Abstract:Purpose: MRI guided radiotherapy is a rapidly growing field; however, current electron accelerators are not designed to operate in the magnetic fringe fields of MRI scanners. As such, current MRI-Linac systems require magnetic shielding, which can degrade MR image quality and limit system flexibility. The purpose of this work was to develop and test a novel medical electron accelerator concept which is inherently robust to operation within magnetic fields for in-line MRI-Linac systems. Methods: Computational s… Show more
“…2). A number of other linac-MR delivery platforms are in development [45–47] and may contribute to advances in the practice of central SABR.Fig. 2Comparative treatment plans for MRI-guided radiotherapy using breath-hold versus a standard free-breathing internal target volume (ITV)-based approach for a central tumor in a patient with interstitial lung disease.…”
Lung cancer is the leading cause of cancer mortality, and radiotherapy plays a key role in both curative and palliative treatments for this disease. Recent advances include stereotactic ablative radiotherapy (SABR), which is now established as a curative-intent treatment option for patients with peripheral early-stage NSCLC who are medically inoperable, or at high risk for surgical complications. Improved delivery techniques have facilitated studies evaluating the role of SABR in oligometastatic NSCLC, and encouraged the use of high-technology radiotherapy in some palliative settings. Although outcomes in locally advanced NSCLC remain disappointing for many patients, future progress may come about from an improved understanding of disease biology and the development of radiotherapy approaches that further reduce normal tissue irradiation. At the moment, the benefits, if any, of radiotherapy technologies such as proton beam therapy remain unproven. This paper provides a critical review of selected aspects of modern radiotherapy for lung cancer, highlights the current limitations in our understanding and treatment approaches, and discuss future treatment strategies for NSCLC.
“…2). A number of other linac-MR delivery platforms are in development [45–47] and may contribute to advances in the practice of central SABR.Fig. 2Comparative treatment plans for MRI-guided radiotherapy using breath-hold versus a standard free-breathing internal target volume (ITV)-based approach for a central tumor in a patient with interstitial lung disease.…”
Lung cancer is the leading cause of cancer mortality, and radiotherapy plays a key role in both curative and palliative treatments for this disease. Recent advances include stereotactic ablative radiotherapy (SABR), which is now established as a curative-intent treatment option for patients with peripheral early-stage NSCLC who are medically inoperable, or at high risk for surgical complications. Improved delivery techniques have facilitated studies evaluating the role of SABR in oligometastatic NSCLC, and encouraged the use of high-technology radiotherapy in some palliative settings. Although outcomes in locally advanced NSCLC remain disappointing for many patients, future progress may come about from an improved understanding of disease biology and the development of radiotherapy approaches that further reduce normal tissue irradiation. At the moment, the benefits, if any, of radiotherapy technologies such as proton beam therapy remain unproven. This paper provides a critical review of selected aspects of modern radiotherapy for lung cancer, highlights the current limitations in our understanding and treatment approaches, and discuss future treatment strategies for NSCLC.
“…Although there have been proposals to redesign electron accelerators to function within magnetic fields, 4,20 at this stage all systems appear to be using conventional equipment in conjunction with magnetic shielding. It is therefore interesting to note that the external dimensions of this triode gun are much larger than the model which was previously investigated for shielding.…”
Section: Model Referencementioning
confidence: 99%
“…Meanwhile, the power from backward-accelerated electrons can be on the order of kilowatts. 20 The fact that the duty cycle of the gun is of the order 0.001 will further mitigate heating of the grid. 6 Nevertheless, the impact of collisions between the emitted electrons and control grid is clearly significant enough in real world applications to have been the subject of a large body of work aimed at reducing them-mainly by utilizing some form of "shadow grid" which reduces the number of electrons intercepting the grid.…”
Section: Model Referencementioning
confidence: 99%
“…This part of the geometry can be very important for mitigating back bombardment, 20 and can have a strong effect on the gun sensitivity to in-line magnetic fields. This is discussed in Sec.…”
Section: A Gun Geometrymentioning
confidence: 99%
“…In-line field sensitivity (G) 20 In order to minimize the amount of back-accelerated electrons reaching the cathode, the drift tube should have a minimum radius corresponding to the minimum beam radius. By choosing the injection point to be close to the beam waist, the minimum anode radius can be made as small as possible, minimizing the amount of back accelerated electrons which can return to hit the cathode.…”
