Polystyrene Slabs Research Articles

Universal slabs for hydro- and heat insulation

The article deals with the creating of a universal material for protecting underground structures from groundwater as well as providing their thermal protection. The article presents a method for waterproofing and drainage of underground walling structures using drainage slabs made of filtration polystyrene foam and considers the possibility of using these slabs for reducing heat losses of heated underground structures. Thermal engineering characteristics of filtration polystyrene foam are determined, and calculations of heat losses of underground walling structures with slabs made of this material are given. The results of studies of the kinetics of water drainage by filtration polystyrene slabs are shown. The dependence of the heat conductivity of filtration polystyrene on humidity and other factors are also presented. Calculations of heat losses through the walls of heated underground premises are given, taking into account the temperature distribution in the ground along the depth of the structure. A comparative analysis of heat losses with the use of filtration polystyrene foam slabs and without them has been carried out. Based on the experimental results obtained, it was revealed that filtration polystyrene foam has not only high filtration characteristics, but is also an effective material for thermal protection of underground structures.

Validation of a commercial deformable image registration for surface-guided radiotherapy using an ad hoc-developed deformable phantom.

The use of optical surface systems (OSSs) for patient setup verification in external radiation therapy is increasing. To manage potential deformations in a patient's anatomy, a novel deformable image registration (DIR) tool has been applied in a commercial OSS. In this study we investigate the accuracy of the DIR as compared to rigid image registration (RR). The positioning accuracy of the DIR and RR implemented in the OSS was investigated using an ad hoc-developed anthropomorphic deformable phantom, named Mary. The phantom consists of 33 slices of expanded polystyrene slabs shaped thus to simulate part of a female body. Anatomical details, simulating the ribs and spinal cord, together with 10 inner targets at different depths are included in thorax and abdominal parts. Mary is capable of realistic body movements and deformations, such as head and arm rotations, body torsion and moderate breast/abdomen swelling. The accuracy of DIR and RR was investigated for four internal targets after deliberately deforming the phantom nine times. Breast and abdomen enlargements and torsions around x, y, and z axes were applied. For reference purposes, rigid displacements (where Mary's anatomy was kept intact) were included. The phantom was positioned on the linac couch under the OSS guidance and for each target and displacement a CBCT was acquired. The accuracy of DIR and RR was assessed evaluating the difference in means of absolute values between CBCT and the OSS registration parameters (lateral, longitudinal, vertical, rot, pitch, and roll), using both a reference surface extracted from CT (CTr) or acquired with the OSS (OSSr). A comparison of the four different combinations, DIR+OSSr, DIR+CTr, RR+OSSr, and RR+CTr, was carried out to evaluate the position accuracy for the various combinations. Finally, the positioning accuracy of the different target positions using only OSSr was investigated for the DIR. A paired sample Wilcoxon signed-rank test (P<0.05) and a two-tailed Mann-Whitney test (P<0.05) were carried out. The DIR in combination with OSSr showed significantly (P<0.05) improved positioning accuracy in the lateral and longitudinal directions and in pitch, compared to RR, when deformations were applied to Mary. The positioning accuracy improved from 1.9±1.5mm, 1.1±0.8mm to 1.1±1.2mm, 0.6±0.5mm in lateral and longitudinal directions, respectively, and from 0.8±0.6° to 0.4±0.4° in pitch, using DIR compared to RR. Both the DIR and RR showed a similar positioning accuracy when rigid displacements of Mary were applied. For DIR, the OSSr generally showed improved calculation accuracy compared to CTr. Independent of the reference image used, the target position influenced the registration accuracy, and hence, one target could not be evaluated using RR due to its inability to calculate the correct position. Improved positioning accuracy was observed for DIR with respect to RR when deformations of Mary's anatomy were applied. For both DIR and RR, improved positioning accuracy was observed using OSSr as compared to CTr. The position of the target inside the phantom influenced the positioning accuracy for DIR.

