SEARCH · PubMed Health

Results for “Particle Accelerators”

Explore indexed PubMed citations for clinical trials, systematic reviews and public health research. Read source abstracts and follow each citation to its original PubMed record.

Quote a phrase for an exact phrase match. Source license links do not imply unrestricted reuse.

At least 1,585 records · Page 88Linked to original sources

PubMedDate not supplied

Stereotactic radiation therapy of intracranial lesions. Methodologic aspects.

A technique for stereotactic radiation therapy of cerebral tumours and arteriovenous malformations using a linear accelerator (6 MV photons) is proposed. Treatment relies on a fixation system that permits a precise use of the coordinates estimated at stereotactic angiography or stereotactic computed tomography. The field of treatment can be exactly outlined in the CT images during repeat examinations, thus facilitating the recognition of changes induced by radiation. The system also allows the extent of the arteriovenous malformation, as seen at angiography, to be accurately traced in the CT sections thus enabling evaluation of possible radiation damage to surrounding brain structures. The precision of the method as well as its hypothetical merits and disadvantages are discussed. The number of patients treated is still small and the follow-up time is too short in the majority of cases to allow definite conclusions. Examples of preliminary results are given.

Aged↗

PubMedDate not supplied

Dose distributions in dynamic stereotactic radiosurgery.

A treatment planning technique for calculation of dose distributions in dynamic stereotactic "radiosurgery" with a 10-MV isocentrically mounted linear accelerator is presented. The treatment planning for dynamic radiosurgery is a three-dimensional problem, since during treatment both the gantry and the couch rotate simultaneously, the gantry from 30 degrees to 330 degrees and the couch from 75 degrees to - 75 degrees. The patient surface and anatomical information is obtained from a family of computed tomography or magnetic resonance scans, and a stereotactic frame is used for target localization, treatment setup, and patient immobilization during the treatment. The dose calculational algorithm follows the gantry and couch rotation in an incremental fashion, and relies on measured stationary beam central axis percentage depth doses and dose profiles to calculate the normalized tissue-maximum-ratio distributions over a matrix of points defined on one of three orthogonal planes (transverse, sagittal, or coronal). The dose calculation algorithm is discussed in detail and calculated dose distributions for single plane and dynamic radiosurgery compared with measured data.

Brain Diseases↗

PubMedDate not supplied

Telescopic measurements of backscattered radiation from secondary collimator jaws to a beam monitor chamber using a pair of slits.

A contribution to field-size dependent output by backscattered radiation (BSR) from secondary collimator jaws to a beam monitor chamber of a linear accelerator was measured with a Farmer ionization chamber, positioned 200 cm from the source behind a low-melting-point alloy slab with a 10-cm wide slit. Another slit was positioned against the collimator jaws. Both slits were in the form of a 6.3-mm-diam hole in the middle and were aligned to the source. The use of a pair of slits was intended to eliminate any influence on the ion chamber readings due to field-size dependent charge contribution from the flattening filter and collimator jaw forward scattering. In addition, the setup permits to observe the degree of field-size dependence on BSR. Charge measurements from the Therac-20 18-MV x rays showed a 7.5% field-size dependence on BSR whereas 6- and 18-MV x rays from Varian Clinac-1800 showed less than 2% dependence on BSR. The telescopic method was found to be easy to use and permitted direct determination of BSR contributions.

Humans↗

PubMed2000

Electron beam modeling and commissioning for Monte Carlo treatment planning.

A hybrid approach for commissioning electron beam Monte Carlo treatment planning systems has been studied. The approach is based on the assumption that accelerators of the same type have very similar electron beam characteristics and the major difference comes from the on-site tuning of the electron incident energy at the exit window. For one type of accelerator, a reference machine can be selected and simulated with the Monte Carlo method. A multiple source model can be built on the full Monte Carlo simulation of the reference beam. When commissioning electron beams from other accelerators of the same type, the energy spectra in the source model are tuned to match the measured dose distributions. A Varian Clinac 2100C accelerator was chosen as the reference machine and a four-source beam model was established based on the Monte Carlo simulations. This simplified beam model can be used to generate Monte Carlo dose distributions accurately (within 2%/2 mm compared to those calculated with full phase space data) for electron beams from the reference machine with various nominal energies, applicator sizes, and SSDs. Three electron beams were commissioned by adjusting the energy spectra in the source model. The dose distributions calculated with the adjusted source model were compared with the dose distributions calculated using the phase space data for these beams. The agreement is within 1% in most of cases and 2% in all situations. This preliminary study has shown the capability of the commissioning approach for handling large variation in the electron incident energy. The possibility of making the approach more versatile is also discussed.

