Operation Of Linear Accelerators Research Articles

Neutron production from a mobile linear accelerator operating in electron mode for intraoperative radiation therapy

Intraoperative electron beam radiotherapy is increasingly performed using mobile linac delivering therapeutic radiation doses in unshielded operating rooms. While no special neutron-shielding problem should arise for operation at 10 MeV or less, it is not clear whether this holds true for operation at higher energies. This paper reports the measured neutron production from a Mobetron mobile electron linac, operated at 12 MeV, and compares the results with those from a conventional linac, also operated at 12 MeV in electron mode. Neutron leakage measurements were performed by means of passive bubble detectors in the scattering foil, patient and floor planes. Neutron dose equivalent rates per unit of electron dose delivered by the Mobetron at its normal treatment distance (50 cm SSD) were 0.33 µSv Gy−1 at the accelerator head, 0.18 µSv Gy−1 in the patient plane at 15 cm from the beam axis and 0.31 µSv Gy−1 at the floor plane, on the beam axis and under the beam stopper. For a weekly workload of 250 Gy, the weekly neutron dose equivalents at 12 MeV for the Mobetron at a distance of 300 cm from the scattering foil were 14.3 and 1.7 µSv/week for floor below and adjoining areas on the same floor, respectively. Neutron dose equivalent rates generated from Mobetron are at least one order of magnitude lower than ones produced by a conventional linac operated at the same energy in electron mode. Mobetron can be used at 12 MeV in an unshielded operating room for a weekly workload of up to 250 Gy if the bremsstrahlung x-rays are shielded to negligible levels.

The Development of a New Basic Treatment Equivalent Model to Assess Linear Accelerator Throughput

The basic treatment equivalent (BTE) model was developed in 1996 in an attempt to improve the measurement of linear accelerator throughput in radiotherapy. This study aimed to assess the effect of treatment set-up and patient characteristics on fraction duration, to update the BTE model and to determine the better throughput measure between fields per hour and BTE per hour. Stopwatch measurements of the duration of each radiotherapy treatment fraction delivered on each linear accelerator in participating New South Wales radiation oncology departments over a 5-day period in 2003 were undertaken. Patient, equipment and staff data were collected to assess the effect of these variables on fraction duration. A new BTE equation was derived, including the most significant variables. Statistical comparison of fields and BTE per unit time was made to assess the better predictor of fraction duration. Data collected on 27 linear accelerators in 13 departments included a total of 135 days of linear accelerator operation, 4316 fractions and 12 892 treatment fields. Seventeen factors significantly influenced fraction duration (P < 0.01). These accounted for 46% of the total variance in the models. The eight most influential predictors of prolonged fraction duration were included in the BTE model. These were as follows: high number of fields, high number of port films/electronic portal imaging, absence of automatic field-sequencing and multi-leaf collimation, high number of junctions, use of bolus and first fraction of a treatment course. The BTE per hour was shown to be a better predictor of throughput than fields per hour. The BTE model is a better measure of linear accelerator throughput. It incorporates weightings for treatment and patient factors that significantly influenced fraction duration. This measure could be routinely collected by the radiation oncology departments and included in the electronic radiotherapy information systems.

Dosimetry of high-energy electron linac produced photoneutrons and the bremsstrahlung gamma-rays using TLD-500 and TLD-700 dosimeter pairs

The neutron and gamma doses are crucial to interpreting the radiation effects in microelectronic devices operating in a high-energy accelerator environment. This report highlights a method for an accurate estimation of photoneutron and the accompanying bremsstrahlung (gamma) doses produced by a 450 MeV electron linear accelerator (linac) operating in pulsed mode. The principle is based on the analysis of thermoluminescence glow-curves of TLD-500 (Aluminium Oxide) and TLD-700 (Lithium Fluoride) dosimeter pairs. The gamma and fast neutron response of the TLD-500 and TLD-700 dosimeter pairs were calibrated with a 60Co (gamma) and a 241Am–Be (α, n) neutron standard-source, respectively. The Kinetic Energy Released in Materials (kerma) conversion factor for photoneutrons was evaluated by folding the neutron kerma (dose) distribution in 7LiF (the main component of the TLD-700 dosimeter) with the energy spectra of the 241Am–Be (α, n) neutrons and electron accelerator produced photoneutrons. The neutron kerma conversion factors for 241Am–Be neutrons and photoneutrons were calculated to be 2.52×10 −3 and 1.37×10 −3 μGy/a.u. respectively. The bremsstrahlung (gamma) dose conversion factor was evaluated to be 7.32×10 −4 μGy/a.u. The above method has been successfully utilised to assess the photoneutron and bremsstrahlung doses from a 450 MeV electron linac operating at DESY Research Centre in Hamburg, Germany.

