Single Beam Profile Research Articles

Propagation of Intense Electron Beams Through a Rectangular Tube

In this study, cylindrical intense electron beams were injected into a rectangular tube. The beam cross section profiles were observed along the propagation path. The single-beam profile was deformed periodically. The center of the beam cross section was shifted when the beam was not injected just on the tube center. When two beams were injected, the beams rotated each other, and the distance between them decreased. Using a simple model with momentum equations of the line density elements for the beam, it was evident that $E\times B$ drift, which originated from the asymmetric electric field and the magnetic field on the beam, caused the previously mentioned phenomena.

Investigating the impact of collimator size variation on the single beam radiation of Gamma Knife PerfexionTM based on Monte Carlo simulation

The Gamma Knife PerfexionTM (Elekta Oncology Systems) is a specialized stereotactic radiotherapy device that uses 192 Cobalt-60 radioactive sources. A dynamic collimator system enables three circular field sizes of 4 mm, 8 mm, and 16 mm diameter to be delivered. This study investigates the dosimetric characteristics of the three different sized collimated beams using Monte Carlo (MC) simulations. The MC simulation made use of previously calculated phase space from a Monte-Carlo model of the encapsulated Cobalt-60 sources. The collimators were modelled using the FLATFILT component module of the BEAMnrc user code. The output of the single beam radiation was stored in a circular scoring plane with a radius of 1.4 cm at source surface distance (SSD) 17.20 cm in the phase space file. The simulation parameters were used 2.1 x 109 particles, electron cut off energy (ECUT) of 0.7 MeV, and photon cut off energy (PCUT) of 0.01 MeV. To investigate the physical full width at half maximum (FWHM) and penumbra width (80-20%), the single beam profiles and mean energy were obtained using BEAMnrc/BEAMDP code. Based on the results that there was an increase of the full width at half maximum (FWHM) with the size of the collimator on the beam profile and the mean energy. The FHWM of the energy fluence profile for the collimator 4, 8, and 16 mm are 2.22, 3.90, and 7.80 mm, respectively. The FHWM of the fluence profile for the collimator sizes 4, 8, and 16 mm are 1.87, 3.87, and 7.80 mm. The FHWM of mean energy as a function of position for the collimator sizes 4, 8, and 16 mm are 2.56, 4.20, and 7.94 mm, respectively. The maximum mean energy for the collimators decreases as much as 0.0185 MeV.

Study the effect of source to diaphragm distance (SDD) on beam penumbra width of Gamma Knife machine model 4C using Monte Carlo simulation

Introduction: Stereotactic radiosurgery (SRS) is one of the conformal radiotherapy methods and Gamma Knife is a non-invasive SRS technique to treat intracranial lesions with 201 Co- 60 sources without need to surgery and bleeding. Beam penumbra phenomenon leads to irradiation outside the field and normal tissues. Therefore, considering it is momentous to attain the acceptable treatment plan. Materials and Methods: In this study, Gamma Knife unit was modeled using EGSnrc/BEAMnrc Monte Carlo code. SDD (Source to Diaphragm Distance) parameter that is one of the affecting factors on physical penumbra width was studied. For this purpose, SDD was increased for 4, 8, 14 and 18 mm collimator sizes in five steps and each step as much as one centimeter. Single beam profiles were calculated using EGSnrc/DOSXYZnrc code at isocentre depth in a cubic Plexiglas phantom. MATLAB and EXCEL software were used to plot profiles and measure physical penumbra width . Results: Based on the results, physical penumbra width (90%-50%) does not show a regular trend with increasing SDD. This may be attributed to scatter radiation which is due to the low distance between the Gamma Knife collimator end and phantom surface. Best results were observed for 4 and 8 mm collimators in 1 cm increase in which penumbra width decreased from 0.785 and 0.790 mm to 0.749 and 0.736 mm for 4 and 8 mm collimator sizes, respectively. Conclusion: It is recommended to increase SDD of Gamma Knife 4C around 1 centimeter to reduce penumbra width for small tumor sizes located near the critical nerves.