Purpose: MRI-linac therapy is a rapidly growing field, and requires that conventional linear accelerators are operated with the fringe field of MRI magnets. One of the most sensitive accelerator components is the electron gun, which serves as the source of the beam. The purpose of this work was to develop a validated finite element model (FEM) model of a clinical triode (or gridded) electron gun, based on accurate geometric and electrical measurements, and to characterize the performance of this gun in magnetic fields. Methods: The geometry of a Varian electron gun was measured using 3D laser scanning and digital calipers. The electric potentials and emission current of these guns were measured directly from six dose matched true beam linacs for the 6X, 10X, and 15X modes of operation. Based on these measurements, a finite element model (FEM) of the gun was developed using the commercial software /. The performance of the FEM model in magnetic fields was characterized using parallel fields ranging from 0 to 200 G in the in-line direction, and 0-35 G in the perpendicular direction.Results: The FEM model matched the average measured emission current to within 5% across all three modes of operation. Different high voltage settings are used for the different modes; the 6X, 10X, and 15X modes have an average high voltage setting of 15, 10, and 11 kV. Due to these differences, different operating modes show different sensitivities in magnetic fields. For in line fields, the first current loss occurs at 40, 20, and 30 G for each mode. This is a much greater sensitivity than has previously been observed. For perpendicular fields, first beam loss occurred at 8, 5, and 5 G and total beam loss at 27, 22, and 20 G. Conclusions: A validated FEM model of a clinical triode electron gun has been developed based on accurate geometric and electrical measurements. Three different operating modes were simulated, with a maximum mean error of 5%. This gun shows greater sensitivity to in-line magnetic fields than previously presented models, and different operating modes show different sensitivity. C 2016 American Association of Physicists in Medicine. [http://dx
Purpose
Recently, high‐precision radiotherapy systems have been developed by integrating computerized tomography or magnetic resonance imaging to enhance the precision of radiotherapy. For integration with additional imaging systems in a limited space, miniaturization and weight reduction of the linear accelerator (linac) system have become important. The aim of this work is to develop a compact medical linac based on 9.3 GHz X‐band RF technology instead of the S‐band RF technology typically used in the radiotherapy field.
Methods
The accelerating tube was designed by 3D finite‐difference time‐domain and particle‐in‐cell simulations because the frequency variation resulting from the structural parameters and processing errors is relatively sensitive to the operating performance of the X‐band linac. Through the 3D simulation of the electric field distribution and beam dynamics process, we designed an accelerating tube to efficiently accelerate the electron beam and used a magnetron as the RF source to miniaturize the entire linac. In addition, a side‐coupled structure was adopted to design a compact linac to reduce the RF power loss. To verify the performance of the linac, we developed a beam diagnostic system to analyze the electron beam characteristics and a quality assurance (QA) experimental environment including 3D lateral water phantoms to analyze the primary performance parameters (energy, dose rate, flatness, symmetry, and penumbra) The QA process was based on the standard protocols AAPM TG‐51, 106, 142 and IAEA TRS‐398.
Results
The X‐band linac has high shunt impedance and electric field strength. Therefore, even though the length of the accelerating tube is 37 cm, the linac could accelerate an electron beam to more than 6 MeV and produce a beam current of more than 90 mA. The transmission ratio is measured to be approximately 30% ~ 40% when the electron gun operates in the constant emission region. The percent depth dose ratio at the measured depths of 10 and 20 cm was approximately 0.572, so we verified that the photon beam energy was matched to approximately 6 MV. The maximum dose rate was measured as 820 cGy/min when the source‐to‐skin distance was 80 cm. The symmetry was smaller than the QA standard and the flatness had a higher than standard value due to the flattening filter‐free beam characteristics. In the case of the penumbra, it was not sufficiently steep compared to commercial equipment, but it could be compensated by improving additional devices such as multileaf collimator and jaw.
Conclusions
A 9.3 GHz X‐band medical linac was developed for high‐precision radiotherapy. Since a more precise design and machining process are required for X‐band RF technology, this linac was developed by performing a 3D simulation and ultraprecision machining. The X‐band linac has a short length and a compact volume, but it can generate a validated therapeutic beam. Therefore, it has more flexibility to be coupled with imaging systems such as CT or MRI and can reduce the bore size of the gantry. In add...
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