bluuebunny/crossref_metadata_2025 huggingface dataset:
Backscatter into the beam monitor chamber: Implications for dosimetry of asymmetric collimators
Authors: Duzenli C. , McClean B. , Field C.
Published: March 2003

Backscatter from the asymmetric collimators of a linac into the beam monitor chamber (BMC) has been investigated for two accelerators having different BMC configurations. The effect has been quantified as a function of field size and collimator jaw position for 6 and 18 MV beams. The results indicate a maximum 2.5% (6 MV) and 4% (18 MV) decrease in output in one case and a negligible effect in the other case. The experiments indicate that the difference can be attributed to the different construction of the BMC's for the two accelerators.
Contrast enhancement of portal images by selective histogram equalization
Authors: Crooks I. , Fallone B. G.
Published: March 2003

Because of the high energy of the treatment beam, contrast of portal verification films is very poor. A simple contrast enhancement technique is described which we have labeled selective histogram equalization (SHE), to improve visualization of double‐exposure portal images and this facilitate the beam verification process. The technique performs separate histogram equalization on the treatment‐ and open‐field sections of double‐exposure portal images. Delineation of the treatment field edge and separation into two regions is performed automatically for off‐line portal radiographs by a strategic combination of Sobel filtration and morphological processes. Analyses of images processed by SHE and other adaptive histogram equalization techniques indicate that SHE produces improved contrast enhancement with minimal addition of noise or artifacts, thus simplifying the beam verification procedure. The simple implementation of an automatic SHE process with on‐line portal systems is also discussed.
Head‐scatter factors in rectangular photon fields
Authors: Tatcher Morris , Bjarngard Bengt E.
Published: March 2003

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.
Comparison of calculated and measured heterogeneity correction factors for 125I, 137Cs, and 192Ir brachytherapy sources near localized heterogeneities
Authors: Williamson Jeffrey F. , Perera Harold , Li Zuofeng , Lutz Wendell R.
Published: March 2003

The influence of tissue and applicator heterogeneities on brachytherapy dose distributions is not well understood, despite widespread use of shielded applicators in intracavitary therapy. Heterogeneity correction factors (HCF) have been measured using a silicon diode detector arising from bounded heterogeneities consisting of lead, steel, titanium, silver, aluminum, and air cylinders near brachytherapy sources of 125I, 137Cs, and 192Ir. In addition, transverse‐axis dose distributions for the three sources in homogeneous water were measured for distances of 0.2 to 16.0 cm. For each point of measurement, relative diode readings were simulated by a Monte Carlo photon transport code utilizing accurate models of the source internal structure, the experimental measure geometry and the source‐strength calibration geometry. Comparison of measured and calculated HCF's reveals excellent agreement (1%–3% average) over a wide range of materials, diameters, and thicknesses. In addition, Monte Carlo simulation not only accurately reproduced the relative transverse‐axis dose distributions in homogeneous medium, but was able to predict the variation of diode response with photon energy with an accuracy of 3% over the range of 30–662 keV. Our measurements demonstrate that HCF's vary by as much as 60%–100% with distance and heterogeneity diameter for a fixed thickness. Finally, silicon diode measurements of HCF (defined as reading with heterogeneity/reading in homogeneous medium) is shown to lead to errors of 5%–30% for 137Cs and 192Ir sources in the presence of high‐atomic number shielding materials. This paper concludes, that Monte Carlo simulation is a powerful, convenient and accurate tool for investigating the long‐neglected area of brachytherapy heterogeneity corrections.
One‐dimensional scatter‐subtraction method for brachytherapy dose calculation near bounded heterogeneities
Authors: Williamson Jeffrey F. , Li Zuofeng , Wong John W.
Published: March 2003

Neglect of tissue and applicator heterogeneities in brachytherapy treatment planning is due in part to lack of accurate, general, and fast three‐dimensional (3D) dose‐computation algorithms. A novel dose‐calculation algorithm that accounts for the lateral dimensions and location of the heterogeneity as well as its thickness has been developed. For simple 2D, water‐equivalent density heterogeneities, the algorithm is shown to be applicable to a wide range of photon energies and is 500–1000 times more efficient than Monte Carlo photon‐transport calculations. The model is based upon reducing the bounded 2D heterogeneity problem to two 1D problems by dividing the scattering volume into two regions: A cone‐shaped region that subtends the heterogeneity with its apex at the source and the complementary cone that contributes scatter dose by diffusion around the heterogeneity. The input data consist of precalculated scatter‐to‐primary ratios (SPRs) for collimated isotropic point sources. The central‐axis “mini‐beam” problem for a slab heterogeneity is solved by a simple 1D scatter integration model that accounts for both the thickness of the heterogeneity and its location relative to the point of interest. The scatter contribution arising outside the mini‐beam is modeled as the difference in SPRs corrected for transmission through the heterogeneity. The algorithm agrees, on average, with sample Monte Carlo calculations within 1% to 7% for 125I, 192Ir, and 100 keV point sources along the axes of water‐equivalent cylindrical heterogeneities (ρ=0–12.6 g/cm3, 3.6, and 24 mm diameters, and dose‐perturbation factors of 0.44–1.33 relative to the homogeneous case). The potential of generalizing the scatter‐subtraction approach to encompass 3D heterogeneities, those consisting of high‐atomic number media, and those of irregular shape, is discussed.
MRI‐guided noninvasive ultrasound surgery
Authors: Hynynen K. , Darkazanli A. , Unger E. , Schenck J. F.
Published: March 2003

