Surface Dose Reduction Research Articles

Radiation protection in dental imaging: Evaluating the impact of the SABA thyroid shield during panoramic and cone beam computed tomography

BackgroundDental panoramic radiography (DPR) and cone beam computed tomography (CBCT) are an imaging modalities in dentistry. However, these procedures involve exposure to ionising radiation, raising concerns about radiation-induced thyroid damage. To mitigate these risks, thyroid shields have been introduced. This study aimed to evaluate the effectiveness of the SABA thyroid shield in reducing thyroid radiation exposure during DPR and CBCT scans. MethodAn experimental study to measure surface dose in the thyroid region during (DPR) and (CBCT) scans using an Anthropomorphic Phantom and a Solid-State Detector. The attenuation percentage was calculated between radiation surface doses with and without shielding. The significance of the Saba thyroid shield was calculated using a t-test. ResultsFor DPR scans, the average surface doses in the thyroid reduced significantly from 301.1 μGy (at 85 kVp) and 170.3 μGy (at 60 kVp) to 64.43 μGy and 12.94 μGy, respectively, when the Saba thyroid shield was used. For CBCT scans, the average surface doses in the thyroid decreased significantly from 814.43 μGy (for 11 × 13 cm2 field size) and 32.40 μGy (for 5 × 5 cm2 field size) to 62.91 μGy and 12.07 μGy, respectively, with the application of the Saba thyroid shield. The maximum attenuation percentages in the thyroid for the DPR and CBCT scans were 92.40% and 92.27%, respectively. Statistically significant differences between the surface dose reduction with the Saba shield and without it was observed in both DPR scans (p < 0.0000) and CBCT scans (p < 0.0003). ConclusionThe study demonstrates that Saba thyroid shields effectively reduce doses in the thyroid region during dental DPR and CBCT scans. The dose reduction depends on tube voltage for DPR scans, field size, and its position for CBCT scans. Findings highlight the importance of using Saba thyroid shields to minimise radiation exposure and protect the thyroid gland during dental scans.

Integration of Hybrid Dendrimers and Their Generations for Modulated Spatio‐Temporal Action

AbstractHybrid dendrimers constitute a unique class of well‐defined complex architecture featured with a central domain and bifurcated branches with two different dendritic sequences for their atomic configuration and functional groups at the periphery. This review article pioneers the field of new hybrid dendrimers using different generations by nanoconjugation with metals, carbohydrates, nucleotides, proteins/peptides, carbosilane, urea, silica, stem cells, guanidine, etc. The smart dendrimers contain desirable electrical, magnetic, optical, and biological attributes to increase surface area, monodisperse behavior, dose reduction, dissolution, permeability, long‐term stability, and significant decrement in nanotoxicity studies. The higher encapsulation of lipid soluble and insoluble moieties explores an excellent platform for the delivery of drugs (ibuprofen, indomethacin, etc.), nucleic acids (oligonucleotide, siRNA, and aptamer), genetic materials , and chemical diagnostic agents (gadolinium chelates and superparamagnetic iron oxide particles) for imaging. Owing to their flexibility in structural adaptability, different health conditions like glaucoma, inflammation, microbial infection, neurodegenerative problems (Alzheimer's disease and Parkinsonism), and cancer are benefited using such long‐lasting drug delivery. Advancements in molecular engineering techniques, 3D printing, artificial intelligence, robotic, green synthesis, and microwave‐assisted methods aid in the development of economically reliable and personalized pharmaceutical hybrid dendritic systems resembling antibodies, globular proteins, stem cells, enzymes, and genetic materials.

Novel multi jet fusion 3D-printed patient immobilization for radiation therapy.

