High-dose Electron Research Articles

Calculation of dose profiles in stereotactic Synchrotron microplanar beam radiotherapy in a tissue-lung phantom

Synchrotron x-ray beams with high fluence rate and highly collimated may be used in stereotactic radiotherapy of lung tumours. A bundle of converging monochromatic x-ray beams having uniform microscopic thickness i.e. (microplanar beams) are directed to the center of the tumour, delivering lethal dose to the target volume while sparing normal cells. The proposed technique takes advantage of the hypothesised repair mechanism of capillary cells between alternate microbeam zones, which regenerate the lethally irradiated endothelial cells. The sharply dropping lateral dose of a microbeam provides low scattered dose to the off-target interbeam volume. In the target volume the converging bundle of beams are closely spaced, and relatively high primary and secondary electron doses overlap and produce a high dose region between the beams. This higher and lower dose margins in the target volume allows precise targeting. The advantages of stereotactic microbeam radiotherapy will be lost as the dose between microbeams exceeds the tolerance dose of the dose limiting tissues. Therefore, it is essential to optimize the interbeam doses in off-target volume. The lateral and depth doses of 100 keV microplanar beams are investigated for a single beam and an array of converging microplanar beams in a tissue, lung and tissue-lung phantoms. The EGS5 Monte Carlo code is used to calculate dose profiles at different depths and bundles of beams. The maximum dose on the beam axis (peak) and the minimum interbeam dose (valley) are compared at different energies, depths, bundle sizes, heights, widths and beam spacings. The interbeam dose is calculated at different depths and an isodose map of the phantom is obtained. An acceptable energy region is found for tissue and lung microbeam radiotherapy and a stereotactic microbeam radiotherapy model is proposed for a 4 cm diameter and 1 cm thick tumour on the lung phantom.

Influence of high energy electron irradiation on the characteristics of polysilicon thin film transistors

The influence of high energy electron (23 MeV) irradiation on the electrical characteristics of p-channel polysilicon thin film transistors (PSTFTs) was studied. The channel 220 nm thick LPCVD (low pressure chemical vapor deposition) deposited polysilicon layer was phosphorus doped by ion implantation. A 45 nm thick, thermally grown, SiO2 layer served as gate dielectric. A self-alignment technology for boron doping of the source and drain regions was used. 200 nm thick polysilicon film was deposited as a gate electrode. The obtained p-channel PSTFTs were irradiated with different high energy electron doses. Leakage currents through the gate oxide and transfer characteristics of the transistors were measured. A software model describing the field enhancement and the non-uniform current distribution at textured polysilicon/oxide interface was developed. In order to assess the irradiation-stimulated changes of gate oxide parameters the gate oxide tunneling conduction and transistor characteristics were studied. At MeV dose of 6×1013 el/cm2, a negligible degradation of the transistor properties was found. A significant deterioration of the electrical properties of PSTFTs at MeV irradiation dose of 3×1014 el/cm2 was observed.

Intraoperative Electron Radiotherapy for Extremity Sarcomas Does Not Increase Acute or Late Morbidity

Intraoperative electron radiotherapy is used to treat surgical sites that potentially harbor occult tumor immediately after limb-sparing surgical resection of extremity soft tissue sarcomas. It is unknown whether single-fraction, high-dose intraoperative electron radiotherapy at the time of surgery increases wound morbidity when combined with preoperative or postoperative external beam radiotherapy. In a retrospective study, we evaluated whether intraoperative electron radiotherapy increased 90-day and late (> 90 days) wound complication rates by comparing patients who had adult extremity soft tissue sarcomas treated by limb-sparing surgery and preoperative (n = 14) or postoperative (n = 13) external beam radiotherapy. The median followup was 36 months. Seven (26%) patients had wound complications occurring within 90 days postoperatively and completion of radiotherapy. Late wound complication rates were similar. Two patients in each of the external beam radiotherapy groups required late subtotal limb amputations for prolonged wound complications. Our findings suggest intraoperative electron radiotherapy during limb-sparing surgery allows radiation dose escalation without increased 90-day or late-wound complication rates when combined with preoperative or postoperative external beam radiotherapy for patients with extremity soft tissue sarcomas. Prognostic Study, Level II (retrospective study). See the Guidelines for Authors for a complete description of levels of evidence.

