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PubMedDate not supplied

Analysis of the bremsstrahlung component in 6-18 MeV electron beams.

The magnitude of the bremsstrahlung component in the 6-18 MeV electron beams from a Varian Clinac 18 accelerator was measured directly using a 0.16 T magnetic field to deflect the electron beams. The central axis depth ionization curves were measured at 130- and 197-cm source-surface distances in a Therados RFA-3 water phantom with and without the magnet being placed between the 4 X 4 cm2 electron cone and the water phantom. Results obtained clearly demonstrate that the bremsstrahlung component is primarily (greater than 90%) generated in the electron scattering-foil/collimation system and not in the water phantom. The x-ray central axis depth ionization curves exhibit buildup and attenuation regions characteristic of megavoltage x-ray beams. Ionization profiles of the x-ray component, measured in a plane perpendicular to the central axis, were also examined. Analysis of bremsstrahlung production as a function of photon collimation jaw opening shows a strong dependence, especially for the smaller openings.

Electrons↗

PubMed1989

Stereotactic radiosurgery with the linear accelerator: treatment of arteriovenous malformations.

An original stereotactic radiosurgical approach coupling a) Talairach's stereotactic methodology, b) a specially devised mechanical system, and c) a linear accelerator is detailed. The authors present their preliminary results on 66 patients with nonsurgical intracranial arteriovenous malformations. The doses delivered for treatment varied from 20 to 70 Gy. Doses of no more than 40 Gy were used in 80% of patients. An angiographic study was performed when the computed tomographic scan controls showed relevant modifications of the lesion volume. Total obliteration was obtained in 27 of the 41 patients (65.8%) who were followed up for at least 24 months. The percentage of the cured patients is significantly higher when a) the entire malformation is included in the 75% isodose (96%) and b) the maximum diameter of the lesion is less than 12 mm (81%). Two patients died of rebleeding at 18 and 29 months after treatment.

Adolescent↗

PubMed1992

Photon beam attenuation for a patient support assembly during arc therapy for a medical linear accelerator.

Arc therapy is one of the treatment techniques for small, centrally located deep-seated tumors. However, care must be taken to remove any components that would interfere with the beam as the gantry rotates around the patient. One such component that may interfere with the beam is the patient support assembly (PSA) or treatment table. Beam attenuation factors due to the presence of the couch side-rails and the centerspine bar of the PSA are presented for both 6 MV and 18 MV photon beams of a Clinac 1800 during a 360 degrees rotation. Dose perturbations arising from these obstructions are displayed using an Alderson Rando phantom. A method is described to calculate the start and stop angles for the largest unobstructed arc for a given field size (FS), centerspine bar to the isocenter height (H), centerspine bar width (W), and SAD. As an illustration, for an SAD of 100 cm and a W of 4 cm, the start and stop angles for arc therapy with a FS of 10 x 10 cm2 and an H of 12.7 cm would be 29 degrees and 331 degrees.

Humans↗

PubMed2003

Accelerator mass spectrometry at Arizona: geochronology of the climate record and connections with the ocean.

There are many diverse uses of accelerator mass spectrometry (AMS). Carbon-14 studies at our laboratory include much research related to paleoclimate, both with 14C as a tracer of past changes in environmental conditions as observed in corals, marine sediments and many terrestrial records. Terrestrial records such as forest fires can also show the influence of oceanic oscillations, whether they are short-term such as ENSO, or on the millennial time scale. In tracer applications, we have developed the use of 129I as well as 14C as tracers for nuclear pollution studies around radioactive waste dump sites, in collaboration with IAEA. We discuss some applications carried out in Tucson for several of these fields and hope to give some idea of the breadth of these studies.

Beryllium↗

PubMed1993

Ion recombination corrections for plane-parallel and thimble chambers in electron and photon radiation.

