bluuebunny/crossref_metadata_2025 huggingface dataset:
Scanner‐induced variability and quality assurance in longitudinal dual‐energy x‐ray absorptiometry measurements
Authors: Sievänen Harri , Oja Pekka , Vuori Ilkka
Published: March 2003

Characteristics of typical malfunctions and scanner‐induced variability observed in dual‐energy x‐ray absorptiometry (DXA), and their potential effects on longitudinal reliability of DXA were evaluated. According to extensive, cumulative quality assurance (QA) data obtained from two successive x‐ray sources during a 3‐yr period, the scanner‐induced variability may derive from long‐term drift (∼0.5%/year), short‐term drift (∼0.2%–2.2%/day), inhomogeneity of the x‐ray beam intensity over the tabletop (∼1%), and changes in internal filtration (∼0.5%). The absolute magnitudes of these effects may be considerable with respect to expected small changes in bone characteristics observed in intervention studies. Furthermore, these effects may not be discriminated from each other. Therefore it may not be possible to correct their cumulative effect using long‐term QA data only. The observed drifts are fortunately negligible with respect to the precision adequate for clinical decision making. In contrast, the evaluation of these multiform scanner‐induced variability is warranted in longitudinal intervention studies using DXA. It is emphasized that this study was performed with a single DXA system, and the results should be considered accordingly; nevertheless, it is believed that the issues raised would apply to other systems too, at least in the sense of stringent QA. In this respect, single daily phantom measurement appeared to be occasionally ineffective, whereas a remeasurement of the phantom after subject measurements significantly improved the effectiveness of QA. Altogether, the QA procedures, which consider both the short‐term and long‐term variability as well as the spatial variability over the tabletop, may provide a more effective method to detect silently degrading scanner performance and evaluate its effects on the subject measurements. High‐quality operator performance is, however, a prerequisite for proper QA in any setting.
Beam profiles along the nonwedged direction for large wedged fields
Authors: Chui Chen‐Shou , LoSasso Thomas
Published: March 2003

Beam profiles along the nonwedged direction of a wedged field produced by a linear accelerator exhibit more “sagging” than that of an open field at the same depth. For large fields, the profiles of open and wedged fields can differ by as much as 7%. The extra “sagging” of wedged profiles is mainly due to the difference in penetration between on‐ and off‐axis rays caused by the variation of beam quality across the field. An algorithm was developed to estimate an “effective” depth such that the profile of a wedged field can be approximated by the open‐field profile at the effective depth. The algorithm was verified by measured beam profiles for 6‐ and 15‐MV x‐ray beams for 15°, 30°, 45°, and 60° wedges.
A formal study of lateral magnification and its influence on mammographic imaging sharpness
Authors: Schiabel Homero , Ventura Aldo , Frère Annie France
Published: March 2003

Radiographic magnification is often employed in mammography to improve detection of small anatomical details of clinical interest, such as microcalcifications. The image size depends on the conventional magnification factor, on the focal spot size, and on the x‐ray angulation. In this paper magnification effects are investigated for an object which lays off the radiation field center, for which there is a “lateral magnification” that makes it possible to obtain a sharper image of the object. This occurs because the true image size (i.e., the shadow) becomes larger as the distance from the beam center to the object increases, while the total blurring (penumbra) which is responsible for the unsharpness remains constant. The statement is valid only in the boundary of the perpendicular axis, which for most mammographic units corresponds to the position where the thickest portion of the breast is placed. Calculations are presented which show that the lateral magnification can be useful for enhancing breast microcalcifications using radiographic magnification.
Computerized three‐dimensional segmented human anatomy
Authors: Zubal I. George , Harrell Charles R. , Smith Eileen O. , Rattner Zachary , Gindi Gene , Hoffer Paul B.
Published: March 2003

Manual segmentation of 129 x‐ray CT transverse slices of a living male human has been done and a computerized 3‐dimensional volume array modeling all major internal structures of the body has been created. Each voxel of the volume contains a index number designating it as belonging to a given organ or internal structure. The original x‐ray CT images were reconstructed in a 512×512 matrix with a resolution of 1 mm in the x,y plane. The z‐axis resolution is 1 cm from neck to midthigh and 0.5 cm from neck to crown of the head. This volume array represents a high resolution model of the human anatomy and can serve as a voxel‐based anthropomorphic phantom suitable for many computer‐based modeling and simulation calculations.
Addendum to “History, current status, and trends of radiation protection standards” by William R. Hendee [Med. Phys. 20, 1303–1314 (1993)]
Authors: Hendee William R.
Published: March 2003

Tables II, III, and IV in this article should be updated with information recently published in Report No. 116: Limitation of Exposure to Ionizing Radiation, National Council on Radiation Protection and Measurements, Bethesda, MD (1993). Updated Tables II, III, and IV were provided to Medical Physics in the early fall, but were not included in the printed review. The updated tables are presented here.
Resolution limits for optical transillumination of abnormalities deeply embedded in tissues
Authors: Gandjbakhche A. H. , Nossal R. , Bonner R. F.
Published: March 2003

Random walk theory is used to calculate the line spread function (LSF) of photons as they cross the midplane of a slab of finite thickness. The relationship between the LSF and the photon transit time in transillumination time‐resolved experiments is investigated. It is found that the LSF is approximately Gaussian distributed, with a standard deviation, σ, which can be used as a criterion of the spatial resolution of the imaging system. Results are substantiated by comparison with actual data in the literature. Any given resolution can be improved by reducing the excess transit time Δt, but heterogeneity of the scattering medium and low levels of detected light enormously complicate the achievement of subcentimeter spatial resolution. The latter point is discussed by using optical parameters of breast tissues for visible and near‐infrared radiation (NIR) light.
Implementation of a three‐dimensional compensation system based on computed tomography generated surface contours and tissue inhomogeneities
Authors: Jursinic Paul A. , Podgorsak Matthew B. , Paliwal Bhudatt R.
Published: March 2003