Thermoplastic immobilizers are used routinely in radiation therapy to achieve positioning accuracy. These devices are variable in quality as they are dependent on the skill of the human fabricator. We examine the potential multi jet fusion (MJF) 3D printing for the production immobilizers with a focus on the surface dosimetry of several MJF-printed PA12-based material candidates. Materials are compared with the goal of minimizing surface dose with comparison to standard thermoplastic. We introduce a novel metamaterial design for the shell of the immobilizer, with the aims of mechanical robustness and low-dose buildup. We demonstrate first examples of adult and pediatric cranial and head-and-neck immobilizers. Three different PA12 materials were examined and compared to fused deposition modeling-printed polylactic acid (PLA), PLA with density lowered by adding hollow glass microspheres, and to perforated or perforated/stretched and solid status quo thermoplastic samples. Build-up dose measurements were made using a parallel plate chamber. A metamaterial design was established based on a packed hexagonal geometry. Radiochromic film dosimetry was performed to determine the dependence of surface dose on the metamaterial design. Full cranial and head-and-neck prototype immobilizers were designed, printed, and assessed with regard to dimensional accuracy. Build-up dose measurements demonstrated the superiority of the PA12 material with a light fusing agent, which yielded a ∼15% dose reduction compared to other MJF materials. Metamaterial samples provided dose reductions ranging from 11% to 40% compared to stretched thermoplastic. MJF-printed immobilizers were produced reliably, demonstrated the versatility of digital design, and showed dimensional accuracy with 97% of sampled points within ±2mm. MJF is a promising technology for an automated fabrication of patient immobilizers. Material selection and metamaterial design can be leveraged to yield surface dose reduction of up to 40%. Immobilizer design is highly customizable, and the first examples of MJF-printed immobilizers demonstrate excellent dimensional accuracy.

Calculation of Lens Exposure Reduction Using Organ-effective Modulation in Pediatric Head CT

Using a pediatric head phantom constructed in our department, we examined a method to reduce exposure by using organ-effective modulation (OEM; Toshiba Medical Systems Corporation, Tochigi) to tilt the gantry during pediatric head computed tomography (CT) scanning. The radiation reduction and CT image standard deviation (SD) were measured at gantry angles at which the orbit was slightly irradiated, partially irradiated, and completely irradiated. The OEM incident surface dose reduction rate was measured using an automatic exposure control (AEC) phantom with a diameter of 6-18 cm. The lens surface dose reduction rate using OEM was 21.2%. When the gantry was tilted and the orbit was completely out of the scanning range, the rate of reduction was 47.8%. OEM incident surface dose reduction rates were 27.4% for a phantom diameter of 18 cm, 22.0% for that of 16 cm, 17.8% for that of 14 cm, 17.2% for that of 12 cm, 8.4% for that of 10 cm, and 0% for that of 8 cm and 6 cm. OEM effectiveness decreased with decreasing phantom diameter. The use of OEM increased the rate of change of SD by 1.25´ when the gantry inclination was 0°, 1.27´ when the gantry inclination was 10°, and 1.27´ when the gantry inclination was 20°in the 12 o'clock position. The degree of reduction in exposure dose to the lens in pediatric head CT imaging was 47.8% by completely removing the lens from the irradiation range using gantry tilt and 21.2% by using OEM. The effect of OEM changed in proportion to tube current. The exposure reduction effect of the OEM decreases with decreasing head size, indicating its reduced effectiveness in head CT scans of smaller infants.

Dose Optimization of Electron Arc Treatment Technique in Chest Wall Beams after Mastectomy

In breast cancer, electron energies are preferred over photon energies because they provide minimal lung exposure when conditions allow for thin chest wall irradiation after mastectomy. In patients with irregular chest wall, it is difficult to perform a homogeneous irradiation with fixed electron beam therapy due to reasons such as thickness differences, irregular contour and lack of tissue, long scar and area joint problems. Electron arc therapy is propose as an alternative method in such patients. The study was carried out with electron energies of 6, 9, 12, 13.5 and 16 MeV. First, in order to be able to use it in electron arc planning in the planning system, after determining the dose characteristics of all available electron energies of the electron arc technique, the accuracy of these dose distributions was verified with film and TLD dosimetry. After the suitability was determined, electron arc plans were made on the CT simulation image of 20 patients selected due to the difficulty of homogeneous irradiation with the classical method. While the chosen reference dose of 85% covered the PTV homogeneously, it was found that the dose was decreased by an average of 50% compared to photon and classical electron therapy in the examination performed in terms of radiation dose to which the lung volume was exposed. During the planning, a homogeneous dose and bolus of different thicknesses were required depending on the energy in most patients to regulate the reduction in surface dose depending on the arc angle. Bolus prevents the lung and heart from overdosing while ensuring that the dose in the deeper parts of the target volume is more uniform. The use of tertiary block in electron-arc dose distributions prevented unwanted dose reduction in the field edges and provided a more homogeneous dose distribution at 85% reference isodose. If the structure of the patient's contour is very irregular, the dose distribution is not smooth due to the depth difference. In this context, it has been determined that during optimization, the isocenter depth should be chosen for the homogeneity of the dose distribution and to be as equal as possible from the surface at all beam angles. In addition, in the study, it was determined that more appropriate dose distributions were obtaine when the isocenter depth is greater than the maximum reach of electrons. Even if multiple electron fields of different energies are used, more homogeneous dose distributions have been achieve by eliminating field combination problems with the use of electron arc therapy.

