bluuebunny/crossref_metadata_2025 huggingface dataset:
Influence of cone design on the electron beam characteristics on clinical accelerators
Authors: Kassaee A. , Altschuler M. D. , Ayyalsomayajula S. , Bloch P.
Published: March 2003

Electron beam characteristics were investigated for different applicator cone designs. Low‐energy scattered electrons from the cone housing can contribute to the skin dose. The depth dose was measured as a function of electron energies and field size for different cones. The measurements were compared with calculated 3‐D dose distributions obtained from Monte‐Carlo‐derived energy deposition kernels [Phys. Med. Biol. 37, 391 (1992)]. This comparison shows that beam‐defining cones can be fabricated to eliminate low‐energy scattered electrons from the cone housing. Eliminating low‐energy electrons from the cones provides depth dose (DD) characteristics for clinical electron beams that are comparable to those (1) from nearly monoenergetic electrons (as determined from the practical range of the electrons), and (2) for which the DD is nearly independent of source‐to‐skin distance. Measurements of DD for small circular fields were also performed. For very small field sizes, corresponding to the width of the lateral spread of a pencil beam, the theoretical and measured DD were found to depend on field size. Three‐dimensional dose calculations of small and irregularly shaped electron fields are necessary to appreciate these effects.
Dual‐energy x‐ray imaging technique for invitro tissue composition measurement
Authors: Moreau M. , Holdsworth D. W. , Fenster A.
Published: March 2003

A dual‐energy in vitro radiographic technique has been developed to study the thickness of tissue and bone within atherosclerotic plaques. Results concerning the accuracy and precision of the thickness measurements using this technique are presented and discussed. Planar radiographs of phantoms were obtained with a low‐energy spectrum (45 kVp, no added filtration) and a high‐energy spectrum (100 kVp, 2.88‐mm copper‐added filtration), and then decomposed into bone‐equivalent and Lucite basis‐material images. Thickness measurements from these images yielded average accuracies of ±750 μm for the Lucite images, and ±25 μm for the bone‐equivalent images. The imprecision (one standard deviation) of the thickness measurements was ±192 and ±47 μm for the Lucite and the bone‐equivalent images, respectively (for thin sections). Although the accuracy and precision of Lucite thickness measurements were not as good as those obtained with other techniques, such as the iodine displacement technique, the accuracy and precision of the bone thickness measurements are shown to be much better. The high accuracy and precision of the bone measurement makes dual energy a very appealing technique for analyzing the physical properties of calcified atherosclerotic plaques in excised arterial specimens.
Segmentation of MRS signals using ASPECT (Analysis of SPectra using Eigenvector deComposition of Targets)
Authors: Roebuck Joseph R. , Windham Joe P. , Hearshen David O.
Published: March 2003

Efforts to minimize the effects of partial volume contamination (PVC) in in vivo magnetic resonance spectroscopy (MRS) have focused upon improving the sensitivity and efficiency of spatially localized MRS measurements. Such improvements may improve spatial resolution and reduce the time required to acquire multiple spectra, however, PVC can affect in vivo spectra at any resolution. In this paper, a model for segmenting in vivo MRS signals compromised by PVC in selected applications is introduced. The segmentation algorithm used is linear and is based on filters originally developed for image processing applications. The model is developed from first principles and evaluated using computer simulations. It is suited for segmenting multivoxel or chemical shift imaging data, and can be used with spectra acquired at any spatial resolution. It is used to estimate the size of the partial volumes contributing to a voxel compromised by PVC and the spatially selective signal components that would be expected to arise from these partial volumes if they could be measured directly. Several spectral perturbants present in in vivo MRS measurements violate the linearity assumptions underlying the model and produce systematic errors that must be accounted for. A number of perturbants are discussed, and the potential in vivo applications of the model are illustrated using solvent‐suppressed 1H–CSI spectra from the normal human brain.
Optimization of metal/phosphor screens for on‐line portal imaging
Authors: Wowk B. , Radcliffe T. , Leszczynski K. W. , Shalev S. , Rajapakshe R.
Published: March 2003

Studies were conducted to determine the optimal metal/phosphor screen for on‐line video verification of radiation treatment portals. Screens were evaluated for luminance and spatial resolution as a function of composition and thickness at 6‐ and 23‐MV x‐ray energies. A new video technique was used to determine modulation transfer functions. Gd2O2S was found to be the most efficient (brightest) phosphor for this application. Luminance was found to vary linearly with phosphor thickness up to a thickness of 500 mg/cm2. Metal plates made of iron, brass, copper, lead, and sintered tungsten of various thicknesses were also tested for luminance and resolution with Gd2O2S phosphor. Brightness peaked at about 2‐mm thickness for most metals. Significant contributions to the brightness were found to come from x rays interacting with the phosphor itself.
A robust morphological algorithm for automatic radiation field extraction and correlation of portal images
Authors: Wang H. , Fallone B. G.
Published: March 2003

