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At least 397 records · Page 22Linked to original sources
PubMedDate not supplied

Off-axis beam quality change in linear accelerator x-ray beams.

The effective energy of the x-ray beam from linear accelerators changes as a function of the position in the beam due to nonuniform filtration by the flattening filter. In this work, the transmittance through a water column was measured in good geometry and the beam quality characterized in units of HVL in water. Measurements were made on a variety of linear accelerators from 4 to 10 MV. The beam energy decreased with increasing distance from the central ray for all accelerators measured.

Particle Accelerators↗
PubMedDate not supplied

Electron beam simulation applicators.

A system for simulating electron beam treatment portals using low-temperature melting point alloy is described. Special frames having the same physical dimensions as the electron beam applicators used on the Varian Clinac 20 linear accelerator were designed and constructed.

Electrons↗
PubMedDate not supplied

Determination of the source position for the electron beams from a high-energy linear accelerator.

We have investigated the energy and field-size dependence of the source position of the electron beams from a Varian Clinac-2,500 accelerator. Three independent experimental methods were used: (1) multipinhole camera (MPC), (2) back projection of the full width at half maximum (FWHM), and (3) the inverse square law (ISL). The positions of the virtual and effective sources were calculated using the multiple Coulomb scattering (MCS) formalism. The results obtained from the MPC agree, within the experimental uncertainties, with the calculated values for the virtual source position. Similarly, the results from the FWHM method agree with the calculations with the exception of those for small field sizes at the lower energies. This is consistent with the fact that both kinds of measurements are not very sensitive to scattering in the photon and electron collimators. In contrast, the source position determined by the ISL method shows strong dependence on field size and energy, and does not agree with the values predicted by the MCS formalism. This is due to contamination from electrons scattered in the x ray and electron collimation system. The techniques and results reported here should be generally applicable to other scatter foil linear accelerators.

Electrons↗
PubMed2001

Cell inactivation by beryllium, boron and carbon ions at the low-energy irradiation facility of the Naples University.

The 3MV HVEC TTT-3 Tandem accelerator at the University of Naples, already used for radiobiological studies with protons and alpha particles, was set up for irradiation of biological samples with low energy carbon, boron, and beryllium beams. Radiobiological characterisation and study of these ion beams is essential in hadrontherapy (correction of hadrotherapy) to understand, for example, the possible biological effect of the target fragmentation products. Furthermore in space radiation biology we need to know the biological effects of heavy ions, a component of cosmic radiation that can contribute to the radiobiological risk when long sojourns in space are concerned. V79 Chinese hamster cells were irradiated with the different ions and the resulting cell inactivation data are reported.

Animals↗
PubMed1988

[Radioactivity in dismantling a betatron].

The authors report on the specific radioactivity of construction components of a 42 MeV betatron measured 28 days after the last run by means of a Ge spectrometer (ultra-pure germanium detector employed in a semiconductor spectroscopic method) recording the gamma spectra of various components of the betatron. This examination served to find out which components would have to be considered as radioactive waste requiring special treatment in accordance with Federal German legislation on radioactive waste disposal (section 47 of the Radiation Protection Ordinance). The examination showed that such special treatment would be required only for the target (main material: platinum) and the Platness-filter (chief constituent: lead) of the decommissioned electron accelerator.

Humans↗
PubMed1986

Quality assurance procedures in radiotherapy--IEC specifications for equipment.

The International Electrotechnical Commission (IEC) worked out international standards for requirements and tests of electrical, mechanical and radiation safety as well as for definition and tests of functional performance characteristics of radiotherapy equipments (medical electron accelerators, gamma beam teletherapy and afterloading equipments, simulators and accessories) and for clinical dosimeters and terminology for medical radiology. A survey is given on the actual state of standardization projects. The problems of such standards are shown for the standard for functional performance characteristics of medical electron accelerators as example.

Humans↗
PubMed1999

Size analysis of a pressurized metered dose inhaler-delivered suspension formulation by the API Aerosizer time-of-flight aerodynamic particle size analyzer.