Learning-based lensless imaging through optically thick scattering media

The problem of imaging through thick scattering media is encountered in many disciplines of science, ranging from mesoscopic physics to astronomy. Photons become diffusive after propagating through a scattering medium with an optical thickness of over 10 times the scattering mean free path. As a result, no image but only noise-like patterns can be directly formed. We propose a hybrid neural network for computational imaging through such thick scattering media, demonstrating the reconstruction of image information from various targets hidden behind a white polystyrene slab of 3 mm in thickness or 13.4 times the scattering mean free path. We also demonstrate that the target image can be retrieved with acceptable quality from a very small fraction of its scattered pattern, suggesting that the speckle pattern produced in this way is highly redundant. This leads to a profound question of how the information of the target being encoded into the speckle is to be addressed in future studies.

Au-nanoparticles strip-loaded channel waveguide

A strip-loaded channel waveguide was fabricated by deposition of Au nanoparticles (AuNPs) in a thin strip via organometallic chemical vapor deposition on top of a polystyrene slab waveguide. The beam confinement is demonstrated experimentally, despite the absence of a classical closed layer for strip loading. The AuNP density conditions for plasmonic-waveguide hybrid transmission devices are discussed.

Verification of Acuros XB dose algorithm using 3D printed low-density phantoms for clinical photon beams.

The transport‐based dose calculation algorithm Acuros XB (AXB) has been shown to accurately account for heterogeneities primarily through comparisons with Monte Carlo simulations. This study aims to provide additional experimental verification of AXB for clinically relevant flattened and unflattened beam energies in low density phantoms of the same material. Polystyrene slabs were created using a bench‐top 3D printer. Six slabs were printed at varying densities from 0.23 to 0.68 g/cm3, corresponding to different density humanoid tissues. The slabs were used to form different single and multilayer geometries. Dose was calculated with Eclipse™ AXB 11.0.31 for 6MV, 15MV flattened and 6FFF (flattening filter free) energies for field sizes of 2 × 2 and 5 × 5 cm2. EBT3 film was inserted into the phantoms, which were irradiated. Absolute dose profiles and 2D Gamma analyses were performed for 96 dose planes. For all single slab configurations and energies, absolute dose differences between the AXB calculation and film measurements remained <3% for both fields in the high‐dose region, however, larger disagreement was seen within the penumbra. For the multilayered phantom, percentage depth dose with AXB was within 5% of discrete film measurements. The Gamma index at 2%/2 mm averaged 98% in all combinations of fields, phantoms and photon energies. The transport‐based dose algorithm AXB is in good agreement with the experimental measurements for small field sizes using 6MV, 6FFF and 15MV beams adjacent to various low‐density heterogeneous media. This work provides preliminary experimental grounds to support the use of AXB for heterogeneous dose calculation purposes.

Synthesis of a Novel Flame Retardant and Study on Its Flame Retardancy

A novel intumescent flame retardant (MAPP-MUF-GO) was prepared by means of the in-situ condensation polymerization of urea and formaldehyde in the presence of graphite oxide (GO), polyvinyl alcohol (PVA) and modified ammonium polyphosphate (MAPP). The synthesis mechanism and chemical construction of the obtained flame retardant were conjectured by Fourier transform infrared spectroscopy (FTIR), and the thermal stability of the flame retardant MAPP-MUF-GO was investigated by Thermogravimetric analysis (TGA) and Micro combustion calorimeter (MCC) measurement. The expandable polystyrene (EPS) slab was coated using the novel flame retardant to prepare flame retardant slab. The flame retardancy of the flame retardant slab was studied by means of limiting oxygen index (LOI) and horizontal burning test (UL-94). The LOI of the slabs coated with 25 (wt%) MAPP-MUF-GO reached up to 32.3% and the slabs passed the standard of HF1 according to UL-94. The thermal conductivity index of the flame retardant slab met the government standard.

Machine-specific quality assurance procedure for stereotactic treatments with dynamic couch rotations.

