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PubMedDate not supplied

Dosimetric study of total skin irradiation with a scanning beam electron accelerator.

The Therac 20 6-MeV scanned electron beam may be used for partial or total skin therapy. The maximum field size at 1 m is 30 X 30 cm defined by a set of primary photon collimators in conjunction with secondary trimmers. We have studied electron beam profiles with and without trimmers at the nominal source-skin distance of 1 m versus extended distances of 3-5 m. We find that the trimmers limit the field size and add little to the beam uniformity at extended distances. Beam energy, dose distributions, and output factors at extended distances were measured for single and multiple field arrangements with and without trimmers. Beam parameters were measured after introducing a degrader that lowered the energy to 3.7 MeV.

Humans↗

PubMed1993

Possible leakage radiation during malfunctioning of a Sagittaire accelerator.

A non-fatal accident with a CGR-Sagittaire accelerator in the Dr Daniel den Hoed Cancer Center (DDHCC) in Rotterdam in 1988 is described. In a period with frequently occurring technical problems, a patient, undergoing fractionated treatment at this accelerator for prostatic cancer, developed severe skin reactions on the right half ventral part of thorax, head and upper arm. The skin reactions suggested an irradiation with a dose of up to 10-20 Gy, so it was likely that a radiation accident had occurred caused by leakage radiation. A number of experiments were performed in attempting to explain the estimated large dose rates of leakage radiation. Under rather extreme malfunctioning conditions, a high leakage dose rate (4 Gy/min) could be obtained during therapy use. This condition might have occurred during this period of technical difficulty.

Accidents↗

PubMed1998

Consistency in quality control programmes for electron accelerators in radiotherapy centres.

BACKGROUND AND PURPOSE: To gain insight into the current practice of quality control (QC) of medical electron accelerators and to reduce possible variations in test frequencies and test procedures. MATERIALS AND METHODS: An extensive questionnaire on QC procedures of medical electron accelerators was distributed and completed by all (21) radiotherapy institutions in The Netherlands. The questions were related to safety systems, mechanical parameters, beam profiles, beam energy, absolute dosimetry, wedge filters, the dose monitor system and radiation leakage. The data of the questionnaire were compared with recommendations given in national and international reports on QC of electron accelerators. RESULTS: Large variations in time spent on QC exist, especially for accelerators having dual energy photon beams and several electron beam energies. This diversity is mainly due to differences in philosophy with regard to QC and the differences in resources and machine time available. Furthermore, large variations in test frequencies and test methodologies were observed. The staffing level involved in the QC measurements was evaluated and compared with recent recommendations provided by EFOMP-ESTRO. CONCLUSIONS: From these recommendations and the results of the questionnaire, a set of minimum guidelines for a QC programme could be formulated and implemented in all radiotherapy institutions in The Netherlands.

Guidelines as Topic↗

PubMed2003

Focal spot motion of linear accelerators and its effect on portal image analysis.

The focal spot of a linear accelerator is often considered to have a fully stable position. In practice, however, the beam control loop of a linear accelerator needs to stabilize after the beam is turned on. As a result, some motion of the focal spot might occur during the start-up phase of irradiation. When acquiring portal images, this motion will affect the projected position of anatomy and field edges, especially when low exposures are used. In this paper, the motion of the focal spot and the effect of this motion on portal image analysis are quantified. A slightly tilted narrow slit phantom was placed at the isocenter of several linear accelerators and images were acquired (3.5 frames per second) by means of an amorphous silicon flat panel imager positioned approximately 0.7 m below the isocenter. The motion of the focal spot was determined by converting the tilted slit images to subpixel accurate line spread functions. The error in portal image analysis due to focal spot motionwas estimated by a subtraction of the relative displacement of the projected slit from the relative displacement of the field edges. It was found that the motion of the focal spot depends on the control system and design of the accelerator. The shift of the focal spot at the start of irradiation ranges between 0.05-0.7 mm in the gun-target (GT) direction. In the left-right (AB) direction the shift is generally smaller. The resulting error in portal image analysis due to focal spotmotion ranges between 0.05-1.1 mm for a dose corresponding to two monitor units (MUs). For 20 MUs, the effect of the focal spot motion reduces to 0.01-0.3 mm. The error in portal image analysis due to focal spot motion can be reduced by reducing the applied dose rate.

Artifacts↗

PubMedDate not supplied

Jaws 2: a description of in-air beam profiles for half blocked and dynamically wedged radiation fields.