Electron Paramagnetic Resonance and Scanning Tunneling Microscopy Investigations on the Formation of F+ and F0 Color Centers on the Surface of Thin MgO(001) Films

The formation of surface color centers (F(S) centers) by electron bombardment of thin MgO(001) films is investigated using electron paramagnetic resonance and low-temperature scanning tunneling microscopy. At low electron doses both techniques indicate the formation of singly occupied color centers (F(S)(+)), whereas at high electron doses the doubly occupied type (F(S)(0)) is dominant. It is suggested that with increasing electron dose F(S)(+) centers are transformed into F(S)(0). Tunneling spectra of individual F(S)(0) centers reveal a large distribution of energetic positions of occupied and unoccupied states, which is caused by local variations of the coordination number of the defects and explains the broad signals usually detected with integrating spectroscopic techniques.

Effect of high-dose electron irradiation on specific heat capacity of high-impact polystyrene

The influence of high-dose electron beam irradiation on polystyrene was investigated using differential scanning calorimetry (DSC). For this purpose, high-impact polystyrene (HIPS) was irradiated with doses up to 10 4 kGy . After each irradiation the specific heat capacity of the polystyrene was measured in the temperature range compatible with polystyrene. The specific heat capacity at a fixed temperature was plotted against the absorbed dose. In this way, the specific heat capacity as a function of dose was determined. Cross linking prevailed up to a dose of 4–5 MGy above which chain scission became dominant.

Cross-linked PMMA on Porous Silicon: An Effective Nanomask for Selective Silicon Etching

Combining a porous silicon (PS) buffer layer with cross-linked poly(methyl methacrylate) (PMMA), we have obtained masks that show high resistance to the electrochemical process usually employed to produce PS. PMMA is normally dissolved in an HF/ethyl alcohol mixture, but it becomes resistant to such solution after cross-linking of the polymer. This can be achieved by high-dose electron irradiation, obtaining a mask for the subsequent electrochemical etching. Due to the strong electric field across the masking layer during the electrochemical process, the time duration of such a mask is limited. We demonstrate that the presence of a highly porous silicon thin film lying under the resist is sufficient for an evident improvement of the masking power during the electrochemical process, allowing for the selective formation of thick PS regions. Furthermore, a final PS removal in alkaline solution leads to the formation of silicon micro- and nanostructures in relief, such as microtips and nanomolds.

Hardening and microstructural evolution in A533B steels under high-dose electron irradiation

Commercial A533B steels (0.12, 0.16wt% Cu) irradiated at 290°C up to 22mdpa with 5MeV electrons were examined by hardness measurements, positron lifetime spectroscopy, the coincidence Doppler broadening (CDB) technique and three-dimensional atom probe microanalysis (3DAP). The radiation-induced hardening increased with electron dose and lay on the same trend of neutron-induced hardening within data scatter. CDB measurements revealed that clustering of Cu atoms occurred at doses over 1mdpa and proceeded with increasing dose. 3DAP results showed that well-defined Cu-rich precipitates with a diameter of less than 2nm were formed at doses of 10 and 22mdpa. The precipitates had a shell structure consisting of a Cu–Fe core region and a surrounding Mn–Ni–Si shell, which are similar to those formed under neutron irradiation. The size and number density of the precipitates were consistent with previous neutron data. Positron lifetime spectroscopy showed that no microvoids were formed. The electron irradiation caused almost the same hardening efficiency and evolution of Cu-rich precipitates as those under neutron irradiation on a dpa basis.