The aim of this work was to investigate the collection efficiency of several popular ionization chambers used in high-energy electron and photon dosimetry. The recombination effect was evaluated in plane-parallel type ion chambers (Markus, NACP, Calcam and Vinten-631) and in a thimble-type chamber (NE2571). The chambers' response was studied in a pulsed accelerator beam, for both photons and electrons, and in continuous radiation from a 60Co machine. The study involved both theoretical and experimental evaluation of the recombination effect utilizing the methods described in the literature. The main conclusions to be drawn from this work are as follows: (i) For the Farmer NE2571 0.6 cm3 chamber used here, the correction factors required are of the order of 0.1% for 60Co radiation, 0.5% for 5 and 8 MV x-rays and 10 and 12 MeV electrons and 0.9% for 15 MV x-rays and 15 and 18 MeV electrons, all at typical clinical dose rates of 200-400 cGy min-1 (0.017 cGy pulse-1). (ii) The recombination effect for the plane-parallel chambers was found to be smaller than for the thimble chamber, with values of < 0.1% in continuous radiation and 0.2-0.4% in pulsed radiation.

Cobalt Radioisotopes↗

PubMed1998

Dose characteristics of in-house-built collimators for stereotactic radiotherapy with a linear accelerator.

Dose characteristics of a stereotactic radiotherapy unit based on a standard Varian Clinac 4/100 4 MV linear accelerator, in-house-built Lipowitz collimators and the SMART stereotactic radiotherapy treatment planning software have been determined. Beam collimation is constituted from the standard collimators of the linear accelerator and a tertiary collimation consisting of a replaceable divergent Lipowitz collimator. Four collimators with isocentre diameters of 15, 25, 35 and 45 mm, respectively, were constructed. Beam characteristics were measured in air, acrylic or water with ionization chamber, photon diode, electron diode, diamond detector and film. Monte Carlo simulation was also applied. The radiation leakage under the collimators was less than 1% at 50 mm depth in water. Specific beam characteristics for each collimator were imported to SMART and dose planning with five non-coplanar converging 140 degrees arcs separated by 36 degrees angles was performed for treatment of a RANDO phantom. Dose verification was made with TLD and radiochromic film. The in-house-built collimators were found to be suitable for stereotactic radiotherapy and patient treatments with this system are in progress.

Biophysical Phenomena↗

PubMed2001

Correction of scatter in megavoltage cone-beam CT.

The role of scatter in a cone-beam computed tomography system using the therapeutic beam of a medical linear accelerator and a commercial electronic portal imaging device (EPID) is investigated. A scatter correction method is presented which is based on a superposition of Monte Carlo generated scatter kernels. The kernels are adapted to both the spectral response of the EPID and the dimensions of the phantom being scanned. The method is part of a calibration procedure which converts the measured transmission data acquired for each projection angle into water-equivalent thicknesses. Tomographic reconstruction of the projections then yields an estimate of the electron density distribution of the phantom. It is found that scatter produces cupping artefacts in the reconstructed tomograms. Furthermore, reconstructed electron densities deviate greatly (by about 30%) from their expected values. The scatter correction method removes the cupping artefacts and decreases the deviations from 30% down to about 8%.

Artifacts↗

PubMed2000

Modeling photon output caused by backscattered radiation into the monitor chamber from collimator jaws using a Monte Carlo technique.