In this study, the feasibility of using magnetic resonance imaging (MRI) to detect tissue necrosis induced by focussed ultrasound beams was investigated. It was shown that lesions produced in dog's thigh muscle in vivo were clearly visible in T2‐weighted images and that the lesion dimensions measured from the images correlated with the postmortem measurements of the visible tissue damage. It was also shown that the sonications can be done in the magnet and that the lesions are visible immediately after the sonications with increasing image contrast as a function of time. These results showed that MRI can be used to direct and monitor on‐line noninvasive ultrasound surgery. This may have a major impact in future patient treatments.
Energy loss of 70 MeV protons in organic polymers
Authors: Hiraoka T. , Kawashima K. , Hoshino K. , Fukumura A. , Bichsel Hans
Published: March 2003

Using Bragg ionization curves, measurements of the average stopping power of organic polymers relative to that of water have been made for 70 MeV protons losing about 30 MeV in the absorbers. By comparing calculated curves to experimental ones, I‐values for the polymers were obtained, relative to the I‐value for Al. For repeated measurements with A‐150 tissue equivalent plastic, the standard deviation of the ratio of the average stopping powers q was ±0.06%. It was found that q depends on the year of production of A‐150. For other absorbers, the uncertainty of q is estimated to be ±0.2%, with a corresponding uncertainty of the I‐values of ±1.5%. Most measured I‐values exceed values calculated with the additivity rule by about 10%. During the course of the measurements, fluctuations of the proton energy from the cyclotron of up to 0.8% were found.
X‐ray sources of medical linear accelerators: Focal and extra‐focal radiation
Authors: Jaffray D. A. , Battista J. J. , Fenster A. , Munro P.
Published: March 2003

A computerized tomography (CT) reconstruction technique has been used to make quantitative measurements of the size and shape of the focal spot in medical linear accelerators. Using this technique, we have measured the focal spots in a total of nine accelerators, including (i) two Varian Clinac 2100c's, (ii) two Atomic Energy of Canada Ltd. (AECL) Therac‐25's, (iii) two AECL Therac 6's, (iv) a Siemens KD‐2, (v) a Varian Clinac 600c (4 MV), and (vi) an AECL Therac‐20. Some of these focal spots were monitored for changes over a 2‐yr period. It has been found that (i) the size and shape of the source spot varies greatly between accelerators of different design ranging from 0.5 to 3.4 mm in full width at half maximum (FWHM); and (ii) for accelerators of the same design, the focal spots are very similar. In addition to the measurements of the focal spot, a new technique for measuring the magnitude and distribution of extra‐focal radiation originating from the linear accelerator head (flattening filter, primary collimator) has also been developed. The extra‐focal radiation produced by a Varian Clinac 2100c accelerator was measured using this technique and it was found that the extra‐focal radiation accounts for as much as 8% of the total photon fluence reaching the isocenter. The majority (75%) of this extra‐focal radiation originates from within a circle 6 cm in diameter at the target plane. The source MTFs for each of the measured focal spots have been calculated in order to assess their influence on the spatial resolution of verification images. The limiting spatial resolution (i.e., 10% modulation) for all the source MTFs is 1.8 mm−1 or greater when used for transmission radiography at a magnification of 1.2. The extra‐focal radiation, which produces a low‐frequency drop in the source MTFs of up to 8%, changes with field size. As a result, the source MTFs of linear accelerators depend not only on the design of individual accelerators and image magnification, but also on the field size used when forming an image.
Dosimetry of 6‐MV x‐ray beam penumbra
Authors: Metcalfe Peter , Kron Tomas , Elliott Andrew , Wong Tony , Hoban Peter
Published: March 2003

The measurement of x‐ray beam dose profiles in the penumbral region, using silicon diode, ionization chamber, TLD, and film dosimetry, has been investigated for a 6‐MV beam defined by independent collimators. Penumbral width (80%–20%) at dmax, as measured by diode, film, and TLD was found to be 3.6, 3.6, and 3.4 mm, respectively. These results reflect the relative sensitive widths of each of the measurement systems (2.5, 2.0, and 1.0 mm, respectively). An empirical forming function was used to relate the penumbral shape measured with a finite‐sized detector to that which would be measured with a point detector, the width of the point detector penumbra calculated from the diode penumbra is 3.4 mm, indicating that the TLD rods are a good approximation to a point detector. An alternative method of determining the width of a point detector penumbra is to extrapolate the penumbral widths obtained using two or more detectors of sensitive width. With this method, using Farmer and RK ionization chambers, a point detector penumbra width of 3.1 mm is obtained. An EGS4 Monte Carlo simulation, where a point source was assumed, gave a penumbral width of 2.8 mm. Negligible differences between the penumbra of beams defined by symmetric and asymmetric collimators was observed.
A versatile method for planning stereotactic brain implants
Authors: Anderson Lowell L. , Harrington Patrick J. , Osian Adrian D. , Arbit Ehud , Leibel Steven A. , Malkin Mark G.
Published: March 2003

