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Per cent depth dose data for a few selected depths for 4 MV roentgen rays from Varian Clinac-4 linear accelerators equipped with lead or uranium field flatteners are presented and compared with published data. A difference by as much as 12 per cent was found.
BACKGROUND: At the University of Lübeck, radiotherapy is delivered by a 6/18-MV linear accelerator. Using the integrated multileaf collimator, irradiation of individually shaped treatment fields is possible in place of alloy blocks. Due to unsatisfactory pretherapeutic review of the radiation-field-specific multileaf collimator (MLC) configuration, we developed a simulation-multileaf collimator (SMLC) and assessed its feasibility at different tumor sites. MATERIAL AND METHODS: The SMLC is made of a perspex carrier with 52 horizontal sliding leaves. The position of each leaf is calculated by a 3D treatment-planning computer. The technician manually adjusts the leaves according to the beams-eye-view plot of the planning computer. Consequently, the SMLC is mounted on the therapy simulator at a distance of 64.8 cm from the focus. The treatment fields and the position of the leaves are documented by X-ray films. RESULTS: Using the SMLC, radiation oncologists are able to review exactly the leaf configuration of each MLC-shaped radiation field and to correlate the MLC-shaped radiation field with the treated volume, the organs at risk and the port films acquired by the Portal Vision system. CONCLUSION: The SMLC is a new tool to review radiation planning that uses an MLC in daily routine. The use of the SMLC improves the documentation and the quality assurance. It accelerates the treatment field review at the linear accelerator by comparing the SMLC simulator films with the portal images.
Tomotherapy presents an evolutionary modality that holds forth the promise of better dose conformation to tumor volumes with a concomitant reduction in radiation-induced damage to surrounding normal structures. This delivery technique also presents a new set of radiation protection challenges that impact upon the design of the shielding vault required to house such a unit. A formalism is presented to determine the requisite amounts of shielding for both the primary beam and leakage radiation associated with a generic tomotherapy unit. A comparison is made with the shielding requirements for a conventional linear accelerator operated in a standard manner. Substantial differences in the amount of both primary and secondary shielding are indicated. A tomotherapy primary beam shield is both reduced in width by a factor of almost 10 and increased in thickness by more than a tenth value layer in comparison to a conventional accelerator. Furthermore, the secondary shielding requirements are enhanced by more than two tenth value layers with respect to conventional shielding demands.
A computerized tomography (CT) reconstruction technique has been used to make quantitative measurements of the size and shape of the focal spot in medical linear accelerators. Using this technique, we have measured the focal spots in a total of nine accelerators, including (i) two Varian Clinac 2100c's, (ii) two Atomic Energy of Canada Ltd. (AECL) Therac-25's, (iii) two AECL Therac 6's, (iv) a Siemens KD-2, (v) a Varian Clinac 600c (4 MV), and (vi) an AECL Therac-20. Some of these focal spots were monitored for changes over a 2-yr period. It has been found that (i) the size and shape of the source spot varies greatly between accelerators of different design ranging from 0.5 to 3.4 mm in full width at half maximum (FWHM); and (ii) for accelerators of the same design, the focal spots are very similar. In addition to the measurements of the focal spot, a new technique for measuring the magnitude and distribution of extra-focal radiation originating from the linear accelerator head (flattening filter, primary collimator) has also been developed. The extra-focal radiation produced by a Varian Clinac 2100c accelerator was measured using this technique and it was found that the extra-focal radiation accounts for as much as 8% of the total photon fluence reaching the isocenter. The majority (75%) of this extra-focal radiation originates from within a circle 6 cm in diameter at the target plane. The source MTFs for each of the measured focal spots have been calculated in order to assess their influence on the spatial resolution of verification images. The limiting spatial resolution (i.e., 10% modulation) for all the source MTFs is 1.8 mm-1 or greater when used for transmission radiography at a magnification of 1.2. The extra-focal radiation, which produces a low-frequency drop in the source MTFs of up to 8%, changes with field size. As a result, the source MTFs of linear accelerators depend not only on the design of individual accelerators and image magnification, but also on the field size used when forming an image.
Wedge transmission factors have been measured for two sets of physical wedges for the 4 MV beam of a new dual energy linear accelerator as a function of field size and depth. The field size dependence of these factors has been compared with the 4 MV beam from another, single-energy linear accelerator and a difference of a factor of approximately 2 has been observed in the relative wedge transmission factors between the two machines.