Accurate extraction of the radiation field boundary from portal images is required in selective histogram equalization techniques for contrast enhancement of portal images, and is required in portal‐simulator image correlation. Local edge detectors are so noise sensitive that delineation of the field from anatomy edges and noise is very difficult. Optimization in edge localization and noise suppression can be achieved with optimal edge detectors, but the required calculation is very time consuming. A portal‐image segmentation algorithm based on morphological edge detection and morphological segmentation techniques is presented. Relying on two predefined criteria that are nonsensitive to variation of portal‐image types, the algorithm can automatically search the optimal threshold value, which is sensitive to the variation of the type and quality of portal images. With two stages of the searching procedure, the algorithm can accommodate a large variation of single and double exposure portal images obtained from different therapy machines. Results of our morphological edge detector are also compared to that of an optimal edge detector. Portal‐simulator image correlation using the inertia moments of the radiation field mask is investigated, and compared to correlation obtained using inertia moments of the radiation field contour. It is shown that the mask method is less sensitive to small variations and distortions in the field shape, resulting in a more accurate correlation. This would substantially simplify the task of treatment verifications.
Measurement of augmentation of 252Cf implant by 10B and 157Gd neutron capture
Authors: Wierzbicki J. G. , Maruyama Yosh , Porter A. T.
Published: March 2003

252Cf has been used as a brachytherapy source since the early 1970s. The dominant mechanism of interactions of 252Cf neutrons with tissue is elastic scattering. The scattered neutrons lose part of their energy, which is released as kinetic energy of the recoiling nuclei. By multiple scattering, neutrons lose their energy and eventually become thermalized (in energetic equilibrium with tissue atoms with an average energy of 0.025 eV) and do not play any role in radiotherapy. These thermal neutrons may interact with hydrogen nuclei or with nitrogen, but the cell killing effects by these reaction products are negligibly small compared to the elastic scattering by fast neutrons or by photons emitted by californium. Nonetheless, these thermal neutrons are still potentially usable for neutron capture therapy and can be used to enhance californium brachytherapy effects. Neutron capture therapy is a two‐part therapy relying on the selective loading of tumor cells with compounds containing 10B or 157Gd and subsequent irradiation with thermal neutrons. To calculate neutron capture doses one has to know thermal neutron flux. This paper presents results of an experimental study of thermal neutron flux and calculations of boron neutron capture and gadolinium neutron capture doses in the vicinity of 252Cf sources.
Diode dosimetry of 103Pd model 200 seed in water phantom
Authors: Fontenla Doracy P. , Ahmad Munir , Chiu‐Tsao Sou‐Tung , Anderson Lowell L.
Published: March 2003

The relative dose distribution around the 103Pd model 200 implant seed was measured with a computerized data acquisition system employing a p‐n junction silicon diode immersed in a water phantom. Data are acquired in polar coordinates by computer control of (1) the diode distance from the seed center and (2) the rotation angle of seed about a transverse axis. Transverse axis data are compared with thermoluminescent dosimeter (TLD) measurements and a Monte Carlo calculation by others.
Pseudocorrelation: A fast, robust, absolute, grey‐level image alignment algorithm
Authors: Radcliffe T. , Rajapakshe R. , Shalev S.
Published: March 2003

A new image alignment algorithm—pseudocorrelation—has been developed based on the application of Monte Carlo techniques to the calculation of a cross‐correlation integral for grey‐scale images. It has many advantages over cross‐correlation: it is at least a factor of 10 faster than fast‐Fourier‐transform‐based cross‐correlation, and requires 8 times less memory. Its high speed allows for the search space of geometric transformations between images to include magnification and rotation as well as translations without the search time becoming too long. It allows noise to be taken into account, making calculation of a robust, absolute probability of good alignment possible. It is relatively insensitive to differences in quality between images. This article describes the pseudocorrelation algorithm in detail and presents the results of tests of the effects of contrast enhancement, resolution differences, and noise on the algorithm's performance. These tests show that the algorithm is well suited to the task of automated alignment of very low contrast images from video electronic portal imaging devices.
Generation of arbitrary intensity profiles by dynamic jaws or multileaf collimators
Authors: Spirou Spiridon V. , Chui Chen S.
Published: March 2003

An algorithm, which calculates the motions of the collimator jaws required to generate a given arbitrary intensity profile, is presented. The intensity profile is assumed to be piecewise linear, i.e., to consist of segments of straight lines. The jaws move unidirectionally and continuously with variable speed during radiation delivery. During each segment, at least one of the jaws is set to move at the maximum permissible speed. The algorithm is equally applicable for multileaf collimators (MLC), where the transmission through the collimator leaves is taken into account. Examples are presented for different intensity profiles with varying degrees of complexity. Typically, the calculation takes less than 10 ms on a VAX 8550 computer.
Soda‐lime‐silica glass for radiation dosimetry
Authors: Ezz‐Eldin F. M. , Abdel‐Rehim F. , Abdel‐Azim A. A. , Ahmed A. A.
Published: March 2003

