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At least 1,081 records · Page 60Linked to original sources

PubMed1998

Production of radioisotopes by direct electron activation.

High-energy electrons bombarded on materials can induce radioactivity by either directly knocking out neutrons or by first converting a fraction of the electron kinetic energy into electromagnetic energy, with subsequent neutron emission induced by the photons produced. The purpose of this paper was to develop a calculation method for estimating neutron emission and radionuclide production by high-energy (15-25 MeV) electrons directly interacting with a nucleus. The reaction (e,n) is considered using the method of virtual photons. The cross section for electron bombardment of lead, tantalum, rhenium, and tungsten targets is calculated. The electron cross sections are roughly 100 times less than the corresponding photon cross sections. The cross section increases monotonically with incident energy. A traveling wave linear accelerator was used for a qualitative test of the magnitude and energy dependence of the calculated cross sections. Tantalum was bombarded with electrons and the resultant emission of neutrons was inferred from the induced activation of 180Ta. The energy dependence and magnitude of the calculated electron cross sections agree with experiment within experimental uncertainties. It is concluded that accurate estimates of electron activation via the direct process is possible.

Electrons↗

PubMed1978

The validity of the approximation method "area/perimeter" as applied to 8 and 25-MV photon beams.

The approximation method of area/perimeter for different portal sizes used in practice for finding the equivalent squares of a rectangular field has been tested on a 35-MV Clinac Linear Accelerator for the purpose of treatment planning. Data were obtained for 8 and 25-MV x-rays at depths of maximum build-up regions, and at 10 cm depth below the surface of a suitable water phantom. Maximum variation of dose calculation from the above method is less than 1.6% for routine clinical usage while less than 3.2% for extreme rectangular field sizes. Dose variation improves as the depth in phantom increases.

Humans↗

PubMed2002

Chromosome instability of HPRT-mutant subclones induced by ionising radiation of various LET.

The induction of HPRT-mutations and survival of Chinese hamster cells (line B11ii-FAF28, clone 431) were studied after irradiation by 4He and 12C-ions of various LET (20-360 keV/micrometers), produced by the U-200 heavy ion accelerator. The RBE increases with LET up to the maximum at 100-200 keV/micrometers and then decreases. Cytogenetic analysis was performed on the HPRT-mutant subclones selected from unirradiated Chinese hamster V-79 cells and from HPRT-mutant subclones that arose after exposure to gamma-rays, 1 GeV protons and 14N-ions (LET-77 keV/micrometers), produced by the synchrophasotron and the U-400M heavy ion accelerator. Slow growing mutant subclones were observed. The cytogenetic properties of individual clones were highly heterogeneous and chromosome instability was observed in both spontaneous and radiation-induced mutants. Chromosome instability was highest among spontaneous mutants and decreased with increasing LET.

Animals↗

PubMed2001

A two-source model for electron beams: calculation of relative output factors.

A two-source model for the calculation of relative output factors (ROF) for clinical applications of electron beams has been developed. The model consists of (1) an effective extended source above the final field-defining aperture (cutout) plane and (2) a source due to scattering from the aperture. Calculations are based on Fermi-Eyges theory and a pencil beam algorithm with parameters determined independently for each major scattering component. The model predicts a modified inverse square law for determining the dose rate for the electron beams. It also generalizes the "square-root method" and "one-dimensional method" that are often used clinically for ROF calculations. A computer program based on the model has been developed to calculate ROF for irregular fields. The predictions of ROF values have been compared with measurements on a Varian CLINAC 2100C/D accelerator for different cutout size, energies, applicators, and SSDs for square fields, rectangular fields, circular fields, and irregular fields. The agreement between prediction and measurement of the ROF for these wide range of conditions is generally within 1% for energies from 6 to 20 MeV. This two-source model can be used for clinical applications and it requires a minimal set of measured input data.

Algorithms↗

PubMedDate not supplied

Characteristics of electron beams from a medical microtron.

