Linear Accelerator Bunkers: Shielding Verification

Strictly speaking, the term megavoltage therapy should be applied to the use of x rays and gamma rays which have an effective energy of 1 million electron volts or more. However, for our purposes the term can be applied to cover all x rays generated by voltages greater than 1 megavolt and terminating, for medical purposes, at 43 megavolts, as well as cobalt 60 gamma radiation. From the practical point of view, the apparatus in regular use for providing high energy radiation are cobalt teletherapy units (1.2 megavoltage gamma radiation), Van de Graaf generators (1 to 2 megavoltage x rays), linear accelerators (4 to 8 megavoltage x rays), and betatrons (18 to 43 megavoltage x rays). Both the linear accelerator and the betatron have advantages over a cobalt unit in the sense that they can produce both electrons and x rays; they are capable of carrying a higher patient load

Focused Impedance Method (FIM) is an innovative and relatively new electrical impedance measurement technique that allows localized measurement of the properties of electrically conducting materials in a volume conductor such as the human body using simple measurement setups. In a previous work, FIM, with dual electrode separation, was used to determine thicknesses of subcutaneous fat layers through experimental study on phantoms and human subjects. The present work, carried out using COMSOL Multiphysics software package, is a finite element simulation study for a similar target object that verifies and extends the results of the previous experimental work. A rectangular box of different heights containing a material of uniform conductivity such as saline together with internally embedded layers of resistive materials of different thicknesses were used for this study. The measured transfer impedance in the FIM method showed marked change with the variation of the thickness of the resistive layer and with electrode separation, showing a point of maximum curvature in the latter. We obtained a calibration curve for the thickness from the electrode separation of this point. This allowed a unique method for the determination of thickness of embedded resistive layers which is more general than what had been done in the previous experimental work. This work will help standardize the application of 4-electrode FIM for determination of the thickness of less-conducting material layer in any finite volume, including determination of fat layer thickness, etc.Bangladesh Journal of Medical Physics Vol.7 No.1 2014 8-23

The Purpose of this paper is to alert radiation therapists to the possibility of progressive source misalignment in certain types of Co60 teletherapy units, to discuss its effects on dose distribution, and to suggest several simple and practicable technics for its early detection. Such misalignment is especially insidious, as the machine shows no obvious signs of malfunction, even though the radiation pattern may be grossly distorted. The present discussion applies primarily to teletherapy units which have the source attached to a wheel that rotates 180 degrees to carry the source from the “off” to the “on” position. The wheel is brought to the “on” position by a motor-driven gear train and to the “off” position by a coiled steel spring. As long as no current flows through the motor, the source is held securely in the “off” position by the spring tension. When the motor is energized, the wheel is rotated half a revolution, its motion being arrested by a lug on the wheel striking an adjustable stop fastene...

Purpose: To characterize the photon spectrum from a Theratronics T1000 cobalt-60 teletherapy unit using Compton spectrometry and compare to primary beam measurement and Monte Carlo (MC) simulation. Methods: A well-shielded high-purity germanium detector was used to measure the photon spectrum scattered at 22.5 and 70 degrees from the central axis of a 10cm×10cm beam of a cobalt-60 teletherapy unit. The central-axis photons were scattered using an aluminum rod (6mm diameter) positioned 100cm from the source, and the scattered photons were admitted to the detector through a 30cm-long collimating aperture (2mm diameter). The measured Compton-scattered spectra were corrected for detector response, determined using a model of the measurement apparatus in the MCNP5 MC code, and Compton relations (energy-angle and Klein-Nishina) to determine the central-axis spectrum at 100cm from the source. The primary photon spectrum was measured with the collimating aperture aligned to the central axis and corrected for detector response. A MC model of the cobalt-60 unit was developed in MCNP5 and used to generate the central-axis spectrum for a 10cmx10cm field. Results: The primary photon peaks from the cobalt-60 source were appropriately shifted to lower energies in the Compton-scattered spectra. The resolution of the shifted peaks degraded with increasing scattering angle. The corrected scattered spectra compared well with the primary measurement and MC simulation for peak position, but disagreed in the lower-energy continuum where source and collimator scatter are present. Differences are likely due to resolution loss with scattering and incomplete measurement correction. Conclusions: The spectrum from a cobalt-60 teletherapy unit was measured using Compton spectrometry. This spectrum was compared to direct measurement and MC simulation of the unit. Results indicate that small scattering angles should be used with this measurement technique and that further work is required to reconcile differences between measurements and simulations for source and collimator scatter.