The color developed in a commercially available soda‐lime‐silica glass when subjected to gamma‐irradiation and the stability of such radiation‐induced color were studied to test its sensitivity to small doses of gamma‐rays (0.0–27 kGy). After irradiation, two absorption bands developed at 400 and 620 nm. The former band exhibited a stronger absorption than the later one. The intensity of both bands showed a gradual increase with increasing irradiation dose and a gradual decrease with increasing fading time after irradiation. The development of these bands is associated with the generation of defects at nonbridging oxygen atoms in the glass lattice and hole centers. The results obtained suggest that this glass simulated the Z of compact bone in terms of gamma rays absorption properties over broad radiation spectra (0.1 to 10 MeV).
Image quality of a digital chest radiography system based on a selenium detector
Authors: Neitzel Ulrich , Maack Ingo , Günther‐Kohfahl Susanne
Published: March 2003

A digital chest radiography system has been developed, with a detector based on the photoelectric properties of amorphous selenium. The selenium layer is deposited on a cylindrical aluminium drum, large enough to cover the full field of view for chest imaging. The electrostatic charge image which is formed on the selenium surface after x‐ray exposure is read out by electrometer probes using fast drum rotation. For a physical evaluation of the attainable image quality, the characteristic curve, the modulation transfer function, and the noise spectra were measured. From these measurements, the signal‐to‐noise properties of the detector in terms of detective quantum efficiency (DQE) and noise equivalent quanta (NEQ) were derived. The results show that the selenium‐based detector has a wide dynamic range and a significantly better DQE than screen‐film and storage phosphor systems for spatial frequencies below the Nyquist limit (2.7 lp/mm). As a consequence, the detectability of small, low‐contrast details is considerably improved.
Recommendations for measurement of tray and wedge factors for high energy photons
Authors: Sharma S. C. , Johnson M. W.
Published: March 2003

Transmission factors for beam modifiers such as wedges and blocking trays in dosimetry calculations are very common, but various methods have been used in measuring these factors. A systematic study to measure these factors as a function of field size and point of measurement was carried out. The beam modifiers were a blocking tray and the standard wedges supplied with a cobalt‐60 unit and a medical electron accelerator (6‐ and 18‐MV x rays). Several points of measurement were selected, using several depths: dmax, TG‐21 calibration depth, 10 cm (wedge angle defining depth, ICRU), 15 cm and in‐air (miniphantom), and various field sizes. The results show a small field size dependence in these transmission factors with more dependence found in the point of measurement.
Voxel‐based localization in frame‐based and frameless stereotaxy and its accuracy
Authors: Lemieux Louis , Kitchen Neil D. , Hughes Stephen W. , Thomas David G. T.
Published: March 2003

The problem of accurate stereotactic localization and registration of targets in computed tomography (CT) data sets is addressed, in particular the effect of using a single transformation matrix to map voxel coordinates onto stereotactic coordinates. An algebraic approach to the calculation of stereotactic target coordinates in tomographic data acquired with conventional stereotactic localizers is presented. The volume transformation matrix (VTM) is discussed, which is useful for the registration of volumetric data sets, and also corresponds to the rigid body transformation matrix used in many so‐called frameless registration methods. The VTM can lead to accuracy degradation, in particular due to patient movement during scanning. Simulations were performed and CT data sets acquired with patients fitted with the CRW or the GTC stereotactic localizer were analyzed. Comparison of STM‐ and VTM‐derived stereotactic coordinates shows an average overall registration error of 0.1 mm for anesthetized patients and in the range 0.6–1.4 mm for nonanesthetized patient. Accuracy maps are described that enable the user to visualize the registration error in relation to the data. It is shown that the effect of fiducial point localization error and patient movement for VTM‐based localization is minimized when all available fiducials in the region of interest are used. The significance of these results is discussed, and methods are proposed to minimize these effects for frame‐based and frameless registration methods.
Incorporation of scattered radiation into dual‐energy radiologic theory and application to mammography
Authors: Johns Paul C. , Beauregard Richard M.
Published: March 2003

A previous analysis of dual‐energy imaging is extended to incorporate scattered radiation. The analysis is general and can include polyenergetic beams and nonideal detectors. In the dual‐material basis plane, scatter generates a displacement of the vectors representing the imaged materials. This changes the target versus background signal difference and the projection angle for cancellation of background contrast due to varying amounts of two materials. Complete cancellation is not possible, because vectors representing various mixtures of the two background constituents are displaced differently and thus no longer lie on a straight line in the dual‐material plane. The effect can be minimized by allowing a typical amount of scatter to reach the image receptor when determining the parameters of the transformation from log attenuation to equivalent materials thicknesses. The analysis has been applied to dual‐energy mammography. The presence of scatter has minimal effect on the optimum beam energies with an ideal detector, which are about 20 and 70 keV.
Correcting for rf inhomogeneities in multiecho pulse sequence MRI dosimetry
Authors: Duzenli C. , Robinson D.
Published: March 2003

