Function Of Beam Incidence Angle Research Articles

A parametric fitting technique for rapid determination of a skin-dose correction factor for angle of beam incidence during image-guided endovascular procedures.

Skin dose is dependent on the incident beam angle and corrections are needed for accurate estimation of the risk of deterministic effects of the skin. To obtain the angular correction factors (ACF's), EGSnrc Monte Carlo (MC) software was used to calculate the skin dose as a function of incident x-ray beam angle at the center of the field for beam energies from 60 to 120 kVp, field sizes from 5 to 15 cm, and thicknesses of Cu beam filters from 0.2 to 0.5 mm. All MC simulations used 3×1010 incident photons. The dose was averaged over a 1 mm depth on the entrance surface of a 40×40 cm by 20 cm thick water phantom and was then normalized to the incident primary dose which was calculated using NIST mass energy absorption coefficients and by integrating over the beam energy spectrum. The Matlab tool, 'cftool', was used to fit these normalized dose values to power law equations as a function of incident beam angle, with coefficients that were fit to polynomials as a function of kVp. Separate fitting was done for different beam sizes and beam filters. The skin dose values calculated using the ACF determined from the fitted functional formulas agreed with that calculated by MC with a mean absolute percentage error (MAPE) less than 3% over the entire range of incident angles and kVp values. This fitting technique allows an ACF to be quickly determined for accurate skin dose calculation.

Theoretical study of electromagnetic and optical properties of periodic conductive networks based on Voronoi diagrams

Abstract This paper is a preliminary report on a theoretical study of transparent shielding materials with potentially very high shielding efficiency based on quasi-periodic conductive networks. Optical transmittance as a function of beam incident angle and mesh geometry is modelled in the geometrical approximation by exact ray-tracing and compared to the analytical solution for ideal hexagonal meshes. An empirical model for rapid calculation of shielding efficiency vs frequency and mesh geometrical parameters is suggested based on numerical simulation results.

Assessing the Validity of Clinician Advice That Patients Avoid Use of Topical Agents Before Daily Radiotherapy Treatments

Radiation dermatitis is common and often treated with topical therapy. Patients are typically advised to avoid topical agents for several hours before daily radiotherapy (RT) out of concern that topical agents might increase the radiation dose to the skin. With modern RT's improved skin-sparing properties, this recommendation may be irrelevant. To assess whether applying either metallic or nonmetallic topical agents before radiation treatment alters the skin dose. A 24-question online survey of patients and clinicians was conducted from January 15, 2015, to March 15, 2017, to determine current practices regarding topical therapy use. In preclinical studies, dosimetric effect of the topical agents was evaluated by delivering 200 monitor units and measuring the dose at the surface and at 2-cm depth in a tissue-equivalent phantom with or without 2 common topical agents: a petroleum-based ointment (Aquaphor, petrolatum 41%) and silver sulfadiazine cream, 1%. Skin doses associated with various photon and electron energies, topical agent thicknesses, and beam incidence were assessed. Whether topical agents altered the skin dose was also evaluated in 24 C57BL/6 mice by using phosphorylated histone (γ-H2AX) immunofluorescent staining and terminal deoxynucleotidyl transferase dUTP nick end labeling (TUNEL) assay. Preclinical studies took place at the University of Pennsylvania. Patient and clinician survey responses; surface radiation dose readings in tissue-equivalent phantom; and γ-H2AX and TUNEL intensity measured in mice. The 133 patients surveyed received RT for cancer and had a median (range) age of 60 (18-86) years; 117 (87.9%) were women. One hundred eight clinicians completed the survey with 105 reporting that they were involved in managing patient skin care during RT. One hundred eleven (83.4%) of the patients and 96 (91.4%) of the 105 clinicians received or gave the advice to avoid applying topical agents before RT treatments. Dosimetric measurements showed no difference in the delivered dose at either the surface or a 2-cm depth with or without a 1- to 2-mm application of either topical agent when using en face 6- or 15-megavoltage (MV) photons. The same application of topicals did not alter the surface dose as a function of beam incident angle from 15° to 60°, except for a 6% increase at 60° with the silver sulfadiazine cream. Surface dose for 6- and 15-MV beams were significantly increased with a thicker (≥3-mm) topical application. For 6 MV, the surface dose was 1.05 Gy with a thick layer of petroleum-based ointment and 1.02 Gy for silver sulfadiazine cream vs 0.88 Gy without topical agents. For 15 MV, the doses were 0.70 Gy for a thick layer of petroleum-based ointment and 0.60 Gy for silver sulfadiazine cream vs 0.52 Gy for the controls. With 6- and 9-MeV electrons, there was a 2% to 5% increase in surface dose with the use of the topical agents. There were no dose differences at 2-cm depth. Irradiated skin in mice showed no differences in γ-H2AX-positive foci or in TUNEL staining with or without topical agents of varying thickness. Thin or moderately applied topical agents, even if applied just before RT, may have minimal influence on skin dose regardless of beam energy or beam incidence. The findings of this study suggest that applying very thick amounts of a topical agent before RT may increase the surface dose and should be avoided.