We present a method in which the treatment couch's accuracy is measured using the electronic portal imaging device (EPID) and a phantom of our own construction. Using this phantom, we were able to quantify the treatment couch walkout, and the rotation angle accuracy for both static and dynamic couch treatments. These measurements were used to provide an accurate measure of the treatment couch isocenter as well as to verify the couch rotation angle recorded in the trajectory log. The phantom was constructed using a polystyrene slab in which five ball bearings of 4 mm diameter are placed on the same plane at varying radii (0, 2.8, 4.4, 5.6, and 6.7 cm). The couch was rotated through its full extent (-90, 90 degrees) while MV images were acquired continuously. The couch rotational accuracy was calculated using a least squares minimization which fit the locations of the BBs to their expected locations relative to reference setup conditions. Using this approach, rotation angle and isocenter walkout was calculated in three dimensions. These measurements were used to quantify the accuracy of the couch as well as to validate the Varian TrueBeam trajectory logs. Additionally, a method for an EPID-based couch star-shot measurement was developed and compared with the traditional film-based method. The measured couch center of rotation consisted of a cloud of points clustered around the room isocenter within 0.7 mm distance. The trajectory log couch angle values agreed with those recorded in the DICOM header of the EPID images to the third significant digit and the couch rotation angles recorded in the trajectory log and DICOM header agreed with the calculated values to 0.08 degrees. Comparison of couch star-shot measurement developed in this study with film-based star-shot measurements gave an agreement to within 0.2 mm. We have developed a quality assurance method for the treatment couch which is simple, accurate, and enables the user to access a multitude of consistent data with a single measurement. Using this method, we have shown that the treatment couch is accurate for both static and dynamic stereotactic deliveries.

Optimal combination of anti-scatter grids and software correction for CBCT imaging.

Cone beam computed tomography (CBCT) has been widely adopted in clinical practice for image-guided radiotherapy. Soft tissue contrast and Hounsfield units are impaired to the presence of scattered radiation. In our previous work, we proposed a high selectivity anti-scatter grid (ASG) as a possible solution to the problem. An alternative approach is the application of iterative scatter correction using deconvolution with scatter point spread function (PSF). The purpose of this work was to compare the performance of ASGs with different selectivity with and without the iterative and uniform scatter corrections in terms of CBCT image quality. A secondary objective of this study was to develop a novel measurement approach to measure the scatter point spread functions. The scatter PSF was modeled as a sum of two bivariate Gaussian functions. The PSF parameters were estimated from a series of transmission measurements through polystyrene slabs of varying thickness with lead partial beam-blocker for three different ASG designs ranging from low (5.6), medium (9), and high (11) selectivity. The scatter correction scheme is based on iterative convolution of the current estimate of the primary with the scatter PSF until the root mean square deviation (RMSD) of the measured projection and the sum of the estimate of primary and scatter falls below a predefined threshold. The image quality was evaluated with the CIRS CBCT Image Quality and Electron Density phantom in a head and neck and pelvis configuration and the CIRS Virtual Male Human Patient. The image quality was quantified by the contrast-to-noise ratio (CNR) relative to the uncorrected scans and the root mean square deviation of the average gray values for different regions with respect to the nominal Hounsfield units and the mean difference of the reconstructed HU between the planning CT and CBCTs of the virtual human phantom. For the head and neck phantom, the CNR increased with more advanced scatter correction algorithm and the ASG selectivity, reaching 3.9, 3.7, 3.5, and 3.1 for the high, medium, light, and with no grid configuration, respectively, combined with the iterative software correction. The same is true for the pelvis phantom with CNR improvement reaching 1.5 for the heavy and medium grid, 1.3 for the light grid, and 1.1 on its own. The HU RMSD for the head and neck phantom was 22HU, 13HU, 12HU, and 6HU for iterative correction without the grid, with the light grid, medium grid and the heavy grid, respectively. For same correction strategies, the values for the pelvis phantom where 170, 120, 34, and 27HU. The average difference with the PCT of the virtual human phantom was 59±48HU and 63±59HU with scans reconstructed with the iterative correction and two higher selectivity grids. Visual inspection revealed similar trends for a head-and-neck and prostate cancer patient. The best scatter mitigation strategy was found to be a combination of a grid with selectivity larger than 9, combined with iterative scatter estimation. None of the investigated grids required increasing the imaging dose. The PSF determined using proposed method leads to image quality improvements results for all but one of the investigated scenarios.