Our approach to planning stereotactic 125I brachytherapy of brain tumors has involved least‐squares optimization of individual seed positions within the target contour, followed by repeated combining of seeds from nearest‐neighbor catheters in order to achieve an acceptably low number of catheters and an acceptable separation of entry points. In one option, the catheters diverge from an extra‐cranial point that can be close to the skull if all catheters are to be placed through a small craniectomy to treat a larger‐diameter target. In another option, catheters converge toward a point beyond the target, to facilitate perpendicularity at the skull surface if a separate opening is to be drilled for each catheter. In either case, the fact that seed orientations are known, permits including anisotropy in dose calculations. Trial seed locations are constrained to a target region defined on a 1‐mm mesh, both in the initial optimization of single‐seed catheters and in subsequent combinations followed by tune‐up optimizations. In the optimization process, sum‐of‐squares contributions are weighted more heavily when the dose rate is lower than the target dose rate; the weighting imbalance falls short of keeping all target points above the target dose rate and requires targeting on a dose rate about 25% higher than the desired minimum dose rate.
High‐resolution x‐ray imaging at soft diagnostic energies using a silicon photodiode array
Authors: Reid B. K. , Cunningham I. A.
Published: March 2003

A linear self‐scanned silicon photodiode array (PDA) has been used as a high‐resolution detector for small object computed tomography using soft diagnostic x rays. The imaging performance of the PDA when used to detect x rays directly has been characterized for x‐ray spectra between 24 and 69 kVp. The modulation transfer function (MTF) was measured to 40 cycles/mm (twice the PDA sampling cut‐off frequency) by double sampling an edge profile using a half‐detector shift. The MTF had no significant energy dependence, a 10% value at 28 cycles/mm, and a 3% low‐frequency drop. The detective quantum efficiency zero‐frequency value ranged from 1.5% with an 18‐keV average‐energy beam to 0.4% at 40 keV, and decreased further with increasing spatial frequency. It was found that the effective thickness of the silicon detector was 22 μm. This is typical of the expected diffusion length of minority carriers in doped silicon. An average of 4.6±1 eV was required to generate one e–h pair in the PDA, consistent with the accepted value of 3.6 eV in silicon. No evidence of radiation damage was observed after an accumulated exposure of over 5000 R. Use of the PDA is demonstrated by generating computed tomographic images using a third‐generation (rotate only) reconstruction algorithm. Images of a resolution test phantom, a pen and pencil, a mouse head, and bone biopsy specimen taken from a patient with osteoporosis are shown.
Observer performance and dose efficiency of mammographic scanning equalization radiography
Authors: Sabol John M. , Soutar Ian C. , Plewes Donald B.
Published: March 2003

The detection of fibrils, microcalcifications, and low contrast lesions is vital to the detection of early breast cancer. It has been shown through sensitometric measures and anthropomorphic phantom images that mammographic scanning equalization radiography (MSER) overcomes the latitude limitations of conventional mammographic techniques. MSER increases image quality by regional modulation of the entrance exposure to suit the local variations in x‐ray transmission within the patient. In order to assess the effect of equalization on the detection of breast lesions, we have compared observer performance in MSER and conventional imaging techniques. The observation tasks were the threshold visualization of fibrils, microcalcifications, and low contrast discs (simulating lesions), located on a uniform background. The performance of the observers was determined for a range of background x‐ray transmission simulating the range of transmission generated by variations in breast composition and thickness. For the conventional images, the threshold visible diameter of the fibrils, microcalcifications, and low contrast discs, increased as the x‐ray transmission of the phantom changed from that for which the film was optimally exposed. For the MSER images, the performance of the observers was almost independent of the background transmission of the object since MSER ensures that the film is optimally exposed for a large range of object transmission. Even with significant changes in object x‐ray transmission, only minor changes in fibril, microcalcification, and disc detection were observed. Utilizing the results of the contrast‐detail experiment, a dose efficiency comparison of conventional and MSER imaging techniques was performed. The dose efficiency analysis showed that MSER varied the incident exposure so as to maintain consistent performance of the observer, over the entire breast. These results suggest that MSER would improve the ability of radiologists to detect early breast cancer in women presenting with mammographically dense breasts, in a very dose efficient manner.
Computer simulation of broad‐beam and narrow‐beam attenuation in lead for diagnostic x‐ray energies
Authors: McLean Donald
Published: March 2003

