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PubMedDate not supplied

Dosimetry characteristics of large wedges for 4- and 6-MV x rays.

Two sets of newly designed large wedge filters for field sizes up to 20 X 20 cm2 have become commercially available for use with 4- and 6-MV linear accelerators. Such field sizes are sometimes required to ensure adequate coverage in certain treatment techniques. This work reports base line data resulting from an investigation of the dosimetric properties of these wedges. Measurements of wedge angles, transmission factors, and beam hardening effects are described, and comparisons are made with the smaller standard wedges.

Humans↗

PubMedDate not supplied

Microdosimetry of 10-15 MeV bremsstrahlung x rays.

Experimental techniques have been developed for obtaining microdosimetric spectra on a hospital-based linear accelerator. Teletherapy beams of 10 and 15 MeV bremsstrahlung x rays from a Varian Clinac-18 and Clinac-20, respectively, have been produced at ultralow dose rates (50-200 microGy/h) which enables direct measurements of lineal energy distributions with a conventional Rossi-type gas proportional counter. Extensive measurements have been made to insure that the dosimetric properties of these low dose rate beams are nearly identical to those produced under high dose rate clinical conditions. Analytical procedures have been developed to correct measured lineal energy spectra for pileup caused by the low duty factor of the linear accelerator. The lineal energy spectra of these megavoltage beams differ significantly from Co-60, with dose averaged lineal energies (yD) being 20%-30% lower than for Co-60. Although such differences may not be important at clinical doses, the theory of dual radiation action does predict a lower biological effectiveness for these beams at very low dose levels.

Humans↗

PubMed1990

[Development of special electron applicators for intraoperative radiotherapy at the Grosshadern Clinic].

A specialized applicator system for intraoperative radiation therapy using high energy electrons of a linear accelerator has been developed and manufactured, regarding the specific situation that there is no dedicated linear accelerator available at the surgery facility. Additionally, long lasting interruptions of the daily routine irradiations are hardly tolerable. A significant improvement of the procedure could be achieved developing applicators, which are divided into two halves. Positioning the lower part of the applicator into the patient at the operating room already, the transportation and the irradiation of the closed patient is possible. Adapted to the linear accelerator in use, a Siemens Mevatron KD having electron energies in the range from 6 to 21 MeV, the dosimetric properties of the system have been optimized by iteration. Excess dose values near the applicator walls could be avoided completely. A rapid decrease of the leakage dose could be realized.

Electrons↗

PubMed1983

[Weekly control measurement at the linear accelerator].

Weekly control measurements taken at the linear accelerator of the Medizinisches Strahleninstitut der Universität Tübingen are described which largely exceed those prescribed by the "Richtlinien Strahlenschutz in der Medizin" (instructions about radioprotection in medicine). Since the determination of the field homogeneity and the energy of electron and X-ray radiation is very time-consuming, a largely automatized procedure has been elaborated which is presented in this study.

Methods↗

PubMed1999

Biomedical applications of accelerator mass spectrometry-isotope measurements at the level of the atom.

Accelerator mass spectrometry (AMS) is a nuclear physics technique developed about twenty years ago, that uses the high energy (several MeV) of a tandem Van de Graaff accelerator to measure very small quantities of rare and long-lived isotopes. Elements that are of interest in biomedicine and environmental sciences can be measured, often to parts per quadrillion sensitivity, i.e. zeptomole to attomole levels (10(-21)-10(-18) mole) from milligram samples. This is several orders of magnitude lower than that achievable by conventional decay counting techniques, such as liquid scintillation counting (LSC). AMS was first applied to geochemical, climatological and archaeological areas, such as for radiocarbon dating (Shroud of Turin), but more recently this technology has been used for bioanalytical applications. In this sphere, most work has been conducted using aluminium, calcium and carbon isotopes. The latter is of special interest in drug metabolism studies, where a Phase 1 adsorption, distribution, metabolism and excretion (ADME) study can be conducted using only 10 nanoCurie (37 Bq or ca. 0.9 microSv) amounts or less of 14C-labelled drugs. In the UK, these amounts of radioactivity are below those necessary to request specific regulatory approval from the Department of Health's Administration of Radioactive Substances Advisory Committee (ARSAC), thus saving on valuable development time and resources. In addition, the disposal of these amounts is much less an environmental issue than that associated with microCurie quantities, which are currently used. Also, AMS should bring an opportunity to conduct "first into man" studies without the need for widespread use of animals. Centre for Biomedical Accelerator Mass Spectrometry (CBAMS) Ltd. is the first fully commercial company in the world to offer analytical services using AMS. With its high throughput and relatively low costs per sample analysis, AMS should be of great benefit to the pharmaceutical and biotechnology industries as well as other life science areas.