The effect of fluorescence on surface dose with superficial X-rays incident on tissue with underlying lead.

An Advanced Markus chamber on the surface of solid water phantom was used to determine surface dose reduction, with either a lead or air interface, as a function of surface-interface separation (t). The beam quality dependence of dose reduction was investigated using the 50kV, 100kV and 150kV beams of an Xstrahl 150 superficial X-ray unit. For each beam the dose correction factor, DCF(t), namely the ratio of surface dose (t) to surface dose (t = 100mm), was determined. Monte Carlo simulations of DCF(t) with a lead interface were compared with corresponding measured values. Simulated spectra were calculated at the phantom surface for full backscatter (t = 100mm) and with either a lead or air interface at 2mm or 8mm depth. For each depth and beam quality lead fluorescent radiation at the surface was evident. The variation of DCF(t) for each beam and field size exhibits a minima at t ≈ 5mm and in the range 1mm ≤ t ≤ 40mm surface dose reduction is larger for 100kV than 150kV. Monte Carlo simulated DCF(t) are consistent with corresponding measured DCF(t). From simulated spectra L-series fluorescent X-rays (≈ 15keV) emanating from lead at t = 2mm are evident for all beams and fluorescent K-series X-rays only occur with 100kV and 150kV beams.

Effects of Self-Made Bismuth Shield Installation on Entrance surface Dose Reduction during Endovascular Treatment of Cerebral Aneurysms

Effects of Self-Made Bismuth Shield Installation on Entrance surface Dose Reduction during Endovascular Treatment of Cerebral Aneurysms

Measured and Monte Carlo simulated surface dose reduction for superficial X-rays incident on tissue with underlying air or bone.

Measurement of surface dose reduction effects for superficial x-rays incident on tissue with underlying air or bone and comparison with Monte Carlo simulations of such effects. Further to investigate the correlation between surface dose reduction and changes in Compton backscatter spectra with tissue-bone separation. An Advanced Markus chamber with entrance window facing downstream on the surface of a solid water phantom was used to investigate changes in surface dose with an underlying air or bone interface located at various depths below the surface. Chamber readings were obtained for interface depths ranging from 1 to 100 mm using the 50 kV, 100 kV and 150 kV beams of an Xstrahl 150 x-ray unit, with field diameters (ϕ) = 2.5 cm and 5 cm. For each beam quality and field size the dose correction factor, DCF(t), namely the ratio of measured dose (t) to dose (t = 100 mm) was determined. Monte Carlo simulations of DCF(t) for air and bone interfaces in tissue are used to validate corresponding measured data. For a given beam and field size, the difference between simulated spectra with an air or bone interface at t = 3 mm was used to determine the Compton backscatter from bone at the surface. For air, DCF(t < 40 mm) is less than unity and for a given t DCF(t) decreases as beam energy increases from 50 kV to 150 kV. Conversely for bone DCF(t < 40 mm) increases for a given t with increasing beam energy. In particular for t = 1 mm, ϕ = 5 cm, DCF for air(bone) are 0.90(0.92) and 0.86(0.96) for 50 kV and 150 kV, respectively. For the same tube potentials corresponding factors, ϕ = 2.5 cm, for air(bone) are 0.94(0.96) and 0.92(0.99). Calculated DCF(t) based on Monte Carlo simulations are consistent with experimental observations to within 2%. Monte Carlo simulations of x-ray spectra demonstrate the presence of Compton backscatter from underlying bone in tissue. With bone at 3 mm depth calculated backscatter spectra at the tissue surface suggest that surface dose is influenced by the proximity of bone and that this effect depends on beam quality. This work demonstrates the feasibility of using an Advanced Markus chamber with entrance window facing downstream to investigate surface dose reduction with underlying air or bone in tissue. As the field size decreases and beam quality increases surface dose with underlying bone tends to full backscatter values even though tissue thicknesses are below those normally associated with full backscatter. Conversely with underlying bone close to the surface dose will increasingly fall below full backscatter values as the beam energy is reduced and field size is increased.

