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At least 1,423 records · Page 79Linked to original sources

PubMed1997

Skin dose reduction by a clinically viable magnetic deflector.

A variable magnetic deflector which attaches onto the treatment head of a linear accelerator has reduced skin dose by as much as 65% for 6MV x-rays. The magnetic deflector is constructed from Neodymium Iron Boron (NdFeB) rare earth magnets. It weighs approximately 15 kg and is designed to easily fit onto the accessory mount of a clinical linear accelerator. All field sizes are attainable up to 35 cm x 35 cm at 100 cm SSD. The gap between the magnetic poles can be adjusted, providing the highest field strength for each field size. Magnetic field strengths up to 0.55 Tesla are attainable. For a 6MV x-ray beam with a 10 mm perspex block tray, surface dose is reduced from 29% to 14% and from 59% to 37% for a 20 cm x 20 cm and 35 cm x 35 cm field size, respectively. Results at varying SSD's have shown at least 10 cm of space must be allowed between the magnets and patient for adequate reduction of skin dose through removal of electron contaminants.

Humans↗

PubMed1997

Leaching of accelerator-produced radionuclides.

Leaching of radionuclides produced in soil and rock by high energy proton-induced radiation was studied for the SSC site. Comparison was made with predictions of a Monte Carlo code CASIM and previous results for the Fermilab site. The principal long-lived radionuclides were 3H and 22Na in agreement with Fermilab results. A few other radionuclides were present at lower concentrations in a subset of the samples. For example, 134Cs was detected in a few SSC water samples. Leaching from SSC chalk was dependent on previous weathering and on leaching time. The more soil-like marl and shale were leached more rapidly. Results of this study, in conjunction with the SSC groundwater model, show that adequate groundwater protection would have been maintained for an accidental loss of the entire proton beam at a point in the SSC Collider tunnel. Early warning techniques developed are directly applicable to soil activation monitoring at other facilities.

Cesium Radioisotopes↗

PubMed1986

An intracavitary cone system for electron beam therapy using a Therac 20 linear accelerator.

The Therac 20 is an AECL medical linear accelerator that produces electron and photon beams. Electron fields are produced by a scanned beam; collimation is provided by two sets of primary collimators and further collimated by external electron trimmers located 11 cm above the plane of isocenter (100 cm). These collimators are not suitable for intracavitary treatment. To overcome this limitation, we have designed an intracavitary cone system that attaches to the electron trimmers. Since the trimmers do not have to be removed while this system is in use, there is no need to bypass the associated interlock system. The apparatus consists of a platform which slides onto the lower set of trimmers, onto which a lead insert is attached. Dosimetry measurements for 9, 13, and 17 MeV electron beams are reported for three different treatment cones.

Humans↗

PubMed1999

A new polymer gel for magnetic resonance imaging (MRI) radiation dosimetry.

New composition polymer gels, the N-vinylpyrrolidone argon (VIPAR) gels, were developed and investigated as MRI dosimeters. VIPAR gels were irradiated in the dose range of 0-12 Gy by a 6 MV x-ray linear accelerator and MR-scanned in a 1.5 T magnetic resonance imager. A linear relationship was found between absorbed dose and spin spin relaxation rate R2. The dose sensitivity was found to be approximately 0.1 s(-1) Gy(-1) for a gel composition of 4% w/w in N-vinylpyrrolidone, 4% w/w in N,N'-methylene-bisacrylamide, 5% w/w in gelatine type A and 87% w/w in water. This dose sensitivity was stable with time and did not deteriorate even when a boost radiation dose of 2.5 Gy was applied 15 days after the first irradiation. Good reproducibility of these results was observed when a new batch of gels was produced and used for corresponding measurements and analysis.

Gels↗

PubMed2006

Evaluation of the radioactivity of the pre-dominant gamma emitters in components used at high-energy proton accelerator facilities.