Etching Silicon Through an Effective Nanomask: An Electrochemical Way to Nanomachining

AbstractWe present a novel approach to silicon nanomachining, based on the electrochemical etching of the material through a nanopatterned mask.Combining a porous silicon (PS) buffer layer with cross-linked poly(methyl methacrylate) (PMMA) we have obtained masks which show high resistance to the electrochemical etching. PMMA is normally dissolved in a HF/EtOH mixture, but it becomes resistant to such a solution after cross-linking of the polymer. This can be achieved by high-dose electron irradiation in a Scanning Electron Microscope (SEM), obtaining a mask for the subsequent etching. Anyway, due to the strong electric field across the masking layer during the electrochemical process, time duration of such a mask is limited. We demonstrate that the presence of a highly porous silicon thin film lying under the resist leads to an evident improvement of the masking power. A final PS removal leads to the formation of silicon micro- and nanostructures in relief, such as microtips and nanomolds. Thus, we have at hand a simple silicon nanomachining process, where the nanofeatures written by the electron beam in the SEM are transferred to the bulk material through a short anodization step in acid solution. This may be a useful alternative method for fabricating nanodevice elements, such as nanofluidic channels or field emitter arrays.

Three-dimensional distributions of elements in biological samples by energy-filtered electron tomography

By combining electron tomography with energy-filtered electron microscopy, we have shown the feasibility of determining the three-dimensional distributions of phosphorus in biological specimens. Thin sections of the nematode, Caenorhabditis elegans were prepared by high-pressure freezing, freeze-substitution and plastic embedding. Images were recorded at energy losses above and below the phosphorus L 2,3 edge using a post-column imaging filter operating at a beam energy of 120 keV. The unstained specimens exhibited minimal contrast in bright-field images. After it was determined that the specimen was sufficiently thin to allow two-window ratio imaging of phosphorus, pairs of pre-edge and post-edge images were acquired in series over a tilt range of ±55° at 5° increments for two orthogonal tilt axes. The projected phosphorus distributions were aligned using the pre-edge images that contained inelastic contrast from colloidal gold particles deposited on the specimen surface. A reconstruction and surface rendering of the phosphorus distribution clearly revealed features 15–20 nm in diameter, which were identified as ribosomes distributed along the stacked membranes of endoplasmic reticulum and in the cytoplasm. The sensitivity of the technique was estimated at <35 phosphorus atoms per voxel based on the known total ribosomal phosphorus content of approximately 7000 atoms. Although a high electron dose of approximately 10 7 e/nm 2 was required to record two-axis tilt series, specimens were sufficiently stable to allow image alignment and tomographic reconstruction.

Biomechanical Characterization of Human Amniotic Membrane Preparations for Ocular Surface Reconstruction

Purpose: To investigate the tensile and elastic properties of both commercially available and experimental human amniotic membrane preparations. Method: Nine preparations of human amniotic membrane were studied. The four dry preparations were untreated (nonirradiated, n = 20), and gamma (n = 25), low-dose (AmbioDry^®, Okto Ophtho Inc., Costa Mesa, Calif., USA, n = 20) and high-dose (n = 20) electron beam sterilized. The same dry membranes were moistened with balanced salt solution (n = 20, 34, 20 and 20, respectively). The ninth group consisted of thawed medium-frozen amniotic membrane (AmnioGraft^®, Bio-Tissue Inc., Miami, Fla., USA, n = 20). The membranes were cut into thin strips, loaded on a gram range load sensor, and stretched incrementally to the point of rupture. The modulus of elasticity, displacement until rupture and maximum tolerated stress were recorded and compared. Results: The dry preparations exhibited higher moduli of elasticity when compared with the moist samples, with the low-dose electron beam-irradiated samples having the greatest mean modulus of elasticity overall and maintaining a high modulus of elasticity as a moist sample (p < 0.05). Moist nonirradiated preparations and thawed medium-frozen preparations stretched the farthest before rupture and experienced the greatest mean stresses at the point of rupture. While 3 of 4 membranes had greater stretch when moistened as compared to their dry counterparts, there was no difference in the membrane stiffness between dry and moistened low-dose electron beam-irradiated samples (p > 0.8). Conclusions: Low-dose electron beam-irradiated amnion appeared to maintain desirable elastic characteristics in transition from a dry to rehydrated state and may thus provide an easy-to-manipulate transplant tissue for ocular surface reconstruction. Moist nonirradiated and thawed medium-frozen tissues, however, may provide surgical advantages as they required greater forces to rupture.