In a recent paper [Med. Phys. 19, 1441 (1992)] it was shown that the field size dependence of on-axis relative output from a linear accelerator can be well described by a model comprising two components. One component represents radiation emanating from a point source and the other represents radiation from a field size dependent distributed source. In this communication the application of the model is extended to off-axis situations and demonstrates that a good description of the dependence of off-axis output on collimator configuration can be obtained. Of particular clinical relevance in this regard is the beam profile of half blocked fields which are adequately reproduced. The model has also been applied to the study of profiles generated by simulated dynamic wedging.

Humans↗

PubMed1990

Linear accelerator radiosurgery of three-dimensional irregular targets.

Current radiosurgical techniques allow concentration of radiation from external sources into spherical targets. Nonspherical target volumes require a modification of the irradiation technique. Moreover, isodose shaping is sometimes necessary to spare nearby important radiosensitive structures. To meet these prerequisites we have introduced some technical refinements: (1) computer-controlled rectilinear translations of the target in combination with different angular positions of the source and (2) computer-controlled rotations of the target around a vertical axis in combination with different angular positions of the source. The first solution has proven efficient for treating irregular targets, while the second one was employed for focusing the radiation dose inside volumes obtained by rotation of ellipses.

Brain Neoplasms↗

PubMed1989

Using different sets of wedges: clinical applications, dosimetry and safety interlocks.

Each accelerator is customarily supplied with one set of wedges, typically for a maximum field width of 15 cm across the wedge slope. Clinically, however, the need arises at times for larger wedged fields. Independent jaws may require wedges which are mounted at 90 degrees from a standard set, and some half-field treatment techniques may benefit from half-field wedges. To optimize treatment conditions, it is therefore desirable to have several sets of wedges available for each treatment machine. We will present clinical examples for different sets of wedges, the results of our dosimetric measurements and will discuss the required safety interlocks.

Humans↗

PubMed1998

A solution for the treatment of small lesions using electron beams from a Saturne linear accelerator with continuous variable trimmers; dosimetrical aspects.

For treatment of superficial lesions with ionizing radiation, a number of treatment techniques can be used. We use a standardized technique by applying electron beams from GE-MS Saturne-41 linear accelerators; this also works in cases where very small target areas have to be irradiated. In our institute linear accelerators of this type are equipped with continuous variable trimmer systems. Very small electron fields show specific dosimetric problems, which in general necessitates performing measurements for each individual patient. A solution is presented, using a specially designed set of 17 templates, which can be placed on the skin of a patient. The trimmers are set to a predefined field size. Output factors, profiles, percentage depth dose (PDD) and dose outside the template are determined for each combination of template and beam energy. Look-up tables are produced, which can be used by the radiation oncologist and in a monitor unit calculation program.

Electrons↗

PubMed2000

[High-dose, stereotactic, one-time radiotherapy with curative intent for peripheral lung cancer].

In peripheral non small cell lung cancer without metastasis, surgical resection achieves 5-year survival rates of at least 65%. In functionally inoperable patients radiation therapy offers the second best changes. However, in cases of severe emphysema with severely limited lung function even conventional radiation therapy is prohibited because of possible fibrotic reactions of the lung parenchyma. For such patients high dose rate stereotactic one-time radiation therapy may be an option. According to the study protocol of the DKFZ Heidelberg a dose rate of 24 Gy at the iso-center is applied with the linear accelerator in a single session. The recognisable tumour is irradiated with 22 Gy (90% isodose included). 20 Gy are applied to the tumour plus 6 millimeters safety margin. Prerequisites of such a therapy are a detailed computer-based planning using CT scans and an exact positioning with immobilisation of the patient. The irradiation is ideally performed under general anesthesia with high-frequency jetventilation to avoid movements due to breathing. We report on this new therapeutic modality in a patient with lung emphysema having a T2 tumour.

Carcinoma, Non-Small-Cell Lung↗

PubMed2003

Neutron and capture gamma along the mazes of linear accelerator vaults.