Algorithms↗

PubMed2007

Comparison of 4 MV photon surface dose among Varian, Siemens, and Elekta linear accelerators for tangential breast treatment: a phantom study.

PURPOSE: We have compared the differences in a 4-MV photon surface dose among Varian, Siemens, and Elekta linear accelerators (linacs) with wedges for tangential breast treatment. MATERIALS AND METHODS: The wedge factor and the surface dose were measured using a solid water phantom and an ion chamber for each linear accelerator with various field sizes and wedge angles. A tangential treatment plan was applied to an elliptical hollow cylinder water phantom with a radiochromic film placed thereon. A dose was delivered to a simulated target in the phantom, and the resulting dose distribution was analyzed using a film scanner. RESULTS: Varian's wedges resulted in the highest wedge factors, ranging from 0.37 to 0.75 depending on the wedge angles. Varian's wedges led to the highest normalized skin doses, ranging between 0.40 and 0.73 depending on the wedge angles and field sizes. In the cylinder phantom test with two tangential beams, the Varian linac provided a nearly 20% higher maximum dose than the Siemens and Elekta linacs. CONCLUSION: The Varian linac resulted in the highest surface doses, and the Elekta linac led to the lowest for nearly all the measurement conditions we employed, including open beams.

Breast Diseases↗

PubMed1995

A method for delivering accurate and uniform radiation dosages to the head and neck with asymmetric collimators and a single isocenter.

PURPOSE: To investigate the use of asymmetric collimators and a single isocenter for delivering a uniform, accurate dose of radiation to the head, neck, and supraclavicular lymph nodes. METHODS AND MATERIALS: A linear accelerator with a pair of asymmetric collimators is required for this technique. An isocenter was placed at the junction of the lateral head and neck fields and the anterior supraclavicular field. The asymmetric collimators were set longitudinally, by collimator rotation if necessary. The collimators split the radiation beam to all portals. Dose uniformity was measured at the junction with films in solid-water phantoms. RESULTS: Film dosimetry showed a uniform dose at the junction without hot or cold regions. A digital display tolerance of +/- 1.0 mm for a field size maintained an acceptable uniform dose (+/- 5% dose variation) at the junction. The single isocenter and asymmetric collimators reduced field setup time by half. No table rotation was required to match fields. CONCLUSION: The asymmetric collimators lead to easy and accurate patient setup. The absence of the trapezoid effect resulted in the complete coverage of the submandibular and cervical nodes without any hot spots.

Head and Neck Neoplasms↗

PubMed2005

Infrastructure of radiation oncology in France: a large survey of evolution of external beam radiotherapy practice.

PURPOSE: To study the structural characteristics of radiation oncology facilities for France and to examine how technological evolutions had to be taken into account in terms of accessibility and costs. This study was initiated by the three health care financing administrations that cover health care costs for the French population. The needs of the population in terms of the geographic distribution of the facilities were also investigated. The endpoint was to make proposals to enable an evolution of the practice of radiotherapy (RT) in France. METHODS AND MATERIALS: A survey designed by a multidisciplinary committee was distributed in all RT facilities to collect data on treatment machines, other equipment, personnel, new patients, and new treatments. Medical advisors ensured site visits in each facility. The data were validated at the regional level and aggregated at the national level for analysis. RESULTS: A total of 357 machines had been installed in 179 facilities: 270 linear accelerators and 87 cobalt units. The distribution of facilities and megavoltage units per million inhabitants over the country was good, although some disparities existed between areas. It appeared that most megavoltage units had not benefited from technological innovation, because 25% of the cobalt units and 57% of the linear accelerators were between 6 and 15 years old. Computed tomography access for treatment preparation was not sufficient, and complete data management systems were scarce (15% of facilities). Seven centers had no treatment planning system. Electronic portal imaging devices were available in 44.7% of RT centers and in vivo dosimetry in 35%. A lack of physicians and medical physicists was observed; consequently, the workload exceeded the normal standard recommended by the French White Book. Discrepancies were found between the number of patients treated per machine per year in each area (range, 244.5-604). Most treatments were delivered in smaller facilities (61.6%). CONCLUSION: On the basis of the findings of this study, measures were taken to update the infrastructure of RT in France. A first evaluation showed an improvement of care supply in RT in the country.