A particle-core-MD model for intrabeam scattering and halo formation in high current beams in a FODO channel

An essential problem for the successful operation of high current linear ion accelerators is the control of beam losses due to halo particles. As a possible mechanism for the formation of such a halo we concentrate on the interplay between intrabeam scattering (IBS) and the incidence of particles which are driven to high amplitudes by resonances with the nonlinear space charge fields of a mismatched beam. Since a fully microscopic numerical treatment including all the mutual Coulomb interactions between the beam ions requires much too high computational effort, we developed an approximative method. These particle-core-molecular-dynamics (PCMD) simulations suitably join the mean-field description of the time evolution of the beam in framework of the envelope equations and a microscopic calculation of the Coulomb interactions between pseudo-particles with a renormalized charge. With this method we studied matched and mismatched continuous KV-beams in a FODO channel. In first simulation runs we observed a significant difference in the formation of a cloud of particles around the beam core between matched and mismatched beams when IBS is present. While for a matched beam with IBS no particles with increasing amplitudes have been found, such particles appear at the same IBS rate for a mismatched beam. These results suggest that in a mismatched beam even rather small IBS rates seem to act as a non-negligible source for a halo formation. To conclude from these observations on intense real beams in a high current linac needs, however, further verification of the proposed PCMD method.

A thin target approach for portal imaging in medical accelerators

A new thin-target method (patent pending) is described for portal imaging with low-energy (tens of keV) photons from a medical linear accelerator operating in a special mode. Low-energy photons are usually produced in the accelerator target, but are absorbed by the target and flattening filter, both made of medium- or high- Z materials such as Cu or W. Since the main contributor to absorption of the low-energy photons is self-absorption by the thick target through the photoelectric effect, it is proposed to lower the thickness of the portal imaging target to the minimum required to get the maximum low-energy photon fluence on the exit side of the target, and to lower the atomic number of the target so that predominantly photoelectric absorption is reduced. To determine the minimum thickness of the target, EGS4 Monte Carlo calculations were performed. As a result of these calculations, it was concluded that the maximum photon fluence for a 4 MeV electron beam is obtained with a 1.5 mm Cu target. This value is approximately five times less than the thickness of the Cu target routinely used for bremsstrahlung production in radiotherapeutic practice. Two sets of experiments were performed: the first with a 1.5 mm Cu target and the second with a 5 mm Al target (Cu mass equivalent) installed in the linear accelerator. Portal films were taken with a Rando anthropomorphic phantom. To emphasize the low-energy response of the new thin target we used a Kodak Min-R mammographic film and cassette combination, with a strong low-energy response. Because of its high sensitivity, only 1 cGy is required. The new portal images show a remarkable improvement in sharpness and contrast in anatomical detail compared with existing ones. It is also shown that further lowering of the target's atomic number (for example to C or Be) produces no significant improvement.

Neutron Contamination From Medical Electron Accelerators

This report was produced by a scientific committee of eight members, chaired by Richard C. McCall. It is a reference manual dealing with the problem of neutron production that results from the operation of high-energy electron linear accelerators used in radiation therapy. As radiation therapy becomes more sophisticated, in many hospitals in the United States and abroad the trend is to higher energy machines. Accelerators operated below 10 meV do not represent a major hazard; however, as accelerators operate in the 10 to 50 meV range, neutron production may be seen. The source of neutron production in the treatment machine and the methods of protection and measurement are exquisitely detailed in this book. There is an extensive reference list to the source information. This pamphlet goes into details concerning facilities design, including wall thickness, shielding requirements, machine placement, and maze design. Information on hazards, patient risks, and operating personnel risks

A Primer on Theory and Operation of Linear Accelerators in Radiation Therapy by C. J. Karzmark and R. J. Morton

<i>A Primer on Theory and Operation of Linear Accelerators in Radiation Therapy</i> by C. J. Karzmark and R. J. Morton

Microwave Measurements of Azimuthal Asymmetries in Accelerating Fields of Disk-Loaded Waveguides

This paper presents microwave measurements of azymuthal asymmetries in the accelerating fields of the SLAC disk-loaded waveguide. These field asymmetries lead to RF phase-dependent beam steering which can be detrimental to operation of linear accelerators in general and of the SLAC Linear Collider in particular.

Radiological Safety Aspects of the Operation of Electron Linear Accelerators, by W. P. Swanson

<i>Radiological Safety Aspects of the Operation of Electron Linear Accelerators</i>, by W. P. Swanson

The use of linear accelerators for gamma radiography

The use of linear accelerators for radiography for ndt is still in its infancy. The author describes the operation of linear accelerators from the point of view of gamma radiography. He discusses the working parameters and limits. He describes installations in operation in France and proposes that hiring the facilities out by the beam-hour would best serve the development of this method of gamma radiography.

Automatic Frequency Control System for a Microwave Linear Electron Accelerator

In the operation of microwave electron linear accelerators, adjustment of the frequency-determining element of the power source is frequently required to optimize performance. These adjustments are necessitated by thermal-mechanical changes of the accelerator structure proper as well as independent changes of the oscillator frequency. An automatic frequency control system has been designed to maximize the x-ray output for an S-band accelerator used in radiation therapy. Frequency modulation of the oscillator produces a phase sensitive amplitude modulation component in the x-ray output. This information is used with a suitable detector and servo system to effect frequency corrections.