Virtual patient 3D dose reconstruction using in air EPID measurements and a back-projection algorithm for IMRT and VMAT treatments

PurposeAt our institute, a transit back-projection algorithm is used clinically to reconstruct in vivo patient and in phantom 3D dose distributions using EPID measurements behind a patient or a polystyrene slab phantom, respectively. In this study, an extension to this algorithm is presented whereby in air EPID measurements are used in combination with CT data to reconstruct ‘virtual’ 3D dose distributions. By combining virtual and in vivo patient verification data for the same treatment, patient-related errors can be separated from machine, planning and model errors. Methods and materialsThe virtual back-projection algorithm is described and verified against the transit algorithm with measurements made behind a slab phantom, against dose measurements made with an ionization chamber and with the OCTAVIUS 4D system, as well as against TPS patient data. Virtual and in vivo patient dose verification results are also compared. ResultsVirtual dose reconstructions agree within 1% with ionization chamber measurements. The average γ-pass rate values (3% global dose/3mm) in the 3D dose comparison with the OCTAVIUS 4D system and the TPS patient data are 98.5±1.9%(1SD) and 97.1±2.9%(1SD), respectively. For virtual patient dose reconstructions, the differences with the TPS in median dose to the PTV remain within 4%. ConclusionsVirtual patient dose reconstruction makes pre-treatment verification based on deviations of DVH parameters feasible and eliminates the need for phantom positioning and re-planning. Virtual patient dose reconstructions have additional value in the inspection of in vivo deviations, particularly in situations where CBCT data is not available (or not conclusive).

Sci‐Sat AM: Radiation Dosimetry and Practical Therapy Solutions ‐ 04: On 3D Fabrication of Phantoms and Experimental Verification of Patient Dose Computation Algorithms

Purpose:Transport based dose calculation algorithm Acuros XB (AXB) has been shown to accurately account for heterogeneities mostly through comparisons with Monte Carlo simulations. This study aims at providing additional experimental verification for AXB for flattened and unflattened clinical energies in low density phantoms of the same material.Materials and Methods:Polystyrene slabs were created using a bench‐top 3D printer. Six slabs were printed at varying densities from 0.23 g/cm3 to 0.68 g/cm3, corresponding to different density humanoid tissues. The slabs were used to form different single and multilayer geometries. Dose was calculated with AXB 11.0.31 for 6MV, 15MV flattened and 6FFF (flattening filter free) energies for field sizes of 2×2 cm2 and 5×5 cm2. The phantoms containing radiochromic EBT3 films were irradiated. Absolute dose profiles and 2D gamma analyses were performed for 96 dose planes.Results:For all single slab, multislab configurations and energies, absolute dose differences between the AXB calculation and film measurements remained &lt;3% for both fields, with slightly poor disagreement in penumbra. The gamma index at 2% / 2mm averaged 98% in all combinations of fields, phantoms and photon energies.Conclusions:The transport based dose algorithm AXB is in good agreement with the experimental measurements for small field sizes using 6MV, 6FFF and 15MV beams adjacent to low density heterogeneous media. This work provides sufficient experimental ground to support the use of AXB for heterogeneous dose calculation purposes.