Gamma Putty shielding effect in megavoltage photon beam

Purpose: Traditionally, lead and Cerrobend have been employed for field shaping in radiation therapy. Lately, another shielding material called Gamma Putty has emerged. The objective of this report is to examine its dosimetric and shielding characteristics in megavoltage photon beam. Methods: All measurements were carried out in a dual energy linac. Data were collected using a calibrated ionization chamber. Percent transmission, linear attenuation, and field size dependence were evaluated for open square fields (4 × 4 cm 2 to 10 × 10 cm 2 ) defined by collimator jaws and for different Gamma Putty thicknesses (t = 0, 0.3, 0.5, 1.0, 1.5, 2.0, and 2.5 cm) at 6 and 18 MV photon beams. The measurements were performed both in air using appropriate acrylic buildup cap and in solid water. Results: The Gamma Putty tray factor (GPTF) increased steadily with field size for both 6 and 18 MV. It was characterized by a half value thickness (HVT) of 2.513 ± 0.101 and 2.855 ± 0.024 cm for 6 and 18 MV, respectively. The reduction in surface dose was about 6%, 14.5%, 22%, 36.37%, and 54% for 6 MV and 2.75 %, 9.36 %, 16.25 %, 28.95 %, and 44.47 % for 18 MV for Gamma Putty thicknesses of 0.3, 0.5, 1.0, 1.5, 2.0, and 2.5 cm. Conclusion: The result of Gamma Putty shielding on the photon beam output increases with thickness, beam energy, and field size. Therefore, clinical use of Gamma Putty tray factors should be tailored for all thicknesses, beam energies, and field sizes.

Influence of standard RF coil materials on surface and buildup dose from a 6 MV photon beam in magnetic field.

Magnetic resonance guided teletherapy systems aspire to image the patient concurrently with the radiation delivery. Thus, the radiofrequency (RF) coils used for magnetic resonance imaging, placed on or close to patient skin and in close proximity to the treatment volume, would be irradiated leading to modifications of radiation dose to the skin and in the buildup region. The purpose of this work is to measure and assess these dose modifications due to standard off-the-shelf RF coil materials. A typical surface coil was approximated as layered sheets of polycarbonate, copper tape, and Teflon to emulate the base, conductor, and cover, respectively. A separate investigation used additional coil materials, such as copper pipe, plastic coil housing, a typical coil padding material, and a thin copper conductor. The materials were placed in the path of a 6 MV photon beam at various distances from polystyrene phantoms in which the surface and buildup doses were measured. The experiments were performed on a clinical Varian linac with no magnetic field and with a 0.21 T electromagnet producing a magnetic field parallel to the beam central axis. The authors repeated similar experiments in the presence of a 0.22 T magnetic field oriented perpendicular to the beam central axis using an earlier linac-MR prototype, with a biplanar permanent magnet. The radiation detectors used for the measurements were two different parallel plate ion chambers and GAFChromic films. A typical open beam surface dose of 20% (relative to open beam Dmax) was increased to 63% by the coil padding material and to >74% by all other materials when placed in direct contact with the phantom, irrespective of magnetic field presence or orientation. Without a magnetic field, the surface dose decreased as the test materials were moved away from the phantom surface toward the radiation source, reaching between 30% and 40% at 10 cm gap between the phantom and the test materials. In the presence of the transverse magnetic field, the surface dose reduction was more rapid reaching a dose level of 30%-40% with only 3-4 cm gap. In the presence of the parallel magnetic field, as expected, the surface dose did not decrease considerably as the gap between the phantom surface and test materials was increased; the surface dose remained >60% at 10 cm gap for all tested materials except for the thin copper conductor. As expected, placing coil materials in direct contact with the phantom surface increases the surface dose considerably. The surface dose is reduced by creating a gap between the coil materials and phantom surface. This dose reduction happens more rapidly in the presence of a transverse magnetic field. However, the surface dose stays relatively large irrespective of the gap in the presence of a parallel magnetic field. Thus, the standard, off-the-shelf RF coils should be used with caution in integrated linac-MR systems, especially those using a parallel magnetic field orientation in which case the RF coils will probably need to be reconfigured to create open ports for the radiation beam.