At a high-energy, high-intensity proton accelerator facility, the accelerator components inside the beam-line tunnel and the concrete wall of the tunnel are activated due to beam loss during operation. During maintenance work inside the tunnel, radiation emitted from these activated materials can cause exposure to radiation workers. In this study, the relation between the surface dose rate of activated materials and the induced radioactivity was considered, and an evaluation of the major radioactivity using the surface dose rate was made. It has been revealed that (22)Na in Al samples and (54)Mn in Fe samples, (60)Co and (54)Mn in Fe samples, and (60)Co in Fe and Cu samples are principal residual radionuclides after a few hundred days of cooling about a few years of cooling and approximately 10 y of cooling, respectively.

Construction Materials↗

PubMed1985

Radiation measurements in a labyrinth penetration at a high-energy proton accelerator.

The efficient design of access penetrations at high-energy proton accelerators is desirable for both economic and personnel protection reasons. This paper reports on a series of measurements made in a personnel access labyrinth which viewed an A1 target bombarded by 400-GeV protons from the Fermilab Tevatron. Measurements of absorbed dose in the labyrinth using tissue-equivalent ion chambers were consistent with theoretical predictions of both the relative attenuation through the penetration and the absolute magnitude near the target. The multisphere technique was used to determine the neutron energy spectrum in one section of the labyrinth. A recombination chamber was used to measure the quality factor of the radiation field in two sections of the labyrinth. Good agreement with the quality factor deduced from the multisphere result was obtained at the same measurement location.

Neutrons↗

PubMedDate not supplied

An accurate temperature controller for linear accelerators.

Linear accelerators use water cooling systems to ensure that certain components operate at specific temperatures. The water cooling system used on a Siemens Mevatron 77 has been found by us to be inadequate for our purposes for three major reasons. First, the controller is not able to regulate the machine temperature accurately when the beam is on for more than 10 min. Second, there is a high risk of failure at our site due to a high level of impurities in the utility water, and finally, changes in the temperature of the utility water necessitate adjustments to the temperature controller. A microprocessor-based temperature controller has been designed and built for use with a Mevatron 77. The three areas of concern mentioned have been eliminated by the use of the new controller.

Humans↗

PubMed1994

Dosimetric parameters of a modified set of wedges for use with asymmetric fields of a 6 MV linear accelerator.

A set of standard wedge filters has been modified for use with half-collimated beams of a 6 MV linear accelerator. The position of the standard size wedge filter has been shifted as far to one side of the wedge plate to ensure optimum half-collimated field coverage (up to 20 x 30 cm) required in certain clinical situations. Dosimetric parameters were normalized at 1.5 cm depth and at an off-axis reference point (3.5 cm from the central axis of the collimator at 100 cm SSD. The shapes of the wedged profile and isodose curves of the modified wedges remained similar to those of standard wedges. Data presented include wedge transmission factors, wedge angles, beam profiles, and isodose distributions. The clinical advantages of using modified wedge filters (larger field size, larger transmission, and smaller weight) over standard large wedges is discussed.

Humans↗

PubMed1995

Dose rates and energy spectra in the maze of a linear accelerator treatment room.

When designing an entrance maze for a linear accelerator facility, it is useful to be able to estimate both the dose rate at the entrance and, in case further shielding is necessary, the energy spectrum of the scattered photons reaching the maze entrance. This study examines the applicability of the commonly used empirical formula contained in the National Council on Radiation Protection and Measurements (NCRP) Report No. 51 for estimating the dose rates at the maze entrance in a typical treatment room, and proposes an improvement allowing for the fact that the initial scattering event occurs most often in the patient at the isocentre. The X-ray energy spectrum at the maze entrance is also measured and shown to have a lower average energy than is assumed in NCRP Report No. 51.

Algorithms↗

PubMed1989

[Conservative treatment of malignant liver tumors].

Conservative treatment was carried out in 119 cases of hepatic malignancy (primary cancer--36 and metastatic tumors--83). Radiation therapy using a Rokus-M installation or a Saturn linear accelerator was performed in 26 cases, chemoradiation treatment--14 and cytostatic therapy alone--in 79 patients. Single focal dose used was 2 Gy, while total dose was as high as 51-70 Gy in cases receiving radiation alone and 35-40 Gy in the combined therapy group. Various combinations of carminomycin, bleomycin, cisplatinum, adriamycin, 5-fluorouracil, allopurinol and cyclophosphamide were employed. Radiation and chemoradiation treatment proved effective in 67.5% of cases. Mean survival time for patients suffering primary cancer was 15.8 +/- 4.3 months, and for those with metastatic tumor--8.3 +/- 1.6 months. Remission was registered in 14, while stabilization--in 31 out of 79 patients who had received chemotherapy. The symptomatic effect was observed in 38%.