Novel scintillation detector design and performance for proton radiography and computed tomography

Proton computed tomography (pCT) will enable accurate prediction of proton and ion range in a patient while providing the benefit of lower radiation exposure than in x-ray CT. The accuracy of the range prediction is essential for treatment planning in proton or ion therapy and depends upon the detector used to evaluate the water-equivalent path length (WEPL) of a proton passing through the object. A novel approach is presented for an inexpensive WEPL detector for pCT and proton radiography. A novel multistage detector with an aperture of 10 × 37.5 cm was designed to optimize the accuracy of the WEPL measurements while simplifying detector construction and the performance requirements of its components. The design of the five-stage detector was optimized through simulations based on the geant4 detector simulation toolkit, and the fabricated prototype was calibrated in water-equivalent millimeters with 200 MeV protons in the research beam line of the clinical proton synchrotron at Loma Linda University Medical Center. A special polystyrene step phantom was designed and built to speed up and simplify the calibration procedure. The calibrated five-stage detector was tested in the 200 MeV proton beam as part of the pCT head scanner, using a water phantom and polystyrene slabs to verify the WEPL reconstruction accuracy. The beam-test results demonstrated excellent performance of the new detector, in good agreement with the simulation results. The WEPL measurement accuracy is about 3.0 mm per proton in the 0-260 mm WEPL range required for a pCT head scan with a 200 MeV proton beam. The new multistage design approach to WEPL measurements for proton CT and radiography has been prototyped and tested. The test results show that the design is competitive with much more expensive calorimeter and range-counter designs.

Anti-biofilm effect of nanometer scale silver (NmSAg) coatings on glass and polystyrene surfaces against P. mirabilis, C. glabrata and C. tropicalis strains.

Nowadays, in order to terminate biofilm associated infections, coating of particular biomaterial surfaces with particular substances, via some nanotechnological tools, is being applied. Therefore, in the present study, investigation of anti-biofilm effects of nanometer scale silver (NmSAg) coatings on glass and polystyrene surfaces against clinical strains of Proteus mirabilis, Candida glabrata and Candida tropicalis was aimed. In this study, glass and polystyrene slabs with 1.5 cm × 1.5 cm × 0.3 mm dimensions were cleaned by using surface plasma technology, covered with NmSAg by using a physical vapor deposition machine, and biofilm inhibition was determined by crystal violet binding assay. According to our results, 32 nm of silver layer on a glass slab decreased biofilm formation of P. mirabilis strain to a maximum amount of 88.1% and caused 20.9% inhibition in biofilm formation of C. glabrata strain. On the other hand, NmS coating of Ag on a polystyrene slab caused 34.4% and 20% inhibitions, respectively, in biofilm formations of C. glabrata and C. tropicalis strains. Although biofilm inhibition of NmSAg layer on polystyrene slab was more (34.4%) than biofilm inhibition caused by NmSAg layer on glass slab (20.9%), C. glabrata strain's biofilm formation on uncoated glass slab was lower than both uncoated and NmSAg-coated polystyrene slabs. Our results show that glass surfaces with NmSAg coatings can be used as a new surface material of various indwelling devices on which P. mirabilis colonizations frequently occur and in order to avoid C. glabrata-associated biofilm infections, it is more useful to choose a surface material of glass rather than choosing a surface material of polystyrene.

