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PubMedDate not supplied

Head-scatter factors in rectangular photon fields.

Head-scatter factors were measured for a range of field sizes of rectangular shape from a linear accelerator that provides 6 and 25 MV photon beams. For a given field length and width, exchange of the inner and outer collimator pairs produces differences in the head-scatter factor of up to about 2% and 3% in open and wedged fields, respectively. Dependence on elongation deviates by up to 2% from that given by the equivalent squares. Such effects can easily be taken into account by using head-scatter data that have been directly measured in rectangular fields or by applying empirical corrections to values for square or equivalent square fields.

Biophysical Phenomena↗

PubMed2000

Dose calculation and verification of intensity modulation generated by dynamic multileaf collimators.

While the development of inverse planning tools for optimizing dose distributions has come to a level of maturity, intensity modulation has not yet been widely implemented in clinical use because of problems related to its practical delivery and a lack of verification tools and quality assurance (QA) procedures. One of the prerequisites is a dose calculation algorithm that achieves good accuracy. The purpose of this work was twofold. A primary-scatter separation dose model has been extended to account for intensity modulation generated by a dynamic multileaf collimator (MLC). Then the calculation procedures have been tested by comparison with carefully carried out experiments. Intensity modulation is being accounted for by means of a 2D (two-dimensional) matrix of correction factors that modifies the spatial fluence distribution, incident to the patient. The dose calculation for the corresponding open field is then affected by those correction factors. They are used in order to weight separately the primary and the scatter component of the dose at a given point. In order to verify that the calculated dose distributions are in good agreement with measurements on our machine, we have designed a set of test intensity distributions and performed measurements with 6 and 20 MV photons on a Varian Clinac 2300C/D linear accelerator equipped with a 40 leaf pair dynamic MLC. Comparison between calculated and measured dose distributions for a number of representative cases shows, in general, good agreement (within 3% of the normalization in low dose gradient regions and within 3 mm distance-to-dose in high dose gradient regions). For absolute dose calculations (monitor unit calculations), comparison between calculation and measurement reveals good agreement (within 2%) for all tested cases (with the condition that the prescription point is not located on a high dose gradient region).

Algorithms↗

PubMedDate not supplied

Radiation leakage through electron applicators on Clinac-1800 accelerators.

Radiation leakage through electron applicators by 6-, 9-, 12-, 16-, and 20-MeV electron beams from Varian Clinac-1800 has been measured with films. High levels of leakage were found under the corners of applicators and the touch-plate mounting port holes. The radiation leakage, relative to the central-axis dose at dmax, was found to be up to 13% on the patient plane [100-cm source-to-film distance (SFD)] and up to 35% beneath the corners of applicators (96-cm SFD). Up to 18% radiation leakage was measured beneath the touch plate near the mounting port holes (96-cm SFD). The extent of radiation leakage in all electron beams was investigated and some simple shielding solutions to reduce the leakage are suggested.

Electrons↗

PubMed1991

A comparison of techniques for stereotactic radiotherapy by linear accelerator based on 3-dimensional dose distributions.

In order to establish the appropriate beam arrangement for use in stereotactic radiotherapy using a linear accelerator, dose volume distributions were calculated for a number of spherical targets in a head phantom and assessment was made by dose sparing of normal tissue outside the target volume. Using a single isocentre, fixed beam arrangements were compared with single and multiple non-coplanar isocentric arc rotations at target sizes from 10 to 55 mm diameter on a 6 MV Philips linear accelerator. From the dose-volume histograms produced, an arrangement of 3 or 4 arcs of rotation proved most suitable, in terms of sparing of normal tissue outside the target volume to high dose irradiation, across the range of target sizes studied. There was little further benefit with increasing the number of arcs beyond this. At target sizes greater than 20 mm diameter an arrangement of 6 static non-coplanar beams achieved sparing equivalent to multiple arc rotations and may have considerable advantages in the treatment of irregular volumes where customised beam shaping could be employed.

Humans↗

PubMed1991

The use of an universal wedge for asymmetric fields.

Beam collimators on newer linear accelerators may be collimated asymmetric to the central axis. The asymmetric beam has a non-flat profile adjusted to yield fields whose half widths are not symmetric about the central axis. While some treatment planning systems modify their programs to mimic the nonuniformity, ideally it is preferred to have a flat profile under the open beam. We have developed a universal wedge that can be used to flatten the field for a variety of jaw sizes and positions and energies for the Varian 2100C. The wedge flattens the field to +/- 3% over 80% of the field.

Humans↗

PubMed2001

Pre-treatment dosimetric verification by means of a liquid-filled electronic portal imaging device during dynamic delivery of intensity modulated treatment fields.