SU‐F‐T‐671: Effects of Collimator Material On Proton Minibeams

Purpose:To investigate the dosimetric effects of collimator material on spatially modulated proton minibeams (pMBRT). Methods: pMBRT holds promise to exhibit shallow depth normal‐tissue sparing effects similar to synchrotron based microbeams while also retaining potential for uniform dose distributions for tumor targets. TOPAS Monte Carlo simulations were performed for a 5cm thick multislit collimator with 0.3mm slits and 1mm center‐to‐center spacing for a 50.5MeV proton minibeam while varying collimator material between brass, tungsten, and iron. The collimator was placed both “flush” at the water phantom surface and at 5cm distance to investigate the effects on surface dose, peak‐to‐valley‐dose‐ratio (PVDR) and neutron contribution.Results:For flush placement, the neutron dose at the phantom surface for the tungsten collimator was approximately 20% higher than for brass and iron. This was not reflected in the overall surface dose, which was comparable for all materials due to the relatively low neutron contribution of &lt;0.1%. When the collimator was retracted, the overall neutron contribution was essentially identical for all three collimators. Surface dose dropped by ∼40% for all collimator materials with air gap compared to being flush with the phantom surface. This surface dose reduction was at the cost of increase in valley dose for all collimator materials due to increased angular divergence of the mini‐beams at the surface and their consequent geometric penumbra at depth. When the collimator was placed at distance from the phantom surface the PVDR decreased. The peak‐to‐entrance‐dose ratio was highest for the iron collimator with 5cm air gap. Conclusion: The dosimetric difference between the collimator materials is minimal despite the relatively higher neutron contribution at the phantom surface for the tungsten collimator when placed flush. The air gap between the collimator and phantom surface strongly influences all dosimetry parameters due to the influence of scatter on the narrow spatial modulation.

Reduction of Entrance Surface Dose Depending on Shielding Methods for Panoramagraphy

Reduction of Entrance Surface Dose Depending on Shielding Methods for Panoramagraphy

Evaluation of Manual Angulation Fixed Focal Approach Using Flat Panel Detector in Digital Radiography of Scoliosis

Scoliosis patient underwent numerous spine radiographs during their monitor and treatment period, thus, may expose them to high accumulative radiation dose. In this study, the diagnostic performance of the flat panel detector (FPD) fixed focal approach with manual angulation was evaluated in term of accuracy and consistency of Cobb angle measurement, spatial accuracy as well as radiation dose to the patients compared to the conventional photostimulable phosphor (PSP) approach. Cobb angle measurements were evaluated with two special constructed human vertebrae phantoms. The intraclass correlation coefficients (ICC) for interobserver variation and percentage of accuracy in Cobb angle measurement were calculated. The significant differences in angle measurement between both approaches were assessed. The spatial accuracy was evaluated with TO.M1 phantom. Dose measurements were performed with radiochromic film and 20 cm polymethyl methacrylate (PMMA) slabs. The FPD fixed focal approach showed excellent interobserver reproducibility (ICC, r = 0.99, p 0.05) for the Cobb angle measured from both approaches. The FPD fixed focal approach resulted in higher accuracy (3%) of angle measurement compared to the conventional PSP approach. The spatial accuracy was within the recommended limit of AAPM report 93. An average entrance surface dose (ESD) reduction of 20% was achieved with FPD fixed focal approach. We developed and evaluated a manual tube angulation method which facilitates the use of existing FPD digital radiography system to produce full spine image. It has good consistency and high accuracy in the measurement of Cobb angle. The acquired images are suitable for angle measurement for scoliosis patients with substantial dose reduction.