Commissioning and Clinical Implementation of Varian Surface Applicators

To validate the dosimetric performance of Varian surface applicators with the source vertically positioned as well as to develop procedures for clinical implementation. Varian surface applicators with the source vertically positioned are circular or ovoid in shape, ranging in diameter from 10 to 45 mm. The variety of applicator sizes that are available make them clinically advantageous, though the orthogonal source orientation and the steepness of the resulting dose gradient in the region of prescription depth (3-4 mm) presents multiple challenges. In the absence of published guidelines, the following tests were designed for commissioning:1) Verification of functional integrity and physical dimensions by visual inspection and measurement;2) Dosimetric measurements to validate data provided by Varian (based on ionization chamber measurements performed in plastic phantoms during the 1990’s) as well as obtained using the Brachyvision Treatment Planning System equipped with the Acuros BV GBBS algorithm for heterogeneity corrected dose calculation. a) Absolute Dose Measurements Two sets of measurements were performed with the source positioned at -10 mm and -15 mm from the center of the first nominal dwell position. Corrections were applied to account for the electrometer, temperature and pressure, polarity effects, as well as recombination. A solid water phantom was scanned and the Acuros algorithm was used to compute the dose at 5 mm depth and at surface for all applicators, using a resolution of 0.05 cm and assuming medium is water. The measurement results were then compared to the vendor’s data and the Acuros calculated dose.i) Measurements were taken at 5 mm depth in a solid water phantom using a Markus chamber in a 2.5-cm polystyrene slab with a 5-cm solid water slab for backscatter, using a calibration factor for 60Co in water. A previously determined phantom factor was then used to convert the results into dose to water.ii) Measurements at the applicator surface were performed in air using an Exradin A20 ion chamber and a Standard Imaging jig designed for such measurements. The calibration factor for 192Ir was obtained through interpolation between the factors for 137Cs and a 250 kV beam. The corrected reading was converted to dose using the AAPM TG 61 formalism. b) Relative Dose Measurements Measurements using GafChromic film were taken at a depth of 4 mm in solid water with the source positioned at -15 mm from the center of the first nominal dwell position. Dwell times were selected to deliver a dose at depth of 1- 2 Gy. The exposed films were scanned and from the digital dataset profiles were generated for comparison with those predicted by the Acuros algorithm. 1) All applicators were found to be functional with physical dimensions within 1 mm of specifications.2) Dosimetric Measurements (excluding the 10 mm applicator):ai) Measurements taken in solid water at a depth of 5 mm were within 15% of the vendor’s data when using a source at -10 mm position and within 10% when using a source at -15 mm position. These measured data were also within 5% and 4% of the Acuros predictions, respectively.aii) Measurements taken at the surface of each applicator were found to be within 8% of the vendor’s data when using a source at -10 mm position and within 6% when using a source at -15 mm position. These measured data were also within 16% and 5% of the Acuros predictions, respectively. It should also be noted that the vendor’s dataset and the Acuros data at surface were within 15% agreement when using a source at -10 mm position and within 5.5% agreement when using a source at -15 mm position. Measurements taken using the 10 mm applicator showed poor agreement with both the vendor’s data as well as the Acuros calculations.b) The full widths of the measured dose profiles were within 2 mm of those predicted by Acuros at the 90% dose level. Based on the dosimetric results, a quality assurance program and procedures for clinical implementation were developed. Treatment planning will be performed using a single dwell position located at -15 mm from the center of the first nominal dwell position. The 10 mm applicator will not be released for clinical use. A prescription depth of 4 mm is recommended, to ensure full coverage at 3 mm and a minimum dose of 90% of prescribed dose at 4 mm depth. As a conservative approach, the applicator selection will be based on the 90% dose level width at 4 mm depth from Acuros plans.

THz wave transmission within the metal film coated double-dielectric-slab waveguides and the tunable filter application

We propose a metal film coated double-dielectric-slab waveguide for guiding THz wave. Detailed comparisons of transmission characteristics of the coated and uncoated double-dielectric-slab waveguides are given. The comparisons of different slab materials are also given. We put forward using the measureable cut-off frequency of the coated waveguides for tunable filter. The tunable sensitivities to the thickness of the slab t and the air interval w are discussed. We find that for Polystyrene slab at t=0.0380mm, the theoretical tunable sensitivity of cut-off frequency on w can be 7.65THz/mm.

Dosimetry audit of radiotherapy treatment planning systems.

In radiotherapy Treatment Planning Systems (TPS) various calculation algorithms are used. The accuracy of dose calculations has to be verified. Numerous phantom types, detectors and measurement methodologies are proposed to verify the TPS calculations with dosimetric measurements. A heterogeneous slab phantom has been designed within a Coordinated Research Project (CRP) of the IAEA. The heterogeneous phantom was developed in the frame of the IAEA CRP. The phantom consists of frame slabs made with polystyrene and exchangeable inhomogeneity slabs equivalent to bone or lung tissue. Special inserts allow to position thermoluminescent dosimeters (TLD) capsules within the polystyrene slabs below the bone or lung equivalent slabs and also within the lung equivalent material. Additionally, there are inserts that allow to position films or ionisation chamber in the phantom. Ten Polish radiotherapy centres (of 30 in total) were audited during on-site visits. Six different TPSs and five calculation algorithms were examined in the presence of inhomogeneities. Generally, most of the results from TLD were within 5 % tolerance. Differences between doses calculated by TPSs and measured with TLD did not exceed 4 % for bone and polystyrene equivalent materials. Under the lung equivalent material, on the beam axis the differences were lower than 5 %, whereas inside the lung equivalent material, off the beam axis, in some cases they were of around 7 %. The TLD results were confirmed with the ionisation chamber measurements. The comparison results of the calculations and the measurements allow to detect limitations of TPS calculation algorithms. The audits performed with the use of heterogeneous phantom and TLD seem to be an effective tool for detecting the limitations in the TPS performance or beam configuration errors at audited radiotherapy departments.

