kV and MV CT imaging and MV radiotherapy beams.MethodsThe CT values of a number of materials were measured on an Aquilion One CT scanner at 80 kVp, 120 kVp and a Tomotherapy Hi Art MVCT imaging beam. Attenuation of the materials in a 6 MV radiotherapy beam was investigated.ResultsPlastic filaments printed with various infill densities have CT values of −743 ± 4, −580 ± 1 and −113 ± 3 in 120 kVp CT images which approximate the CT values of low-density lung, high-density lung and soft tissue respectively. Metal-infused plastic filaments printed with a 90% infill density have CT values of 658 ± 1 and 739 ± 6 in MVCT images which approximate the attenuation of cortical bone. The effective relative electron density RED_eff is used to describe the attenuation of a megavoltage treatment beam, taking into account effects relating to the atomic number and mass density of the material. Plastic filaments printed with a 90% infill density have RED_eff values of 1.02 ± 0.03 and 0.94 ± 0.02 which approximate the relative electron density RED of soft tissue. Printed resins have RED_eff values of 1.11 ± 0.03 and 1.09 ± 0.03 which approximate the RED of bone mineral.Conclusions3D printers can model a variety of body tissues which can be used to create phantoms useful for both imaging and dosimetric studies.     

Low contrast detectability performance of model observers based on CT phantom images: kVp influence

This paper studies low contrast detectability (LCD) performance of two model observers in CT phantom images acquired at different kVp levels and compares the results with humans in a 2-alternative forced choice experiment (2-AFC). Images of the Catphan phantom with objects of different contrasts (0.5 and 1%) and diameters (2–15 mm) were acquired in an Aquilion ONE 320-detector row CT (Toshiba Medical Systems, Tokyo, Japan), in two experiments, selecting (80–100–120–135 kV) with fixed mAs and varying the mAs to keep the dose constant, respectively. Four human observers evaluated the objects visibility obtaining a proportion correct (PC) for each case. LCD was also analyzed with two model observers (non-prewhitening matched filter with an eye filter, NPWE, and channelized Hotelling observer with Gabor channels, CHO).Object contrast was affected by kV, with differences up to 17% between the lowest and highest kV. Both models overestimated human performance and were corrected by efficiency and internal noise factors. The NPWE model reproduced better the human PC values trends showing Pearson's correlation coefficients ≥0.976 (0.954–0.987, 95% CI) for both experiments, whereas for CHO they were ≥0.706 (0.493–0.839). Bland–Altman plots showed better agreement between NPWE and humans being the average difference Δ and the range of the differences Δ±2σ (σ, standard deviation) of Δ=−0.3%, Δ±2σ = [−4.0%,4.5%]. For CHO, Δ=−1.2%, Δ± 2σ= [−10.7%,8.3%]. The NPWE model can be a useful tool to predict human performance in CT low contrast detection tasks in a standard phantom and be potentially used in protocol optimization based on kV selection.     

Real-time estimation of surface dose based on incident air kerma in diagnostic radiology

PurposeTo estimate the surface dose in diagnostic radiology in real time based on the relationship between the incident air kerma and the surface dose.MethodsThe air kerma for 20 X-ray beams with tube voltages of 50–140 kV and a half-value layer (HVL) of 2.27–9.65 mm Al was measured using an ionization chamber. The beam quality was classified based on the quality indexes (QIs) of 0.4, 0.5, and 0.6, which are defined as the ratio of the effective energy to the maximum energy corresponding to the tube potential. The surface dose for 20 X-ray beams was evaluated based on the measured air kerma, backscatter factor, and ratio of the mass–energy absorption coefficients of water to air, which were calculated using the Monte Carlo method. Finally, the relationship between the air kerma and the surface dose was investigated for X-ray beams with the specific QI values.ResultsThe surface dose at a water phantom was represented by a linear approximation of R² > 0.98, with the air kerma, regardless of the X-ray beam quality. The surface dose estimated based on a linear approximation with the air kerma indicated an agreement within 8% with that evaluated by the chamber measurements at HVL > 3.4 mm Al.ConclusionIt is possible to estimate the surface dose in real time using the linear relationship between the incident air kerma and the surface dose regardless of the X-ray beam quality by accepting ±10% uncertainty in the surface dose estimation.     

Determination of backscatter factors based on the quality index for diagnostic kilovoltage x-ray beams

PurposeThis study aims to investigate the relationship between backscatter factors and Al-half-value-layers (Al-HVL) by making the quality index (QI) a parameter for diagnostic kilovoltage x-ray beams.MethodsBackscatter factors, B_w, for x-ray fluence spectra were calculated from the weighted average of B_w for monoenergetic photons of between 8 and 140 keV with field sizes of 10 cm × 10 cm to 40 cm × 40 cm. The value of B_w for monoenergetic photons was calculated from the ratio of the water kerma at the surface of a water phantom and that at the same point free-in-air using the EGSnrc/cavity code. The weighted averaged backscatter factors were validated by comparing them with those of direct Monte Carlo calculations for the x-ray fluence spectra. The B_w for the x-ray fluence spectra were classified by a QI of 0.35, 0.4, 0.5, 0.6, and 0.7 specified by the ratio of the effective energy and maximum energy. The relationship between B_w and Al-HVL was evaluated for the given QI values. The x-ray fluence spectra were generated for tube voltages of 40–140 kVp with Al-HVLs of 0.5–13.2 mm using the SpekCalc program.ResultsThe weighted averaged backscatter factors for x-ray fluence spectra agreed within 0.7% with those of the direct Monte Carlo calculations. The backscatter factors were represented by the fitting curves of R² > 0.99 with Al-HVL for the given QI values.ConclusionsIt is possible to obtain B_w more accurately by using QI specified by the measured Al-HVL.     

