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PubMed2000

Dose calculations for asymmetric fields defined by independent collimators using symmetric field data.

Several methods have been developed for the dosimetry of asymmetric radiation fields formed by independently moving collimator jaws. Three of these methods, based on different principles and modified to comply with our set of available data, are utilized for the calculation of asymmetric field dose profiles. All three methods use output factors and per cent depth doses or tissue maximum ratios of symmetric fields. In the first method, calculation of the off-centre ratio (OCR) of the asymmetric field is based on the symmetric field from which the asymmetric is originated, by setting the one jaw in an asymmetrical position. In the second method the OCR of the symmetric field is used for the OCR calculation of the asymmetric field of the same size; whereas the third method does not allow for the asymmetric OCR calculation. The results obtained using data for the 6 MV photon beam of a Philips SL-20 linear accelerator indicate that both the first and second method can accurately reproduce asymmetric field profiles from symmetric field data; the third method does not allow for penumbra reproduction, but it is accurate at the central part of the asymmetric field. The problems encountered in the application of the three methods are reported and their accuracy is compared.

Humans↗

PubMed2002

[Depth dose characteristics of photon beams released from a scanning-type racetrack microtron].

The MM50 is a racetrack microtron that can emit photon beams or electron beams up to 50 MeV. The MM22 using the scanning beam method and the MM22 using a flattening filter method both to flatten the emission field and a water phantom with particular function measurable of PDD etc. in an accelerator using the scanning beam method to make up the PDD curve of photon beams from the linear accelerator. The Clinac21EX was thus employed. The maximum depth of beam flux was shallow, the gradient of the flux decrement large, the surface dose large, and the estimated nominal energy low to the same nominal energy. From these findings, it can be said that thorough comprehension of the characteristics of beam flux properties for these units is necessary when photon beams are to be used.

Particle Accelerators↗

PubMed1989

Stereotactically guided convergent beam irradiation with a linear accelerator: localization-technique.

A stereotactic convergent beam irradiation technique using a linear accelerator has been developed in order to precisely apply single high doses of up to 50 gray and more to brain lesions (radiosurgery). Accurate positioning of the patient and the target point of irradiation is an absolute requirement for this method. The stereotactic localization system developed for this purpose is described.

Humans↗

PubMed1989

The University of Florida radiosurgery system.

A new, linear accelerator based radiosurgical system has been designed and tested. It incorporates a mechanical system of precision bearings to control all patient and accelerator movements. The dosimetry system allows near real-time examination of the isodose distributions in any computed tomography plane such that any treatment plan can be quickly optimized. Extensive testing shows a radiation beam accuracy of .2 +/- .1 mm. The dose gradient between the 90% and 10% isodose distributions compares favorably with any previously published radiosurgical method.

Angiography↗

PubMed2000

Fractionated stereotactic radiation therapy and single high-dose radiosurgery for acoustic neuroma: early results of a prospective clinical study.

PURPOSE: To prospectively assess the local control and toxicity rate in acoustic neuroma patients treated with linear accelerator-based radiosurgery and fractionated stereotactic radiation therapy. METHODS AND MATERIALS: We evaluated 37 consecutive patients treated with stereotactic radiation therapy for acoustic neuroma. All patients had progressive tumors, progressive symptoms, or both. Mean tumor diameter was 2.3 cm (range 0.8-3.3) on magnetic resonance (MR) scan. Dentate patients were given a dose of 5x4 Gy or 5x5 Gy and edentate patients were given a dose of 1x10 Gy or 1x12.50 Gy prescribed to the 80% isodose. All patients were treated with a single isocenter. RESULTS: With a mean follow-up period of 25 months (range 12-61), the actuarial local control rate at 5 years was 91% (only 1 patient failed). The actuarial rate of hearing preservation at 5 years was 66% in previously-hearing patients. The actuarial rate of freedom from trigeminal nerve toxicity was 97% at 5 years. No patient developed facial nerve toxicity or other complications. CONCLUSION: In this unselected series, fractionated stereotactic radiation therapy and linear accelerator-based radiosurgery give excellent local control in acoustic neuroma. It combines a high rate of preservation of hearing with a very low rate of other toxicity, although follow-up is relatively short.