Evaluation of the first commercial Monte Carlo dose calculation engine for electron beam treatment planning.

The purpose of this study is to perform a clinical evaluation of the first commercial (MDS Nordion, now Nucletron) treatment planning system for electron beams incorporating Monte Carlo dose calculation module. This software implements Kawrakow's VMC++ voxel-based Monte Carlo calculation algorithm. The accuracy of the dose distribution calculations is evaluated by direct comparisons with extensive sets of measured data in homogeneous and heterogeneous phantoms at different source-to-surface distances (SSDs) and gantry angles. We also verify the accuracy of the Monte Carlo module for monitor unit calculations in comparison with independent hand calculations for homogeneous water phantom at two different SSDs. All electron beams in the range 6-20 MeV are from a Siemens KD-2 linear accelerator. We used 10,000 or 50,000 histories/cm2 in our Monte Carlo calculations, which led to about 2.5% and 1% relative standard error of the mean of the calculated dose. The dose calculation time depends on the number of histories, the number of voxels used to map the patient anatomy, the field size, and the beam energy. The typical run time of the Monte Carlo calculations (10,000 histories/cm2) is 1.02 min on a 2.2 GHz Pentium 4 Xeon computer for a 9 MeV beam, 10 x 10 cm2 field size, incident on the phantom 15 x 15 x 10 cm3 consisting of 31 CT slices and voxels size of 3 x 3 x 3 mm3 (total of 486,720 voxels). We find good agreement (discrepancies smaller than 5%) for most of the tested dose distributions. We also find excellent agreement (discrepancies of 2.5% or less) for the monitor unit calculations relative to the independent manual calculations. The accuracy of monitor unit calculations does not depend on the SSD used, which allows the use of one virtual machine for each beam energy for all arbitrary SSDs. In some cases the test results are found to be sensitive to the voxel size applied such that bigger systematic errors (>5%) occur when large voxel sizes interfere with the extensions of heterogeneities or dose gradients because of differences between the experimental and calculated geometries. Therefore, user control over voxelization is important for high accuracy electron dose calculations.

Multiwalled Nanotubes: Their Formation by Irradiating Graphite with High Doses of Electrons

We report the production of carbon nanotubes by high dose of electron irradiation. The irradiation was performed with a 2 MeV Van de Graaff accelerator, while the irradiation conditions were the following: voltage 1.3 MeV, current 5 µA, dose rate 25 kGy/min, and total dosage 1000 kGy. The samples were analyzed in a high‐resolution transmission electron microscope. The main features observed on the samples, were huge nanotubes of several nanometer long and few nanometer wide, which are capped at one end. It is good to point out, that at this level of irradiation, we were not able to find either onion‐like or particle structures throughout the material, as it is usual in similar hexagonal structures. This behavior could be attributed to the level of irradiation used to create the nanotubes under investigation.

Structural evolution and dielectric relaxation behavior of electron-irradiated poly(vinylidene fluoride–trifluoroethylene) 80/20 mol % copolymers

The effect of high-energy electron irradiation on poly(vinylidene fluoride-trifluoroethylene) 80/20 mol % copolymers has been studied in a broad dose ranging from 0 to 120 Mrad. It was found that the copolymers transformed from a normal ferroelectric to a relaxor ferroelectric phase (RFE) at high electron doses. For copolymers irradiated at the dose above 90 Mrad, they exhibited typical dielectric behavior of RFE, e.g., whose dielectric constant peaks show strong frequency dispersion. X-ray diffraction patterns showed that the irradiation induced a coexistence of the polar and nonpolar phase in the crystalline regions, leading to nanosized coherent regions in the irradiated copolymer films. Differential scanning calorimetery and polarization hysteresis loops also revealed the reduction in the crystallinity and stability of the ferroelectric state, reflecting significant changes in the ferroelectric-to-paraelectric phase transition due to the effect of high-energy electron irradiation.

Detecting single atoms of calcium and iron in biological structures by electron energy-loss spectrum-imaging.