Validation of GEANT4 simulations for percentage depth dose calculations in heterogeneous media by using small photon beams from the 6-MV Cyberknife: Comparison with photon beam dosimetry with EBT2 film

Percentage depth dose (PDD) distributions in heterogeneous phantoms with lung and soft bone equivalent media are studied by using the GEANT4 Monte Carlo code. For lung equivalent media, Balsa wood is used, and for soft bone equivalent media, a compound material with epoxy resin, hardener and calcium carbonate is used. Polystyrene slabs put together with these materials are used as a heterogeneous phantom. Dose measurements are performed with Gafchromic EBT2 film by using photon beams from the 6-MV CyberKnife at the Seoul Uridul Hospital. The cone sizes of the photon beams are varied from 5 to 10 to 30 mm. When the Balsa wood is inserted in the phantom, the dose measured with EBT2 film is found to be significantly different from the dose without the EBT2 film in and the dose beyond the Balsa wood region, particularly for small field sizes. On the other hand, when the soft bone equivalent material is inserted in the phantom, the discrepancy between the dose measured with EBT2 film and the dose without EBT2 film can be seen only in the region of the soft bone equivalent material. GEANT4 simulations are done with and without the EBT2 film to compare the simulation results with measurements. The GEANT4 simulations including EBT2 film are found to agree well with the measurements for all the cases within an error of 2.2%. The results of the present study show that GEANT4 gives reasonable results for the PDD calculations in heterogeneous media when using photon beams produced by the 6-MV CyberKnife

Thermal and fire risk analysis of typical insulation material in a high elevation area: Influence of sidewalls, dimension and pressure

Small-scaled experiments and modeling are performed to investigate the downward flame spread over extruded polystyrene (XPS) slabs at different widths with and without sidewalls in a city (Lhasa, China) at high elevation. This study provides an understanding of the thermal characteristics and fire risk of XPS under the influence of various factors. It is found that the flame pulsation is more significant for wider samples. The average flame height (H) rises as the width increases. Compared to the results obtained without sidewalls, the flame pulsation is less considerable and H is higher for samples with sidewalls. When sidewalls are present, the average maximum flame temperature (Tmax) increases with the rise of sample width, and it is lower than that without sidewalls. Tmax in the combustion zone besides the sidewalls is larger than that in the middle zone, while the reverse is true for samples without sidewall. The solid-phase temperature curve is comprised of three stages. The temperature growth rate is low during the melting stage. The duration of the melting stage without sidewalls is longer than that with sidewalls. This duration at a low elevation is shorter than that at the high elevation. The shorter duration of the melting stage corresponds to the higher temperature at which the melting stage occurs. A three dimensional downward flame spread model taking into account the effects of sample width, thickness, ambient pressure and sidewall is established. The predicted trends of Vf using the model agree well with the experimental results. The model could also predict XPS fire hazard. The thermal and risk analysis of XPS provides a guidance to fire safety design of energy saving system of buildings.