The control system of the multi-strip ionization chamber for the HIMM

Heavy Ion Medical Machine (HIMM) is a carbon ion cancer treatment facility which is being built by the Institute of Modern Physics (IMP) in China. In this facility, transverse profile and intensity of the beam at the treatment terminals will be measured by the multi-strip ionization chamber. In order to fulfill the requirement of the beam position feedback to accomplish the beam automatic commissioning, less than 1ms reaction time of the Data Acquisition (DAQ) of this detector must be achieved. Therefore, the control system and software framework for DAQ have been redesigned and developed with National Instruments Compact Reconfigurable Input/Output (CompactRIO) instead of PXI 6133. The software is Labview-based and developed following the producer–consumer pattern with message mechanism and queue technology. The newly designed control system has been tested with carbon beam at the Heavy Ion Research Facility at Lanzhou-Cooler Storage Ring (HIRFL-CSR) and it has provided one single beam profile measurement in less than 1ms with 1mm beam position resolution. The fast reaction time and high precision data processing during the beam test have verified the usability and maintainability of the software framework. Furthermore, such software architecture is easy-fitting to applications with different detectors such as wire scanner detector.

Forcing lateral electron disequilibrium to spare lung tissue: a novel technique for stereotactic body radiation therapy of lung cancer

Stereotactic body radiation therapy (SBRT) has quickly become a preferred treatment option for early-stage lung cancer patients who are ineligible for surgery. This technique uses tightly conformed megavoltage (MV) x-ray beams to irradiate a tumour with ablative doses in only a few treatment fractions. Small high energy x-ray fields can cause lateral electron disequilibrium (LED) to occur within low density media, which can reduce tumour dose. These dose effects may be challenging to predict using analytic dose calculation algorithms, especially at higher beam energies. As a result, previous authors have suggested using low energy photons (<10 MV) and larger fields (>5 × 5 cm2) for lung cancer patients to avoid the negative dosimetric effects of LED. In this work, we propose a new form of SBRT, described as LED-optimized SBRT (LED-SBRT), which utilizes radiotherapy (RT) parameters designed to cause LED to advantage. It will be shown that LED-SBRT creates enhanced dose gradients at the tumour/lung interface, which can be used to manipulate tumour dose, and/or normal lung dose. To demonstrate the potential benefits of LED-SBRT, the DOSXYZnrc (National Research Council of Canada, Ottawa, ON) Monte Carlo (MC) software was used to calculate dose within a cylindrical phantom and a typical lung patient. 6 MV or 18 MV x-ray fields were focused onto a small tumour volume (diameter ∼1 cm). For the phantom, square fields of 1 × 1 cm2, 3 × 3 cm2, or 5 × 5 cm2 were applied. However, in the patient, 3 × 1 cm2, 3 × 2 cm2, 3 × 2.5 cm2, or 3 × 3 cm2 field sizes were used in simulations to assure target coverage in the superior–inferior direction. To mimic a 180° SBRT arc in the (symmetric) phantom, a single beam profile was calculated, rotated, and beams were summed at 1° segments to accumulate an arc dose distribution. For the patient, a 360° arc was modelled with 36 equally weighted (and spaced) fields focused on the tumour centre. A planning target volume (PTV) was generated by considering the extent of tumour motion over the patient's breathing cycle and set-up uncertainties. All patient dose results were normalized such that at least 95% of the PTV received at least 54 Gy (i.e. D95 = 54 Gy). Further, we introduce ‘LED maps’ as a novel clinical tool to compare the magnitude of LED resulting from the various SBRT arc plans. Results from the phantom simulation suggest that the best lung sparing occurred for RT parameters that cause severe LED. For equal tumour dose coverage, normal lung dose (2 cm outside the target region) was reduced from 92% to 23%, comparing results between the 18 MV (5 × 5 cm2) and 18 MV (1 × 1 cm2) arc simulations. In addition to reduced lung dose for the 18 MV (1 × 1 cm2) arc, maximal tumour dose increased beyond 125%. Thus, LED can create steep dose gradients to spare normal lung, while increasing tumour dose levels (if desired). In the patient simulation, a LED-optimized arc plan was designed using either 18 MV (3 × 1 cm2) or 6 MV (3 × 3cm2) beams. Both plans met the D95 dose coverage requirement for the target. However, the LED-optimized plan increased the maximum, mean, and minimum dose within the PTV by as much as 80 Gy, 11 Gy, and 3 Gy, respectively. Despite increased tumour dose levels, the 18 MV (3 × 1 cm2) arc plan improved or maintained the V20, V5, and mean lung dose metrics compared to the 6 MV (3 × 3 cm2) simulation. We conclude that LED-SBRT has the potential to increase dose gradients, and dose levels within a small lung tumour. The magnitude of tumour dose increase or lung sparing can be optimized through manipulation of RT parameters (e.g. beam energy and field size).