The growing interest in using MRI to measure distributions of dose in specialized materials has prompted the need to improve existing methods for measuring NMR relaxation rates throughout large volume phantoms. The change in transverse relaxation rate (R2) has proven to be a more sensitive measure of dose than longitudinal relaxation rate (R1) in some materials; however, the accuracy of R2 measurements is limited by the presence of artifacts resulting from imperfect MRI tip angles throughout large phantoms. A novel echo quotient technique allowing accurate measurement of changes in R2 (ΔR2) in regions of imperfect tip angle is presented. Application of this technique to the measurement of a dynamically wedged photon beam dose distribution demonstrates the improved accuracy of the echo quotient technique compared with other methods. Agreement of the MRI data with film and ion chamber data is within 2.5% throughout the volume of interest, indicating the potential for ΔR2 MRI dosimetry to become a clinically useful tool.
Calculation of x‐ray spectra for radiosurgical beams
Authors: Sixel Katharina E. , Faddegon Bruce A.
Published: March 2003

As conformal radiosurgery using micromultileaf collimators gains feasibility, dose calculation algorithms based on Monte Carlo or convolution techniques may become necessary. These require radiosurgical x‐ray spectra. The most accurate method currently available to estimate clinical radiosurgery spectra is the Monte Carlo method. In this study the EGS4 Monte Carlo system was used to simulate the thick target of a 6 MV linear accelerator used for radiosurgery in our center. The calculated spectrum was attenuated through any significant mass thickness of material downstream from the target. The attenuated thick‐target spectral distributions calculated both with and without the flattening filter were compared to the attenuated, thin target spectrum based on the small angle Schiff analytical spectrum calculated for the same target and attenuator material, as well as with a published spectrum from a full Monte Carlo simulation of a treatment head with a flattener in place. The Schiff spectrum neglects contributions from lower‐energy scattered electrons that significantly degrade the quality of the beam. The flattener is removed from our accelerator during radiosurgery to increase the dose rate to approximately 750 cGy/min for a 10×10 cm2 field at the depth of dose maximum. This leaves a substantial fluence of photons below 1 MeV that are not observed in published spectra calculated for accelerators with flattening filters. Removal of the flattening filter has a measurable effect on the central axis depth dose, reducing the percentage dose at 10 cm depth from 59.2% to 54.3% for a 10 mm diam field. Radiosurgical off‐axis ratios and percentage depth dose distributions calculated from these spectra with the EGS4 Monte Carlo code were compared to measured data. Measured and calculated dose distributions both with and without flattener were in good agreement. The dose distributions were found to be insensitive to the differences in the various calculated spectral distributions. Thus, although the attenuated Schiff spectrum is significantly harder than the clinical beam, it is adequate for dose calculations of radiosurgical beams.
Photoacoustic ultrasound (PAUS)—Reconstruction tomography
Authors: Kruger Robert A. , Liu Pingyu , Fang Yuncai “Richard” , Appledorn C. Robert
Published: March 2003

The theoretical underpinnings of photoacoustic ultrasound (PAUS) reconstruction tomography are presented. A formal relationship between PAUS signals and the heterogeneous distribution of optical absorption within the object being investigated is developed. Based on this theory, a reconstruction approach, analogous to that used in x‐ray computed tomography, is suggested. Initial experimental results suggest that this approach produces “reasonable” reconstructions for absorbers distributed within a narrow plane embedded within a highly scattering medium.
The development of conformal radiation therapy
Authors: Fraass Benedick A.
Published: March 2003

The development and clinical use of conformal radiation therapy, and the application of modern computer‐controlled radiation therapy treatment techniques to the delivery of conformal therapy, has been a major research area in the field of radiation oncology in recent years. The introduction of three‐dimensional (3‐D) patient imaging, 3‐D treatment planning systems, computer‐controlled treatment machines equipped with multileaf collimators, and the continuing increase in computer power and software sophistication has finally allowed the clinical implementation of the kinds of conformal treatment planning and delivery that were envisioned decades ago. The use of 3‐D treatment planning in conjunction with conformal treatments and the analysis of clinical normal tissue complications has also begun to give the field the clinical complication (and eventually tumor control) probability data which can be used to truly optimize radiation treatments. At this centennial anniversary of the discovery of the x ray, it is appropriate to look back at the contributions which have led to the current developments in conformal therapy. Few if any of the “new developments” associated with modern computer‐controlled conformal therapy (CCRT) are actually new. However, modern technology has finally progressed to the point that we can integrate all of the features that are necessary to really make the concepts work in the clinic. This paper traces some of the developments which have led to the current interest and progress in CCRT treatments. A brief summary of the current status of conformal therapy and the work still to be done is also included.
Collimator scatter in photon therapy beams
Authors: Ahnesjö Anders
Published: March 2003