SU-E-T-533: Surface Dosimetry in the Presence of a Bone in Skin Radiotherapy: A Monte Carlo Study Using KV Photon Beams

Purpose: This study examined the variation of surface dose in the presence of a bone located under the surface of patient (e.g. sites of forehead, chest wall and kneecap) in skin radiotherapy. A 105 kVp photon beam was used, and simulated by Monte Carlo method using the EGSnrc code. Methods: Heterogeneous phantoms containing a water layer (thickness = 0.5−5 mm) over a bone with thickness equal to 1 cm were irradiated by a clinical 105 kVp photon beam (Gulmay D3225 x-ray machine). The field size was 5 cm diameter and the source-to-surface distance of the beam was 20 cm. Phase-space file of the photon beam was generated using the BEAMnrc code and verified by measurements. Surface doses for different phantom geometries were calculated using the DOSXYZnrc code. For comparison, all Monte Carlo simulations were repeated in a water phantom with the same dimension of the corresponding heterogeneous phantom. Results: With the presence of a bone under a water layer of 1 mm thickness, surface dose was found decreased 6.3% using the 105 kVp photon beam. This is due to the loss of backscatter from the bone in the beam irradiation. When the water thickness increased to 3 and 5 mm, the surface dose reduction decreased to 4.7% and 3.4%, respectively. This shows that the dosimetric impact due to the presence of bone decreased, when the bone was located further away from the phantom surface. Conclusion: Surface dose reduction was found in the presence of a bone under the skin tissue. Since such dose reduction is not considered in the dose calculation based on the absolute dose calibration using a homogeneous water phantom, an overestimation of dose occurs when it is prescribed. This results in a dosimetric uncertainty with a variation of tissue thickness in skin radiotherapy.

Surface dose measurements from air gaps under a bolus by using a MOSFET dosimeter in clinical oblique photon beams

This study is intended to investigate the effects of surface dose from air gaps under the bolus in clinically used oblique photon beams by using a Markus parallel-plate chamber and a metal-oxide semiconductor field-effect transistor (MOSFET) dosimeter. To evaluate the performances of the two detectors, the percentage surface doses of the MOSFET dosimeters in without an air gap under the bolus material were measured and compared with those of the Markus parallel-plate chamber. MOSFET dosimeters at the surface provided results mostly in good agreement with the parallelplate chamber. The MOSFET dosimeters seemed suitable for surface dose measurements having excellent accuracy for clinical used photon beams. The relative surface doses were measured with air gaps (2, 5, 10 mm) and without an air gap under 3 different bolus setups: (1) unbolused (no bolus), (2) 5-mm bolus, and (3) 10-mm bolus. The reductions in the surface dose substantially increased with small field size, thick bolus, and large air gap. The absolute difference in the reductions of the surface dose between the MOSFET dosimeter and the Markus parallel-plate chamber was less than 1.1%. Results at oblique angles of incidence showed larger reductions in surface dose with increasing angle of incidence. The largest reduction in surface dose was recorded for a 6 × 6 cm2 field at a 60° angle of incidence with an 10-mm air gap under a 10-mm bolus. When a 10-mm bolus was used, a reduction in the surface dose with an air gap of up to 10.5% could be achieved by varying the field size and the incident angle. Therefore, air gaps under the bolus should be avoided in radiotherapy treatment, especially for photon beam with highly oblique angles of incidence.