The Instrument AB Scanditronix MM22 medical microtron provides ten electron beam energies from approximately 3 to 22 MeV. Isodose curves, depth dose curves, field uniformity, and other characteristics were measured in water and in polystyrene. The method of acceleration, dual scattering foil system, and collimation technique produce beams having features superior to many other medical electron accelerators. Maximum dose rates at isocenter varied from about 500 to over 900 cGy min-1, photon contamination from 0.6% to 4.1%, and surface doses from 70% to 95% of the maximum. Depth dose curves were indistinguishable from those with identical practical ranges of a scanned beam linac at energies less than 18 MeV, and field flatness was clearly superior to the scanned beam linac at standard treatment distances.

Electrons↗

PubMedDate not supplied

Measurements in high-intensity beams from medical linear accelerators.

There is a potential to produce very high doses and dose rates on dual mode (electron and x-ray capability) medical electron accelerators [C. J. Karzmark, Br. J. Radiol. 40, 697 (1967)]. Measurements were taken on two accelerators to determine the size of the doses which were possible. The experiments demonstrate the limitations of many of the commonly used dosimeters when they are used outside of the dose and dose rate ranges normally encountered in a radiotherapy department. Dose rates from 110 to 165 cGy/pulse were measured using ionization chamber, ferrous sulphate and thermoluminescent dosimetry, and water calorimetry.

Calorimetry↗

PubMed1997

Calculating output factors for photon beam radiotherapy using a convolution/superposition method based on a dual source photon beam model.

A realistic photon beam model based on Monte Carlo simulation of clinical linear accelerators was implemented in a convolution/superposition dose calculation algorithm. A primary and an extra-focal sources were used in this beam model to represent the direct photons from the target and the scattered photons from other head structures, respectively. The effect of the finite size of the extra-focal source was modeled by a convolution of the source fluence distribution with the collimator aperture function. Relative photon output in air (Sc) and in phantom (Scp) were computed using the convolution method with this new photon beam model. Our results showed that in a 10 MV photon beam, the Sc, Sp (phantom scatter factor), and Scp factors increased by 11%, 10%, and 22%, respectively, as the field size changed from 3 x 3 cm2 to 40 x 40 cm2. The variation of the Sc factor was contributed mostly by an increase of the extra-focal radiation with field size. The radiation backscattered into the monitor chamber inside the accelerator head affected the Sc by about 2% in the same field range. The output factors in elongated fields, asymmetric fields, and blocked fields were also investigated in this study. Our results showed that if the effect of the backscattered radiation was taken into account, output factors in these treatment fields can be predicted accurately by our convolution algorithm using the dual source photon beam model.

Computer Simulation↗

PubMed1999

115In as a probe for the characterization of therapy bremsstrahlung beams and the detection of photoneutrons.

Variation of the photoactivation rate across radiation fields of three different bremsstrahlung beams of two medical accelerators has been measured, making use of the photonuclear reactions in natural indium probes: 115In(y,y')115mIn and 115In(y,n)114mIn. The third nuclear reaction, 115In(n,y)116mIn, was used to detect the presence of neutrons in the photon beam and to estimate the spatial distribution of thermal and fast neutrons in the patient plane as a function of collimator opening.

Calibration↗

PubMed1999

Boron neutron capture therapy (BNCT): implications of neutron beam and boron compound characteristics.

The potential efficacy of boron neutron capture therapy (BNCT) for malignant glioma is a significant function of epithermal-neutron beam biophysical characteristics as well as boron compound biodistribution characteristics. Monte Carlo analyses were performed to evaluate the relative significance of these factors on theoretical tumor control using a standard model. The existing, well-characterized epithermal-neutron sources at the Brookhaven Medical Research Reactor (BMRR), the Petten High Flux Reactor (HFR), and the Finnish Research Reactor (FiR-1) were compared. Results for a realistic accelerator design by the E. O. Lawrence Berkeley National Laboratory (LBL) are also compared. Also the characteristics of the compound p-Boronophenylaline Fructose (BPA-F) and a hypothetical next-generation compound were used in a comparison of the BMRR and a hypothetical improved reactor. All components of dose induced by an external epithermal-neutron beam fall off quite rapidly with depth in tissue. Delivery of dose to greater depths is limited by the healthy-tissue tolerance and a reduction in the hydrogen-recoil and incident gamma dose allow for longer irradiation and greater dose at a depth. Dose at depth can also be increased with a beam that has higher neutron energy (without too high a recoil dose) and a more forward peaked angular distribution. Of the existing facilities, the FiR-1 beam has the better quality (lower hydrogen-recoil and incident gamma dose) and a penetrating neutron spectrum and was found to deliver a higher value of Tumor Control Probability (TCP) than other existing beams at shallow depth. The greater forwardness and penetration of the HFR the FiR-1 at greater depths. The hypothetical reactor and accelerator beams outperform at both shallow and greater depths. In all cases, the hypothetical compound provides a significant improvement in efficacy but it is shown that the full benefit of improved compound is not realized until the neutron beam is fully optimized.