BACKGROUND AND PURPOSE: Although intensity modulated radiation therapy is characterized by three-dimensional dose distributions which are often superior to those obtained with conventional treatment plans, its routine clinical implementation is partially held back by the complexity of the beam verification. This is even more so when a dynamic multileaf collimator (dMLC) is used instead of a segmented beam delivery. We have therefore investigated the possibility of using a commercially available, liquid-filled electronic portal imaging device (EPID) for the pre-treatment quality assurance of dynamically delivered dose distributions. METHODS AND MATERIALS: A special acquisition mode was developed to optimize the image acquisition speed for dosimetry with the liquid-filled EPID. We investigated the accuracy of this mode for 6 and 18 MV photon beams through comparison with film and ion chamber measurements. The impact of leaf speed and pulse rate fluctuations was quantified by means of dMLC plans especially designed for this purpose. Other factors influencing the accuracy of the dosimetry (e.g. the need for build-up, remanence of the ion concentration in the liquid and bulging of the liquid at non-zero gantry angles) were studied as well. We finally compared dosimetric EPID images with the corresponding image prediction delivered without a patient in the beam. RESULTS: The dosimetric accuracy of the measured dose distribution is approximately 2% with respect to film and ion chamber measurements. The accuracy declines when leaf speed is increased beyond 2 cm/s, but is fairly insensitive to accelerator pulse rate fluctuations. The memory effect is found to be of no clinical relevance. When comparing the acquired and expected distributions, an overall agreement of 3% can be obtained, except at areas of steep dose gradients where slight positional shifts are translated into large errors. CONCLUSIONS: Accurate dosimetric images of intensity modulated beam profiles delivered with a dMLC can be obtained with a commercially available, liquid-filled EPID. The developed acquisition mode is especially suited for fast and accurate pre-treatment verification of the intensity modulated fields.

Dose-Response Relationship, Radiation↗

PubMed1995

Characteristics of the chemical forms of 11C, 13N, and 15O induced in air by the operation of a 100 MeV electron linear accelerator.

To characterize airborne radioactivity induced by the operation of high-energy accelerators, the fractions of aerosol and gaseous components, and the chemical forms of 11C, 13N, and 15O produced in the air of a target room of a 100 MeV electron linear accelerator were studied. Measurements of radioactivity using a particulate air sampling filter and a gas flow-through ionization chamber showed that more than 98% of 11C, 13N, and 15O were present as gaseous forms. Their chemical forms, detected by means of radio-gas chromatography, were 11C as CO2; 13N as N2 and NO; and 15O as O2 and NO. Machine operating conditions, which affect the compositions of the induced radionuclides and of their chemical forms, and the resulting effect on the estimation of internal doses are discussed.

Aerosols↗

PubMed2002

Experimental determination of beam quality conversion factors k(Q) in clinical photon beams using ferrous sulphate (Fricke) dosimetry.

The implementation of protocols based on absorbed dose to water standards requires beam quality conversion factors, k(Q). Calculated values of k(Q) are available for ionization chambers used for reference dosimetry. Ideally, k(Q) should be experimentally determined at the same beam qualities as that of the user. In this work we measure k(Q) factors in clinical photon beams and compare them with calculated and measured values. Beam quality conversion factors are determined for clinical photon beams of nominal energies 4 MV, 6 MV, 15 MV, and 25 MV, for commonly used cylindrical ionization chambers. Twelve chambers of eight different types are used. For three of them, no experimental data have previously been available. The experimental procedure is based on measurements with ionization chambers and Fricke dosimetry in the reference beam (60Co gamma radiation) and in clinical linear accelerator beams. The k(Q) values determined in this work generally agree within 0.5% with previously reported experimental values both when %dd(10)x and TPR2010 are used for beam quality specification. The agreement with calculated data is generally within 0.5%, except for the 15 MV beam. For this beam the measured values are usually between 0.5% and 1% lower than the data taken from the TG-51 protocol or the TRS-398 code of practice. For three NE2571 chambers and three NE2581 chambers, the maximum observed deviation of individual k(Q) values is 0.2% and 0.4%, respectively.

Cobalt Radioisotopes↗

PubMed1978

A method for rapid determination of the energy of electron beams from medical linear accelerators.

A method has been developed using a Varian Clinac-18 LINAC for the rapid determination of electron beam energy for clinically used linear accelerators. The method involves measuring the ionization values in a water or polystyrene phantom with an ion chamber at two predetermined depths. Then with predetermined Bremsstrahlung "tail" values which are presented here or which can be developed by the user, the practical range, Rp can be measured. With the Rp, the effective surface energy Eo of the electron beam can be determined by the Markus range-energy formula. Thus, by measuring just two depth ionization values, one is quickly able to determine the Eo of an electron beam without plotting a full depth ionization curve.

Electrons↗

PubMed2006

Spinal implants and radiation therapy: the effect of various configurations of titanium implant systems in a single-level vertebral metastasis model.

