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PubMedDate not supplied

Dose computations for asymmetric fields defined by independent jaws.

Asymmetric fields defined by independent jaws can be used to split a beam or to match adjacent fields. We have extended a method originally developed for symmetric fields to calculate the dose for asymmetric fields. The dose to a point is computed as the product of the tissue maximum ratio (TMR), the off center ratio (OCR), and the inverse square factor. The TMR is computed from the measured central axis depth doses for symmetric fields. The OCR is obtained by multiplying the primary OCR (POCR) and the boundary factors (BF's) for the four jaws. The POCR's and BF's were derived from measured beam profiles, which include the effect of off-axis beam quality variations. Using this method, the beam profiles and isodose distributions for asymmetric fields of a 6-MV accelerator were calculated and compared with the measured data. The agreement is within experimental errors both in the penumbra region and along the central ray of the asymmetric field.

Humans↗

PubMedDate not supplied

[Management of electron accelerators for radiotherapy. Data as of 31 December 1984].

Several italian radiotherapeutic centers have supplied the data analysed, regarding problems connected with the management of electron accelerators in the years 1982-1983-1984. The data concern 27 out of the 35 accelerators settled in Italy. The average cost of purchase has proved to be of 940 M pounds with an expense of annual allowance of 94 M pounds. We have then analysed the incidences of the allowance of the purchase expenditure of the machine (23%), the allowance of the expenses for the construction of the bunker (22%), the expenses of maintenance of the machine (5%), the cost of the staff (48%) and the cost of the power supply (2%) on the management costs (about pounds 820.000 per each handled patient). The duration of the various phases of the machinery installment, from the purchase to the beginning of the treatment, the charge of work supported in the three considered years, the daily use of the machinery and the problems deriving from days of time out of services due to failures or to periodical maintenance have then been studied. The analysis of the average values of the parameters studied and of the range of their variability allows us to find out the sector in which it is possible, also at present, to intervene in order to ameliorate the employment of a linear accelerator, regarding the costs and the performance.

Cost-Benefit Analysis↗

PubMedDate not supplied

[Action of secondary radiation from 70-Gev protons on the cellular DNA of mammals].

A study was made of the effect of secondary radiation of 70 GeV protons on DNA of Chinese hamster cells. With a reference to fibroblast DNA, lymphoid cell DNA, and the lethal effect of radiation on the survival of Chinese hamster cells the RBE was 1.6-7.6, 1.1-3.8 and 1.14-1.7, respectively. DNA breaks were repaired to an equal level after exposure to secondary radiation from the accelerator and gamma-radiation from 60Co in equally effective doses.

Animals↗

PubMed2002

Proposed double-layer target for the generation of high-quality laser-accelerated ion beams.

In order to achieve a high-quality, i.e., monoenergetic, intense ion beam, we propose the use of a double-layer target. The first layer, at the target front, consists of high-Z atoms, while the second (rear) layer is a thin coating of low-Z atoms. The generation of high-quality proton beams from the double-layer target, irradiated by an ultraintense laser pulse, is demonstrated with three-dimensional particle-in-cell simulations.

Ions↗

PubMed2004

The linear accelerator mechanical and radiation isocentre assessment with an electronic portal imaging device (EPID).

Regular checks on the performance of radiotherapy treatment units are essential and a variety of protocols has been published. These protocols identify that the determination of isocentre must be accurate and unambiguous since both the localization of a radiation field on a patient and positioning aids are referenced to it. An EPID (BIS 710) with a combined light and radiation scintillation detector screen was used to assess mechanical and radiation isocentres for different collimator and gantry angles. Crosshair positions within light field images were determined from fitted Gaussian intensity profiles and then used to calculate the displacement of the mechanical isocentre. For comparison, the position of the crosshair was also recorded on a graph paper. The radiation field centre was first calculated from the set up geometry for given gantry/collimator angles and then compared with measured values to assess the displacement of the radiation isocentre. The radiation isocentre was also checked by locating a marker, positioned on the couch, on the EPID radiation images for different treatment couch angles. The mechanical and radiation isocentres were determined from the EPID light field and radiation images respectively with an accuracy of 0.3 mm using simple PC based programs. The study has demonstrated the feasibility of using the EPID to assess mechanical and radiation isocentres of a linear accelerator in a quick and efficient way with a higher degree of accuracy achieved as compared to more conventional methods, e.g. the star shot.