Significant differences between broad‐ and narrow‐beam transmission measurements for lead protective barriers have been noted for x‐ray beams in the diagnostic energy range. Work reported in this paper indicates that these differences in transmitted exposure between broad‐ and narrow‐beam geometry measurements are primarily the result of two independent interaction processes; the transmission of K‐characteristic photons produced in the barrier, and coherently scattered photons. The magnitude of these effects was studied for x‐ray spectra between 70 and 150 kVp. The K‐characteristic component was found to be maximal at 150 kVp, while the coherently scattered contribution was found to be maximal for thick barriers and at lower energies. The effect of beam field size on relative transmitted intensities was also studied and found to increase with increasing field size up to 30 cm. A detector‐to‐barrier distance of 10 cm was found to maximize the K‐characteristic component. For a typical diagnostic x‐ray potential of 100 kVp, the K‐characteristic component was found to be less than 7% of the total transmitted beam. Voltage wave form considerations were found to be significant as has been previously reported. The contribution of K‐characteristic photons has been mathematically modeled and a computer program written to allow the rapid computation of broad beam transmission exposures for lead barriers, once the appropriate beam specifications have been entered.
Radio frequency eddy current losses for an annular conductor in MRI: Theory and applications
Authors: McRobbie D. W.
Published: March 2003

A theoretical derivation of the equivalent loading resistance of an annular volume conductor within a magnetic resonance imaging radio frequency (RF) coil is presented. Theoretical predictions of the magnitude of the load resistance agree well with measurement over a range of frequencies. The loading resistance is proportional to the sample conductivity, the frequency and coil sensitivity squared, and depends strongly upon the sample dimensions. The orientation of the magnetic vector potential for the specific coil geometry is also important. An experimental comparison of loading by annular and cylindrical objects with the human head over the imaging frequency range is made. Cylindrical test objects are adversely affected by the skin effect. The optimum means of simulating RF induced losses for quality assurance and performance evaluation is discussed in the light of these results.
Aperture size to therapeutic volume relation for a multielement ultrasound system: Determination of applicator adequacy for superficial hyperthermia
Authors: Moros Eduardo G. , Myerson Robert J. , Straube William L.
Published: March 2003

Three‐dimensional acoustic and thermal models were developed to simulate superficial hyperthermia treatments using a new multielement planar ultrasonic system. Typical power density and steady‐state temperature distributions are presented. A parametric study was performed to determine the relationship between therapeutic volume (volume at and above 42 °C) and aperture size (number of active elements). The parameters investigated were: maximum allowable temperature, skin surface temperature, blood perfusion (thermal diffusion length), acoustic absorption, and frequency. Results showed that this device produces well distributed sound beams with lateral dimensions comparable to the aperture size. These simulated results were in agreement with experimental measurements. The simulated temperature distributions were uniform at each depth across the applicator's aperture. The main heating characteristics found were: (1) the therapeutic volume was directly proportional to the aperture size; (2) the lateral dimensions of the therapeutic volume were independent of the parameters studied and remained practically constant with depth for several centimeters, with a very rapid increase near the skin surface and a very rapid fall off at depth; and (3) therapeutic penetration was strongly dependent on maximum allowable temperature, frequency, and acoustic absorption; and weakly dependent on blood perfusion and skin surface temperature. These heating characteristics are new in commercial systems for superficial hyperthermia. Despite the well‐distributed beams, it was found that in order to produce adequate hyperthermia with this device the lateral dimensions of tumors must be smaller that the applicator's active aperture and that thermal depth coverage must be monitored during treatments. Guidelines for aperture selection and thermometry strategies are discussed.
Wall‐correction and absorbed‐dose conversion factors for Fricke dosimetry: Monte Carlo calculations and measurements
Authors: Ma Chang‐ming , Rogers D. W. O. , Shortt K. R. , Ross C. K. , Nahum A. E. , Bielajew A. F.
Published: March 2003

For megavoltage radiotherapy photon beams, EGS4 Monte Carlo calculations show, and experimental measurements confirm with an accuracy of 0.2%, that glass or quartz‐walled vials used in Fricke dosimetry increase the dose in the Fricke solution. This is mainly caused by increased electron scattering from the glass which increases the dose to the Fricke solution. The dose perturbation is shown to vary from nothing in a 60Co beam up to 2% in a 24‐MV beam. For plastic vials of similar shapes, calculations demonstrate that the effect is in the opposite direction and even at high energies it is much less (0.2% to 0.5%).
Shaping of photon beams from electron linear accelerators in radiation therapy
Authors: Izewska Joanna
Published: March 2003

The method of calculation of the bremmstrahlung yield and angular distributions based on the semianalytical model described by Kovalev [Vtoricnoe izlucenie uskoritelej elektronov (Atomizdat, Moscow, 1979)] was analyzed. The specially written computer program was used to optimize the geometry of the collimators and the flattening filter design for radiation therapy purposes. The x‐ray beam shaping system thus designed was installed in the 4‐MeV linear Limex accelerator working at the Cancer Center in Warsaw (Poland). By comparing the calculated beam distributions with the experimental data, the agreement between theory and measurements was found to be within about ±3%.
Experimental and theoretical x‐ray imaging performance comparison of iodine and lanthanide contrast agents
Authors: Cardinal H. Neale , Holdsworth David W. , Drangova M. , Hobbs Barry B. , Fenster Aaron
Published: March 2003