As techniques for electron energy-loss spectroscopy (EELS) reach a higher degree of optimization, experimental detection limits for analysing biological structures are approaching values predicted by the physics of the electron scattering. Theory indicates that it should be possible to detect a single atom of certain elements like calcium and iron contained in a macromolecular assembly using a finely focused probe in the scanning transmission electron microscope (STEM). To test this prediction, EELS elemental maps have been recorded with the spectrum-imaging technique in a VG Microscopes HB501 STEM coupled to a Gatan Enfina spectrometer, which is equipped with an efficient charge-coupled device (CCD) array detector. By recording spectrum-images of haemoglobin adsorbed onto a thin carbon film, it is shown that the four heme groups in a single molecule can be detected with a signal-to-noise ratio of approximately 10 : 1. Other measurements demonstrate that calcium adsorbed onto a thin carbon film can be imaged at single atom sensitivity with a signal-to-noise ratio of approximately 5 : 1. Despite radiation damage due to the necessarily high electron dose, it is anticipated that mapping single atoms of metals and other bound elements will find useful applications in characterizing large protein assemblies.

High beta and electron dose from 192Ir: implications for “gamma” intravascular brachytherapy

Purpose : Trains of multiple 192Ir seeds are used in many clinical trials for intravascular brachytherapy. 192Ir source is commonly considered as a gamma emitter, despite the understanding that this radionuclide also emits a wide range of electron and beta energies, with a similar range of energy. The high dose from betas and electrons in the submillimeter range due to unsealed ends of seed sources should be precisely quantified to fully understand the backdrop for complications associated with 192Ir coronary artery brachytherapy. Methods and Materials : Monte Carlo simulations (MCNP4C code) were performed for a model 5-seed 192Ir train used in SCRIPPS, GAMMA, and the Washington Radiation for In-Stent Restenosis (WRIST) randomized clinical trials. A stack of radiochromic films was also used to measure the dose distributions for an actual 6-seed train. Results : In the submillimeter range very close to the source, Monte Carlo results show that betas and electrons deposit a higher dose than 192Ir photons (gamma and X-rays) over the interseed gap. A high luminal dose from the combined effects of betas, electrons, and photons emitted from 192Ir can be deposited, particularly between seeds. When prescribing 15 Gy at 2 mm, the combined dose can be as high as 160 Gy at 0.5 mm. Different peak doses near the interseed gaps were noted, which may be due to variability of seed-end surfaces and nonuniformity of seed activity within a real multiseed train. Dose-volume histograms (DVH) of lumen surfaces were evaluated for an eccentric seed train. The DVH parameters indicating the extent of hot spots in the lumen wall, DV 10, DV 5, DV 2, and DV 1 (dose received by 10, 5, 2, 1% respectively of the total lumen surface), can be as high as 55, 76, 81, and 155 Gy for a lumen with 3-mm diameter, and 75, 80, 110, and 158 Gy for a narrow 2-mm lumen. Conclusion : 192Ir multiple seed trains used in the SCRIPPS, GAMMA, and WRIST trials can deposit a very high dose to the luminal wall. A particularly high electron and beta dose can be delivered near the interseed gap if the source is not centered in the catheter and lumen. The dose from 192Ir betas and electrons may partially explain adverse outcomes reported from 192Ir multiseed clinical trials. Improvement of the encapsulation design to filter out the betas and electrons should be seriously considered.

Analytical Electron Microscopy Study of Electron Radiation Damage in Iron Phosphate Glass Waste Forms

ABSTRACTA series of iron phosphate glass waste forms with compositions of 45Fe2O3-55P2O5, 20Fe2O3-80P2O5, and 20Fe2O3-20Na2O-60P2O5, namely FeP-1, FeP-2 and FeP-3, was studied by analytical electron microscopy (AEM). Transmission electron microscopy (TEM) bright-field (BF) imaging showed that under the electron irradiation, phase segregation occurred in both the FeP-1 and FeP-2 samples at high electron doses (3.84×1026 e/m2). In contrast, bubbles formed in the FeP-3 sample, even at a relatively low dose (2.88×1025 e/m2), which may be attributed to the migration of Na under irradiation as in the case in sodium borosilicate glass. Series electron energy-loss spectroscopy (EELS) analysis showed that the glass materials experienced mass-loss and composition variation. No obvious Fe valence state changes under irradiation were observed within the irradiation period.