Australia↗

PubMed1994

Relative output factors of asymmetric megavoltage beams.

A new method for calculating output factors of asymmetric therapy fields is presented. The method uses the output factors of symmetric fields, as well as off-axis ratios measured in air, to calculate the output factor for an arbitrary asymmetric field. Calculations have been checked by measurements in four photon beams (4-18 MV) of three different linear accelerators. The accuracy between the theory and the measurements is generally better than 1%. According to the preliminary results the method may also be suitable for megavoltage electron beams.

Algorithms↗

PubMed1999

Intensity modulation with the "step and shoot" technique using a commercial MLC: a planning study. Multileaf collimator.

PURPOSE/OBJECTIVE: For complex planning situations where organs at risk (OAR) surrounding the target volume place stringent constraints, intensity-modulated treatments with photons provide a promising solution to improve tumor control and/or reduce side effects. One approach for the clinical implementation of intensity-modulated treatments is the use of a multileaf collimator (MLC) in the "step and shoot" mode, in which multiple subfields are superimposed for each beam direction to generate stratified intensity distributions with a discrete number of intensity levels. In this paper, we examine the interrelation between the number of intensity levels per beam for various numbers of beams, the conformity of the resulting dose distribution, and the treatment time on a commercial accelerator (Siemens Mevatron KD2) with built-in MLC. METHODS AND MATERIALS: Two typical, clinically relevant cases of patients with head and neck tumors were selected for this study. Using the inverse planning technique, optimized treatment plans are generated for 3-25 evenly distributed coplanar beams as well as noncoplanar beams. An iterative gradient method is used to optimize a physical treatment objective that is based on the specified target dose and individual dose constraints assigned to each organ at risk (brain stem, eyes, optic nerves) by the radiation oncologist. The intensity distribution of each beam is discretized within the inverse planning program into three to infinitely many intensity levels or strata. These stratified intensity distributions are converted into MLC leaf position sequences, which can be subsequently transferred via computer link to the linac console, and can be delivered without user intervention. The quality of the plan is determined by comparing the values of the objective function, dose-volume histograms (DVHs), and isodose distributions. RESULTS: Highly conformal dose distributions can be achieved with five intensity levels in each of seven beams. The merit of using more intensity levels or more beams is relatively small. Acceptable results are achievable even with three levels only. On average, the number of subfields per beam is about 2-2.5 times the number of intensity levels. The average treatment time per subfield is about 20 s. The total treatment time for the three-level and seven-beam case with a total of 39 subfields is 13 min. CONCLUSION: Optimizing stratified intensity distributions in the inverse planning process allows us to achieve close to optimum results with a surprisingly small number of intensity levels. This finding may help to facilitate and accelerate the delivery of intensity-modulated treatments with the "step and shoot" technique.

Brain Stem↗

PubMed1995

Calculation of dose, dose equivalent, and relative biological effectiveness for high charge and energy ion beams.

The Green's function for the transport of ions of high charge and energy is utilized with a nuclear fragmentation database to evaluate dose, dose equivalent, and RBE for C3H10T1/2 cell survival and neoplastic transformation as a function of depth in soft tissue. Such evaluations are useful to estimates of biological risk for high altitude aircraft, space operations, accelerator operations, and biomedical applications.

Aircraft↗

PubMed2003

Dose linearity and uniformity of a linear accelerator designed for implementation of multileaf collimation system-based intensity modulated radiation therapy.