A new multi-strip ionization chamber used as online beam monitor for heavy ion therapy

A multi-strip ionization chamber has been built for precise and fast monitoring of the carbon beam spatial distribution at Heavy Ion Researched Facility of Lanzhou Cooling Storing Ring (HIRFL-CSR). All the detector's anode, cathode and sealed windows are made by 2μm aluminized Mylar film in order to minimize the beam lateral deflection. The sensitive area of the detector is (100×100)mm2, with the anode segmented in 100 strips, and specialized front-end electronics has been developed for simplifying the data acquisition and quick feedback of the relevant parameters to beam control system. It can complete one single beam profile in 200μs.

The Dnepr Canyon: evidence for a continuous submarine channel link between the outer shelf and the deep-sea basin of the northwestern Black Sea

Multibeam bathymetric surveys and single-beam profiles were collected in 2003–2010 from aboard the Ukrainian RV Professor Vodyanitskiy (cruises PV-58 and PV-60, 2003 and 2004), and the German RV Meteor (cruise M-72, legs 1 and 4, 2007) and RV Maria S. Merian (cruise MSM-15, leg 2, 2010) along the continental margin of the NW Black Sea. Integrating published, reprocessed and novel data has revealed the existence of a major continuous channel extending from the Dnepr paleo-delta into greater water depths. It is more than 90 km long, 1.1 km wide and up to 125 m deep. On the upper slope (120–960 m water depth), a number of smaller channels merge into the large, Y-shaped Dnepr Canyon, which then continues obliquely downslope via this submarine channel to at least 1,815 m water depth off the Crimean continental margin, NW Black Sea. The channel could be an important, hitherto unknown link between the shallow oxic and deep anoxic environments of the Black Sea, along which sediment and organic matter could be funneled into the deep-sea basin. This would have far-reaching implications for investigations dealing with marine geology and biology, climate change, as well as oil and natural gas exploitation. The unusual alignment of the channel along the margin of the basin, as well as the location and mode of channel termination in deeper waters deserve future research.

Geomorphology of submerged reefs on the shelf edge of the Great Barrier Reef: The influence of oscillating Pleistocene sea-levels