Surface dose reduction from bone interface in kilovoltage X‐ray radiation therapy: a Monte Carlo study of photon spectra

This study evaluated the dosimetric impact of surface dose reduction due to the loss of backscatter from the bone interface in kilovoltage (kV) X‐ray radiation therapy. Monte Carlo simulation was carried out using the EGSnrc code. An inhomogeneous phantom containing a thin water layer (0.5–5 mm) on top of a bone (thickness=1 cm) was irradiated by a clinical 105 kVp photon beam produced by a Gulmay D3225 X‐ray machine. Field sizes of 2, 5, and 10 cm diameter and source‐to‐surface distance of 20 cm were used. Surface doses for different phantom configurations were calculated using the DOSXYZnrc code. Photon energy spectra at the phantom surface and bone were determined according to the phase‐space files at the particle scoring planes which included the multiple crossers. For comparison, all Monte Carlo simulations were repeated in a phantom with the bone replaced by water. Surface dose reduction was found when a bone was underneath the water layer. When the water thickness was equal to 1 mm for the circular field of 5 cm diameter, a surface dose reduction of 6.3% was found. The dose reduction decreased to 4.7% and 3.4% when the water thickness increased to 3 and 5 mm, respectively. This shows that the impact of the surface dose uncertainty decreased while the water thickness over the bone increased. This result was supported by the decrease in relative intensity of the lower energy photons in the energy spectrum when the water layer was with and over the bone, compared to without the bone. We concluded that surface dose reduction of 7.8%–1.1% was found when the water thickness increased from 0.5–5 mm for circular fields with diameters ranging from 2–10 cm. This decrease of surface dose results in an overestimation of prescribed dose at the patient's surface, and might be a concern when using kV photon beam to treat skin tumors in sites such as forehead, chest wall, and kneecap.PACS number: 87.55.K‐; 87.55.ne; 87.57.uq

Dose Reduction to Anterior Surfaces With Organ-Based Tube-Current Modulation: Evaluation of Performance in a Phantom Study

The purpose of this study was to evaluate in phantoms the dose reduction to anterior surfaces and image quality with organ-based tube-current modulation in head and thoracic CT. Organ-based tube-current modulation is designed to reduce radiation dose to superficial radiosensitive organs, such as the lens of the eye, thyroid, and breast, by decreasing the tube current when the tube passes closest to these organs. Dose and image quality were evaluated in phantoms for clinical head and thorax examination protocols with and without organ-based tube-current modulation. Surface dose reduction as a function of position was measured using a 32-cm CT dose index (CTDI) phantom, an anthropomorphic adult phantom, and ion chambers. Surface dose reduction as a function of patient size was investigated using three semianthropomorphic phantoms with posteroanterior dimensions of 14, 25, and 31 cm. Image noise (the SD of CT numbers in regions of interest) was evaluated for the anthropomorphic and the semianthropomorphic phantoms. For equivalent scanner output (volume CTDI), the dose to the midline of the anterior surface was reduced by 27-50%, depending on the anatomic region (head or thorax) and phantom size, and the dose to the posterior surface was correspondingly increased. Image noise was not significantly different between scans with and without organ-based tube-current modulation (p = 0.85). Organ-based tube-current modulation can reduce the dose to the anterior surface of patients without increasing image noise by commensurately increasing the dose to the posterior surface. This technique can reduce the dose to anterior radiosensitive organs for head and thoracic CT scans.

SU‐E‐T‐693: Surface Dose Reduction from Bone Interface in Superficial X‐Ray Radiation Therapy: A Monte Carlo Study

Purpose: This study evaluated the dosimetric impact of surface dose reduction due to the loss of backscatter from the bone interface in superficial x‐ray radiation therapy. Monte Carlo simulation was carried out using the EGSnrc code. Methods: An inhomogeneous phantom containing a thin layer of tissue (1,3 and 5 mm) on top of a bone (thickness = 1 cm) was irradiated by a clinical 105 kVp photon beam produced by a Gulmay D3225 kV x‐ray machine. The field size and SSD was equal to 5 cm diameter and 20 cm, respectively. Surface doses for different phantom configurations were calculated using the DOSXYZnrc code. Photon energy spectra at the phantom and bone surface were determined according to phase‐space files at the particle scoring planes. For comparison, all Monte Carlo simulations were repeated in a phantom with the bone replaced by soft tissue. Results: Surface dose reduction was found when a bone was underneath the tissue layer. When the tissue thickness = 1 mm, a surface dose reduction of 3.5% was found. The dose reduction decreased to 2.1% and 1.8% when the tissue thickness increased to 3 and 5 mm, respectively. This shows that the impact of the surface dose uncertainty decreased while the tissue thickness on top of the bone increased. This result was supported by the decrease of intensity in the photon energy spectrum, when the tissue layer was with and over the bone, compared to without the bone. Conclusions: Surface dose reduction of 3.5%–1.8% was found when the tissue layer increased from 1 to 5 mm. This decrease of surface dose results in an overestimation of prescribed dose at the patientˈs surface, and should be a concern when using superficial x‐ray to treat skin tumours in sites such as forehead, chest wall and kneecap.