It is possible to model the time-intensity profile of solar particles expected in space after the occurrence of a significant solar flare on the sun. After the particles are accelerated in the flare process, if conditions are favorable, they may be released into the solar corona and then into space. The heliolongitudinal gradients observed in the inner heliosphere are extremely variable, reflecting the major magnetic structures in the solar corona which extend into space. These magnetic structures control the particle gradients in the inner heliosphere. The most extensive solar particle measurements are those observed by earth-orbiting spacecraft, and forecast and prediction procedures are best for the position of the earth. There is no consensus of how to extend the earth-based models to other locations in space. Local interplanetary conditions and structures exert considerable influence on the time-intensity profiles observed. The interplanetary shock may either reduce or enhance the particle intensity observed at a specific point in space and the observed effects are very dependent on energy.
This paper describes a facility to produce positron labeled radiopharmaceuticals with a tandem Van de Graaff accelerator. 11C labeled carbon monoxide (11CO), 15O labeled H2O (H2 15O) and 18F labeled 2-deoxy-2-fluoro-D-glucose ([18F]2FDG), were chosen so that physiological measurements of cerebral blood volume, cerebral blood flow, and the cerebral metabolic rate for glucose could be made routinely in our PET center. This facility uses a tandem Van de Graaff accelerator which is relatively remote from the clinical area. Measurements of radionuclide yield and beam penetration showed that it was feasible to deliver sufficient amounts of radioactivity for the measurements. Both 11CO and H2 15O presented significant delivery problems due to their shorter half-lives. For H2 15O a continuous delivery system for water vapour and an infusion system for the administration of H2 15O in saline were designed and constructed. 11CO and [18F]2FDG were hand carried. This facility has demonstrated that a tandem Van de Graaff accelerator can routinely supply a PET center with adequate quantities of these three radiopharmaceuticals.
This paper describes the status of the design study of the Austrian Ion Therapy and Cancer-Research Centre Project MedAustron. This work was performed during the last two years by the MedAustron study group. The team was spread out over several research institutes and University clinics, with full-time members at the Medical University of Vienna, Innsbruck Medical University and Fotec, Wiener Neustadt in collaboration with the Medical University Graz, the Hospital of Wiener Neustadt, the Vienna University of Technology and the research institutes CERN, PSI, the Slovak University of Technology in Bratislava and the Jozef Stefan Institute, Ljubljana. The study group has also worked in cooperation with GSI, Darmstadt and the CNAO foundation, Milan. The agreed aim of this study was to investigate a conceptual design of an accelerator facility which provides optimum treatment conditions for high-precision active beam scanning of cancer tumours with both proton and carbon ions.
The final stage in the production of a radiotherapy treatment plan must always be an independent check that the linear accelerator settings given on the plan do in fact deliver the required dose distribution. A tool is described that enables rapid checking of diaphragm settings in relation to the patient.
Electron beam radiotherapy treatment planning systems need to be fed with the characteristics of the high-energy electron beams (4-50 MeV) from the specifically applied accelerator. Beams can be characterized by their mean initial energy, effective initial angular variance, virtual source position and the resulting central axis depth dose distribution in water. This information is the only input to pencil beam dose calculation models. Newer calculation models like macro Monte Carlo, voxel Monte Carlo and phase space evolution require as input the full initial phase space or a parametrization of that initial phase space, generally consisting of a primary beam component and one or more scatter components. This primary beam component is often characterized by initial energy, primary beam initial angular variance and virtual source distance. The purpose of the present investigation was to investigate to what extent standard values can be used both for the effective initial angular variance as input to pencil beam models and for the primary beam initial angular variance. Comprehensive benchmark data were obtained on the initial angular variance of various types of accelerator, for various energies and field sizes. The initial angular variance sigma2theta(x) has been derived from penumbra measurements in air by means of film dosimetry at various distances from the lower collimator. For the types of accelerator used in radiotherapy nowadays the measurements show values for sigma2theta(x)/T(E) of around 13 cm where T(E) is the ICRU-35 linear angular scattering power in air. This value can be chosen as standard value for the primary beam initial angular variance, only slightly compromising the dose calculation accuracy. As input to pencil beam models, an effective sigma2theta(x)/T(E) should be used incorporating the scatter from the lower collimator. For the case that the air gaps between lower collimator and patient are small (5-10 cm) an effective sigma2theata(x)/T(E) of 20 cm has been found and is recommended as the standard input for pencil beam models. Of the accelerators investigated, a different value was found only for the Elekta SL15, i.e. 50% higher for the effective sigma2theta(x)/T(E).
Basic dosimetric properties of 6 MV and 18 MV photon beams from a Varian Clinac 21EX accelerator operating without the flattening filter have been measured. These include dose rate data, depth dose dependencies and lateral profiles in a water phantom, total scatter factors and transmission factors of a multileaf collimator. The data are reviewed and compared with measurements for the flattened beams. The unflattened beams have the following: a higher dose rate by factors of 2.3 (6 MV) and 5.5 (18 MV) on the central axis; lower out-of-field dose due to reduced head scatter and softer spectra; less variation of the total scatter factor with field size; and less variation of the shape of lateral dose profiles with depth. The findings suggest that with a flattening filter free accelerator better radiation treatments can be developed, with shorter delivery times and lower doses to normal tissues and organs.