The dose linearity and uniformity of a linear accelerator designed for multileaf collimation system-(MLC) based IMRT was studied as a part of commissioning and also in response to recently published data. The linear accelerator is equipped with a PRIMEVIEW, a graphical interface and a SIMTEC IM-MAXX, which is an enhanced autofield sequencer. The SIMTEC IM-MAXX sequencer permits the radiation beam to be " ON" continuously while delivering intensity modulated radiation therapy subfields at a defined gantry angle. The dose delivery is inhibited when the electron beam in the linear accelerator is forced out of phase with the microwave power while the MLC configures the field shape of a subfield. This beam switching mechanism reduces the overhead time and hence shortens the patient treatment time. The dose linearity, reproducibility, and uniformity were assessed for this type of dose delivery mechanism. The subfields with monitor units ranged from 1 MU to 100 MU were delivered using 6 MV and 23 MV photon beams. The doses were computed and converted to dose per monitor unit. The dose linearity was found to vary within 2% for both 6 MV and 23 MV photon beam using high dose rate setting (300 MU/min) except below 2 MU. The dose uniformity was assessed by delivering 4 subfields to a Kodak X-OMAT TL film using identical low monitor units. The optical density was converted to dose and found to show small variation within 3%. Our results indicate that this linear accelerator with SIMTEC IM-MAXX sequencer has better dose linearity, reproducibility, and uniformity than had been reported.

Particle Accelerators↗

PubMed1979

The vacuum cassette for improved linear accelerator portal films.

The use of a commercially available vacuum cassette with lead screens for linear accelerator portal films is described. The vacuum cassette technique provides improved quality with no increase in patient discomfort or cost and avoids gemoetric distortion, compared to other currently available methods.

Humans↗

PubMed2005

[Improvement of the first cerebral stereotactic radiosurgery system of Hungary].

OBJECTIVE: The first cerebral stereotactic radiosurgery system in Hungary was built in 1991. This system was based on a Leksell stereotactic head frame and a Neptun 10p linear accelerator. We performed 624 radiosurgery treatments with this system between 1991 and 2000. Our objective was to increase the reliability of operation and to extend the applicability of our radiosurgery system. METHODS AND MATERIALS: We modified our stereotactic floor stand with specially designed adapter plates to make it compatible with the Mevatron KD and Neptun 10p linear accelerators and other stereotactic head frames (Riechert-Mundiger, CRW and BrainLab). We made a new tertiary collimator holder attachable to the Mevatron KD linac. The range of treatable cerebral lesion was increased from 10-30 mm to 5-42.5 mm with additional collimator inserts. With the above modifications our radiosurgery system is compatible simultaneously with the Neptun 10p and the Mevatron KD linear accelerators. This way we were able to increase the reliability of operation of the system, as the treatment can be performed with the Neptun 10p linac in case of breakdown of the Mevatron KD linac after fixation of the head frame to a patient's skull. RESULTS: The measured diameter of the radiation isocenter defined by the new radiosurgery collimator was less than 1 mm with the Mevatron KD linac. According to the Lutz-test the distance between the radiosurgery isocenter and the rotation axis of ZIV treatment table was less than 0.5 mm. Results of phantom test showed that the overall spatial precision of our modified radiosurgery system was better than 1.3 mm with Leksell head frame. CONCLUSIONS: On the basis of experiences with 662 patients' radiosurgery treatments, the extension of our first cerebral radiosurgery system to Mevatron KD linear accelerator resulted in a more reliable operation. In accordance with our phantom tests the extension of the original system did not worsen its overall spatial precision.

Brain Neoplasms↗

PubMed1984

Radiation dosimetry of 12 MV photons from a CGR Therac 20 MeV Saturne linear accelerator.

Typically useful clinical radiation dosimetry characteristics of 12 MV photon beams from a CGR Therac 20 MeV Saturne linear accelerator are briefly outlined. Central axis percent depth dose data are compared with other published data. Beam profiles for small, medium and large fields are delineated to show the uniformity of beams at various depths.

Particle Accelerators↗

PubMed2001

Monte Carlo simulation of a clinical linear accelerator.