Dose per monitor unit in photon fields generated by clinical linear accelerators can be affected by the backscattered radiation into the monitor chamber from collimator jaws. Thus, it is necessary to account for the backscattered radiation in computing monitor unit setting for a treatment field. In this work, we investigated effects of the backscatter from collimator jaws based on Monte Carlo simulations of a clinical linear accelerator. The backscattered radiation scored within the monitor chamber was identified as originating either from the upper jaws (Y jaws), or from the lower jaws (X jaws). From the results of Monte Carlo simulations, ratios of the monitor-chamber-scored dose caused by the backscatter to the dose caused by the forward radiation, R(x,y), were modeled as functions of the individual X and Y jaw positions. The amount of the backscattered radiation for any field setting was then computed as a compound contribution from both the X and Y jaws. The dose ratios of R(x,y) were then used to calculate the change in photon output caused by the backscatter, Scb(x,y). Results of these calculations were compared with available measured data based on counting the electron pulses or charge from the electron target of an accelerator. Data from this study showed that the backscattered radiation contributes approximately 3% to the monitor-chamber-scored dose. A majority of the backscattered radiation comes from the upper jaws, which are located closer to the monitor chamber. The amount of the backscatter decreases approximately in a linear fashion with the jaw opening. This results in about a 2% increase of photon output from a 10 cm x 10 cm field to a 40 cm x 40 cm field. The off-axis location of the jaw opening does not have a significant effect on the magnitude of the backscatter. The backscatter effect is significant for monitor chambers using kapton windows, particularly for treatment fields using moving jaws. Applying the backscatter correction improves the accuracy of monitor-unit calculation using a model-based dose calculation algorithm such as the convolution method.

Algorithms↗

PubMed1994

Production of [13N]ammonia applicable to low energy accelerators.

We have developed a technique for the rapid conversion of the nitrogen-13 induced in a graphite target into nitrogen oxides. This was accomplished by heating the graphite target in a stream of pure oxygen at 800 degrees C. Less than 20% of the radioactivity was found in the form of [13N]nitrogen. The rest of the radioactivity was efficiently trapped in a solid-phase medium that consisted of an aqueous solution of 5% NaOH dispersed in silica gel. The radioactivity from this solid-phase medium was eluted with water (94% recovery) and found to be in the form of 13NO2- (99%). This was subsequently converted to [13N]ammonia with Raney-nickel, either by a conventional liquid-phase reduction with an overall conversion efficiency to ammonia of 45%, or by an incorporation of the Raney-nickel into the solid-phase medium. The latter system resulted in an overall conversion efficiency to ammonia of 37 +/- 9%, with a radiochemical purity of nearly 100% and a synthesis time under 17 min.

Ammonia↗

PubMed1994

Stereotactic radiotherapy: a technique for dose optimization and escalation for intracranial tumors.

Stereotactic radiosurgery offers the ability to treat relatively small volume intracranial lesions with single fraction, high dose radiotherapy while sparing surrounding tissue due to rapid fall off of dose outside of the treatment volume. Conventional radiotherapy takes advantage of the sparing effects of dose fractionation, but includes relatively large amounts of normal brain in the treatment volume the tolerance of which is dose-limiting. For some intracranial lesions it may not be optimal to treat with large single fractions due to tumor location or size. Conventional fractionated radiotherapy may not be optimum in all cases due to the necessary inclusion of normal structures. Through the development of relocatable head frames, the precision of stereotactic techniques and the biologic advantages of fractionation may be combined in stereotactic radiotherapy (SRT). We report on the treatment of 68 patients with intracranial lesions using a dedicated stereotactic linear accelerator to deliver SRT between June 1992 and June 1993. SRT was used either in order to optimize dose distribution and spare normal tissues in patients with excellent prognosis or in order to increase the dose to tumor while keeping doses to normal tissues below tolerance levels in patients with poorer prognosis (dose escalation). Histologies treated included meningioma, low grade astrocytoma, pituitary adenoma and acoustic neuroma. The most common treatment sites were the parasellar region and cavernous sinuses. Most patients (79%) had surgical debulking prior to SRT. 10-12 patients were treated daily. Patient positioning using relocatable stereotactic frames was highly precise. Acute and subacute side effects were minimal and radiographic responses have been similar to those expected with conventional radiotherapy.(ABSTRACT TRUNCATED AT 250 WORDS)

Adolescent↗

PubMed2003

Capacity and demand models for radiotherapy treatment machines.