The submerged reefs on continental margins and surrounding oceanic islands are important archives of past climates, eustatic sea-level oscillations and oceanographic processes. Australia's north eastern submerged shelf edge reefs have previously been explored, but all studies have been limited by either low-resolution single beam profiles or by their limited spatial extent. The aim of this study is to improve the understanding of the origin and development of submerged shelf edge reefs, terraces and related features on the Great Barrier Reef (GBR), and the influence of Pleistocene sea-level oscillations on their distribution and geomorphology. We investigated the first large-scale (1400 km 2) 3D bathymetry and acoustic backscatter dataset of four widely-spaced sites located across ca. 650 km along the shelf edge at Ribbon Reefs 1–5, Noggin Pass, Viper Reef and Hydrographers Passage. We provide a comprehensive description and systematic classification of the shelf edge features, and constrain their spatial distribution and geomorphologic variability using a wide range of quantitative morphometrics. Fifteen features are defined, four of which are the focus of this study, including terraces, pinnacles, ridges and channels. Using modern reefs within the GBR for comparison, we interpret these as fringing reefs, patch and barrier reefs and tidal channels respectively. The influence of gross shelf geomorphology and oceanographic factors driving the observed variability within and between sites is discussed in conjunction with Mid-Late Pleistocene sea-level oscillations. We observed two distinct types of reef growth characterised by the degree of development, which we interpret as analogous to juvenile and mature modern reefs of the GBR. Juvenile reefs are very common and occur within each site at depths of less than 50 m to greater than 100 m. They are defined as discontinuous peaked barrier reefs, peaked patch reefs and fringing reefs. Mature reefs are less common and develop exclusively at a depth range of ca. 50–70 m. These reefs are defined as flat-topped continuous barrier and double barrier reefs. Through a comparison with published Mid-Late Pleistocene sea-level oscillations, we note that the depth range of mature reefs is consistent with the depth of repeated low-amplitude (< 20 m) eustatic sea-level oscillations, and independent of the relative time spent at sea-level. We conclude that repeated low-amplitude sea-level rises provide the much-needed accommodation space for vertical accretion that is not provided during stillstands, without inducing reef drowning, which can be caused by high-amplitude sea-level rise. Low-amplitude sea-level falls are also less detrimental than high-amplitude falls, in terms of habitat compression. Together, the effects of enhanced reef accretion and limited habitat compression at depths of 50–70 m have lead to a concentrated net gain in vertical relief that is not observed elsewhere on the shelf edge.

SU‐GG‐T‐452: Attenuation of Single Beam in Dose Calculation of Gamma Knife (GK) Treatment Planning

Purpose: In Gamma Knife (GK) stereotactic radio‐surgery, highest accuracy of dose calculation is important. The attenuation of each single beam in GK is determined by their paths. The path, where each beam passes through the skull to the tumor, is determined by the measurement of skull radius. To estimate the path attenuation effect on the dose calculation is important for clinicians in deciding selection of the gamma angle, blocking beam and helmet size, etc. The purpose of this study was to design and test a new method to assess the beam path attenuation effects on dose computation in GK. Method and Materials: A simulation of GK C‐model was employed for this study. The skull was initially assumed to be a spherical bulb. Shot locations in the center and in the periphery of the bulb were tested. Shot center dose and each beam's contribution was calculated using the tissue maximum ratio (TMR) formula for each path. The simulation of the inhomogeneity calculation is an approach similar the procedure of finding the different paths from different beams. The dose point at different beam contribution is quantitatively calculated with variation difference. Results: The data from several typical plug locations showed that the path length difference could be over 50% and ensuring TMR changes could be as much as 20% of the average value. The single beam profiles at the shot center were variable; however, the composite profiles from the total beams did not show significant changes. Conclusion: Our method to calculate the different path length of single beam from GK treatment planning simulation model shows that the beam path parameters affect GK treatment planning. The beam profile changes based on path length variations. These findings suggest the necessity of considering dose contributions to the normal tissue from different beam paths.

Dosimetric verification of the 95% isodose surface for a conformal irradiation technique

In the treatment planning of conformal radiotherapy, field shapes are often designed in such a way that a high-value isodose surface fully encompasses the target volume. Therefore, knowledge about the accuracy with which the treatment planning system calculates the position of that isodose surface is essential to prevent field shapes which are either too large or too small. To determine this accuracy for a conformal multi-field technique, the dose in the high-dose region must be measured with a high spatial resolution. A method is presented to reconstruct and evaluate the experimental high-dose region from a set of water phantom scans. This method, which assesses combined dose profiles for multi-field irradiation techniques, can be used for the commissioning and/or quality assurance of a 3-D treatment planning system. For a specific conformal technique, the measured and calculated 95% isodose positions along lines in several directions have been compared. It is shown that different dose values of single beam profiles determine the resulting 95% isodose position, which is important to recognize for quality assurance of treatment planning calculations. It is further found that the uncertainty in the calculated 95% isodose surface can be described by a standard deviation in dose value, which relates to a positional uncertainty through the local dose gradient. Thus the confidence region of the calculated 95% isodose can be indicated in the treatment plan by plotting isodoses at the 95% level plus and minus its standard deviation. Such a procedure is recommended instead of plotting the 95% isodose with a constant width. In addition, restrictions for the cumulative dose-volume histogram of acceptable treatment plans can be formulated, based on the sensitivity of the actual target coverage on the uncertainty with which the prescribed isodose surface is calculated.