Contrast agents based on the lanthanide elements gadolinium and holmium have recently been developed for magnetic resonance imaging (MRI). Because of the increased atomic number of these elements relative to iodine, these new compounds, used as x‐ray contrast agents, may yield higher radiographic contrast, and hence improved x‐ray image quality, relative to conventional iodinated compounds, for clinically useful x‐ray spectra. This possibility has been investigated, in independent experimental and theoretical studies, for two x‐ray imaging systems: a digital radiographic system, using an x‐ray image intensifier (XRII) and charge‐coupled device (CCD) detector; and a conventional screen/film system, using a Lanex Regular screen. Iodine, gadolinium, and holmium contrast agents were investigated over a wide range of concentration‐thickness products (0.1–0.6 M cm) and diagnostic x‐ray spectra (60–120 kVp). A simple theoretical model of x‐ray detector response predicts the experimental radiographic contrast measurements with a mean absolute error of 8.0% for the XRII/CCD system and 5.9% for the screen/film system, and shows that the radiographic contrast for these two systems is representative of all XRII and screen/film systems. An index of image quality is defined, and its dependence on radiographic contrast, x‐ray fluence per unit dose, and detective quantum efficiency (DQE) is shown. Theoretical values of the index, predicted by our model, are then used to compare the performance of the three contrast agents for the two systems investigated. In general, iodine performance decreases steadily with increasing kVp, gadolinium performance has a broad maximum near 85 kVp, and gadolinium outperforms holmium. Gadolinium outperforms iodine for spectra above (and vice versa below) about 72 kVp, depending slightly on spectrum filtration, object thickness, and detector type. Thus, raising the kVp to shorten exposure times or reduce x‐ray tube heat loading results in a loss of image quality with iodine, but not with gadolinium. Similarly, beam‐hardening artifacts in performing video densitometry with iodine would be reduced with gadolinium. Gadolinium‐based contrast agents are thus shown to offer several practical advantages over conventional iodinated contrast agents.
Calibration of high‐energy photon and electron beams for radiotherapy using AAPM 1983 and IAEA 1987 dosimetry protocols
Authors: Saiful Huq M. , Agostinelli A. G. , Nath Ravinder
Published: March 2003

To follow up on the theoretical comparison of the IAEA 1987 and AAPM 1983 protocols for dosimetry calibration of high‐energy photons and electrons [Med Phys. 18, 26–35 (1991)], results of a set of dosimetric measurements made with a Farmer type PTW and Capintec ionization chambers in solid water, PMMA, and polystyrene phantoms and exposed to a 4 MV photon beam from a Varian Clinac 4S at Yale, a 10 MV photon beam and 6 and 15 MeV electron beams from a Varian Clinac 1800 at Phelps Radiation Center, University of Connecticut, and a 25 MV photon beam from a Sagittaire at Yale, are presented. Because different methods are used for the determination of electron beam energies, the values of mean electron energy determined by the two protocols are different by up to 8%. However, for dose intercomparison, the overall agreement between the two protocols is within 1% in most cases, with a maximum discrepancy of 3.3% in one case. For photons, the IAEA results are smaller than the AAPM results by 0.7% on the average, while maximum discrepancies are in the range of −0.4%–−1%. In the case of 15 MeV electrons, the discrepancies between the two protocols are found to be in the range of −0.1%–1% and have an average value of 0.5%. In contrast to the above, a large discrepancy is observed between the two protocols for 6 MeV electrons. Depending upon the choice of phantom and ion chamber, this discrepancy is found to be in the range of −0.1%–−3.3%. A largest discrepancy of −3.3% is observed when a Capintec chamber is used in a polystyrene phantom. A most likely cause for this discrepancy is in the values of fluence correction factors used in the two protocols. Until better values for these parameters are available, a Farmer type chamber should not be used in a polystyrene phantom for the calibration of low‐energy electrons (6 MeV). In contrast to the theoretical intercomparison reported earlier, the present results of dosimetry measurements show improved agreement (by up to 1.4%) between the two protocols for electron beams, and about the same level of overall agreement (within 0.5%) for photon beams. The standard deviation of dose calibration for electron beams determined by the two protocols using different chambers and phantoms are about the same, approximately equal to ±1.5% for both protocols. It is recommended that either the AAPM should revise and update its 1983 protocol or adopt the international protocol produced by the IAEA. The IAEA protocol [Technical Report Series No. 277 (IAEA, Vienna, 1987), pp. 1–98] has fewer erroneous equations, uses more recent interaction data, and has an international appeal.
Recombination correction factors for an ionization chamber exposed to discrete patterned pulsed swept beams
Authors: Chang K. S.
Published: March 2003

An equation for a recombination correction factor for a pulsed swept beam of electrons was derived by Boag. This equation is based on an integration technique, which assumes that a large number of spot beams cover the radiation field, that the field size is much larger than the spot beam size, and that the spot beam size is much larger than the chamber size. However, for computer‐controlled pulsed swept beams of electrons, the spot beam pattern can be altered, may not cover all of the field area, and the locations are reproducible. In this report, a summation method is proposed for this type of beam. Calculations for two such beams are demonstrated with the chamber located in the center of the field, and with the chamber half‐way to and at the edge of the field for a linear accelerator. The results lie between the integration pulsed swept and pulsed beam curves. Moving the ionization chamber from the center toward the edge of the field produces curves closer to the pulsed swept beam curve. Increasing spot beam size produces curves closer to the pulsed beam curves. It is therefore concluded that the pulsed swept beam cannot be characterized by a single recombination correction factor curve. The actual curve will be bounded by the integration pulsed swept and pulsed beam curves.
Neutron fluxes in radiotherapy rooms
Authors: Agosteo Stefano , Para Armando Foglio , Maggioni Bruno
Published: March 2003