Adult↗

PubMed2001

[The Plexitom, a new quality assurance tool for the linear accelerator].

Continuous depth dose measurements are generally performed in water phantoms. It is now possible to make continuous depth dose measurements in the solid PMMA phantom Plexitom, developed by our group and manufactured by PTW. The mechanical construction of this phantom allows the continuous movement of the ionisation chamber within the solid phantom. The Plexitom is light and very handy. It is very useful for quality assurance, especially to check the radiation quality of the linac.

Equipment Design↗

PubMedDate not supplied

Phosphorus activation neutron dosimetry and its application to an 18-MV radiotherapy accelerator.

Neutron fluxes and dose rates in and near the 18-MV x-ray beam of a Therac-20 accelerator were determined with measured activities from the nuclear reactions 31P(n, rho)31Si (fast neutrons) and 31P(n, gamma)32P (thermal neutrons), published cross sections, and neutron energy spectra from Monte Carlo calculations. Measurements were made in the patient plane in air and at a 10-cm depth in a tissue-similar phantom, and in a plane containing the x-ray target. Orthophosphoric acid solution was identified as a suitable and convenient phosphorus dosimeter material. In the 31P activation method, fluxes and dose rates are determined as the product of measured saturation activity per 31P atom and a conversion factor, which depends on the shape of the assumed neutron spectrum. For fast neutrons, which deliver most of the dose, the accuracy error in the saturation activity determinations was shown to be approximately less than 25%. An inconsistency resulting from neglect of the accelerator's adjustable collimator in the Monte Carlo calculations was demonstrated between the measured saturation activities and the theoretical neutron spectra. The maximum neutron dose equivalent rate observed was 5.9 mSv/Gy of x-ray absorbed dose at the accelerator calibration point. Surface dose equivalent rates of the present study are less than those of fluxmeter and remmeter studies at sites outside Therac-20 treatment fields by as much as factors of 2.4 and 2.8, respectively. The phantom study showed that at 18 MV internally produced neutrons have a negligible effect on the neutron field within the patient.

Neutrons↗

PubMedDate not supplied

Dosimetric aspects of a 3.3-MV linear accelerator.

Both the design considerations and the dosimetric properties of the Siemens Model 5800 linear accelerator are discussed. This unit is of such an energy (3.3 MV) as to imitate Cobalt-60 teletherapy depth doses. A linear relation of dmax to depth dose at low energies was found for various wave guides and targets. The energy of the unit can be characterized by its nominal accelerating potential of 2.70 MV, its d80 of 5.3 cm, its first half-value layer of 0.8 cm lead and the measured energy of the electron beam at 3.3 MeV. The following selected commissioning aspects are reported: central axis depth dose, relative output factors, beam profiles, wedge factors, virtual source position, back scatter factors, penumbra and build-up region.

Humans↗

PubMed1991

Dosimetric accuracy at low monitor unit settings.

Dosimetric accuracies at low monitor units are evaluated for linear accelerators from various manufacturers. A large error is observed in the majority of the accelerators. The error can be positive or negative. Although the error can exceed 20% for the first few monitor units, it is usually less than 5% when more than 10 monitor units are delivered. When low doses are required proper precautions should be taken for dosimetric accuracy including the beam energy, beam flatness and dose per monitor unit.

Humans↗

PubMed1999

Origins of the short latency vestibular evoked potentials (VsEPs) to linear acceleration impulses.