A model for collimator photon scatter calculations is presented. The scatter from a collimating block is separated into two categories, one being the scatter released from photons entering the block through the surface facing the source, and one from photons striking the block tangentially through the side tangential to the beam. Both sources of scatter are analyzed by means of scatter kernels, defined as the scatter fluence distribution from a narrow line beam striking a collimator block. Kernels for both cases are calculated analytically using first scatter models based on Klein–Nishina cross sections including corrections for binding effects and coherent scattering. For 1‐MeV primary photons, Monte Carlo calculations (EGS4) are used to show that the collimator scatter is dominated by first scatter. The kernels for the two types of scattering geometries are shown to be related and thus the total collimator scatter kernel can be derived from the kernel for the photons which have entered the block through the source facing side. Using a set of photon beam spectra, the kernels are parametrized as functions of beam energy in a form suitable for implementation in treatment planning systems. The parametrization is used to derive collimator scatter distributions for broad beam geometries of clinical interest. It is shown that scatter from the primary collimator, due to its location close to the beam source, is a major source of scatter that amounts to several percent of the primary fluence. The scatter from beam shaping collimators, however, generally accounts for less than 1% of the primary fluence. Although the beam‐shaping collimators generate little scatter, the model used to calculate the resulting dose component is simple to implement and separates the different sources of scattered photons from the accelerator head. The latter provides generality and accuracy in dose per monitor unit calculations in treatment planning. Due to the low magnitude of collimator scatter it is recommended to use the phrase “head scatter” instead of “collimator scatter” when addressing the overall subject of extra‐focal radiation from medical accelerators.
Controlled mold geometry for surgical deficit treatment planning
Authors: Anderson Lowell L. , Zlotolow Ian M. , McCormick Beryl , Harrison Louis B.
Published: March 2003

The relatively precise placement of brachytherapy sources afforded by stereotactic frames for brain implants is not generally achievable for other sites, which lack the fixed geometry of the cranium and its contents. An exception is a source‐containing rigid mold that delivers brachytherapy when inserted securely in a surgical defect. A technique has been developed in which an acrylic mold of the region to be treated is suspended in a demountable aluminum box, which is then filled to a suitable level with dental stone to form a casting that supports the mold and that can be removed intact. First, the box is aligned on a mill table and a ball mill is used to drill three parallel holes in the acrylic mold, with precisely known locations and depths and as widely separated as possible. The spherical air cavities that result from plugging these holes with ball‐milled acrylic rods become reference markers in subsequent computed tomography (CT) scans. After optimum CT‐coordinate locations have been planned for 125I seeds in catheters, they are transformed to mill coordinates using a matrix developed from the known marker coordinates in the two systems. Catheter holes are then drilled with the mold in the reassembled casting and box. The method has been used to treat both recurrent maxillary cancer and recurrent orbital rhabdomyosarcoma.
Collimator optimization for lesion detection incorporating prior information about lesion size
Authors: Moore Stephen C. , deVries Daniel J. , Nandram Balgobin , Kijewski Marie Foley , Mueller Stefan P.
Published: March 2003

A Bayesian estimator has been developed as a paradigm for human observer performance in detecting lesions of unknown size in a uniform noisy background. The Bayesian observer used knowledge of the range of possible lesion sizes as a prior; its predictions agreed well with the results of a six‐observer perceptual study. The average human response to changes in collimator resolution, as measured by the detectability index, dA, was tracked by the Bayesian detector's signal‐to‐noise ratio (SNR) somewhat better than by two other estimation models based, respectively, on lesser and greater degrees of lesion size uncertainty. As the range of possible lesion sizes increased, the Bayesian detector's SNR decreased and the optimal collimator resolution shifted towards better resolution. An analytic approximation for the variance of lesion activity estimates (which included the same prior) was shown to predict the variance of the Bayesian estimator over a wide range of collimator resolution values. Because the bias of the Bayesian estimator was small (<1%), the analytic variance estimate permitted a rapid and convenient prediction of the Bayesian detection SNR. This calculation was then used to optimize the geometric parameters of a two‐layer tungsten collimator being constructed from crossed grids for a new imaging detector. A Monte Carlo program was first run to estimate all contributions to the radial point‐spread function for collimators of differing tungsten contents and spatial resolution values, imaging 140‐keV photons emitted from the center of a 15‐cm‐diameter, water‐filled attenuator. The optimal collimator design for detecting lesions with unknown diameters in the range 2.5–7.5 mm yielded a system resolution of ∼8.5‐mm FWHM, a geometric collimator efficiency of 1.21×10−4, and a single‐septum penetration probability of 1%.
Accuracy requirements of the primary x‐ray spectrum in dose calculations using FFT convolution techniques
Authors: Zhu Yunping , Van Dyk Jake
Published: March 2003