The spatial distribution of the neutron flux, originated in an electron accelerator therapy room when energies above the threshold of (γ,n) and (e,e′n) reactions are employed, is physically due to a direct flux, coming from the accelerator head, and to a flux diffused from the walls. In this work, the flux is described to a high degree of approximation by a set of functions whose spatial behavior is univocally determined by the angular distributions of the neutrons emitted from the shield of the accelerator head and diffused from the walls. The analytical results are verified with an extended series of Monte Carlo simulations obtained with the MCNP code.
A geometrically accurate vascular phantom for comparative studies of x‐ray, ultrasound, and magnetic resonance vascular imaging: construction and geometrical verification
Authors: Frayne Richard , Gowman Linda M. , Rickey Daniel W. , Holdsworth David W. , Picot Paul A. , Drangova Maria , Chu Ken C. , Caldwell Curtis B. , Fenster Aaron , Rutt Brian K.
Published: March 2003

A technique for producing accurate models of vascular segments for use in experiments that assess vessel geometry and flow has been developed and evaluated. The models are compatible with x‐ray, ultrasound, and magnetic resonance (MR) imaging systems. In this paper, a model of the human carotid artery bifurcation, is evaluated that has been built using this technique. The phantom consists of a thin‐walled polyester‐resin replica of the bifurcation through which a blood‐mimicking fluid may be circulated. The phantom is surrounded by an agar tissue‐mimicking material and a series of fiducial markers. The blood‐ and tissue‐mimicking materials have x‐ray, ultrasound, and MR properties similar to blood and tissue; fiducial markers provide a means of aligning images acquired by different modalities. The root‐mean‐square difference between the inner wall geometry of the constructed model and the desired dimensions was 0.33 mm. Static images were successfully acquired using x‐ray, ultrasound, and MR imaging systems, and are free of significant artifacts. Flow images acquired with ultrasound and MR agree qualitatively with each other, and with previously published flow patterns. Volume‐flow measurements obtained with ultrasound and MR were within 4.4% of the actual values.
Dosimetry of 125I sources in a low‐density material using scaling
Authors: Rashid H. , Bjarngard B. E. , Chin L. M. , Rice R. K.
Published: March 2003

One may apply O'Connor's scaling theorem to dose measurements with brachytherapy sources in order to overcome the difficulties associated with the need for high spatial accuracy. This possibility has been evaluated by measuring the dose distribution around 125I sources in a low‐density styrofoam phantom and comparing it with the dose distributions in water and solid water. Some generalization of the scaling theorem is proposed to allow for the minor differences in atomic composition between styrofoam and water, and the distances are scaled according to the ratio of the linear attenuation coefficients, instead of the physical densities, of the two media. The validity of this application of the scaling theorem has also been tested using Monte Carlo calculations. The results indicate that the scaling of the styrofoam measurements to water is a useful approximation in brachytherapy dosimetry.
Ultrasound contrast‐detail analysis: A preliminary study in human observer performance
Authors: Hall Timothy J. , Insana Michael F. , Soller Nannette M. , Harrison Linda A.
Published: March 2003

Results of a human observer performance study using a new ultrasound contrast‐detail (CD) phantom are reported. The new phantom allows estimates of the fundamental statistical uncertainties involved in CD analysis. Results of these experiments show that observers generally considered to be “trained” require experience at the specific task of low‐contrast target detection under signal known exactly (SKE) conditions, and that observers require an orientation period at the beginning of each observation session to obtain stable responses. The results obtained in this study can be used to estimate the number of independent images, observers, and repeat observation sessions required for a desired uncertainty in CD curves, and show that 10% fractional standard deviations are obtainable with modest observer effort.
New method for shielding electron beams used for head and neck cancer treatment
Authors: Farahani M. , Eichmiller F. C. , McLaughlin W. L.
Published: March 2003