OBJECTIVE: To verify the vestibular origin of the short latency (t < 12.5 msec) vestibular evoked potentials (VsEPs) in response to linear acceleration impulses (L-VsEPs) and to differentiate between the contributions of the otolith organs and the semi-circular canals (SCCs) to their initiation. DESIGN AND METHODS: L-VsEPs (stimulus intensity, 3 g; rise time, 1.0 to 1.5 msec) were recorded in fat sand rats (Psammomys obesus) before and after unilateral labyrinthectomy, plugging of the SCCs in the remaining ear, and bilateral labyrinthectomy. Auditory nerve brainstem evoked responses (ABRs) and VsEPs to angular acceleration impulses (A-VsEPs) were also recorded. Wave amplitudes and latencies were statistically analyzed (MANOVA, repeated t-tests). RESULTS: In the intact animal, the linear VsEPs consisted of 5 to 6 waves, several mV in amplitude, with short latencies. The latency of the first wave was 2.0 msec. These waves were abolished after bilateral labyrinthectomy. Before and after plugging of the SCCs, linear acceleration VsEP wave latencies did not change, although amplitudes were slightly reduced. Similar results were obtained with respect to ABRs recorded from the same ear. Angular acceleration VsEPs were abolished after SCC plugging. CONCLUSIONS: These and other results confirm that the linear VsEPs are compound action potentials of the vestibular pathway, the first wave is the response of the vestibular nerve, and they are initiated mainly in the otolith organs.

Animals↗

PubMed2000

Simulations of a meter-long plasma wakefield accelerator.

Full-scale particle-in-cell simulations of a meter-long plasma wakefield accelerator (PWFA) are presented in two dimensions. The results support the design of a current PWFA experiment in the nonlinear blowout regime where analytic solutions are intractable. A relativistic electron bunch excites a plasma wake that accelerates trailing particles at rates of several hundred MeV/m. A comparison is made of various simulation codes, and a parallel object-oriented code OSIRIS is used to model a full meter of acceleration. Excellent agreement is obtained between the simulations and analytic expressions for the transverse betatron oscillations of the beam. The simulations are used to develop scaling laws for designing future multi-GeV accelerator experiments.

Journal Article↗

PubMed1992

Custom beam profiles in computer-controlled radiation therapy.

A computer-controlled radiation therapy technique is demonstrated which uses multiple concurrent boost fields to modify the beam profile of a conventional treatment beam. A principal field, identical to that of a corresponding conventional treatment plan, delivers the major component of the prescribed dose. Dose increments given from boost fields placed within this principal field compensate for variations in patient anatomy, for variations in target volume shape, and/or for imperfect beam characteristics, such as excessive off-axis dose or inadequate beam wedge angle. This concurrent boost field technique is demonstrated for several treatment sites. It produces significant improvement in uniformity of dose delivered to the target compared to conventional treatment. Implementation of these treatments requires a computer-controlled linear accelerator with independently-movable collimator jaws, an automatic beam set-up procedure, and a patient prescription database. Since all fields are delivered under computer control, concurrent boost technique treatment times are not much longer than those of conventional treatments.

Feasibility Studies↗

PubMed1994

Estimation of peripheral dose from two linacs: Mevatron MX6700 and Mevatron KDS.

An empirical calculation method for high-energy beam peripheral dose estimation is described. The peripheral dose has been measured for a Siemens Mevatron MX6700 (6 MV) and a Siemens Mevatron KDS (6 and 18 MV) linear accelerators. The dose distribution is parameterized for each beam energy as a function of depth, distance from the edge of the field, and field size. A simple algorithm has been developed for dose calculation up to 100 cm from the field central axes. Predictions by this algorithm are compared with measurements in an Alderson phantom.

Particle Accelerators↗

PubMed1997

Dosimetric comparison of treatment planning systems in irradiation of breast with tangential fields.