Scattered radiation from linear accelerator and cobalt-60 collimator jaws

The distribution of ionization in the first few millimeters of tissue-like material was studied with a cobalt-60 teletherapy unit (1, 2). It was found that for short cone-to-skin distance (C.S.D.) the effect of electrons from the collimating system could be reduced appreciably by thin filters which did not noticeably affect the gamma-ray dosage pattern. Optimum results were obtained with the filter mounted over the lower end of the treatment cone. With this arrangement, however, the filter obscured the beam of light from the light localizer unless it was removed with each use. The interference of a filter with light localization is not encountered with x-ray generators in which the filter employed to harden the beam is located above the mirror of the localizer. Below the collimating system of an x-ray generator, filters to reduce electron contamination are generally not needed, for several reasons. For units operating below the supervoltage region, the average energy of secondary electrons is low, and they are removed from the beam through air scatter even for short diaphragm-skin distances. Further, the focal spot of an x-ray generator is usually small in comparison with the size of a teletherapy source. Therefore, for a given focal skin distance, long diaphragm-skin distances can be used without resulting in wide penumbras. Filters to reduce electron contamination are not needed when the diaphragm-skin distance is large. The low-activity sources employed in some teletherapy units necessitate treatment with short source-to-skin distances (S.S.D.) if the treatment time is to be kept within reasonable limits. If the penumbra width is to be kept small, this requires treatment at a short cone-to-skin distance, for which the electron contamination is appreciable. To minimize the skin dose, it was felt advisable to use an electron filter for treatment dose to the cone. It was also considered desirable to retain the light localizer without having to remove and replaces the filter with each use. Since the purpose of a light localizer is to mark the outer extremities of the field, it was found possible to leave the filter attached to the cone without hindering this function of the localizer. In fact, the method developed is believed to have improved light localization. Method of Mounting Filters and Results A Lucite cap, with bottom and walls of 1.5 mm. thickness, was constructed to fit over the end of the cone (Fig. 1). A sheet of cadmium, 0.25 mm, thick, was then attached to the Lucite so as to cover the opening of the cone except for a narrow slit (about 1 mm.) around the edges. With this arrangement, the filter can easily be removed if desired. The light pattern from the localizer is shown in Figure 2. This pattern was made with the light from the bulb of the light localizer collimated so that its image in the mirror reproduced the source in both shape and position.

Fused deposition modelling (FDM) is a popular additive manufacturing process used for rapid prototyping and the production of complex geometries. Despite its popularity, FDM’s susceptibility to variations in numerous process parameters can significantly impact the quality, design, functionality, and mechanical properties of 3D printed parts. This study explores thirteen FDM process parameters and their influence on the mechanical properties of polylactic acid (PLA) polymer, encompassing surface roughness, warpage, tensile and bending strength, elongation at break, deformation, and microhardness. The optimum parameters were identified alongside key contributors by applying the Taguchi method, signal-to-noise ratios, and analysis of variances (ANOVA). Notably, specific FDM parameters significantly affect the surface profile, with layer thickness contributing 32.65% and fan speed contributing 8.59% to the observed variations. Similarly, warping values show notable influence from nozzle temperature (29.53%), wall thickness (16.74%), layer thickness (16.56%), and retraction distance (12.80%). Tensile strength is primarily determined by wall thickness (31.83%), followed by infill percentage (26.73%) and infill pattern (16.18%). Elongation at break predominantly correlates with wall thickness (44.82%), with a supplementary contribution from nozzle temperature (10.90%). Microhardness lacks a dominant parameter. Bending strength variations primarily arise from layer thickness (38%), wall thickness (37.6%), and infill percentage (9.17%). Deformation tendencies are influenced by layer thickness (19.20%), print speed (11.37%), wall thickness, and fan speed (10.9% each). The optimized dataset of FDM process parameters was then employed in two prediction models: multiple-regression and artificial neural network (ANN). Evaluation based on the correlation coefficient (R2) and root mean squared error (RMSE) indicates that the ANN model outperforms the multiple-regression approach. The results indicate that precise control of FDM parameters, coupled with ANN predictions, facilitates the fabrication of 3D printed parts with the desired mechanical characteristics.