Biophysics↗

PubMed1993

Implementation of stereotactic focal radiotherapy for intracranial arteriovenous malformations using a linear accelerator.

A system of stereotactic focal radiotherapy using a linear accelerator has been developed in cooperation with a neurosurgeon. The treatment is delivered using a carefully calibrated 10 MV machine and the Cosman-Roberts-Wells (CRW) system. The precision of the method as well as its quality assurance is described. Eight patients with intracranial arteriovenous malformations (AVM) received irradiation from August 1990 to November 1991. The prescribed dose at the periphery of the AVM was 8 Gy per session, with six patients receiving two sessions and two patients receiving one session. The field size, encompassing the 90% isodose, ranged from 20 mm to 35 mm. In four patients, follow-up angiography was performed one year after the full course of therapy; total obliteration of the AVM was noted in three (75%) with a partial response in the other. In the other four patients, follow-up angiography was not performed; one patient, who had only one session of irradiation, experienced rebleeding six months later and died, and the other three patients had no further episodes of bleeding during their follow-up of 28, 18 and 14 months, respectively. Linear accelerator-based stereotactic focal radiotherapy can attain a precisely defined and reproducible dose distribution. The effects of this treatment may take one to two years to develop. Our preliminary study suggests that it is an effective alternative treatment for surgically inaccessible lesions. Patients with a small cavernous sinus dural AVM appear to have a better and more rapid response.

Adult↗

PubMed1991

[Mounting devices for screening blocks on the linear accelerator--the drawbacks and modifications].

Block mounting devices for linacs--drawbacks and modifications: Shielding blocks are often used in radiotherapy. They are fixed on the linac's head by means of standard accessory mounting devices. The disadvantages of these parts are discussed and two modifications are presented: One device with adjustable focus-support distance for standard lead blocks and another with gliding planes to insert easily the heavy individually formed shielding blocks. Errors in mounting of all these devices are very dangerous, so we developed a simple locking lever providing a high security.

Equipment Design↗

PubMed1994

Optimization of irradiation conditions for photon activation analysis of biological and environmental samples using a high power electron accelerator.

Using the bremsstrahlung flux density data of Tanaka et al. (1983), the PHOTAC code has been developed to calculate the activity induced in a sample during photon activation as a function of the geometrical arrangement of the electron energy and the beam profile. Some results are given for sample volumes from 1 to 1000 cm3 and sample distances from 1 to 10 cm at 25 MeV. Examples for the application of the PHOTAC code to estimate the maximum activity output and to minimize the heating up of the sample during irradiation are discussed.

Activation Analysis↗

PubMed1990

Monte Carlo and convolution dosimetry for stereotactic radiosurgery.

The dosimetry of small photon beams used for stereotactic radiosurgery was investigated using Monte Carlo simulation, convolution calculations, and measurements. A Monte Carlo code was used to simulate radiation transport through a linear accelerator to produce and score energy spectrum and angular distribution of 6 MV bremsstrahlung photons exiting from the accelerator treatment head. These photons were then transported through a stereotactic collimator system and into a water phantom placed at isocenter. The energy spectrum was also used as input for the convolution method of photon dose calculation. Monte Carlo and convolution results were compared with the measured data obtained using an ionization chamber, a diode, and film.

Brain Neoplasms↗

PubMed1980

Step-rotation therapy: a technique for linear accelerators.