Geometry optimisation of graphite energy degrader for proton therapy

Introduction: Cyclotron-based proton therapy facilities use an energy degrader of variable thickness to deliver beams of the different energies required by a patient treatment plan; scattering and straggling in the degrader give rise to an inherent emittance increase and subsequent particle loss in the downstream energy-selection system (ESS). Here we study alternative graphite degrader geometries and examine with Monte-Carlo simulations the induced emittance growth and consequent particle transmission.Methods: We examined the conventional multiple-wedge degrader used in the Paul Scherrer Institute PROSCAN proton therapy system, the equivalent parallel-sided degrader, and a single block degrader of equivalent thickness. G4Beamline Monte-Carlo tracking of protons was benchmarked against measurements of the existing degrader for proton energies from 75 to 230 MeV, and used to validate simulations of the alternative geometries.Results: Using a careful calculation of the beam emittance growth, we determined that a single-block degrader placed close to the collimators of the ESS is expected to deliver significantly larger transmission, up to 17% larger at 150 MeV. At the lowest deliverable of 75 MeV there is still a clear improvement in beam transmission.Conclusions: Whilst dose rates are not presently limited on the PROSCAN system at higher energies, a single-block degrader offers the ability to access either lower energies for treatment or a larger dose rate at 75 MeV in case transmission optimisation is desired. Single-block degraders should be considered for the delivery of low-energy protons from a cyclotron-based particle therapy system.     

Investigation of the dose distribution for a cone beam CT system dedicated to breast imaging

Cone-beam breast Computed Tomography (bCT) is an X-ray imaging technique for breast cancer diagnosis, in principle capable of delivering a much more homogeneous dose spatial pattern to the breast volume than conventional mammography, at dose levels comparable to two-view mammography. We present an investigation of the three-dimensional dose distribution for a cone-beam CT system dedicated to breast imaging. We employed Monte Carlo simulations for estimating the dose deposited within a breast phantom having a hemiellipsoidal shape placed on a cylinder of 3.5 cm thickness that simulates the chest wall. This phantom represents a pendulant breast in a bCT exam with the average diameter at chest wall, assumed to correspond to a 5-cm-thick compressed breast in mammography. The phantom is irradiated in a circular orbit with an X-ray cone beam selected from four different techniques: 50, 60, 70, and 80 kVp from a tube with tungsten anode, 1.8 mm Al inherent filtration and additional filtration of 0.2 mm Cu. Using the Monte Carlo code GEANT4 we simulated a system similar to the experimental apparatus available in our lab. Simulations were performed at a constant free-in-air air kerma at the isocenter (1 μGy); the corresponding total number of photon histories per scan was 288 million at 80 kVp. We found that the more energetic beams provide a more uniform dose distribution than at low energy: the 50 kVp beam presents a frequency distribution of absorbed dose values with a coefficient of variation almost double than that for the 80 kVp beam. This is confirmed by the analysis of the relative dose profiles along the radial (i.e. parallel to the “chest wall”) and longitudinal (i.e. from “chest wall” to “nipple”) directions. Maximum radial deviations are on the order of 25% for the 80 kVp beam, whereas for the 50 kVp beam variations around 43% were observed, with the lowest dose values being found along the central longitudinal axis of the phantom.     

On-site audits to investigate the quality of radiation physics of radiation therapy institutions in the Republic of Korea

PurposeTo investigate and improve the domestic standard of radiation therapy in the Republic of Korea.MethodsOn-site audits were performed for 13 institutions in the Republic of Korea. Six items were investigated by on-site visits of each radiation therapy institution, including collimator, gantry, and couch rotation isocenter check; coincidence between light and radiation fields; photon beam flatness and symmetry; electron beam flatness and symmetry; physical wedge transmission factors; and photon beam and electron beam outputs.ResultsThe average deviations of mechanical collimator, gantry, and couch rotation isocenter were less than 1 mm. Those of radiation isocenter were also less than 1 mm. The average difference between light and radiation fields was 0.9 ± 0.6 mm for the field size of 20 cm × 20 cm. The average values of flatness and symmetry of the photon beams were 2.9% ± 0.6% and 1.1% ± 0.7%, respectively. Those of electron beams were 2.5% ± 0.7% and 0.6% ± 1.0%, respectively. Every institutions showed wedge transmission factor deviations less than 2% except one institution. The output deviations of both photon and electron beams were less than ±3% for every institution.ConclusionsThrough the on-site audit program, we could effectively detect an inappropriately operating linacs and provide some recommendations. The standard of radiation therapy in Korea is expected to improve through such on-site audits.