PURPOSE: The objectives of this study are: (1) to investigate the dosimetric differences of the different treatment planning systems (TPS) in breast irradiation with tangential fields, and (2) to study the effect of beam characteristics on dose distributions in tangential breast irradiation with 6 MV linear accelerators from different manufacturers. METHODS AND MATERIALS: Nine commercial and two university-based TPS are evaluated in this study. The computed tomographic scan of three representative patients, labeled as "small", "medium" and "large" based on their respective chest wall separations in the central axis plane (CAX) were used. For each patient, the tangential fields were set up in each TPS. The CAX distribution was optimized separately with lung correction, for each TPS based on the same set of optimization conditions. The isodose distributions in two other off-axis planes, one 6 cm cephalic and the other 6 cm caudal to the CAX plane were also computed. To investigate the effect of beam characteristics on dose distributions, a three-dimensional TPS was used to calculate the isodose distributions for three different linear accelerators, the Varian Clinac 6/100, the Siemens MD2 and the Philips SL/7 for the three patients. In addition, dose distributions obtained with 6 MV X-rays from two different accelerators, the Varian Clinac 6/100 and the Varian 2100C, were compared. RESULTS: For all TPS, the dose distributions in all three planes agreed qualitatively to within +/- 5% for the "small" and the "medium" patients. For the "large" patient, all TPS agreed to within +/- 4% on the CAX plane. The isodose distributions in the caudal plane differed by +/- 5% among all TPS. In the cephalic plane in which the patient separation is much larger than that in the CAX plane, six TPS correctly calculated the dose distribution showing a cold spot in the center of the breast contour. The other five TPS showed that the center of the breast received adequate dose. Isodose distributions for 6 MV X-rays from three different accelerators differed by about +/- 3% for the "small" patient and more than +/- 5% for the "large" patient. For two different 6 MV machines of the same manufacturer, the isodose distribution agreed to within +/- 2% for all three planes for the "large" patient. CONCLUSION: The differences observed among the various TPS in this study were within +/- 5% for both the "small" and the "medium" patients while doses at the hot spot exhibit a larger variation. The large discrepancy observed in the off-axis plane for the "large" patient is largely due to the inability of most TPS to incorporate the collimator angles in the dose calculation. Only six systems involved agreed to within +/- 5% for all three patients in all calculation planes. The difference in dose distributions obtained with three accelerators from different manufacturers is probably due to the difference in beam profiles. On the other hand, the 6 MV X-rays from two different models of linear accelerators from the same manufacturer have similar beam characteristics and the dose distributions are within +/- 2% of each other throughout the breast volume. In general, multi-institutional breast treatment data can be compared within a +/- 5% accuracy.

Body Constitution↗

PubMed1999

A radiographic and tomographic imaging system integrated into a medical linear accelerator for localization of bone and soft-tissue targets.

PURPOSE: Dose escalation in conformal radiation therapy requires accurate field placement. Electronic portal imaging devices are used to verify field placement but are limited by the low subject contrast of bony anatomy at megavoltage (MV) energies, the large imaging dose, and the small size of the radiation fields. In this article, we describe the in-house modification of a medical linear accelerator to provide radiographic and tomographic localization of bone and soft-tissue targets in the reference frame of the accelerator. This system separates the verification of beam delivery (machine settings, field shaping) from patient and target localization. MATERIALS AND METHODS: A kilovoltage (kV) x-ray source is mounted on the drum assembly of an Elekta SL-20 medical linear accelerator, maintaining the same isocenter as the treatment beam with the central axis at 90 degrees to the treatment beam axis. The x-ray tube is powered by a high-frequency generator and can be retracted to the drum-face. Two CCD-based fluoroscopic imaging systems are mounted on the accelerator to collect MV and kV radiographic images. The system is also capable of cone-beam tomographic imaging at both MV and kV energies. The gain stages of the two imaging systems have been modeled to assess imaging performance. The contrast-resolution of the kV and MV systems was measured using a contrast-detail (C-D) phantom. The dosimetric advantage of using the kV imaging system over the MV system for the detection of bone-like objects is quantified for a specific imaging geometry using a C-D phantom. Accurate guidance of the treatment beam requires registration of the imaging and treatment coordinate systems. The mechanical characteristics of the treatment and imaging gantries are examined to determine a localizing precision assuming an unambiguous object. MV and kV radiographs of patients receiving radiation therapy are acquired to demonstrate the radiographic performance of the system. The tomographic performance is demonstrated on phantoms using both the MV and the kV imaging system, and the visibility of soft-tissue targets is assessed. RESULTS AND DISCUSSION: Characterization of the gains in the two systems demonstrates that the MV system is x-ray quantum noise-limited at very low spatial frequencies; this is not the case for the kV system. The estimates of gain used in the model are validated by measurements of the total gain in each system. Contrast-detail measurements demonstrate that the MV system is capable of detecting subject contrasts of less than 0.1% (at 6 and 18 MV). A comparison of the kV and MV contrast-detail performance indicates that equivalent bony object detection can be achieved with the kV system at significantly lower doses (factors of 40 and 90 lower than for 6 and 18 MV, respectively). The tomographic performance of the system is promising; soft-tissue visibility is demonstrated at relatively low imaging doses (3 cGy) using four laboratory rats. CONCLUSIONS: We have integrated a kV radiographic and tomographic imaging system with a medical linear accelerator to allow localization of bone and soft-tissue structures in the reference frame of the accelerator. Modeling and experiments have demonstrated the feasibility of acquiring high-quality radiographic and tomographic images at acceptable imaging doses. Full integration of the kV and MV imaging systems with the treatment machine will allow on-line radiographic and tomographic guidance of field placement.