Shields and stents of metals with high atomic number, which are custom cast in molds from the melt, are the materials most widely used to protect surrounding tissues during treatment of skin or oral lesions with therapeutic electron beams. An improved fabrication method is to mix a polysiloxane–metal composite, which is readily cast at room temperature by combining a metal‐powder/polysiloxane resin mixture with a hardening catalyst. The purpose of the present study is to compare the shielding effectiveness of two different metal–polysiloxane composites with that of conventional cast Lipowitz metal (50.1% Bi, 26.6% Pb, 13.3% Sn, 10% Cd). Also, a 23 factorial experiment was run to investigate the effects and interactions of metal particle size (20‐μm vs 100‐μm diameter), the atomic weight of the metal (304 stainless steel vs 70% Ag, 30% Cu alloy), and the presence or absence of a layer of unfilled polymer added to the forward‐scatter side of the shield. The composites of different thicknesses were made by blending 90% (w/w) metal powder separately with 10% polysiloxane base and catalyst. A thin GafChromicTM dosimeter film was placed between the shielding material and a polystyrene base to measure the radiation shielding effect of composite disc samples irradiated with a 6‐MeV electron beam normal to the flat surface of the disc. The results show that composite shields with the metal of higher atomic weight and density (Ag–Cu) combined with an additional unfilled layer are more effective than the stainless‐steel composite with a similar additional unfilled layer, in terms of diminishing the dose at the surface of the polystyrene backing material. In general, the unfilled layer on the side of the shield away from the radiation source helps attenuate secondary forward‐scattered electrons from the higher‐density composite shielding layer. Although the Ag–Cu composite shield requires about double the thickness (∼7 mm) to achieve the same shielding effectiveness as the Lipowitz metal stent (∼3 mm) for 6‐MeV electrons, the polymer–metal composite will be more comfortable for the patient and more easily and quickly cast to custom shapes and sizes.
Investigation of the convolution method for polyenergetic spectra
Authors: Papanikolaou Nikos , Mackie T. Rockwell , Meger‐Wells Carol , Gehring Mark , Reckwerdt Paul
Published: March 2003

The distribution of absolute dose per unit fluence from polyenergetic photon beams impinging upon a water phantom was calculated using two convolution approaches that properly account for beam hardening effects. Dose deposition kernels calculated previously using the EGS4 Monte Carlo code are convolved with the primary terma to give the dose for monoenergetic photon beams of energies ranging from 100 kev to 50 MeV. A polyenergetic dose distribution is composed of separately calculated monoenergetic components, which are appropriately weighted with the fluence spectrum to yield the polyenergetic dose distribution. Alternatively, a single convolution for the polyenergetic beam is considered, where a composite polyenergetic kernel is convolved with the respective polyenergetic terma. The effects of the polyenergetic kernel variance due to beam hardening as well as the effect of tilting the kernels for a diverging beam geometry were also examined. The depth dose data produced using the two proposed methods were compared with measured data and Monte Carlo simulations and showed good agreement.
The penumbra of a 6‐MV x‐ray beam as measured by thermoluminescent dosimetry and evaluated using an inverse square root function
Authors: Kron Tomas , Elliott Andrew , Metcalfe Peter
Published: March 2003

Data on the dose distribution in the penumbral region of megavoltage x rays are of importance for most radiotherapy planning systems. For medical linear accelerators the distance between the points representing 20% and 80% of the central axis dose (20/80) is typically only a few millimeters. To achieve good spatial resolution a radiation detector with small physical size has to be used for penumbra measurements. The penumbra in a 6‐MV therapeutic x‐ray beam was investigated for field sizes of 10×10 cm2 and 20×20 cm2 at depth of maximum dose (dmax=1.5 cm) and at 10‐cm depth in a solid water phantom. In addition, the field edge of an independent jaw driven to the center of the axis of the primary collimator was investigated. LiF thermoluminescent (TL) ribbons and rods were used embedded in solid water in different geometries resulting in relative detector sizes of 1, 3.1, and 6 mm. A forming function based on an inverse square root function was used to fit the experimental penumbra measurements. For the asymmetric field an amendment to the function is proposed to give a better fit for the experimental data. From the penumbra measured with the three different detector sizes, a penumbra can be extrapolated for an infinitesimal small detector. The extrapolated penumbral width (20/80) was found to be 2.3, 3.2, and 2.7 mm at dmax for the 10×10‐cm2 symmetric, 10×10‐cm2 asymmetric, and 20×20‐cm2 field sizes, respectively. The 20/80 values at 10‐cm depth in the solid water phantom for the same radiation fields were 4.2, 4.3, and 4.1 mm, respectively.
Dosimetric characteristics of a double wall 125I source for interstitial brachytherapy
Authors: Nath Ravinder , Melillo Anthony
Published: March 2003

Recently, a newly designed encapsulated source of 125I has become commercially available for use in permanent and temporary interstitial brachytherapy. The 125I sources in current use come in two different configurations: the Model 6711 source (Medi Physics/Amersham) for permanent implants has radioactive iodine adsorbed on the surface of a silver wire, and the Model 6702 (Medi Physics/Amersham) source for temporary implants has radioactive iodine absorbed in three spherical resin balls. Both of these iodine sources are encapsulated in a thin‐walled shell (0.05‐mm thick) made of titanium. The newly designed 125I source (Best Industries Model 2300 series) contains radioactive iodine adsorbed on a tungsten wire that is encapsulated by two walls of titanium. This double‐walled 125I source offers the following potential advantages: (i) Because it contains radioactive iodine on the ends as well as the circular surface of the tungsten wire, it can produce a more isotropic dose distribution than the sources in current use; (ii) because it is available in a wider range of source strengths, it is suitable for both temporary and permanent implantation; (iii) because it has a tungsten radiographic marker, source localization is considerably easier than the 125I Model 6702 source that has no radiographic marker; and (iv) because it uses a double‐walled encapsulation the risk of radioactive contamination due to source rupture is considerably reduced. In this work, dose distributions produced by the new design 125I source (Model 2300) for interstitial brachytherapy have been measured using LiF TLD's in a Solid Water phantom. Dosimetric characteristics of the new 125I sources are compared with those of the currently available 125I sources. Radial dose function for the Model 2300 source is found to be similar to that for the 6702 source, as expected by the lack of silver characteristic x rays in the photon spectrum from the 2300 source. Using the calibration of source strength based upon the 6702 standard, the dose‐rate constant for the 2300 source was determined to be 0.86 cGy h−1 U−1 [equal to 1.10 cGy h−1 mCi−1 (app)]. From the measured two‐dimensional dose distributions around the source, the anisotropy function for the new source was determined as a function of radial distance and angle. The dose distribution produced by the Model 2300 source was considerably more isotropic than those produced by the 6711 and 6702 sources.
Photon beam characteristics on the MM50 racetrack microtron and a new approach for beam quality determination
Authors: Karlsson Mikael , Nyström Håkan , Svensson Hans
Published: March 2003