Cancer Care Facilities↗

PubMed2001

The use of the Australian Basic Treatment Equivalent (BTE) workload measure for linear accelerators in Canada.

The Inter Society of Radiation Oncologists of North America (ISRON) workload standard for linear accelerators is the one most widely used; it regards the treatment of 250 or more patients per year as an acceptable limit. Nevertheless, there is concern that this standard does not represent the current workload of linear accelerators, given that the complexity of techniques and equipment has increased significantly since the ISRON model was developed in the late 1980s. Delaney et al. recently validated a workload indicator for Australian (AUS) centres, known as the basic treatment equivalent (BTE). They showed that this was a better predictor of workload and that there was less variation between centres using this model than there would have been by using fields/hour. This centre attempted to validate this model for use in a Canadian centre, by collecting treatment data on all linear accelerator-treated patients during February 1998. The linear accelerators at this centre delivered 2,295 fractions (6,928 fields) in 662 hours during February 1998. When 15 minutes was used as a denominator, the BTE model functioned as a better workload indicator than simple measures such as fields/hour. It also had better performance in reducing variability between machines. A BTE of 3,403 was calculated for these machines. The mean value for fields/hour, BTE/hour and BTE/fraction for this centre fell within the range of values quoted by AUS centres. The BTE/fraction value for this centre was relatively high compared with the AUS mean, indicating this centre's reliance on the use of a high number of complex techniques. We recommend that the model should be further refined for the Canadian context by developing BTE values with the use of local time and motion studies, including factors such as multileaf collimators and enhanced dynamic wedges.

Australia↗

PubMed2005

The 12B counter: an active dosemeter for high-energy neutrons.

High-energy accelerators can produce strong time-structured radiation fields. Such dose shots are generated at linear machines with low duty cycles as well as at circular machines when complete fills are instantaneously lost. The main dose component behind thick shielding is due to high-energy neutrons occurring at that time structure. Dosemeters based on Geiger-Mueller tubes or proportional counters fail here completely. The 12B counter, a novel dosemeter made of a plastic scintillator using carbon activation for event-like exposure, has been introduced. High-energy neutrons activate the carbon nuclei by three inelastic reactions. The decay patterns with half-lives between 20 ms and 20 min can be exploited depending on the time structure of the radiation field. The response of the 12B counter was measured along with some other dosemeters, both active and passive, in the radiation field behind the lateral concrete shielding of a 7.5 GeV proton transfer line.

Boron↗

PubMed1993

Optimisation of dose distribution for linear accelerator-based stereotactic radiosurgery.

The work presented in the paper addresses a method for obtaining the optimal dose distribution for LINAC-based stereotactic radiosurgery. As many targets have nonspherical or irregular shapes and three-dimensional dose calculations included in dose optimisation, long computation times are required to determine the optimum isocentre separation and collimator sizes to shape the irregular target using the multiple-isocentre approach, by trial-and-error types of method. The simple approach, using a computer-aided design optimisation technique and a fast approximate dose model, has been developed to find the optimum isocentre positions and collimator sizes quickly and automatically. A spherical dose model has been developed to represent the dose for a standard arc system with a single isocentre. The implementation of computer-aided design algorithms with the spherical dose model and their application to several cases are discussed. It is shown that the spherical dose model gives dose distribution similar to that of the exact dose model, which makes this simple dose model more efficient, with computer-aided design optimisation, in finding optimum isocentre positions and collimator sizes used in stereotactic radiosurgery.

Humans↗

PubMed1985

The influence of field size, treatment modality, commissure involvement and histology in the treatment of early vocal cord cancer with irradiation.

One hundred and thirty-seven patients with TNM staged T1 glottic cancer were treated from 1966 to 1980. The patient characteristics were no different from those noted in previous studies. Patients were treated either with 60Co teletherapy units or 4 MeV linear accelerators with different daily (180-225) and weekly (4 times vs. 5 times) fractionation schemes. A recurrent-free survival was attained in 80% of the patients 2 years after primary treatment with radiation therapy. There were 27 local recurrences, of which 82% were salvaged with surgery for an overall adjusted survival of 95%. Patients treated on 60Co units with field sizes less than or equal to 30 sq. cm had a 15% increase in local recurrence compared with field size greater than 30 sq. cm, which could not be attributed to lower doses. Poorly differentiated tumors and those involving the anterior commissure were more likely to develop a local recurrence. Doses above 1900 ret were associated with a decrease in local recurrence in patients having anterior commissure involvement. Salvage with limited surgery for failure was effective in selected cases.