The expression ‘linear accelerator technology’ usually addresses all technological topics related to linear accelerators’ (linac) realization. These topics comprise, for instance, accelerating structures, Radio Frequency (RF) sources, magnets, diagnostics systems, controllers, power supplies, vacuum systems, particle sources, etc. Obviously, the extent of all these subjects is too wide to be covered in a single one-hour lecture and, therefore, this paper will only present an verview of the RF structures (including their fabrication technology) with a few examples of particle sources, RF sources and waveguide components. Moreover, the focus will be on linac technology for Free-Electron Lasers (FELs) and Energy Recovery Lasers (ERLs), which are electron accelerators. Electrons, in fact, even at very low energies (a few MeV) travel very close to the speed of light, and this fact has a direct consequence in the accelerating structures design, since they can be identical all along the linac.

Purpose: The impact of removing the flattening filter on absolute dosimetry based on IAEA's TPR-398 and AAPM's TG-51 was investigated in this study using Monte Carlo simulations. Methods: The EGSnrc software package was used for all Monte Carlo simulations performed in this work. Five different ionization chambers and nine linear accelerator heads have been modeled according to technical drawings. To generate a flattening filter free radiation field the flattening filter was replaced by a 2 mm thick aluminum layer. Dose calculation in a water phantom were performed to calculate the beam quality correction factor kQ as a function of the beam quality specifiers %dd(10)x, TPR20,10 and mean photon and electron energies at the point of measurement in photon fields with (WFF) and without flattening filter (FFF). Results: The beam quality correction factor as a function of %dd(10)x differs systematically between FFF and WFF beams for all investigated ionization chambers. The largest difference of 1.8% was observed for the largest investigated Farmer-type ionization chamber with a sensitive volume of 0.69 cm3. For ionization chambers with a smaller nominal sensitive volume (0.015 – 0.3 cm3) the deviation was less than 0.4% between WFF and FFF beams for %dd(10)x > 62%. The specifier TPR20,10 revealed only a good correlation between WFF and FFF beams (< 0.3%) for low energies. Conclusion: The results confirm that %dd(10)x is a suitable beam quality specifier for FFF beams with an acceptable bias. The deviation depends on the volume of the ionization chamber. Using %dd(10)x to predictkQ for a large volume chamber in a FFF photon field may lead to not acceptable errors according to the results of this study. This bias may be caused by the volume effect due to the inhomogeneous photon fields of FFF linear accelerators.

Chapter 15 - Microencapsulation: Air suspension technique

A new type of Beam Loss Monitor (BLM) system is being developed for use in the European Spallation Source (ESS) linac, primarily aiming to cover the low energy part (proton energies between 3-100 MeV). In this region of the linac, typical BLM detectors based on charged particle detection (i.e. Ionization Cham-bers) are not appropriate because the expected particle fields will be dominated by neutrons and photons. Another issue is the photon background due to the RF cavities, which is mainly due to field emission from the electrons from the cavity walls, resulting in brems-strahlung photons. The idea for the ESS neutron sensi-tive BLM system (ESS nBLM) is to use Micromegas detectors specially designed to be sensitive to fast neutrons and insensitive to low energy photons (X and gammas). In addition, the detectors must be insensitive to thermal neutrons, because those neutrons may not be directly correlated to beam losses. The appropriate configuration of the Micromegas operating conditions will allow excellent timing, intrinsic photon back-ground suppression and individual neutron counting, extending thus the dynamic range to very low particle fluxes.

The ever-growing death rate and the high recurrence of bladder cancer make the early detection and appropriate followup procedure of bladder cancer attract more attention. Compare to optical cystoscopy, image-based studies have revealed its potentials in non-invasive observations of the abnormities of bladder recently, in which MR imaging turns out to be a better choice for bladder evaluation due to its non-ionizing and high contrast between urine and wall tissue. Recent studies indicate that bladder wall thickness tends to be a good indicator for detecting bladder wall abnormalities. However, it is difficult to quantitatively compare wall thickness of the same subject at different filling stages or among different subjects. In order to explore thickness variations at different bladder filling stages, in this study, we preliminarily investigate the relationship between bladder wall thickness and bladder volume based on a MRI database composed of 40 datasets acquired from 10 subjects at different filling stages, using a pipeline for thickness measurement and analysis proposed in our previous work. The Student’s t -test indicated that there was no significant different on wall thickness between the male group and the female group. The Pearson correlation analysis result indicated that negative correlation with a correlation coefficient of -0.8517 existed between the wall thickness and bladder volume, and the correlation was significant(p <0.01). The corresponding linear regression equation was then estimated by the unary linear regression. Compared to the absolute value of wall thickness, the z-score of wall thickness would be more appropriate to reflect the thickness variations. For possible abnormality detection of a bladder based on wall thickness, the intra-subject and inter-subject thickness variation should be considered.