Adult↗

PubMed1995

Analytical representation of head scatter factors for shaped photon beams using a two-component x-ray source model.

An analytic representation proposed for the relative intensity distribution of the extra-focal source in a two-component x-ray source model serves as the basis for calculation. From this representation, a closed-form expression for head scatter factors defined on the central beam axis is derived by integrating over the area of the extra-focal source plane visible from the measurement point. The resulting expression is applicable to photon beams from different Varian accelerators and different photon energies, and includes effects arising from beam shaping with cerrobend blocks or multileaf collimators (MLCs). For 6- and 15-MV photon beams from Varian 600 C and 2300 CD accelerators (Varian Oncology Systems, Palo Alto, CA), 361 measured head scatter factors for square, rectangular, asymmetric, and arbitrarily shaped fields, formed by either the X and Y jaws, the MLC and Y jaws, or by the MLC alone, were compared with model calculations. Results show that 93.4% of calculated values match corresponding experimental points to within 0.5%, the average deviation being 0.23% and the maximum deviation 0.9%. Thus, as a consequence of this work, the different influence of the X jaws, the Y jaws, and the MLC on head scatter factors is quantitatively described. In particular, it is demonstrated that in the case of radially symmetric scattering, the collimator exchange effect arises as a result of the different distances of the X and Y jaws from the focal spot.

Biophysical Phenomena↗

PubMed1996

Dosimetric considerations for a multileaf collimator system.

A commercial linear accelerator with a factory-fitted multileaf collimator (MLC) was commissioned for clinical use. Measurements made of dosimetric parameters included central axis depth-dose, field-size factors, wedge factors, penumbra, and leaf leakage for the 6-MV and 15-MV photon beams available on this accelerator. The depth-dose characteristics, output factors, and transmission factors were similar to those reported in the literature for a machine by the same manufacturer with a standard treatment head. Because of scalloping, the effective penumbra for the MLC was 3 to 4 mm wider than that for the conventional collimator jaws. The output for the fields shaped by the MLC was generally lower than that for similar fields shaped with Lipowitz's metal (Cerrobend). The magnitude of the difference was field-size dependent and ranged from 0.5% to 4.5% for open shaped fields, increasing to 1% to 5% in the presence of wedges. Further analysis of this observation has shown it to be primarily due to differences in the scattered radiation from the collimator head.

Biophysical Phenomena↗

PubMed1996

Analytical representation of head scatter factors for shaped photon beams using a two-component x-ray source model.

An analytic representation proposed for the relative intensity distribution of the extra-focal source in a two-component x-ray source model serves as the basis for calculation. From this representation, a closed-form expression for head scatter factors defined on the central beam axis is derived by integrating over the area of the extra-focal source plane visible from the measurement point. The resulting expression is applicable to photon beams from different Varian accelerators and different photon energies, and includes effects arising from beam shaping with cerrobend blocks or multileaf collimators (MLCs). For 6- and 15-MV photon beams from Varian 600C and 2300CD accelerators (Varian Oncology Systems, Palo Alto, CA), 361 measured head scatter factors for square, rectangular, asymmetric, and arbitrarily shaped fields, formed by either the X and Y jaws, the MLC and Y jaws, or by the MLC alone, were compared with model calculations. Results show that 93.4% of calculated values match corresponding experimental points to within 0.5%, the average deviation being 0.23% and the maximum deviation 0.9%. Thus, as a consequence of this work, the different influence of the X jaws, the Y jaws, and the MLC on head scatter factors is quantitatively described. In particular, it is demonstrated that in the case of radially symmetric scattering, the collimator exchange effect arises as a result of the different distances of the X and Y jaws from the focal spot.

Biophysical Phenomena↗

PubMed2004

Propagation characteristics of neutrons leaking from the accelerator facilities.