BACKGROUND: The combination of surgery and radiation therapy is a common clinical practice in the treatment of spinal tumors. Although it is known that metallic implants disturb radiation therapy beams, it is not known what kind of dose distributions appear with spinal irradiation in the presence of a spinal implant. The aim of the present study was to investigate the effect of various spinal implant constructs on the dose of radiation delivered to the spinal canal in a single-level metastasis model. METHODS: We performed four spinal implant reconstructions on standard sawbones spine models: posterior instrumentation without anterior column reconstruction, posterior instrumentation with anterior column reconstruction with use of a titanium cage, anterior instrumentation with anterior column reconstruction with use of a titanium cage, and anterior instrumentation with anterior column reconstruction with use of chest tubes filled with bone cement. Irradiation with two different radiation therapy units (a cobalt-60 teletherapy unit and a linear accelerator) was performed twice for each model in a posterior-to-anterior direction, and thermoluminescent dosimeters were used to measure the dose changes in the anterior, middle, and posterior portions of the spinal canal. RESULTS: Compared with the sawbones-only model, the posterior instrumentation reconstructions resulted in a 5% to 7% decrease in the radiation dose delivered to the spinal canal with both radiation therapy units, whereas the anterior instrumentation reconstructions resulted in a 1% decrease in the dose delivered with the linear accelerator unit and a < or = 2% increase in the dose delivered with the cobalt-60 teletherapy unit. When thermoluminescent dosimeters in the middle of the spinal canal were evaluated individually, anterior instrumentation with anterior column reconstruction with use of bone cement-filled chest tubes resulted in a 5.5% increase in the radiation dose delivered with the cobalt-60 teletherapy unit, whereas all of the other instrumentation models resulted in a <1% disturbance in the radiation dose delivered with both radiation therapy units. CONCLUSIONS: The posterior instrumentation systems did not result in the delivery of an increased dose of radiation to the spinal cord, suggesting that current radiation therapy regimens may be performed without additional harm. The anterior instrumentation systems also appeared to be relatively safe when irradiation was performed with the linear accelerator unit. However, when irradiation was performed with use of the cobalt-60 teletherapy unit, there was an increase in the dose of radiation delivered to the spinal canal in the presence of the anterior instrumentation systems, particularly the anterior column reconstruction with use of bone cement-filled chest tubes. These dose-perturbation characteristics might be important to consider during the calculation of radiation therapy protocols for patients who are going to receive high doses or recurrent treatments that would reach the tolerance limits of the spinal cord.

Humans↗

PubMedDate not supplied

Biomedical applications of cyclotrons and review of commercially available models.

The growing use of cyclotrons in biomedicine, both for clinical and research purposes and in particular for the production of short-lived radionuclides which are extremely useful in nuclear medicine diagnosis, has reached a stage in which commercial companies are able to offer several models with different performances, in order to satisfy the demand of different users. Many of these commercially produced accelerators are installed all over the world and some of them have been operating for several years, demonstrating that this category of machine has reached a high degree of reliability. A brief description of the operating principle of the cyclotron is presented, together with an illustration of its possible applications in the medical field. A list of the models presently available on the market is given and the installation problems and the criteria to be followed in the choice of a model are discussed. Finally, likely future developments in the field are briefly discussed.

Fast Neutrons↗

PubMed1984

[Possibilities and limits of practice-oriented daily control measurements of linear accelerators].

Daily control measurements of the dose taken at a linear accelerator are described in order to present a practice-oriented measurement and evaluation method rendering possible a long-term quality control. This method allows to establish sudden parameter variations as well as to determine in a simple and precise manner long-term drifting effects with an amplitude within the range of measurement variations and a period of many months. During the time of six months, not only a drifting effect with a long period and an amplitude within the range of pro mille, but also a variation coefficient smaller than 0,3% could be determined.

Humans↗

PubMed2002

[Development and evaluation of QA software for the monitor chamber of an accelerator].

The monitor chamber of a radiotherapy system needs to be calibrated once a week. Because the calibration procedure requires a large variety of complicated calculations, we have developed software that facilitates calculation and enables comparison and storage of data. According to the standard measurement of absorbed dose, we used Visual Basic 6.0 (Microsoft Corp.) to establish the calibration method. This new technique has simplified the conventional intricate calculation required for calibration of the monitor chamber and enabled automatic processing of calculated results. We have confirmed the usefulness of this software in calibrating the monitor chamber. In the routine inspection, we can compare the current data with former results. Because of this advantage, it is possible to eliminate serious accidents such as overdosing and underdosing.

Calibration↗

PubMedDate not supplied

Monte Carlo design study for in vivo bone aluminum measurement using a low energy accelerator beam.