Treatment of Moving Lesions With Helical Intensity Modulated Radiation Therapy (HIMR): A Phantom Study

Treatments performed with Helical Intensity-Modulated Radiation therapy (HIMR) on moving lung lesions might be inaccurate due to the simultaneous gantry rotation, patient couch translation and lesion movement. Moreover depending on the technique used to acquire/reconstruct the planning CT data (Free-Breathing (FB), Breath-Hold (BH) or Average Intensity Projection (AIP) from 4DCT) different results are obtained during HIMR planning. In this study the outcome of how these factors interplay and their effects on dose distribution are explored using a moving phantom and radiochromic EBT3 films. A phantom made of two pieces of tissue equivalent material embedded in two adjacent polystyrene slabs, slid in a cranio-caudal direction using a motor-driven motion platform (3cm and 16cycle/min movement extension and rate). A CT scanner was used to acquire/reconstruct FB and AIP images of the moving phantom and BH images of the static phantom. The three data sets were sent to the HIMR planning station where three plans for delivering 2Gy to the central tissue equivalent material were prepared. The phantom was put on the patient couch, radiochromic EBT3 films were inserted between the two phantom parts and the three treatments were delivered. In order to compare planning and delivered dose distributions radiochromic films, sandwiched between the two phantom parts, were also fixed to an external support that enable the phantom to slit while films stand still. A significant difference between planned and delivered dose distributions was observed in the moving phantom. Gamma index passing rates (3%, 3mm, 10% threshold, local criterion) changed from 89,48% for the static phantom and BH CT data set to 63,89% and 61.38% for the moving phantom using AIP and FB CT data set respectively. Dose delivered by treatments planned on FB and AIP CT data sets shows under dosages (up to 4%) and over dosages (up to 50%) in the target region and surrounding regions, respectively, compared to that delivered to the static target. The over dosage effect is more pronounced in caudal direction. The most evident effect on dose distribution was dose blurring. This effect also impacted dose gradients which tended to decrease in the peripheral region. Treatments performed with HIMR on moving targets showed a significant difference between planned and delivered dose distributions. The use of Average Intensity Projection (AIP) CT images, adequate for dose calculation in most circumstances, does not seem to provide better results. Contouring strategies that keep into account these effects should be explored.

Theoretical study of light localization in photonic bandgaps of organic octagonal quasiperiodic photonic crystal slabs

Based on the polystyrene material of low refractive index, light localization in photonic bandgaps of two kinds of 2D octagonal quasiperiodic photonic crystal slabs are investigated in theory, including the air-rod polystyrene slab and polystyrene-rod slab. The properties of bandgaps and localized modes in both two defect-free patterns are analyzed in detail. When a single-point defect is introduced into two quasiperiodic structures, the position of emerging defect modes and the red-shifting of resonant modes in wavelength are observed quite differently when the defect microcavity is increased in size. This difference is caused by the competition of two physical mechanisms, which are the effect of defect energy levels caused by defects introduced into photonic crystals and the resonance of modes in the defect cavity. These results will provide theoretical support for experimental fabrication of organic light-emitting quasiperiodic photonic crystal devices.

Effects of Sample Width and Sidewalls on Downward Flame Spread over XPS Slabs

To study the effects of sample width and sidewalls on downward flame spread over extruded polystyrene (XPS) slabs, a series of laboratory-scale experiments were conducted. Flame shape, flame spread rate, mass loss rate and temperature were recorded. For XPS without sidewalls, the average maximum flame height (H) and average flame area per unit of width (A/w) rise linearly with an increase in sample width (w) and mass loss rate per unit of width. When sidewalls are absent, flame spread rate first drops and then rises with an increase in width. This trend is determined by gas-phase heat transfer. When sidewalls are present, flame spread rate increases with a rise in width, and solid-phase heat conduction determines the trend. Sidewall effects are comprised of four aspects: oxygen concentration near the sidewalls and gypsum board is low, which leads to reduced flame heat flux; upward and front air flow is intensified; the flame is stretched, and the surface flame is weakened; and molten XPS mass decreases. For narrow samples, H and A/w with sidewalls are higher than those without sidewalls, while the reverse was observed in wider samples. The mass loss rate, preheating length and average flame spread rate with sidewalls are smaller than those obtained without sidewalls. Flame spread acceleration with sidewalls occurs at a broader width than that without sidewalls. The experimental results agree well with the theoretical analysis.