Avoiding skin creams right before radiation: Myth or sound advice?

51 Background: Radiation dermatitis in breast cancer patients is common and often treated with topical creams. Patients are traditionally advised to avoid lotions for several hours before radiotherapy (RT) based on concern that creams might increase skin dose. With modern RT’s improved skin sparing, this recommendation may be irrelevant. We hypothesize that applying either metallic or non-metallic creams before treatment would have minimal effect on skin dose. Methods: We conducted an online, 24-question OncoLink survey of patients and providers to determine current skin cream practices. To evaluate the dosimetric effect of creams, we delivered 200 MU at 100 cm SSD and measured the dose at the surface and 2 cm depth in a tissue equivalent phantom, with or w/o 2 common skin creams, Aquaphor and silver sulfadiazine. We assessed the effect of various photon and electron energies, cream thicknesses and beam incidence on dose. The effect of creams on skin dose was also evaluated in C57BL/6 mice using γ-H2AX IHC staining. Results: 27 of 33 surveyed breast cancer patients (82%) and 43 of 45 providers (96%) either received or gave the advice to avoid applying creams prior to RT treatments. Measurements showed no difference in dose at the surface or 2 cm depth with or w/o a 1-2 mm application of either cream when using enface 6 or 15 MV photons. The same application of cream had no effect on surface dose as a function of beam incident angle from 15-60°, except for a 7% increase at 60° with the silver cream. Surface dose for 6 & 15 MV beams were significantly increased with a thicker (≥3 mm) layer of cream. For 6 MV, the surface dose was 105 cGy (Aquaphor), 102 cGy (silver cream) and 88 cGy (controls). For 15 MV, the doses were 70, 60 and 52 cGy. With 6 and 9 MeV electrons, there was only a 2-5% increase in surface dose with use of creams. There were no dose differences at 2 cm depth. Irradiated skin in mice showed no difference in γ-H2AX positive foci with or w/o creams. Conclusions: This is the first dosimetric assessment of the effect of skin creams for RT dermatitis. Our findings suggest that thin or moderately applied creams, even if applied just prior to RT, have minimal impact on skin dose, regardless of beam energy or beam incidence. Applying very thick amounts of cream just prior to RT increased surface dose and should be avoided.

SU-E-T-84: Characterization of a Three-Dimensional Radiochromic Film Stack Dosimeter for Measurements of 6 MV Photon Beams

Purpose: To investigate the energy response, orientation dependence, and water equivalence of a novel three-dimensional radiochromic film stack dosimeter (FSD) and verify its accuracy for measurements of 6 MV photon beams. Methods: The FSD consists of 22 films, 3.8 cm in diameter, separated by 1 mm-thick water-equivalent-plastic spacers. A model of the FSD was created using MCNP5. The photon energy spectrum through each film in the FSD from a 6 MV photon source was simulated to ascertain any changes that would produce an energy-dependent response. The absorbed dose within the FSD was simulated for incident beam angles of 0° to 90° relative to the axis normal to the film plane to investigate the orientation dependence of the dosimeter. To determine the water equivalence of the FSD, the percent-depth-dose (PDD) profile within a cylindrical water phantom was simulated and compared with PDD measurements with the FSD within a cylindrical, water-equivalent-plastic phantom. Separate exposures of the FSD were performed with a 6 MV slit field incident normal and parallel to the film plane. The results of these exposures were compared with TLD microcube measurements to verify the accuracy of the FSD. Results: Variations in the photon energy spectrum throughout the FSD are minimal, producing an absorbed-dose energy response of less than 0.1%. The absorbed dose within the FSD varies less than 1.5% as a function of incident beam angle. PDD measurements with the FSD agree with Monte Carlo simulations within 2%. Differences between FSD and TLD measurements within the slit field are less than the respective expanded overall measurement uncertainties of 2.5% and 5.2%. The FSD and TLD measurements agree within 0.5 mm in the field penumbra. Conclusion: For 6 MV photons, the FSD is energy independent, orientation independent, and water equivalent within 2%, and has been verified for three-dimensional dosimetry.

Technique for measuring forward light scatter in intraocular lenses

To develop a technique for measuring the forward light scattering of intraocular lenses (IOLs). The Scatter Works, Tucson, Arizona, USA. Evaluation of diagnostic technique. A scatterometer with laser sources of 488 nm and 633 nm was used to directly measure the forward-scattered light of IOLs. These sources illuminated balanced salt solution-immersed IOLs within a cylindrical wet cell. The wet cell was placed at the center of rotation of a goniometer arm. On the end of the arm was a detector that measured the amount of laser light scattered from the IOL as a function of incident-beam angle. The measurements provided a profile of the scatter distribution for regions outside the directly transmitted beam. Forward and back light scatter was measured in new and artificially aged IOLs. Forward light scatter was increased in artificially aged IOLs compared with that in new unaged IOLs. The artificially aged IOLs developed sub-wavelength-diameter water nanoglistenings in the bulk material just below the surface. The measured scatter profiles were consistent with these subsurface droplets, suggesting Rayleigh-type scatter in the aged IOLs. The amount of light scatter from nanoglistenings does not appear to be sufficient to impair vision. Although the severely aged IOLs showed increased scatter, the level of increase was within 1 standard deviation of what is found in the normal population. A technique was developed enabling quantification of forward-scattered and back-scattered light from IOLs.