The need for aluminum monitoring exists in occupational medicine, as well as for the clinical monitoring of patients with renal dysfunction. After the development of an appropriate neutron source card, Monte Carlo simulations were made to design moderator/reflector assembly consisting of a polyethylene moderator (2 cm) and graphite reflector (30 cm), surrounded by a boronated (5%) wax (20 cm) and lead (1 cm) shield. This design should allow for the bone aluminum measurement of healthy subjects, but prior to that detailed microdosimetry is necessary to address a noticed disagreement between theoretical and experimental dose data.

Aluminum↗

PubMed1998

A thin target approach for portal imaging in medical accelerators.

A new thin-target method (patent pending) is described for portal imaging with low-energy (tens of keV) photons from a medical linear accelerator operating in a special mode. Low-energy photons are usually produced in the accelerator target, but are absorbed by the target and flattening filter, both made of medium- or high-Z materials such as Cu or W. Since the main contributor to absorption of the low-energy photons is self-absorption by the thick target through the photoelectric effect, it is proposed to lower the thickness of the portal imaging target to the minimum required to get the maximum low-energy photon fluence on the exit side of the target, and to lower the atomic number of the target so that predominantly photoelectric absorption is reduced. To determine the minimum thickness of the target, EGS4 Monte Carlo calculations were performed. As a result of these calculations, it was concluded that the maximum photon fluence for a 4 MeV electron beam is obtained with a 1.5 mm Cu target. This value is approximately five times less than the thickness of the Cu target routinely used for bremsstrahlung production in radiotherapeutic practice. Two sets of experiments were performed: the first with a 1.5 mm Cu target and the second with a 5 mm Al target (Cu mass equivalent) installed in the linear accelerator. Portal films were taken with a Rando anthropomorphic phantom. To emphasize the low-energy response of the new thin target we used a Kodak Min-R mammographic film and cassette combination, with a strong low-energy response. Because of its high sensitivity, only 1 cGy is required. The new portal images show a remarkable improvement in sharpness and contrast in anatomical detail compared with existing ones. It is also shown that further lowering of the target's atomic number (for example to C or Be) produces no significant improvement.

Algorithms↗

PubMedDate not supplied

Quantitative evaluation of a portal film contrast enhancement technique.

A study was conducted to evaluate the subjective improvement in portal film image quality resulting from the contact copy contrast enhancement technique which was introduced six years ago. Five observers were asked to identify and orient polyvinyl chloride cylinder images on both original and contrast-enhanced portal films taken with a 10-MeV linear accelerator. Fixed reviewing periods (T) were alloted of 20, 40, and 60 s as well as unlimited viewing time in order to increase the clinical relevance of this comparison. A scoring system and a probability representation were used to compare the original and enhanced films as a function of T. The results show a substantial increase in object detectability for the enhanced films at the short viewing times (T = 20, 40, and 60 s). For longer times (T greater than or equal to 80 s) the object detectability for enhanced and original films is not statistically different.

Humans↗

PubMed1980

Measurements on the radiation protection of patients irradiated with an 18-MeV electron linear accelerator.

In order to quantify the influence of some physical and geometrical treatment parameters on the stray radiation dose absorbed by patients undergoing radiotherapy on an 18-MeV linear accelerator, dose distributions were measured in a polystyrene phantom by means of LiF-dosimeters. For 10-MV X-rays, 6-MeV and 18-MeV electron radiation dose profiles, depth dose and isodose curves were obtained at various field sizes and treatment couch positions. By covering the phantom with 1 or 2 mm thick lead sheets a remarkable dose reduction was found. A two-exponential expression was derived to estimate stray radiation dosage in routine radiotherapy. The validity of the calculation method was demonstrated by comparing the gonadal load measured in an Alderson phantom with the calculated data.

Lead↗

PubMed2005

Radiological safety assessment of Brazilian industrial facilities with electron accelerators.

Industrial electron accelerators are used by eight installations in Brazil, with a total of 14 machines generating electron beams. These facilities are classified into categories I or II, according to the International Atomic Energy Agency (IAEA) system. In category I are included the facilities with an integrally shielded unit with interlocks, where human access during operation is not physically possible owing to the configuration of the shielding. In category II are included the facilities with a unit housed in a shielded room that is kept inaccessible during operation by an entry control system. Of the 14 accelerators operational in Brazil, 11 belong to category I and three to category II. In the present work a methodology for the assessment of the radiological safety of these accelerator facilities was developed and applied, mainly on the basis of specific recommendations from the IAEA. The main safety items were evaluated at those eight installations. According to the results obtained here, no inadequacies were observed at the three installations in category II, from the radiological safety and radioprotection points of view. Nevertheless, two out of the five installations in category I showed several deficiencies. Most of these inadequacies have been corrected during this work, and the rest are in the course of being corrected.

Brazil↗