The first of a new generation of microtron accelerators has been installed and tested. It is currently in use for multisegment conformal radiotherapy at our institution. The unit produces x rays and electrons from 10 to 50 MeV in 5 MeV increments. It incorporates a 64 leaf, doubly focused multileaf collimator (MLC), which can be used to shape x-ray and electron beams. Both x-ray and electron beams are produced by magnetically scanning the electron beams from the accelerator. The new generation unit incorporates a purging magnet to sweep away any primary or secondary electrons that pass through the target(s). In this paper, the beam characteristics of the accelerator that were studied during acceptance testing are described. Representative examples of depth doses, beam profiles, output factors, and elementary beam distributions are presented and discussed, in comparison with the earlier generation of microtron accelerators and with other radiotherapy machines.
Because of the capability of free movement in the treatment room, we recently introduced a Hercules treatment couch on one of our linear accelerators. One of the advantages of this couch is that it allows for a more flexible way of patient setup and that it can be moved entirely out of the way to enable treatment with a hospital bed. A disadvantage, however, is that the couch can hit a wall or a cover of the accelerator accidentally. A limitation system has been developed to protect both the table and the accelerator against such collisions.
Radiation treatment of cervical and uterine cancer (intracavitary and combined irradiation using the ATAT-B, Rokus and 15M1 accelerator) is discussed to illustrate the effectiveness of mathematic procedures for choosing optimal dose regimens to suit for the anatomic peculiarities of a patient and the clinical setting.
Radiation-induced hemopoietic death was measured in mice exposed to photons of four different energies: 250-kVp X rays, 60Co gamma rays (1.25 MeV), and 6- and 25-MV photons from a linear accelerator. For each radiation source, the lethal dose which killed 50% of the population in 30 days (LD50/30) associated with the hemopoietic syndrome was determined in groups of mice exposed to graded doses from 600 to 1150 cGy at dose rates of 20, 40, and 80 cGy/min. The calculated LD50/30 values for 25 and 6 MV were significantly different from each other at all exposure rates while no difference was observed between 6 MV and 60Co. Using 60Co gamma rays as the standard, the relative biologic effectiveness was as follows: 250 kVp greater than 25 MV greater than 6 MV = 60Co. The data suggest that there may be a greater damage to tissue within the marrow cavities following exposure to very high megavoltage radiation, a factor which must be considered with the increasing utilization of linear accelerators in the clinic and laboratory.
Electron beam central axis depth dose distribution can be transformed to fluence distributions with geometric depth replaced by a measure of the state of angular dispersion of the beam. A transformed central axis depth dose distribution was called a 'fluence curve.' The transformation was applied to a set of central axis depth dose curves calculated by Monte Carlo code for broad electron beams of energies ranging from 1 to 60 MeV in homogeneous phantoms of water, aluminium, and copper. For the energies and compositions likely to be encountered in external beam radiation therapy, the resulting fluence curves, were found to belong to a single parameter family. A collimator scatter parameter was introduced to take into account the initial angular dispersion produced by the collimator of an accelerator. Given the energy of the beam, the medium in which the beam is passing through, and the collimator scatter parameter, the fluence curve associated with the beam can easily be transformed back to a calculated depth dose curve. The collimator scatter parameter necessary to fit the depth dose curves measured on different accelerators was investigated. The results for the Clinac 18, LMR-13, Mevatron XII, Mevatron 80, Microtron, Sagittaire, Siemens Betatron, and the Therac 20 are presented.
A semiempirical method for the calculation of the relative crossbeam dose profiles at depth is described. The parameters required to set up the formulae and their dependence with field size and depth are investigated. Using the above method, measured crossbeam dose profiles at depth from two linear accelerators, Philips (SL-18) and AEC (Therac-6) are reproduced. The results indicate that this method is applicable within a wide range of depths and field sizes.
A method for the radiochemical extraction of 129I from atmospheric charcoal filters and its measurement by accelerator mass spectrometry is presented. Either the 129I concentration or the 129I/127I atom ratio can be determined in the sample. With this method, air filters from Seville, in the Southwest of Spain (37.4 degrees N, 6 degrees W) have been analyzed. Sensitivities in the order of 10(4) atoms/m3 for 129I concentrations and 10(-10) for 129I/127I atom ratios are obtained. AMS measurements are performed with the 6 MV tandem accelerator at the ETH-Hönggerberg in Zurich.