In this study spatial and time distribution of neutrons leaking from Hokkaido University 45 MeV Electron Linac facility have measured and compared with the Monte Carlo simulations. The neutron transport processes inside and outside the facility building has been simulated using MCNP. The neutrons have measured by BF3 counters and 3He counters with polyethylene moderators up to the distance of 330 m from the facility. The spatial distribution of ambient dose equivalent converted from the counts has been compared with the simulation. The distribution estimated from the counts by the BF3 counter has been shown fairly good agreement with the calculation. The spatial distribution of counts obtained at the 45 MeV Electron Linac facility has been compared with that obtained at the Fusion Neutronics Source (FNS) facility of JAERI. The difference between the propagation characteristics of neutrons leaking from those facilities has been discussed.

Algorithms↗

PubMedDate not supplied

Evaluation of electron-beam uniformity during commissioning of a linear accelerator.

Electron-beam profiles measured during acceptance testing and commissioning of a linear accelerator were analyzed according to manufacturer's specifications and more stringent AAPM TG-25 recommendations. The difference between specifications of the AAPM report and those experienced commercially is discussed. Although all the electron beams met the manufacturer's specifications, the uniformity parameters for some of them were outside the limits suggested by the AAPM report. Profiles acquired at depths larger than dmax were extremely valuable for evaluation and adjustment of the beams. The following suggestions for practical machine commissioning are made: Profiles taken deeper than at dmax are extremely useful indicators of the beam nonuniformity; acquisition of these profiles should be a part of the routine acceptance testing procedure; and beam uniformity should be evaluated against AAPM recommendations, and this should be included in the purchase specifications.

Biophysical Phenomena↗

PubMed1994

Dosimetric accuracy of a dual photon energy linac at low monitor setting for various pulse repetition frequencies.

Accuracy of dose delivery at low monitor unit setting is studied for a dual photon energy linear accelerator. Dose delivered per MU is found to be constant for both the photon beams for MU settings above 30. For lower MUs there is definite deviation from the calibrated value and the error is found to be increasing as fewer MUs are set for dose delivery. This dose/MU ratio at low MU setting is found to be dose-rate dependent, showing an increasing trend with pulse repetition frequency (PRF). Also, the dosimetric ratio is observed to be mode dependent; its value for an 18 MV beam is almost double that observed in the case of a 6 MV beam at very low MU setting. The magnitude of this error should be determined for each energy so that appropriate corrections can be applied if very low MUs are to be used.

Particle Accelerators↗

PubMed2001

[Constancy check of the radiation output of Linac: comparison with diodes and ionization chamber].

For routine QA measurements of the radiation output of a linear accelerator, an ionization chamber is usually used, the type of which may depend on the radiation quality. In this study, a comparison of the standard ionization chamber and the central diodes of an array, was performed both used simultaneously for the constancy check of the radiation output. It was investigated to which degree the two detectors deviate from each other over a long period of time for different radiation qualities. The aim of the study was to show whether the radiation output can be checked using the diode array, which is usually used for measuring beam profiles, by averaging the values of the central diodes within a reasonable accuracy. The results showed a maximum deviation of the two detectors of 0.9-2.2%, whereas the mean deviation is 0.4-0.8%, both depending on the radiation quality.

Humans↗

PubMed1996

Implementation of the Varian EDW into a commercial RTP system.

An enhanced dynamic wedge system (EDW) has been installed onto our Clinac 600C (6 MV) linear accelerator. This paper addresses and describes the measurements taken and subsequent analysis required to enable the planning of EDW using our commercial radiotherapy treatment planning (RTP) system. This implementation into a 'closed' commercial system is performed without the use of specialized measurement equipment or the necessity of introducing new calculation algorithms and is therefore independent of the RTP manufacturer. We consider that, for incorporation of techniques such as EDW into routine clinical use, a simple verifiable method of inclusion into the RTP system must be achieved. This paper also presents a methodology for quality control of dynamic fields chosen for clinical use which is quick and easy to use by virtue of its use of film dosimetry. We present the method of film calibration used in this work.

Equipment Design↗