SU‐E‐I‐62: Dose Reduction to Anterior Surface with Organ‐Based Tube Current Modulation: Evaluation of Performance in a Phantom Study

Purpose: To evaluate in a phantom study image quality and dose reduction to the anterior surface with organ‐based tube current modulation in head and thoracic CT. Methods: Organ‐based tube current modulation is designed to reduce dose to superficial radiosensitive organs such as the lens of the eye, thyroid and breast by decreasing the tube current when the tube passes closest to these organs. Dose and image quality were evaluated in phantoms for clinical head and thorax exam protocols with and without organ‐based tube current modulation. Surface dose reduction as a function of position was measured using a 32‐cm CT Dose Index (CTDI) phantom, an anthropomorphic adult phantom and ion chambers. Surface dose reduction as a function of patient size was investigated using three semi‐ anthropomorphic phantoms with posteroanterior dimensions of 14, 25 and 31 cm. Image noise (the standard deviation of CT numbers in regions of interest) was evaluated for the anthropomorphic and the semi‐ anthropomorphic phantoms. Results: For equivalent scanner output (Volume CTDI), dose to the midline of the anterior surface was reduced by 27–50%, depending on anatomic region (head or thorax) and phantom size; dose to the posterior surface was correspondingly increased. Image noise was not significantly different between scans with and without organ‐based tube current modulation (p=0.35). Conclusions: Organ‐based tube current modulation can reduce dose to the anterior surface of patients, without increasing image noise, by commensurately increasing dose to the posterior surface. This can reduce dose to anterior radiosensitive organs for head and thoracic CT scans.

Ｐｅｒｃｕｔａｎｅｏｕｓ Ｃｏｒｏｎａｒｙ Ｉｎｔｅｒｖｅｎｔｉｏｎ（ＰＣＩ）における補償フィルタを用いた照射野重複部の防護による患者皮膚線量の低減

Our study was involved with entrance surface dose reduction and irradiation field by the filter use of PCI, and insertion in place of an effective compensating filter to maximize entrance surface dose reduction, which we verified. The radiation dosimetry put a 6cc ion chamber on the back side of the thorax phantom, and changed the filter of the four corners (a: upper left, b: upper right, c: lower right, d: lower left) of the monitor confirmed with fluoroscopy [(0) no filter, (1) one filter, (2) two filters]. The angle of C arm was assumed to be eight directions and 0 degrees adopted by this hospital. It was compared with a corrective rate of which one was no filter. Next, the presence of filter and irradiation field overlaps on the area in monitor in the angle of C arm was verified by this hospital's classic example. As for corrective rate, (1) becomes 0.41 and (2) become 0.25 at fluoroscopy, (1) becomes 0.26 and (2) become 0.16 at exposure. Irradiation field overlaps on the area (+) compensating filter (-) was many with d of RAO/CAU, a of RAO and c of CAU at left CAG, c of LAO at right CAG, b of LAO/CRA (left CAG), b of CRA (right CAG) and a and d of RAO (right CAG) at both CAG. Irradiation field overlaps on the area (+) compensating filter (+) was many with b of CRA at left CAG, a of LAO/CRA at right CAG, b of CRA (left CAG) and b of RAO (right CAG) at both CAG. When the compensating filter is used the entrance surface dose reduction effect was great. If automatic exposure control protects the part of irradiation field overlaps on the area in the range without operating excessively, the radiological risk can be reduced, and it is conceivable as useful clinical setting.