Neutron and photon dose equivalents at various points along the mazes of the vaults for two 15 MV linear accelerators were measured. The measurements were made with the machines set at various gantry angles with and without the scattering phantom, and with the collimators set at the maximum and the minimum field sizes. Neutron dose equivalent measurements were made for five other accelerator vaults. Empirical equations were used to fit the dose data at points along the center of the maze, at 1 m above floor level, with the primary radiation beam pointing downward. It is reported here that both the capture gamma and the neutron dose attenuations along the maze are in agreement with the literature. The neutron dose is dependent on the square root of the ratio of the cross-sectional areas of the inner maze entrance and the maze. The tenth value distance (T(N)) is proportional to the square root of the cross-sectional area of the maze.

Gamma Rays↗

PubMed1998

Murine in vivo total body nitrogen by photoactivation.

A facility to measure total body nitrogen (TBN) by photoactivation analysis in rats, using the 14N(gamma, n)13N reaction, was designed, calibrated and evaluated. TBN provides a quantitative estimate of nutritional status. TBN in humans can be measured by neutron activation, however the small absolute amount of nitrogen in rats causes difficulties with this technique. Photoactivation analysis involved irradiating the specimen with high energy photons from an 18 MV Medical Linear Accelerator to a dose of approximately 2 Gy. The technique was calibrated using urea solution rat phantoms with 2.0-3.0% nitrogen content. By analysing the measured decay curve, the initial nitrogen count rate of 0.51 MeV photons was determined and the initial oxygen count rate, being an interfering source of 0.51 MeV photons, was shown to be constant. Within the range of nitrogen concentrations studied, the response is linear (r2 = 0.9) with a CV of 3.5%. Thus, photoactivation provides a relatively simple, in vivo technique for determining total body nitrogen in rats.

Animals↗

PubMed1994

Field size dependence of effective source distance and field factors of a 9 MeV electron beam.

Effective source distances and field factors were measured for a number of square and rectangular fields using a 9 MeV electron beam from a Varian 2100-C linear accelerator. Mathematical relationships between effective source distance and field size, and field factor and field size were derived. For 9 MeV electrons, effective source distance is dependent on the field area and applicator cone, while field factors are dependent on the elongation of the field, the field area and applicator cone. Measured field factor data for rectangular fields were compared to field factors obtained using the "square root" method and found to agree to within 0.5%. In some instances, field factors can vary up to 10% for the same field size in different electron applicator cones.

Electrons↗

PubMed1998

Accelerator mass spectrometry as a bioanalytical tool for nutritional research.

Accelerator Mass Spectrometry is a mass spectrometric method of detecting long-lived radioisotopes without regard to their decay products or half-life. The technique is normally applied to geochronology, but is also available for bioanalytical tracing. AMS detects isotope concentrations to parts per quadrillion, quantifying labeled biochemicals to attomole levels in milligram-sized samples. Its advantages over non-isotopic and stable isotope labeling methods are reviewed and examples of analytical integrity, sensitivity, specificity, and applicability are provided.

Animals↗

PubMed1998

Induction of asymmetrical type of chromosomal aberrations in cultured human lymphocytes by ion beams of different energies at varying LET from HIMAC and RRC.

Frequencies of asymmetrical type of chromosome aberration were scored in cultured human blood lymphocytes irradiated with carbon and neon beams. Blood cells were irradiated with various doses to establish dose response curves for chromosome aberration frequency vs. dose, and chromosome preparation was made by conventional method. Dose response curves for the per cell frequencies of the dicentrics and centric rings as well as the excess amount of acentric fragments were described for 7 different qualities (LET = 22.4, 40.0, 41.5, 69.9, 70.0, 100.0 and 150 KeV/micrometer) of carbon and neon beams with three different energies, 135, 290 and 400 MeV/u. From the analysis of those dose response curves, the maximum effect was found in the region of LET value at near 70 KeV/micrometer together with linear expression in the response from all endpoints examined. The 135 MeV/u of carbons (69.9 KeV/micrometer) and neons(70.0 KeV/micrometer) showed linear response. The 290 MeV/u of carbons (100 KeV/m) and neons (150 KeV/micrometer) showed medium effects with different shape of response, linear with a plateau and upward concavity. The 2 carbon beams (41.5 and 40 KeV/micrometer) from 2 different accelerators showed much discrepancy in the response. RBE-LET relationship was also described by comparing the coefficient alpha of the 7 different dose responses. The peak (near 70 KeV/m) was localized close to that (80 KeV/m) for the survivals of dsb repair deficient cells (Eguchi-Kasai et al. 1998), but in different position from that previously reported in many other studies (100-200 KeV/mm). Identification of the RBEmax in the present study has yet to be definitive.