Models to predict the number of linear accelerators (linacs) required usually assume that capacity needs to equal demand. Queuing theory shows that capacity needs to exceed mean demand, to avoid the build-up of waiting times. A model has been developed, using Monte-Carlo modelling, to calculate the percentage of spare capacity required to keep waiting times to treatment short. For a matched pair of linacs, in a department that closes on bank holidays and compensates for category 1 patients by treating twice before or after the break, about 10% spare capacity is required to ensure that 86% of patients are able to start radiotherapy within a week of completing the treatment planning process. If a machine is booked as a single (unmatched machine), an additional 3% spare capacity is needed. If all bank holidays in the year are worked, then about 3% less is needed.

Algorithms↗

PubMed2005

Spatial distributions of residuals produced inside a spallation target.

The spallation target model of an accelerator driven system (ADS), consisting of six 5 cm thick and 16 cm in diameter Pb segments, was constructed. Three sets of 17 Bi samples (1/2 inch in diameter and 1 mm thick) were placed in 3 Pb disc-shaped holders inside the target at 5, 10 and 15 cm from its front. After irradiation with 660 MeV proton beam gamma-spectra of radioisotopes produced in Bi were collected several times for each sample with the use of HPGe detectors in order to identify the radioisotopes and to determine their absolute activities. Their spatial distributions were then compared with respective values obtained in the calculations made with the use of FLUKA and/or MCNPX code. A fair agreement with the experiment has been observed.

Bismuth↗

PubMed2005

Shielding analysis at the upper section of the accelerator-driven system.

The proton beam duct of the accelerator-driven system (ADS) acts as a streaming path for spallation neutrons and photons and causes the activation of the magnets and other devices above the subcritical core. We have performed a streaming analysis at the upper section of the lead-bismuth target/cooled ADS (800 MWth). MCNPX was used to calculate the radiation dose from streamed neutrons and photons through the beam duct. For the secondary photon production calculation, cross sections for several actinides were substituted with plutonium because of the lack of gamma production cross section. From the results of this analysis, the neutron dose from the beam duct is seen to be about 20 orders higher than that of the bulk shield. The magnets and shield plug are heavily irradiated by streaming neutrons according to the DCHAIN-SP analysis.

Computer Simulation↗

PubMedDate not supplied

Energy and angular distributions of photons from medical linear accelerators.

For accurate three-dimensional treatment planning, new models of dose calculations are being developed which require the knowledge of the energy spectra and angular distributions of the photons incident on the surface of the patient. Knowledge of the spectra is also useful in other applications, including the design of filters and beam modifying devices and determination of factors to convert ionization chamber measurements to dose. We have used Monte Carlo code (EGS) to compute photon spectra for a number of different linear accelerators. Both the target and the flattening filter have been accurately modeled. We find the mean photon energy to have a value lower than the generally perceived value of one-third the maximum energy. As expected, the spectra become softer as the distance from the central axis increases. Verification of the spectra is performed by computing dose distributions and half-value layers in water using the calculated spectra and comparing the results with measured data. We also examined the angular distributions of photons incident on the surface of the phantom. In currently used models of dose computations, it is assumed that the angular distribution of photons with respect to fan lines emanating from the source is negligible. Although the angular spread of photons with respect to the incident direction has been found to be small, its contribution to the diffuseness of the beam boundaries is significant.

Energy Transfer↗

PubMed1996

Electron contamination in clinical high energy photon beams.

The electron contamination in photon beams has been investigated by means of contaminating lepton depth doses and dose profiles in different geometries with two 20 MV beams. Different components of this contamination have been investigated separately by systematically adding contamination to a "clean" reference field. At 20 MV, the air generated electrons were found to be almost negligible compared to the electrons originating from the accelerator head when measurements were performed in standard fields at SSDs between 80 and 120 cm. The total electron part of the depth dose curve was then almost the same, i.e., independent of SSD, when the collimator opening was held fixed. However, when different accessories such as a shaping block and different attenuating plates were located in the beam path below the collimators, a large SSD dependence of the electron contamination was noticed. A comparison was also made between two machines, one equipped with a multileaf collimator, with similar beam qualities at 20 MV. These measurements indicate that the interior view of the treatment head seen by the detector (mainly the flattening filter, monitor chamber, or other electron generating material) influences the magnitude of the electron contamination. When the collimator opening is decreased the electron contamination will also decrease as parts of the electron source will be shielded by the collimator blocks.