Aerosizer time-of-flight (TOF) aerodynamic particle size analyzers (TSI-Amherst, Amherst, MA) are widely used for the rapid assessment of aerosols from a wide variety of drug delivery devices, including pressurized metered dose inhalers (pMDIs). This technique offers significant advantages in terms of rapid measurement times in comparison with the more time-consuming compendial methods such as the cascade impactor or multistage liquid impinger. Particle size analysis takes place by determining the TOF of individual particles following acceleration to supersonic velocity. No drug assay is performed; thus, the resulting size distribution also includes particles that do not contain any medication such as the excipients and surfactant that are present in most pMDI-based formulations. Illustrative data are presented for one particular formulation (Pulmicort: 200 micrograms of budesonide per dose; Astra Draco; Lund, Sweden) and demonstrate that bias from this source can significantly shift the reported particle distribution to finer sizes compared with impactor-based analysis in which direct assay for drug has taken place. In this case, the mass median aerodynamic diameter (MMAD) determined by an Aerosizer-LD was close to 2.4 microns, but was found to be approximately 4 microns using the cascade impactor-based procedure. Such a shift results in an overestimation of the fine particle fraction of the emitted dose, which may lead to misleading conclusions about the therapeutic benefit of a particular drug delivery system when making use of this formulation. TOF aerosol measurement techniques appear to be vulnerable to this type of bias for any suspension formulation in which the drug content is not homogeneously distributed within all particle sizes.

Bronchodilator Agents↗
PubMed2000

Dynamics of inelastic collapse.

We consider a particle randomly accelerated by Gaussian white noise on the half-line x>0. The collisions of the particle with the wall at x=0 are inelastic. The velocities just before and after reflection are related by v(f)=-rv(i), where r is the coefficient of restitution. Cornell, Swift, and Bray have shown that for r<r(c)=e(-pi/sqrt[3])=0.163, there is inelastic collapse, i.e., boundary collisions localize the particle at x=0. The probability that the particle is not yet absorbed at the boundary after a time t decays, for long times, as t(-straight theta(r)). The exponent straight theta(r) is calculated exactly. We also consider the case of "partial-survival" boundary conditions, i.e., elastic reflection with probability p and absorption with probability 1-p, and derive the analogous persistence exponent straight phi(p). The exact exponents satisfy the Swift-Bray conjecture straight theta(r)=straight phi(r(2straight theta(r))).

Journal Article↗
PubMed2002

Incorporation of realistic delivery limitations into dynamic MLC treatment delivery.

The clinical implementation of IMRT involves the use of a number of complex software-based systems, typically including an inverse planning system, a leaf sequencer, and a computer-controlled treatment delivery system. The inverse planning system determines the desired fluence patterns, the leaf sequencer translates those fluence maps into leaf trajectories, and the control system delivers those trajectories. While verification of intensity-modulated treatment fields has focused primarily on the dosimetric aspects of delivery, accurate delivery of the intended fluence distribution is dependent upon both the leaf sequencer and delivery control systems. Leaf sequencing algorithms typically do not incorporate many control system limitations, and this can lead to discrepancies between planned and delivered sequences. In this work, simple and complex fields were sequenced for the dynamic sliding window technique using different leaf speeds and tolerance settings to identify various limitations of the accelerator control system. This work was conducted on a Varian 2100 EX equipped with a Millennium 120 leaf MLC. The identified limitations were then incorporated into the sequencing algorithm using a limiting leaf velocity (less than the maximum leaf velocity), the leaf position tolerance, and the communications delay in the control system. Collision avoidance in leaf pairs was found to depend on a control system-enforced minimum gap between leaves and led to acceleration effects. By incorporating these effects into the leaf sequencing algorithm, dynamic sliding-window leaf sequences were produced which did not require beam interruptions or dose rate modulations for the parameter values used in calculating the sequence (dose rate, tolerance, leaf speed, and total monitor units). Incorporation of control system limitations into the leaf sequencing algorithm results in IMRT fields that are delivered with the prescribed constant dose rate, require less time to deliver, and have well-defined, calculable transmission dose characteristics.

Acceleration↗