The effects of the physical parameters of an electron beam from a Siemens PRIMUS clinical linear accelerator (linac) on the dose distribution in water were investigated by Monte Carlo simulation. The EGS4 user code, OMEGA/BEAM, was used in this study. Various incident electron beams, for example, with different energies, spot sizes and distances from the point source, were simulated using the detailed linac head structure in the 6 MV photon mode. Approximately 10 million particles were collected in the scored plane, which was set under the reticle to form the so-called phase space file. The phase space file served as a source for simulating the dose distribution in water using DOSXYZ. Dose profiles at Dmax (1.5 cm) and PDD curves were calculated following simulating about 1 billion histories for dose profiles and 500 million histories for percent depth dose (PDD) curves in a 30 x 30 x 30 cm3 water phantom. The simulation results were compared with the data measured by a CEA film and an ion chamber. The results show that the dose profiles are influenced by the energy and the spot size, while PDD curves are primarily influenced by the energy of the incident beam. The effect of the distance from the point source on the dose profile is not significant and is recommended to be set at infinity. We also recommend adjusting the beam energy by using PDD curves and, then, adjusting the spot size by using the dose profile to maintain the consistency of the Monte Carlo results and measured data.

Computer Simulation↗

PubMed1990

A non-invasive method for fractionated stereotactic irradiation of brain tumors with linear accelerator.

A new technique for fractionated stereotactic irradiation of intracranial lesions is described. The treatment is based on a versatile, non-invasive interface for stereotactic localization of the brain target imaged by computed tomography (CT), angiography or magnetic resonance tomography (MRT), and subsequent repetitive stereotactic irradiation of the target using a linear accelerator. The fractionation of the stereotactic irradiation was intended to meet the requirements of the basic principles of radiobiology. The radiophysical evaluation using phantoms, and the clinical results in a small number of patients, demonstrated a good reproducibility between repeated positionings of the target in the isocenter of the accelerator, and a high degree of accuracy in the treatment of brain lesions.

Adult↗

PubMed2000

Reconstruction of megavoltage photon spectra from electronic portal imager derived transmission measurements.

Three variants of the Schiff equation are investigated to model the spectra produced by megavoltage linear accelerators. These models are tested against well-validated Monte Carlo (MC) generated spectra on the central axis of large-area fields, and show excellent agreement. Numerical reconstructions of 6 and 10 MV spectra using the same models are then presented, using experimental attenuation data derived from an electronic portal imager. The process of deriving spectra from experimental attenuation data is shown to be inherently badly constrained mathematically, with the derived spectrum being highly sensitive to noise in the source data, and non-unique. By placing a priori constraints on the Schiff model from both physical knowledge of the construction of the accelerator and MC data, physically useful results are gained and presented for both the energy dependence and off-axis behaviour of photon spectra.

Calibration↗

PubMed2001

Remote control for a stand-alone freely movable treatment couch with limitation system.

One of our linear accelerators is equipped with a free-movable treatment couch. An additional projects was to develop a system that first protects the free-movable couch against collisions, secondly build a remote control for moving the couch from outside the treatment room and finally implement this remote control/limitation system in an automatic position algorithm using an electronic portal image. The latter has been the subject of an on-going departmental investigation on intra-fractional correction of set-up errors. A few years ago, we developed a limitation system to protect both the table and the accelerator against collisions. In this paper we describe the second part of this project, the remote control system.

Algorithms↗

PubMedDate not supplied

Dosimetry of a dual photon energy linear accelerator.

We describe the dosimetric characteristics of a newly introduced dual photon energy linear accelerator, the Varian Clinac 1800. Depth doses are compared with other accelerators of the same nominal accelerating potentials (6 and 10 MV). Field flatness at dmax and at 10 cm depth, depth of dmax, wedge characteristics, output factors, and doses in the buildup region are presented.

Particle Accelerators↗

PubMed1998

Measurement of backscatter to the monitor chamber of medical accelerators using target charge.

A simple noninvasive method is described for determining the backscatter to a monitor chamber of a medical accelerator based on the measurement of charge deposited in the target. This method is compared quantitatively to the more elaborate telescopic method for photon beams of 6 MV and 15 MV on linear accelerators having mica and Kapton monitor chambers. The new target charge method gives results consistent with the telescopic method to within 0.3%.

Equipment Design↗