Air↗

PubMed2005

Chromosomal intrachanges induced by swift iron ions.

We measured the induction of structural aberrations in human chromosome 5 induced by iron ions using the novel technique of multicolor banding in situ hybridization (mBAND). Human lymphocytes isolated from whole blood were exposed in vitro to 500 MeV/n (LET=200 keV/micrometers, doses 1 or 4 Gy) Fe nuclei at the HIMAC accelerator in Chiba (Japan). Chromosomes were prematurely condensed by calyculin A after 48 h in culture and slides were painted by mBAND. We found a frequency of 0.11 and 0.57 residual breakpoints per chromosome 5 after 1 and 4 Gy Fe-ions, respectively. Inter-chromosomal exchanges were the prevalent aberration type measured at both doses, followed by terminal deletions, and by intra-chromosomal exchanges. Among intra-chromosomal exchanges, intra-arm events were more frequent than inter-arm, but a significant number of intra-changes was associated to inter-changes involving the same chromosome after 4 Gy of iron ions. These events show that the complexity of chromosomal exchanges induced by heavy ions can be higher than expected by previous FISH studies.

Chromosome Aberrations↗

PubMed1998

Inactivation of C3H10T1/2 cells by low energy protons and deuterons.

PURPOSE: To determine the RBE-LET relationship for C3H10T1/2 cell inactivation by protons in the LET range 11-33 keV/microm and to compare inactivation frequencies induced in C3H10T1/2 cells by protons and deuterons at two matching LET values in the range 11-20 keV/microm. MATERIALS AND METHODS: C3H10T1/2 cells were irradiated with protons and deuterons at the radiobiological facility set up at the 7MV Van de Graaff accelerator at the LNL, Legnaro, Padova. Gamma rays from 60Co were used as reference radiation. RESULTS: Proton RBE values (alpha/alphagamma) for inactivation of C3H10T1/2 cells are constant around a value of 2 between 11 and 20 keV/microm and then rise sharply to reach a value of 4.2+/-1.0 at 33 keV/microm. Deuteron RBE values are 1.7+/-0.4 and 2.2+/-0.6 at LET values of 13 and 18 keV/microm respectively. CONCLUSIONS: Proton RBE values with C3H10T1/2 cells are significantly larger than unity at LET values as low as 11 keV/microm. No difference in effectiveness for inactivation of C3H10T1/2 has been found between protons and deuterons at two LET values in the range 10-20 keV/microm.

Animals↗

PubMedDate not supplied

The dosimetric properties of an applicator system for intraoperative electron-beam therapy utilizing a Clinac-18 accelerator.

A prototype electron applicator system providing circular and rectangular fields for use in intraoperative electron beam therapy with a Varian Clinac 18 linear accelerator has been fabricated. The dosimetric properties of this system for a variety of electron-beam energies, applicator sizes, and x-ray collimator settings was documented. Significant findings include: (a) surface dose values are in excess of 90% for electron energies of 12 MeV and above; (b) for the 18-MeV beam, the deepest depth where the central axis dose in 90% of its maximum value is in excess of 50 mm for circular applicators whose diameters are in excess of 5 cm; and (c) the treatment time to deliver 1000 rads "given dose" (at given dose rate of 300 MU/min) is on the order of 3-4 min. Cross-field behavior is acceptable for the intended application and x-ray contamination is less than 4% for any applicator/electron energy combination. A system for irregular field blocking and TLD verification dosimetry has been developed.

Electrons↗