The photon beams of the MM50 racetrack microtron have special characteristics which make them more suitable than conventional photon beams for precision radiation therapy with good dosimetric control. The beam flattening is obtained by the scanning of an elementary beam instead of using a flattening filter. This will give a number of advantages such as the possibility to optimize field flattening to individual field forms and field sizes. The radiation quality is the same across the whole beam, which gives smaller changes in dose profiles with depth and also makes it easier to perform careful dose planning. Beam collimation is mainly performed by a multileaf collimator and the special design of the treatment head gives nearly ideal characteristics for dose determination in an arbitrary point in the treatment fields. The output factor has been shown to depend almost solely on scattering within the treatment field. The conventional methods for beam quality characterization have been found less suitable at high energies and a new method based on HVL measurements in water is proposed.
Calculation of absorbed dose ratios using correlated Monte Carlo sampling
Authors: Ma Chang‐ming , Nahum Alan E.
Published: March 2003

A correlated sampling variance reduction technique has been implemented in the EGS4 (Electron Gamma Shower version 4) Monte Carlo code system in order to calculate correction factors for radiation dosimeters. These correction factors are calculated as the ratios of absorbed doses in various geometries irradiated by photon and electron beams. In this paper the efficiency and accuracy of the calculation of the absorbed dose ratios with correlated Monte Carlo sampling is discussed. Theoretical analysis shows that the calculation efficiency and accuracy increases with the degree of correlation between the calculated doses in the individual geometries. Calculations of the absorbed doses in thin slab geometries and in air‐filled ionization chambers have been performed. The results show that the gain in computing efficiency is up to a thousand fold for the calculations of the effect of the central electrode material and size on the response of cylindrical ionization chambers. The calculation accuracy (systematic uncertainty) is significantly improved when good correlation exists between the calculated doses in similar thin‐slab geometries.
The effect of lead, gold, and silver backings on dose near 125I seeds
Authors: Meli Jerome A. , Motakabbir Kazi A.
Published: March 2003

Brachytherapy for ocular melanoma uses 125I seeds backed by a gold shield. Conflicting results are reported in the literature on the effect of the gold on dose close to the seeds. In this work, a small lucite jig was constructed such that the seed‐to‐detector separation remained fixed as high‐Z materials of lead, silver, and gold were moved in and out of position behind the seed. The jig was clamped in place in the water filled tank of a beam scanning system. The response of two p‐type silicon diodes was measured at several distances from the seed with and without the high‐Z backings. The response with the high‐Z backing relative to water, found to be the same for each diode and the same for lead and gold, decreased from about 1.01 at 1.5 mm to about 0.92 at 20 mm. It has been suggested in the literature that L‐shell fluorescent x rays of approximately 10 keV from the gold backing might contribute significantly to the dose within 7 mm of the seed. To test this, the response with the gold backing relative to water was measured with an aluminum cap of 1‐mm wall thickness covering the diode. The cap transmits about 70% of the 125I fluence but is essentially infinitely thick to 10‐keV photons. The relative response (gold/water) was the same with and without the cap showing that the contribution of 10‐keV x rays is negligible. Compared to water, the silver backing was found to enhance the diode response by about 14% between 5 to 10 mm from the seed.
Feasibility study for positron emission mammography
Authors: Thompson C. J. , Murthy K. , Weinberg I. N. , Mako F.
Published: March 2003

A feasibility study is presented for a small, low‐cost, dedicated device for positron emission mammography. Two detector arrays above and below the breast would be placed in a conventional mammography unit. These detectors are sensitive to positron annihilation radiation, and are connected to a coincidence circuit and a multiplane image memory. Images of the distribution of positron‐emitting isotope are obtained in real time by incrementing the memory location at the intersection of each line of response. Monte Carlo simulations of a breast phantom are compared with actual scans of this phantom in a conventional PET scanner. The simulations and experimental data are used to predict the performance of the proposed system. Spatial resolution experiments using very narrow bismuth germanate BGO crystals suggest that spatial resolutions of about 2 mm should be possible. The efficiency of the proposed device is about ten times that of a conventional brain scanner. The scatter fraction is greater, but the scattered radiation has a very flat distribution. By designing the device to fit in an existing mammography unit, conventional mammograms can be taken after the injection of the radio‐pharmaceutical allowing exact registration of the emission and conventional mammographic images.