TH‐D‐224C‐08: Dose Variation Within Lung Tumor and in Its Fingerlike Extension as a Function of Incident Photon Angle

Purpose: Report the dose variation within a lung tumor and its finger‐like extension as a function of incident photon beam angle. Method and Materials: A phantom with 3‐cm diameter acrylic sphere (tumor) inside a cork medium was designed so that the sphere could be rotated. TLDs were placed at various locations within the sphere and just outside the sphere. The outer TLDS simulated the tumor's finger like extension. Two 2.8cm thick acrylic plates (chest wall) sandwiched the phantom. The TLDS were exposed to a 6 MV beam of 10×10 cm2 with the sphere centered at isocenter (100 SAD). The field size was such that the dose at the periphery of the acrylic sphere was not influenced by the beam penumbra. The exposures were repeated at every 30° of the fingerlike extension with respect to the incident beam angle. The doses at the TLDs were also calculated using MCNPX Monte Carlo codes and CC convolution algorithm. Results: The measured and calculated doses were normalized to the dose at the center of the sphere. Then ratios of MC and convolution values to measured values were calculated and plotted as a histogram. The standard deviations for MC and convolution to TLD ratios were 3.5% and 1.1% respectively. The standard deviations of random error in TLD, MC and convolution methods were 2, 3 and 1.5% respectively. Conclusions: MC and CC convolution values agreed well with measured data. The data would be presented as function of incident beam angle. In addition, analysis of a PB algorithm and an 18 MV beam data would be presented.

Mathematical modeling and measurement of etching profile for junction shape control in MR read heads

A computer model was used to predict the junction profile between magnetoresistive (MR) read sensor and hard bias element which is known to have an influence on head performance. Ion beam etching (IBE) rates as a function of incident beam angle for each material were experimentally determined for this model. Key process parameters such as stencil structure (e.g. height, shape, and etch rate of the photoresist), ion beam angle, and MR element structure were varied. A series of experiments have been performed to confirm the model prediction by employing atomic force microscopy. When the sidewall of stencil was vertical, the junction angle became small resulting from the shadowing of incident ion beams. It is also found that when a thin Ta layer was inserted between the MR element and bottom gap layer, the junction angle was increased.

Structure of hydrogen on the Si{100} surface in the (2 x 1)-H monohydride, (1 x 1)-H dihydride, and c(4 x 4)-H phases.

The hydrogen atom adsorption sites on the Si{100} surface in the (2×1)-H monohydride, (1×1)-H dihydride, and c (4×4)-H phases have been studied by time-of-flight (TOF) scattering and recoiling spectrometry. The experimental procedures and methods employed were similar to those of paper I [Y. Wang, M. Shi, and J. W. Rabalais, Phys. Rev. B 48, 1678 (1993)]. Structural analyses were obtained by measuring TOF spectra of neutrals plus ions for H atoms recoiled from 4-keV Ar + as a function of crystal azimuthal angle and Ne atoms scattered by a 4-keV Ne + beam as a function of beam incident angle to the surface

Surface dose under aquaplast as a function of incident beam angle for 4 MV photons

Surface dose under aquaplast as a function of incident beam angle for 4 MV photons

Surface-plasmon excitation by electrons in microlithographically produced channels.

Energy-loss spectra have been measured for low-energy (900 eV) electrons transmitted through cylindrical microchannels in silver foils. Arrays of such channels were produced by a microlithographic process. Inelastic scattering of the electrons results in a peak in the energy-loss spectrum in the vicinity of the silver surface-plasmon energy. The probability of surface-plasmon excitation was measured as a function of incident-beam angle and compared with theory. The results indicate that inelastic scattering occurs principally in a narrow region close to the metal surface where roughness features are important. Calculations in which these roughness structures are modeled as small spheres give excellent agreement with the measured shape and position of the energy-loss peak.

An experimental study of the penetration of a water–sediment interface by a parametric beam

A recent theoretical study [D. J. Wingham, J. Acoust. Soc. Am. 76, 1192–1200 (1984)] presented calculations of the farfield secondary pressure in sediment due to a parametric array incident on a water–sediment interface, assuming the array to be completely truncated at the interface. An experimental investigation of this theory is undertaken and the measured secondary pressure contours throughout the sediment are compared with the theoretical estimates. The theory is shown to provide good estimates of the secondary field. The measured secondary pressure variation as a function of beam incidence angle at various ranges is presented and it is apparent that only when the array is truncated in its primary nearfield does the performance of the parametric array differ significantly from a conventional array. It is concluded that the anomalous behavior of parametric arrays, at postcritical angles, relies on virtual sources which are both close to the interface and suitably phased to radiate into the sediment.