Rotation therapy has been accepted as a standard method of treatment for many years, but is use has been confined almost entirely to telecobalt therapy. This has been due to the difficulty in maintaining a constant output, and therefore a constant dose per degree of gantry rotation, when using a linear accelerator. This paper presents a new form of step-rotation therapy avoids this difficulty and is suitable for use with any modern linac to which remote gantry drive has been added. Apart from the advantages of smaller penumbra and shorter treating time on linear accelerator, in this technique both computation and delivery of dose is done in steps, and therefore the computed dose distribution matches the actual expected distribution. A further advantage is the ability to change the weighting at each step, which gives more control over the final distribution. Other problems which have been inherent in telecobalt rotation therapy are also avoided, such as matching treatment time and arcing time, and correcting for the discrepancies resulting from mismatch.

Particle Accelerators↗

PubMed1998

Quality assurance of the dose delivered by small radiation segments.

The use of intensity modulation with multiple static fields has been suggested by many authors as a way to achieve highly conformal fields in radiotherapy. However, quality assurance of linear accelerators is generally done only for beam segments of 100 MU or higher, and by measuring beam profiles once the beam has stabilized. We propose a set of measurements to check the stability of dose delivery in small segments, and present measured data from three radiotherapy centres. The dose delivered per monitor unit, MU, was measured for various numbers of MU segments. The field flatness and symmetry were measured using either photographic films that are subsequently scanned by a densitometer, or by using a diode array. We performed the set of measurements at the three radiotherapy centres on a set of five different Philips SL accelerators with energies of 6 MV, 8 MV, 10 MV and 18 MV. The dose per monitor unit over the range of 1 to 100 MU was found to be accurate to within +/-5% of the nominal dose per monitor unit as defined for the delivery of 100 MU for all the energies. For four out of the five accelerators the dose per monitor unit over the same range was even found to be accurate to within +/-2%. The flatness and symmetry were in some cases found to be larger for small segments by a maximum of 9% of the flatness/symmetry for large segments. The result of this study provides the dosimetric evidence that the delivery of small segment doses as top-up fields for beam intensity modulation is feasible. However, it should be stressed that linear accelerators have different characteristics for the delivery of small segments, hence this type of measurement should be performed for each machine before the delivery of small dose segments is approved. In some cases it may be advisable to use a low pulse repetition frequency (PRF) to obtain more accurate dose delivery of small segments.

Biophysical Phenomena↗

PubMed1999

Ionization chamber, electrometer, linear accelerator, field size, and energy dependence of the polarity effect in electron dosimetry.

Plane-parallel ionization chambers are the instrument of choice for use in electron calibration and dosimetry, but these chambers may exhibit large polarity effects. This study concentrates on measuring the dependence of the polarity error at various mean energies using different linear accelerator, field size, ion chamber, and electrometer combinations. The polarity error was shown to increase for all four ionization chambers as the mean energy at depth decreased, but was always less than one percent at d(max). Polarity error dependence was also observed for similar plane-parallel chambers, varying field sizes, and different linear accelerators, but no polarity error dependence was observed for similar cylindrical chambers and different electrometers. Measurements of the polarity error can be used to develop correction factors for future measurements that will help minimize the time spent performing electron dosimetry and calibrations. These correction factors can be used to calculate the correct reading without the need to reverse the chamber bias, thus reducing the number of measurements required.

Calibration↗

PubMed1990

[Hyperfractionation--a new challenge for medical electron linear accelerators?].

In radiotherapy to an increasingly degree radiobiological aspects of the tumor behaviour during therapy are realized in daily routine. An example for this is the so called hyperfractionation of the total dose: that means multiple small fractions are applied. Doses of as low as circa 40 cGy per field are necessary. This investigation affects the constancy and the behaviour of the following beam parameters of various treatment units during the initial stage of operation (circa ten seconds): energy of bremsstrahlung, symmetry, flatness and proportionality of dose.

Electrons↗

PubMed1982

[New investigations about the activation of air by the 42-Mev continuous radiation of a betatron (author's transl)].

Measurements of the activity of urea are a simple and reproducible method to determine the activation of air produced by the continuous radiation of a medically used accelerator [3,4]. The authors studied the activity produced by photonuclear induction using a 42 MeV betatron under unfavorable irradiation conditions. The activity measured per individual irradiation is 1.56 x 10(5) Bq 13N and 4.93 x 10(5) Bq 15O. The limit values indicated in the Radioprotection Regulation are not reached if an intensive air exchange and a waiting-time between the end of the irradiation and the entering of the irradiation room is guaranteed.

Air↗