EELS analysis of PMMA at high spatial resolution

We performed the electron energy-loss spectroscopic (EELS) analysis of electron-sensitive polymers in the analytical transmission electron microscope in order to evaluate the possibility to obtain chemical information on polymers at a nanometre scale (i.e. at 2.4nm diameter probe). In the acquired spectra, we propose an identification of the ELNES fine structure to the different chemical bonding in agreement with molecular orbital calculations (EHT) and with previous XANES experiments.The main results confirm that poly(methyl mettacrylate) (PMMA) is very sensitive to electrons when a large probe size is used, with a critical dose of about 102Cm−2. However a high dose rate in a nanometre diameter electron beam is less destructive and the EELS spectra of far less degraded PMMA could be obtained even at 107Cm−2. Irradiation damage was however thought to be the main limitation of the field-emission gun microscope, since high electron doses are required to acquire an EELS spectrum. This surprising behaviour was already observed in the case of poly(ethylene terephthalate), which is however more resistant to the electron beam (Varlot et al., 1997. Ultramicroscopy 68 (2), 123–133). Several possible explanations were studied, such as the influence of the accelerating voltage, a wrong calculation of the electron dose, the excitation delocalisation and the electron dose rate.

New high‐DQE Imaging Plate scanner using the reflected readout laser signal for noise corrections

Imaging Plates (IPs) are in principle ideal electron detectors combining a large active layer area with a high sensitivity, linear dynamic range detection over 5 orders of magnitude. A moderate resolution and a decreasing detection quantum efficiency (DQE) for higher electron doses limit their use so far. The decrease of the DQE results from linear noise contributed by readout laser instabilities and inhomogeneities of the IP active layer. Here we present data on a new IP drum scanner prototype. This scanner combines twin channel amplification electronics with a new type of readout laser which allows a smaller readout focus and increased stability. The current nominal pixel size is 25 μm, and the measured modulation transfer function (MTF) indicates that further reduction of the scanning step size down to pixel sizes in the range of 12–15 μm should be possible. A unique feature of the new scanner is the simultaneous recording of the reflected readout laser light. The reflected light signal can be used for a posteriori alignment of repeated scans of one individual IP and for a correction of one part of the high spatial frequency noise contribution (reflected light correction). The posteriori alignment now allows an easy conventional gain normalization of the luminescence signal without using special markers on the IP. Both corrections lead to an increase of the DQE for high electron doses. Microsc. Res. Tech. 49:281–291, 2000. © 2000 Wiley-Liss, Inc.

New high-DQE imaging plate scanner using the reflected readout laser signal for noise corrections.

Imaging Plates (IPs) are in principle ideal electron detectors combining a large active layer area with a high sensitivity, linear dynamic range detection over 5 orders of magnitude. A moderate resolution and a decreasing detection quantum efficiency (DQE) for higher electron doses limit their use so far. The decrease of the DQE results from linear noise contributed by readout laser instabilities and inhomogeneities of the IP active layer. Here we present data on a new IP drum scanner prototype. This scanner combines twin channel amplification electronics with a new type of readout laser which allows a smaller readout focus and increased stability. The current nominal pixel size is 25 microm, and the measured modulation transfer function (MTF) indicates that further reduction of the scanning step size down to pixel sizes in the range of 12-15 microm should be possible. A unique feature of the new scanner is the simultaneous recording of the reflected readout laser light. The reflected light signal can be used for a posteriori alignment of repeated scans of one individual IP and for a correction of one part of the high spatial frequency noise contribution (reflected light correction). The posteriori alignment now allows an easy conventional gain normalization of the luminescence signal without using special markers on the IP. Both corrections lead to an increase of the DQE for high electron doses.

High energy electron dose-mapping using optically stimulated luminescent films

High energy electron dose mapping is a complex subject. Experiments using optically stimulated luminescent films behind various shielding structures and dual inline packages are presented. The dose-depth deposition in an epoxy package is also investigated. These results are compared with PENELOPE transport code calculations.