Adult↗

PubMed2001

Characteristics of bremsstrahlung in electron beams.

Clinical electron beams contain an admixture of bremsstrahlung produced in structures in the accelerator head, in field-defining cerrobend or lead cutouts, and in the irradiated patient or water phantom. Accurate knowledge of these components is important for dose calculations and treatment planning. In this study, the bremsstrahlung components are separated for electron beams (energy 6-22 MeV, diameter 0-5 cm) using measurements in water and calculations. The results show that bremsstrahlung from the accelerator head dominates and increases with field size for electron beams generated by accelerators equipped with scattering foils. The bremsstrahlung from the field-defining cerrobend accounts for 10% to 30% of the total bremsstrahlung and decreases with increasing beam radius. The bremsstrahlung is softer than the x-ray beams of corresponding nominal energy since the latter are hardened by the flattening filter. For the 6, 12, and 22 MeV electron beams, the effective attenuation coefficients in water for the bremsstrahlung are 0.058, 0.050, and 0.043 cm(-1). The depths of maximum dose at 100 cm SSD are 0.8, 1.7, and 3.0 cm. The position of the virtual source of the bremsstrahlung shifts downstream from the nominal source position by 20, 13, 5.6 cm, respectively. The lateral bremsstrahlung dose distribution is more forward-peaked for higher electron energy. The bremsstrahlung components could be described for any machine by a set of simple measurements and can be modeled by an analytical expression.

Dose-Response Relationship, Radiation↗

PubMed2002

[New method of verification of the position of target points in stereotactic radiosurgery by linear accelerator].

A method for quantitatively checking the position to be used for stereotactic radiosurgery was devised using a digital reconstruction radiogram (DRR) and electric portal images (EPI). On each image, the radiation field and head contour were extracted by binary-coded processing. The center of gravity position vector of the extracted image was calculated, and the vector from the head contour to the radiation field was obtained. By comparing the difference in DRR and EPI, an index was made of positional error. The error in the center of gravity position coordinate in changing 40~80 of 256 image tones in the binary operation was 0.5mm or less. The effect on the center of gravity position vector caused by the image distortion of EPI was within 0.5mm in the 200x200mm(2) region. Statistical processing was carried out on this index value, and a 95% confidence interval was estimated. The index value of the Z component of the lateral image became -1.43+/-0.66, and it shifted to the negative side. Error was evaluated by the verification method devised for the target point position. Results indicate the usefulness of the verification method using center of gravity for the target point position.

Humans↗

PubMed2003

Dynamic conical conformal radiotherapy using a C-arm-mounted accelerator: dose distribution and clinical application.

PURPOSE: The aim of this study was to solve anisotropy in the dose distributions from rotational conformal radiotherapy (RCRT) by using a C-arm-mounted accelerator. MATERIALS AND METHODS: The linac head was designed to move along the C-arm with a maximum angle of 60 degrees (from a vertical position toward the gantry). Simultaneous rotation of the gantry creates a dynamic conical irradiation technique. Dynamic conical conformal radiation therapy (Dyconic CRT) was developed by combining the technique with continuous motion of a multileaf collimator. Dose distributions were measured in phantoms using film densitometry and compared with conventional RCRT. Dose distributions in actual radiation therapy patients are also presented. RESULTS: Dyconic CRT enabled the precise delivery of noncoplanar beams without rotating the table. The measurements showed that three-dimensionally isotropic dose falloff was achieved with Dyconic CRT. Dose inhomogeneity in the sagittal direction with Dyconic CRT was compensated for by use of wedge filters. CONCLUSIONS: The drawbacks of the dose distributions produced by RCRT were overcome with the use of Dyconic CRT.

Dose-Response Relationship, Radiation↗

PubMed1998

Design considerations for the sides of multileaf collimator leaves.