Berechnung der Dosisverteilung für asymmetrische Felder bei einem 4 MV-Linearbeschleuniger

This paper describes a program for the calculation of dose distributions for asymmetric photon fields of a 4 MV linear accelerator. The algorithm allows dose computations using basic data measured for symmetric fields. The only additional data needed are a single beam profile in air of the largest symmetric field and the beam attenuation coefficient on the central axis of an asymmetric field as a function of the offset angle. Comparison between calculated and measured dose distributions in a water phantom for different field sizes and field positions shows good agreement.

Influence of hip prostheses on high energy photon dose distributions

Radiotherapy treatment of patients having a hip prosthesis is a common problem facing dosimetrists and physicists when the treatment plan requires irradiation of the pelvic area. To quantify the perturbation of these devices, attenuation studies were done with 6 and I8 MV photon beams using various hip prostheses models with varying size and composition. These studies have shown that an attenuation of as much as 50% can be found in a single beam profile under the prosthesis. We have studied the capability of a dose planning system to predict the transmission of these devices as compared with measurements.

The development of the chlorinity/salinity concept in oceanography

The submerged reefs on continental margins and surrounding oceanic islands are important archives of past climates, eustatic sea-level oscillations and oceanographic processes. Australia's north eastern submerged shelf edge reefs have previously been explored, but all studies have been limited by either low-resolution single beam profiles or by their limited spatial extent. The aim of this study is to improve the understanding of the origin and development of submerged shelf edge reefs, terraces and related features on the Great Barrier Reef (GBR), and the influence of Pleistocene sea-level oscillations on their distribution and geomorphology. We investigated the first large-scale (1400 km2) 3D bathymetry and acoustic backscatter dataset of four widely-spaced sites located across ca. 650 km along the shelf edge at Ribbon Reefs 1–5, Noggin Pass, Viper Reef and Hydrographers Passage. We provide a comprehensive description and systematic classification of the shelf edge features, and constrain their spatial distribution and geomorphologic variability using a wide range of quantitative morphometrics. Fifteen features are defined, four of which are the focus of this study, including terraces, pinnacles, ridges and channels. Using modern reefs within the GBR for comparison, we interpret these as fringing reefs, patch and barrier reefs and tidal channels respectively. The influence of gross shelf geomorphology and oceanographic factors driving the observed variability within and between sites is discussed in conjunction with Mid-Late Pleistocene sea-level oscillations. We observed two distinct types of reef growth characterised by the degree of development, which we interpret as analogous to juvenile and mature modern reefs of the GBR. Juvenile reefs are very common and occur within each site at depths of less than 50 m to greater than 100 m. They are defined as discontinuous peaked barrier reefs, peaked patch reefs and fringing reefs. Mature reefs are less common and develop exclusively at a depth range of ca. 50–70 m. These reefs are defined as flat-topped continuous barrier and double barrier reefs. Through a comparison with published Mid-Late Pleistocene sea-level oscillations, we note that the depth range of mature reefs is consistent with the depth of repeated low-amplitude (< 20 m) eustatic sea-level oscillations, and independent of the relative time spent at sea-level. We conclude that repeated low-amplitude sea-level rises provide the much-needed accommodation space for vertical accretion that is not provided during stillstands, without inducing reef drowning, which can be caused by high-amplitude sea-level rise. Low-amplitude sea-level falls are also less detrimental than high-amplitude falls, in terms of habitat compression. Together, the effects of enhanced reef accretion and limited habitat compression at depths of 50–70 m have lead to a concentrated net gain in vertical relief that is not observed elsewhere on the shelf edge.