The photon beam dose calculation methods using fast Fourier transform (FFT) convolution require that the primary beam energy spectrum is accurately known; however, a quantitative assessment of the required accuracy is not yet provided. In this study, the sensitivity of various parameters such as central axis depth dose and dose under attenuators to the incident photon spectrum was simulated and evaluated for 6‐ and 18‐MV photon beams. The central axis depth–dose data using the modified spectrum were compared with those calculated using the original spectrum. Maximum errors on the central axis between these two sets of depth–dose data were found by changing the weighting of each energy bin of the primary spectrum. The maximum change in percent depth dose is proportional to the change in average incident energy relating to the quality of the x ray. To keep the maximum depth–dose error to less than 1%, the relative change in average energy brought about by changes in single spectral bins should be less than 2%. The accuracy required for each energy bin varies dramatically, dependent on its deviation from the original average energy and its relative weighting. For example, for the 6‐MV x‐ray spectrum, the energy bins centered around 0.76 MeV (first bin) and 6.8 MeV (fifth bin) need to have accuracies of 5% and 40%, respectively, to obtain a 1% accuracy in percent depth dose.
Calculations of x‐ray and neutron transmission through multirod arrays
Authors: Maughan Richard L. , Kruger David G. , Blosser Gabe F. , Blosser Henry G.
Published: March 2003

Multirod arrays can be used to produce irregularly shaped irradiation fields for use in external beam photon and neutron radiation therapy. Two prototype multirod collimators have been built for use with high energy photon beams. A practical multirod collimator is in routine use with the fast neutron beam at a superconducting cyclotron based neutron therapy facility. A simple computer program has been written for calculating the transmission of photon and neutron beams through multirod arrays. The results of calculations for both close packed and spaced rod arrays are presented, and compared with available photon and neutron transmission data. The transmission through a regularly packed array exhibits a pattern of maxima and minima which occur with a spacing corresponding to the rod radius. The program predicts the positions and magnitudes of the transmission peaks. The rod diameter and spacing, the source size, and the position of the multirod collimator and the measurement plane relative to the source, all effect the exact nature of the measured transmission pattern. The transmission of 15‐MV photons through close packed and spaced rod arrays was calculated using the program and compared with measurements made in a close packed tungsten rod array and with a prototype multirod collimator. Calculations for the transmission of a p(42)‐Be neutron beam through a close packed tungsten rod array were compared with previously published data. Good agreement between calculations and measured data was obtained in all cases. The program was used to design a practical multirod collimator for a d(50)‐Be fast neutron beam. An improvement to the collimator design in which half of the rod rows in the array are displaced by one quarter rod diameter relative to the rest of the array leads to reduced peak transmission along the central beam axis.
Rutherford's curriculum vitae, 1894–1907
Authors: Cohen Montague
Published: March 2003

A single page, handwritten document was discovered when the Macdonald Physics building of McGill University in Montreal was gutted in 1978. This proved to be the draft of Ernest Rutherford's curriculum vitae (C.V.) covering the years 1894–1907, probably written in the autumn of 1906 when Rutherford was preparing to leave McGill. The C.V. contains 21 headings in chronological order, referring to research and other activities of Rutherford and his coauthors (especially Soddy and Barnes), plus a further set of headings relating to the associated investigations of Rutherford's team, including Eve and Hahn. A transcript of the document is provided, although in several places Rutherford's handwriting is difficult to interpret, and the significance of his abbreviations is not always clear. Each of the items in the C.V. is discussed briefly in this review, in the light both of Rutherford's personal career and of the contribution of his team to the development and understanding of radioactivity. This contribution included the cause and nature of radioactivity (with Soddy), energy aspects of radioactive decay (with Barnes), elucidation of the uranium–radium, thorium and actinium series (Godlewski and Hahn), the radioactivity of the earth and atmosphere (Eve), the nature of the γ rays (Eve) and, perhaps most important of all, the nature and properties of the α particle (Rutherford himself). The latter investigations led directly to Rutherford's later work in Manchester, including the nuclear model of the atom and artificial disintegration of the nucleus.
Radiation dose distribution within the bone marrow of patients receiving holmium‐166‐labeled‐phosphonate for marrow ablation
Authors: Bayouth J. E. , Macey D. J. , Boyer A. L. , Champlin R. E.
Published: March 2003