Carbon↗

PubMed1984

A new approach to overcome the inconveniences in electron dosimetry associated with the beam scanning technique in linacs.

The therapeutic advantages of scanned electron beams from medical linear accelerators are accompanied by difficulties in electron dosimetry. Owing to short-time dose inhomogeneities in the treatment field, implicit in the system, time-consuming point-by-point measurements of dose distributions are normally required. However, by triggering the measuring time synchronously with the slow scan frequency, the time necessary to record reliable dose distributions can be shortened considerably. The instrumentation for the computer-controlled dosimetry system developed for this task is described. The results of our measurements show that reproducibility in dose within +/- 1% is achievable in less than 6.3 s per point for scanned electron beams.

Electrons↗

PubMed1997

Practical approach to electron beam dosimetry at extended SSD.

This paper presents a practical approach to clinical dosimetry for electron beams at extended source-to-surface distance (SSD). Characteristics of electron beams from a Siemens MD-2 accelerator are presented at a nominal SSD of 100 cm and an extended SSD of 115 cm. Relative output factors are measured at both 100 and 115 cm SSDs for a range of square field sizes for each beam energy. The change of output with SSD does not follow the inverse square law, if the nominal SSD is used. This deviation is larger for lower beam energy and smaller field sizes. An effective SSD, SSDeff, has been determined at dmax for each beam energy as a function of field size. A comprehensive approach to treatment time calculations is proposed.

Electrons↗

PubMed2005

Shielding evaluation of a medical linear accelerator vault in preparation for installing a high-dose rate 252Cf remote afterloader.

In support of the effort to begin high-dose rate 252Cf brachytherapy treatments at Tufts-New England Medical Center, the shielding capabilities of a clinical accelerator vault against the neutron and photon emissions from a 1.124 mg 252Cf source were examined. Outside the clinical accelerator vault, the fast neutron dose equivalent rate was below the lower limit of detection of a CR-39 etched track detector and below 0.14 +/- 0.02 muSv h(-1) with a proportional counter, which is consistent, within the uncertainties, with natural background. The photon dose equivalent rate was also measured to be below background levels (0.1 muSv h(-1)) using an ionisation chamber and an optically stimulated luminescence dosemeter. A Monte Carlo simulation of neutron transport through the accelerator vault was performed to validate measured values and determine the thermal-energy to low-energy neutron component. Monte Carlo results showed that the dose equivalent rate from fast neutrons was reduced by a factor of 100,000 after attenuation through the vault wall, and the thermal-energy neutron dose equivalent rate would be an additional factor of 1000 below that of the fast neutrons. Based on these findings, the shielding installed in this facility is sufficient for the use of at least 5.0 mg of 252Cf.

Brachytherapy↗

PubMed2002

A performance comparison of flat-panel imager-based MV and kV cone-beam CT.

The use of cone-beam computed tomography (CBCT) has been proposed for guiding the delivery of radiation therapy, and investigators have examined the use of both kilovoltage (kV) and megavoltage (MV) x-ray beams in the development of such CBCT systems. In this paper, the inherent contrast and signal-to-noise ratio (SNR) performance for a variety of existing and hypothetical detectors for CBCT are investigated analytically as a function of imaging dose and object size. Theoretical predictions are compared to the results of experimental investigations employing largearea flat-panel imagers (FPIs) at kV and MV energies. Measurements were performed on two different FPI-based CBCT systems: a bench-top prototype incorporating an FPI and kV x-ray source (100 kVp x rays), and a system incorporating an FPI mounted on the gantry of a medical linear accelerator (6 MV x rays). The SNR in volume reconstructions was measured as a function of dose and found to agree reasonably with theoretical predictions. These results confirm the theoretically predicted advantages of employing kV energy x rays in imaging soft-tissue structures found in the human body. While MV CBCT may provide a valuable means of correcting 3D setup errors and may offer an advantage in terms of simplicity of mechanical integration with a linear accelerator (e.g., implementation in place of a portal imager), kV CBCT offers significant performance advantages in terms of image contrast and SNR per unit dose for visualization of soft-tissue structures. The relatively poor SNR performance at MV energies is primarily a result of the low x-ray quantum efficiencies (approximately a few percent or less) that are currently achieved with FPIs at high energies. Furthermore, kV CBCT with an FPI offers the potential of combined volumetric and radiographic/fluoroscopic imaging using the same device.

Dose-Response Relationship, Radiation↗