Biotechnology↗

PubMed1987

A radiation safety survey on a Clinac-20 linear accelerator.

The radiation protection problems associated with a 20 MV linear accelerator are discussed, in particular those due to the considerable neutron production accompanying the 18 MV photon beam. These include the potential neutron dose to the patient, the special shielding requirements and the neutron activation of the accelerator and its surroundings. The results of the structural protection survey are described. In particular, the methods recommended by the NCRP for predicting the neutron dose equivalent at the end of a maze are shown to be reliable. Dose rates due to activation of the treatment room and various parts of the accelerator were measured, and potential doses to staff were assessed. Neutron dose equivalents were measured in the specified two metre diameter patient plane using the standard NRPB neutron badges and also with the recently developed CR-39 dosimeters. The latter appear to give sensible results.

Equipment Design↗

PubMed2005

A cargo inspection system based on pulsed fast neutron analysis (PFNA).

A cargo inspection system based on pulsed fast neutron analysis (PFNA) is to be used at a border crossing to detect explosives and contraband hidden in trucks and cargo containers. Neutrons are produced by the interaction of deuterons in a deuterium target mounted on a moveable scan arm. The collimated pulsed fast neutron beam is used to determine the location and composition of objects in a cargo container. The neutrons produce secondary gamma rays that are characteristic of the object's elemental composition. The cargo inspection system building consists of an accelerator room and an inspection tunnel. The accelerator room is shielded and houses the injector, accelerator and the neutron production gas target. The inspection tunnel is partially shielded. The truck or container to be inspected will be moved through the inspection tunnel by a conveyor system. The facility and radiation source terms considered in the shielding design are described.

Computer-Aided Design↗

PubMed1986

Use of a gated counting method for radiation monitoring at a 45-MeV electron LINAC facility.

A fixed-point radiation monitoring system based on the gated counting method, which was previously reported by us, was constructed for a 45-MeV electron LINAC facility. It was tested under different beam intensities of the accelerator both at the current and repetition rates. It showed excellent dynamic response (from 2.5 X 10(-11) to 1.0 X 10(-8) Sv h-1) to the intensity variation of the radiation source. It was demonstrated that the system was very useful in monitoring an extremely low-level radiation from a pulsed source.

Electrons↗

PubMedDate not supplied

A prototype beam delivery system for the proton medical accelerator at Loma Linda.

A variable energy proton accelerator was commissioned at Fermi National Accelerator Laboratory for use in cancer treatment at the Loma Linda University Medical Center. The advantages of precise dose localization by proton therapy, while sparing nearby healthy tissue, are well documented [R. R. Wilson, Radiology 47, 487 (1946); M. Wagner, Med. Phys. 9, 749 (1982); M. Goitein and F. Chen, Med. Phys. 10, 831 (1983)]. One of the components of the proton therapy facility is a beam delivery system capable of delivering precise dose distributions to the target volume in the patient. To this end, a prototype beam delivery system was tested during the accelerator's commissioning period. The beam delivery system consisted of a beam spreading device to produce a large, uniform field, a range modulator to generate a spread out Bragg peak (SOBP), and various beam detectors to measure intensity, beam centering, and dose distributions. The beam delivery system provided a uniform proton dose distribution in a cylindrical volume of 20-cm-diam area and 9-cm depth. The dose variations throughout the target volume were found to be less than +/- 5%. Modifications in the range modulator should reduce this considerably. The central axis dose rate in the region of the SOBP was found to be 0.4 cGy/spill with an incident beam intensity of 6.7 x 10(9) protons/spill. With an accelerator repetition rate of 30 spills/min and expected intensity of 2.5 x 10(10) protons/spill for patient treatment, this system can provide 50 cGy/min for a 20-cm-diam field and 9-cm range modulation.(ABSTRACT TRUNCATED AT 250 WORDS)

Equipment Design↗

PubMedDate not supplied

Head-scatter factors and effective x-ray source positions in a 25-MV linear accelerator.