Purpose This paper aims to focus on an experimental study of surface roughness, dimensional accuracy and time of fabrication of parts produced by fused deposition modelling (FDM) technique of additive manufacturing. The fabricated parts of acrylonitrile butadiene styrene (ABS) material have pyramidal and conical features. Influence of five process parameters of FDM, namely, layer thickness, wall print speed, build orientation, wall thickness and extrusion temperature is studied on response characteristics. Furthermore, regression models for responses are developed and significant process parameters are optimized. Design/methodology/approach Comprehensive experimental study is performed using response surface methodology. Analysis of variance is used to investigate the influence of process parameters on surface roughness, dimensional accuracy and time of fabrication in both outer pyramidal and inner conical regions of part. Furthermore, a multi-response optimization using desirability function is performed to minimize surface roughness, improve dimensional accuracy and minimize time of fabrication of parts. Findings It is found that layer thickness and build orientation are significant process parameters for surface roughness of parts. Surface roughness increases with increase in layer thickness, while it decreases initially and then increases with increase in build orientation. Layer thickness, wall print speed and build orientation are significant process parameters for dimensional accuracy of FDM parts. For the time of fabrication, layer thickness and build orientation are found as significant process parameters. Based on the analysis, statistical non-linear quadratic models are developed to predict surface roughness, dimensional accuracy and time of fabrication. Optimization of process parameters is also performed using desirability function. Research limitations/implications The present study is restricted to the parts of ABS material with pyramidal and conical features only fabricated on FDM machine with delta configuration. Originality/value From the critical review of literature it is found that some researchers have made to study the influence of few process parameters on surface roughness, dimensional accuracy and time of fabrication of simple geometrical parts. Also, regression models and optimization of process parameters has been performed for simple parts. The present work is focussed on studying all these aspects in complicated geometrical parts with pyramidal and conical features.

Sensitivity analysis of low-velocity impact response of laminated plates

The incremental elastic modulus (Einc), which is the slope of the relationship between stress and strain of arteries, is a marker of vascular wall material stiffness. Isobaric Einc is reduced at the site of the radial artery in patients with essential hypertension and increased at the site of the common carotid artery in subjects with end-stage renal disease (ESRD). Whether the changes in Einc are influenced by the topography of the vessels, the composition of the arterial wall, and/or by the presence of ESRD is largely ignored. Radial artery Einc was measured in 19 patients with ESRD and compared with the Einc of 89 subjects with essential hypertension and 20 normotensive control subjects. Transcutaneous measurements of radial artery internal diameter and wall thickness (echo-tracking device) and digital pulse pressure (Finapres) were allowed to calculate Einc under operational (ie, at the mean arterial pressure of each group) and isobaric (100 mm Hg) conditions, as well as for a given wall stress. Internal diameter and pulsatile changes in diameter were identical in the three groups. Wall thickness and mean blood pressure were significantly elevated in subjects with hypertension but not in ESRD patients. Circumferential wall stress was identical in the three groups. For the same operational wall stress, and therefore at the operational mean arterial pressure of each group, Einc (kPa x 10[3]) was increased in patients with ESRD (5.53+/-4.0 versus 3.3+/-2.4 in control subjects; P<.05) and normal in subjects with essential hypertension (3.87+/-4.0). Under isobaric conditions, Einc was also significantly lower in subjects with hypertension and elevated in patients with ESRD. Thus, at the site of a medium-sized muscular artery constantly devoid of atherosclerosis, the stiffness of wall material is increased in patients with ESRD. The demonstrated alterations of the arterial wall are independent of the level of blood pressure and tensile stress and should be related to the status ESRD.