The primary objective of this work was to estimate the absorbed dose distribution to the bone marrow of six multiple myeloma patients who received holmium‐166 (166Ho) DOTMP (1,4,7,10‐tetraazacyclododecane‐1,4,7,10‐tetramethylene‐phosphonic acid) for the purpose of bone marrow ablation. A methodology based on gamma camera images was developed to estimate the regional absorbed dose distributions delivered to the bone marrow, and this was compared with values calculated from the MIRD technique and bone marrow biopsies. The activity concentration in various skeletal regions was calculated from the activity in the region of interest (ROI) drawn on whole body gamma camera images, and the mass of bone in each ROI was derived from a dual x‐ray absorptiometry image. The radiation absorbed dose to the bone marrow was calculated from this activity concentration using an adaptation of Bragg–Gray cavity theory. The radiation absorbed dose delivered to the bone marrow in the six patients calculated from the MIRD “S” factors ranged from 15.0 to 46.3 Gy. The gamma camera measured activity concentration in skeletal regions predominantly composed of trabecular bone was approximately five to six times higher than that in cortical regions. The skeletal activity concentration in each patient ranged from highs in such regions as the ribs to lows in such regions as the shin and foot by a factor of nearly 20, producing a heterogeneous distribution of absorbed dose within the marrow. Dose volume histograms calculated for these patients indicated that 15%–20% of the marrow received an absorbed dose significantly larger than the average value, while 5%–10% of the marrow received a substantially lower dose. Weighted mean dose estimates from the regional technique were typically 30% greater than the average dose estimates calculated with the MIRD “S” factors. Finally, absorbed dose estimates for the marrow calculated from the regional technique correlated more closely with the clinical response of blood cells and abnormal proteins measured in bone marrow aspirates and peripheral blood samples than estimates from the MIRD “S” factors.
Fetal dose from radiotherapy with photon beams: Report of AAPM Radiation Therapy Committee Task Group No. 36
Authors: Stovall Marilyn , Blackwell Charles Robert , Cundiff Jackson , Novack Devorah H. , Palta Jatinder R. , Wagner Louis K. , Webster Edward W. , Shalek Robert J.
Published: March 2003

Approximately 4000 women per year in the United States require radiotherapy during pregnancy. This report presents data and techniques that allow the medical physicist to estimate the radiation dose the fetus will receive and to reduce this dose with appropriate shielding. Out‐of‐beam data are presented for a variety of photon beams, including cobalt‐60 gamma rays and x rays from 4 to 18 MV. Designs for simple and inexpensive to more complex and expensive types of shielding equipment are described. Clinical examples show that proper shielding can reduce the radiation dose to the fetus by 50%. In addition, a review of the biological aspects of irradiation enables estimates of the risks of lethality, growth retardation, mental retardation, malformation, sterility, cancer induction, and genetic defects to the fetus.
A spreadsheet technique for dosimetry of transperineal prostate implants
Authors: Feygelman Vladimir , Noriega Brian K. , Sanders Robert M. , Friedland Jay L.
Published: March 2003

Although conceptually straightforward, dosimetry of permanent 125I seed prostate implants is not necessarily easy to implement in clinical practice, especially for institutions that are unwilling or unable to modify their commercial RTP systems. Spreadsheet techniques that aid in both preimplant treatment planning and post‐implant dosimetric evaluation have been developed. The first spreadsheet converts the seed distribution expressed in terms of template grid coordinates to the format suitable for input into the RTP system, and determines the positions and loading patterns of individual needles. The second spreadsheet macroprogram is designed, as a modification of the Roy et al. [Int. J. Radiat. Oncol. Biol. Phys. 26, 163–169 (1993)] technique, to interactively determine physical seed locations from the post‐implant CT images. Although less automated than described elsewhere, this approach was found acceptable for clinical practice at our institution.
Physicists in mammography—A historical perspective
Authors: Rothenberg Lawrence N. , Haus Arthur G.
Published: March 2003

Medical physicists and engineers, working with radiologists and technologists, have made significant contributions in the design of mammographic x‐ray units and image receptors, as well as in the development of methods for evaluating mammographic image quality and procedures for quality control. More accurate methods of measuring radiation exposure in the energy range of mammography and more relevant calculations of radiation dose to breast tissue at risk have also been realized. This article will discuss some of the major contributions made by medical physicists for the benefit of mammography. Contributions of radiologists in mammography have been published elsewhere [Bassett, Gold, and Kimme‐Smith (1994)]. All contributions cited in this article are based on referenced publications and citations in the following: Medical Physics; Radiology; NCRP Report No. 85; Quality Determinants in Mammography; AAPM Report No. 29; Reduced Dose Mammography, W. W. Logan and E. P. Muntz (editors); RSNA Categorical Course: Technical Aspects of Breast Imaging, A. Haus and M. Yaffe (editors); Film Processing in Medical Imaging, A. G. Haus (editor); Screen‐Film Mammography: Imaging Considerations in Medical Physics, G. T. Barnes and G. Donald Frey (editors). The article is divided into six sections: (1) x‐ray equipment and receptor development, (2) image quality, (3) radiation dose, (4) phantoms, (5) quality assurance, (6) digital mammography, and (7) reports and committees.
Physical and biological doses produced from neutron capture in a 235U foil
Authors: Liu Hungyuan B. , Brugger Robert M. , Laster Brenda H. , Greenberg Dennis D. , Gordon Chris R. , Warkentien Lynne S.
Published: March 2003