Animals↗

PubMed1997

Blocked field effects on collimator scatter factors.

In routine dosimetry we assume separability of the collimator (Sc) and phantom (Sp) scatter components that together comprise the total scatter factor (Sc,p). In practice, the addition of blocking also affects the photon fluence attributable to the treatment head and flattening filter in a complicated way. The reduced aperture blocks out some of the head scatter contribution, while the block and tray add back secondary scatter. In the following we present techniques for directly measuring the aperture effect on Sc in air or in a full-scatter phantom. The change in Sc is found to be a scaleable quantity that can be modelled as a simple linear fit to the ratio of projected open-to-blocked equivalent square fields. Measurements have been made for 6, 18 and 24 MV photon beams on one Varian 2500 and two Varian 2100c accelerators. Results indicate a progressive loss of collimator scatter contribution with increased field blocking that is amplified with increasing energy. Block and tray scatter only contribute significantly to Sc for large fields and treatment distances of 80 cm or less. Application of these corrections in monitor unit calculations is presented.

Least-Squares Analysis↗

PubMed2005

Gelina neutron target optimisation.

A study is being performed on the properties of the Geel Electron Linear Accelerator (GELINA), a powerful white neutron source, designed for the high-energy resolution time-of-flight measurements. The main aim of this study is to reduce the time spread of neutrons of the given energy without compromising the neutron yield. Both time spread and neutron intensity influence the experimental accuracy of high-resolution neutron cross section measurements, which are particularly important in the resonance region. The quantities of interest have been simulated with coupled electron-photon-neutron steady state and transient MCNP4C3 calculations. Following benchmarking of the code to the properties of the existing target, neutron yield, energy spectra, resolution functions, and neutron and heat spatial distributions have been determined for various alternative geometries and materials. At a fixed accelerator power, actinides deliver the highest neutron yield and a small target provides the best time resolution. The resulting high-power density requires a joint optimisation of the thermal hydraulics and neutronics properties.

Computer-Aided Design↗

PubMed1982

Air activation by an electron synchrotron.

The activation of 13N and 15O in air of the room housing the INS electron synchrotron was estimated. The total induced activity of 13N and 15O in the room was measured with an NaI(T1) scintillator to be 119.7 and 47.0 muCi, respectively, immediately after the operation of the electron synchrotron at 725-MeV electron energy and 176 W beam power. The operation was long enough for the steady-state concentrations of 13N and 15O to be reached. The generation of 13N and 15O was also evaluated, with only about 20% underestimation using a model calculation. In this calculation the emitted photon flux and distribution were calculated by the Monte-Carlo code, EGS, for electromagnetic cascade showers, and these photon distributions were multiplied by the (gamma, n) photoneutron reaction cross sections to calculate the production of 13N and 15O. From this analysis, it is clear that the usual method of estimating the photon spectrum at an electron accelerator, by using the forward bremsstrahlung spectrum of a thin target, is quite inaccurate when electrons are incident on a bulk structure or impinge on a magnet at a small angle of incidence. In such a case, one must consider that the photons are emitted mainly by backscattering, and have a considerably softer spectrum than the thin-target bremsstrahlung.

Air↗