The behavior of the effective source position and the correction factor associated with the collimator opening (head-scatter factor) were investigated for the 6- and 25-MV x-ray beams of a linear accelerator. The primary photon fluence was measured in air for square field sizes from 5 x 5 cm to 40 x 40 cm at distances from the nominal source of 80 to 140 cm, for open and wedged fields (wedge angle 60 deg). An inverse-square analysis shows that, for open fields, the effective source position of the accelerator is about the same (approximately 1 cm downstream) at 6 and 25 MV, for all field sizes. For the wedged fields, the effective source position depends on field size and ranges from about 2 to 4 cm. The head-scatter correction factors for given collimator settings were found to be essentially independent of distance at both energies.

Mathematics↗

PubMed1990

The radiotherapeutical use of high energy electrons in West Germany.

Germany has always deserved a great interest to electron beam radiotherapy (betatron, linear accelerator). At St. Vincentius Hospital in Karlsruhe the percentage of electron treated patients grew up in the years from 15% to 45%. Electron energies between 7 and 12 MeV are the most commonly used. Major RT Centres need a wider range of energies (5/7-20 MeV), but the minor ones should have at least electron beams of 5-12/15 MeV.

Electrons↗

PubMed1985

Proton radiotherapy with the Uppsala cyclotron. Experience and plans.

From 1957 to 1968, the 230-cm synchrocyclotron at the Gustaf Werner Institute was used for clinical tests with a 185 MeV proton beam. The radiotherapeutic research was part of an extensive research programme in physics, chemistry, biology and medicine. Only a small series of patients were treated. A brief review of the early development and clinical experience of the cyclotron activities at Uppsala from 1957 to 1968 is given. The former accelerator is now being converted. Beams are expected to be available in the new radiotherapy treatment rooms in 1986. Plans for the new facilities with special reference to alternative methods of proton acceleration and beam transport, i.e. fixed beams or an gantry system are presented. The corresponding activities at the Institute of Theoretical and Experimental Physics (ITEP) in Moscow are also referred to thanks to a bilateral research programme which has existed in the past and from which the Uppsala group has benefited greatly.

Brain Neoplasms↗

PubMed1994

Design of a non-docking intraoperative electron beam applicator system.

A development of a non-docking system is described which enables collimation of an electron beam for intraoperative radiation therapy. This system, adapted to a linear accelerator (SATURNE 43-CGR MeV), has been designed to minimize the mechanical, electrical and tumor visualization problems associated with a docking system. A number of dosimetric considerations and technical innovations have been used in the design of this system. Among them are the central axis of the beam alignment with the axis of the cone via a laser system and the clamping method of the intraoperative cone to the treatment couch by a rigid system. This collimation system can be adapted for different makes of linear accelerator. The dose distribution in this new design system shows a better homogeneity in the patient's target volume and small (thus accessable) leakage radiation dose to tissues outside the intraoperative cone. The design concept and dosimetric characteristics of this novel applicator system are presented in this paper.

Acrylic Resins↗

PubMedDate not supplied

Accelerator mass spectrometry radiocarbon ages of amino acid extracts from Californian palaeoindian skeletons.

A decade ago, aspartic acid racemization ages were determined for some skeletal remains found in California, near La Jolla, Del Mar and Sunnyvale, suggesting that people were present in North America during the Upper Pleistocene. These ages were obtained from the aspartic acid racemization rate, which was calibrated using a radiocarbon date of 17,150 +/- 1,470 yr BP determined for a skeleton found in Laguna Beach, California. These studies generated an intense controversy not only about the antiquity of human beings in the New World but also about the validity of racemization-based ages, and prompted efforts to date the finds by other means. Here we have used accelerator mass spectrometry (AMS) to determine the radiocarbon ages of the amino acid extracts used in the original racemization studies. Our studies indicate that some of the controversial Californian skeletons, which had been assigned to the Upper Pleistocene, are in fact Holocene.

Amino Acids↗

PubMed1982

Backscattering in electron beam therapy for energies between 3 and 35 MeV.