As a follow‐on study to the feasibility of neutron capture therapy (NCT) with 235U brachytherapy seeds, physical doses were calculated and measured for the radiation from a 235U foil in a lucite phantom which was irradiated at the epithermal neutron irradiation port of the Brookhaven Medical Research Reactor. In addition, cell survival experiments were performed to obtain the relative biological effectiveness (RBE) for the neutron part of the radiation. The calculated absorbed doses agree with the measured ones. From cell survival experiments, it is deduced that the fission neutrons from the 235U foil have a RBE of 3.0 while the fast neutrons in the beam have a RBE of 3.8. Also observed is that, with the cells 7 mm from the foil, a significant amount of absorbed dose comes from the β rays of 235U fission events. This absorbed dose from β rays is a significant addition to the therapeutic dose. Due to the limited ranges of β rays in tissue, this absorbed dose is restricted to the vicinity of the foil. This is the first demonstration of β rays as part of NCT.
Tests for evaluating laser film digitizers
Authors: Meeder R. J. J. , Jaffray D. A. , Munro P.
Published: March 2003

A number of tests to evaluate the image transfer characteristics of laser film digitizers have been developed and these tests have been used to evaluate the performance of a Konica KFDR‐S laser film digitizer. These tests were designed to be simple in nature and to use materials readily available in clinical departments. The tests examined (i) the geometric accuracy of the laser film digitizer; (ii) the linearity of the digitizer's characteristic curve; (iii) the temporal/spatial response of the digitizer to abrupt changes in optical density; and (iv) the noise added by the digitizer to the digitized images. The tests were easy to use and could be performed in a short period of time. Results of the tests for the Konica laser film digitizer revealed several problems including minor geometric inaccuracies, structural noise added by a shading correction circuit in the digitizer, and spread of the laser beam before reaching the film plane, possibly caused by the f‐theta lens of the digitizer. The light spread made the optical density of small, high density structures measured by the digitizer dependent upon the optical density of the surrounding regions. Therefore, under some circumstances, film digitization with the Konica laser film digitizer could lead to erroneous results. In addition, our measurements suggest that these problems are not unique to the Konica digitizer. Thus, the testing of laser film digitizers is recommended, especially if the digitizer is to be used for quantitative measurements.
Considerations for superficial photon dosimetry
Authors: Stern Robin L. , Kubo Hideo D.
Published: March 2003

Dose measurements at superficial energies required special considerations. First, care must be taken in selecting appropriate phantom materials. Materials that are adequate tissue substitutes at megavoltage energies might not be adequate at superficial energies. The suitability of a material can be judged by comparing its mass attenuation and mass energy absorption coefficients at superficial energies to those of the tissue of interest. Second, very low energy x‐ray and electron contaminants must be removed from the superficial beam before they reach the detector. For detectors with a very thin window, this can be achieved by placing thin film on top of the detector. Failure to properly eliminate contaminants can result in a large increase in dose measured directly at the surface.
A computational model for electron backscattering in electron dosimetry
Authors: Kellogg T. , Ray A. K.
Published: March 2003

A one‐dimensional electron transport algorithm based on the ideas of invariant imbedding has been developed and applied to the problem of electron backscattering in electron dosimetry. Results have been compared with experimental and Monte Carlo results. Agreement appears to be reasonable at low incident energies (0.5 to 10 MeV) and low Z(Z ≤65) materials.
Application of solid state detectors for dosimetry of therapeutic proton beams
Authors: Vatnitsky Stanislav , Miller Daniel , Siebers Jeffrey , Moyers Michael
Published: March 2003

A PTW Riga diamond detector and LiF TLDs have been evaluated for use in proton beam dosimetry by comparing results of proton beam calibration with those obtained using thimble ionization chambers. The thimble ionization chambers were calibrated in terms of exposure while the TLDs and diamond detector were calibrated in terms of absorbed dose in a 60Co beam. Absorbed doses to muscle in proton beams for ionization chambers were derived using the TG 20 charged particle protocol. Absorbed doses to muscle for solid state detectors were derived using absorbed dose proton beam quality correction factors. Differences between the derived doses for ionization chambers and solid state detectors were found to be within the uncertainties of measurements: 4.5% for ionization chambers and 5% for solid state detectors.
X‐ray imaging using amorphous selenium: Photoinduced discharge (PID) readout for digital general radiography
Authors: Rowlands J. A. , Hunter D. M.
Published: March 2003

Digital radiographic systems based on photoconductive layers with the latent charge image readout by photoinduced discharge (PID) are investigated theoretically. Previously, a number of different systems have been proposed using sandwiched photoconductor and insulator layers and readout using a scanning laser beam. These systems are shown to have the general property of being very closely coupled (i.e., optimization of one imaging characteristic usually impacts negatively on others). The presence of a condensed state insulator between the photoconductor surface and the readout electrode does, however, confer a great advantage over systems using air gaps with their relatively low breakdown field. The greater breakdown field of condensed state dielectrics permits the modification of the electric field during the period between image formation and image readout. The trade‐off between readout speed and noise makes this system suitable for instant general radiography and even rapid sequence radiography, however, the system is unsuitable for the low exposure rates used in fluoroscopy.