The use of multileaf collimators (MLCs) in radiation therapy is becoming increasingly widespread as more commercial vendors offer them as options on medical linear accelerators. In most of the commercially available multileaf collimators, the sides of the leaves have a 'tongue and groove' (TG) feature, which is necessary for blocking the straight through leakage when the leaves are staggered together. The dosimetric effects of the TG design have not been fully studied for applications in block replacement or in dynamic intensity modulation. This article analytically explains the TG effect by calculating a deficit in photon flux when the side of a leaf is exposed. It is shown that the maximum photon deficit occurs when the leaf thickness is shared equally by the tongue and groove of adjacent leaves. New designs are proposed based on the theoretical analysis and improvements of the new design in leakage and the TG effect are verified with Monte-Carlo simulations. Measurements of the TG effect on an MLC from one manufacturer are also conducted. The results of the measurements confirm the analytical explanations of the TG effect.

Computer Simulation↗

PubMedDate not supplied

Application of Fermi scattering theory to a magnetically scanned electron linear accelerator.

This paper uses a solution to the Fermi electron transport equation for an isotropic point source to characterize the magnetically scanned broad electron beams from the Sagittaire Therac 40 accelerator in the air space above patients. Thick lead collimation is shown to be adequately modeled by an infinitely thin absorbing plate when used to predict penumbra shape. A relationship between broad beam penumbra width and the value of the root-mean-square spatial Gaussian spread sigma (z) of an elementary pencil beam is derived. This relationship is applicable for any rectangular field size. Measurement of the variation in broad beam penumbra width with source-surface distance (SSD) for a 7-MeV beam locates the isotropic source to be coincident with the exit window of the accelerator and indicates that the scattering effect of the monitor chamber may be considered negligibly small. Using this source location accurate predictions of beam profile shape for any clinically used beam energy, SSD, or field size are made in the presence of lead trimmer collimation. Field penumbra beyond the photon collimation system is formed in each lateral direction by two lead blocks whose faces are aligned along a diverging ray emanating from the source. The photon collimator closest to the source restricts the field size causing a variation of both fluence and the mean square angle spread of the electrons across the plane at the level of the lower collimator. This variation is accounted for by introducing an empirical perturbation factor into the mathematical formalism. An interesting feature of this perturbation factor is that it is field size dependent and its effect on penumbra width may be scaled for both beam energy and SSD to accurately predict beam profile shape.

Humans↗

PubMed1998

Lung cancer: intermittent irradiation synchronized with respiratory motion--results of a pilot study.

PURPOSE: To test the feasibility of a system for intermittent irradiation synchronized with respiratory motion in a clinical setting. MATERIALS AND METHODS: A newly developed gate pulse controller that starts and stops irradiation at a chosen phase of the respiratory cycle by controlling a linear accelerator was used in six patients with lung cancer. A laser displacement sensor was used for the detection of respiratory motion. Three patients underwent radiation therapy during the cycles between 50% expiration and 50% inspiration (step 1), and three patients underwent radiation therapy during the cycles between 70% expiration and 30% inspiration (step 2). RESULTS: The system functioned well; irradiation was verified with portal verification radiography in all six patients. The range of the tumor position during synchronized irradiation was detectable with fast portal localization radiography. The treatment times for steps 1 and 2 were 1.38-1.71 and 2.03-2.18 times longer, respectively, than those for conventional irradiation. CONCLUSION: Synchronized irradiation with the authors' system allowed convenient and reliable reduction of the target volume. Further study is needed to standardize the system for clinical use.

Clinical Protocols↗

PubMed1998

[Stereotactic conforming irradiation of choroid metastases].

UNLABELLED: The aim of our study was to develop noninvasive stereotactic radiotherapy for patients with choroidal metastases. METHODS: The head of the patient was immobilized by an individual cast mask. The target volume and adjacent critical structures were three-dimensionally segmented based on ophthalmological findings and MRI. The beam angles were optimized by a beam's eye view technique with a micro-multileaf collimator. We use a linear accelerator with 6-MeV photons. A patient was treated in a phase I/II trial with a single dose of 22.5 Gy. RESULTS: The accuracy of stereotactic positioning of the eye was 0.8 mm. The dose gradients were 15% mm. It was possible to spare the lens and the lacrimal gland in all cases. The treatment with four to six fields took 35-50 min and was tolerated without acute complications. CONCLUSIONS: We introduce a new method to treat choroid metastases by stereotactic radiotherapy. This method allows to concentrate the dose to the target and to spare adjacent critical structures. One of the advantages of this noninvasive treatment is the short treatment time. The clinical role of this method has to be evaluated based on long-term results of tumor control and visual outcome in these patients.

Adenocarcinoma↗