Whenever a heterogeneity is present in an electron beam treatment field during radiotherapy, there is the possibility of tissue overdosage at the tissue-heterogeneity interface due to electrons backscattered from the heterogeneity. Measurements of this effect were made in a polystyrene phantom using a purpose-built thin-window parallel-plane ionisation chamber. Materials of various atomic numbers were used as scatterers and the investigations were made over a wide range of electron energies. Electron backscatter factor (EBF), defined as the ratio fo dose at the interface surface with and without the scatterer present, was found to increase with increasing atomic number and decrease with increasing beam energy. Both of these relationships were found to be non-linear. The EBF dependence on the scatterer thickness was also investigated. All data in this work were expressed in relation to the beam energy incident on the scatterer in preference to the nominal beam energy set on the accelerator. This approach enables the dose enhancement at an interface to be predicted from a knowledge of the heterogeneity (atomic number and thickness,), its depth in tissue and the beam energy being used for treatment. The results of this work were compared with the published data and an explanation is offered to account for the difference.

Electrons↗

PubMed2005

Respiratory correlated cone beam CT.

A cone beam computed tomography (CBCT) scanner integrated with a linear accelerator is a powerful tool for image guided radiotherapy. Respiratory motion, however, induces artifacts in CBCT, while the respiratory correlated procedures, developed to reduce motion artifacts in axial and helical CT are not suitable for such CBCT scanners. We have developed an alternative respiratory correlated procedure for CBCT and evaluated its performance. This respiratory correlated CBCT procedure consists of retrospective sorting in projection space, yielding subsets of projections that each corresponds to a certain breathing phase. Subsequently, these subsets are reconstructed into a four-dimensional (4D) CBCT dataset. The breathing signal, required for respiratory correlation, was directly extracted from the 2D projection data, removing the need for an additional respiratory monitor system. Due to the reduced number of projections per phase, the contrast-to-noise ratio in a 4D scan reduced by a factor 2.6-3.7 compared to a 3D scan based on all projections. Projection data of a spherical phantom moving with a 3 and 5 s period with and without simulated breathing irregularities were acquired and reconstructed into 3D and 4D CBCT datasets. The positional deviations of the phantoms center of gravity between 4D CBCT and fluoroscopy were small: 0.13 +/- 0.09 mm for the regular motion and 0.39 +/- 0.24 mm for the irregular motion. Motion artifacts, clearly present in the 3D CBCT datasets, were substantially reduced in the 4D datasets, even in the presence of breathing irregularities, such that the shape of the moving structures could be identified more accurately. Moreover, the 4D CBCT dataset provided information on the 3D trajectory of the moving structures, absent in the 3D data. Considerable breathing irregularities, however, substantially reduces the image quality. Data presented for three different lung cancer patients were in line with the results obtained from the phantom study. In conclusion, we have successfully implemented a respiratory correlated CBCT procedure yielding a 4D dataset. With respiratory correlated CBCT on a linear accelerator, the mean position, trajectory, and shape of a moving tumor can be verified just prior to treatment. Such verification reduces respiration induced geometrical uncertainties, enabling safe delivery of 4D radiotherapy such as gated radiotherapy with small margins.

Algorithms↗

PubMedDate not supplied

Extended expression for tissue-maximum ratio fitted to the Varian Clinac-4 and Clinac-6 accelerators.

An expression which generates tissue maximum ratio (TMR) data for the Varian Clinac-4 and Clinac-6 accelerators is presented. This semiempirical expression can be programmed easily for radiotherapy computer systems or the new generation of scientific calculators. Our technique leads to good agreement with the published data, as demonstrated by a total average percent deviation of the measured TMR values of -0.115% for the Clinac-4 and -0.002% for the Clinac-6.

Particle Accelerators↗

PubMedDate not supplied

Improving precision and safety in the use of beam modifying devices in radiation therapy.

Reliable and safe implementation of beam modifying devices such as wedges and block trays requires careful design and construction. Inappropriate design may pose problems ranging from user-hostile operation to hard-to-track, but significant variations in actual position in a beam. This may cause variation in actual wedge output factors, or variation in the position of a block tray. In case of simple mechanical failure or personnel mistake, design related mechanical conditions may result in injury to either a patient or a staff member. This paper is based on experience with linear accelerators from one manufacturer, but similar conditions are likely to exist with other radiation machines. A simple technical modification is offered which improves both accuracy and reproducibility in the placement of wedge-type filters. For our machines the solution also provides improved safety in the use of both wedge trays and block trays.

Equipment Design↗