A computed tomography (CT) based system that compensates for patient surface contour and internal tissue inhomogeneity was implemented in our clinic. The compensators are fabricated with a mixture of tin granules and bee's wax. The tin/wax mixture was optimized for tin granule size and tin granule to wax ratio. The narrow beam attenuation coefficients were measured for 4‐, 6‐, 10‐, and 24‐MV photon beams. The compensator design and fabrication methodology were verified by measuring the dose distribution for a known surface contour irradiated with a compensated beam and for a known inhomogeneity that was submerged in a water phantom and irradiated with a compensated beam. For the surface contour, the uncompensated isodose levels varied by as much as 10% in the compensation plane and the compensator restored the isodose level to a variation of less than 1.3%. Measured and calculated doses for this surface contour were found to differ by less than 3.4%. For the inhomogeneity, the uncompensated isodose levels varied by 27% in the compensation plane and the compensator restored the isodose level to a variation of less than 1.5%. Measured and calculated doses for the known inhomogeneity were found to differ by less than 2%. Measurements of depth‐dose curves indicate that the presence of the compensator in the beam does not significantly increase the surface dose. Twenty‐six compensators have now been fabricated for clinical cases. In these patients, dose variations as great as 19% occurred in the plane of compensation prior to placing the compensator in the beam. Measured and calculated dose profiles with the compensators in place have been found to agree within 2.3%.
Determination of beam orientation in radiotherapy planning
Authors: Gokhale P. , Hussein E. M. A. , Kulkarni N.
Published: March 2003

A computer‐assisted method for selecting beam orientation in a one‐step procedure is presented. Inverse and adjoint techniques are developed to obtain the best beam directions. Both methods rely on determining the “path of least resistance” to radiation from the tumor location to the surface of the section. The effectiveness of beam directions is then determined by monitoring the dose distribution along the section boundary. The inverse and adjoint calculations are performed for three tumor cases using a two‐dimensional discrete ordinates transport code. The proposed treatment plans from these calculations are verified against typical treatment plans. The new techniques improved the dose distribution in the treated section. The inverse calculations are useful in sections involving low‐density tissues. The adjoint technique can effectively deal with multiple target volumes and/or sections with complex geometry. The proposed method is potentially useful in selecting beam orientations for three‐dimensional planning systems and in determining beam intensities in rotational and conformal therapy.
Buildup region and depth of dose maximum of megavoltage x‐ray beams
Authors: Sixel Katharina E. , Podgorsak Ervin B.
Published: March 2003

The depth of dose maximum, dmax, of megavoltage x‐ray beams was studied as a function of beam energy and field size for 6‐, 10‐, and 18‐MV x‐ray beams and field sizes ranging from 1×1 to 30×30 cm2. For a given beam energy, dmax increases rapidly with increasing field size at small fields, reaches a maximum around 5×5 cm2 and then gradually decreases with increasing field size for large fields. Monte Carlo simulations combined with measurements verified that the effect observed at small field sizes is caused by in‐phantom scatter, while at large fields the effect is due to scatter contamination of the primary beam from the linac head. A comparison between the dmax behavior of flattened beams to that of unflattened beams indicates that the dmax decrease at large fields for flattened beams is caused mainly by contamination electrons which are produced in the flattening filter and further scattered by collimator jaws and air.
Digital image subtraction of temporally sequential chest images for detection of interval change
Authors: Kano Akiko , Doi Kunio , MacMahon Heber , Hassell Dayne D. , Giger Maryellen L.
Published: March 2003

An automated digital image subtraction technique for temporally sequential chest images has been developed in order to aid radiologists in the detection of interval changes. A number of small regions of interest (ROIs) are selected automatically in the lung areas of two temporally sequential chest images. A local matching, based on a cross‐correlation method, is performed on each pair of corresponding ROIs in order to determine shift values for the coordinates of two images. A proper warping of x,y coordinates is obtained by fitting two‐dimensional polynomials to the distributions of shift values. One of the images is warped and then subtracted from the other. Forty six pairs of chest images (42 with interval changes and 4 without interval change) were processed using this method. The subtraction images were able to enhance various important interval changes, such as differences in the size of tumor masses, changes in heart size, and changes in pulmonary infiltrates or pleural effusions. Approximately 70% of the pairs showed reasonably good registration.
The measured electrical properties of normal and malignant human tissues from 50 to 900 MHz
Authors: Joines William T. , Zhang Yang , Li Chenxing , Jirtle Randy L.
Published: March 2003

The electrical conductivity and relative permittivity of malignant and normal human tissues were measured at frequencies from 50 to 900 MHz. The measurements were made between 23 and 25 °C using a network analyzer connected to a flat‐ended coaxial probe that was pressed against the freshly excised tissue samples. The malignant tissues were of the following normal tissue origin: bladder, colon, kidney, liver, lung, lymph nodes, mammary gland, spleen, and testes. The normal tissues included: colon, kidney, liver, lung, mammary gland, and muscle. Normal tissue samples of bladder, lymph, spleen, and testes were not available. In general, at all frequencies tested, both conductivity and relative permittivity were greater in malignant tissue than in normal tissue of the same type. For tissues of the same type, the differences in electrical properties from normal to malignant were the least for kidney (about 6% and 4% average differences over the frequency range in permittivity and conductivity, respectively), and these differences were the greatest for mammary gland (about 233% and 577% average differences in permittivity and conductivity, respectively). To illustrate a potential use of these data in hyperthermia applications, frequency‐selective heating of malignant tissue (modeled as a sphere) surrounded by host normal tissue is calculated from the measured electrical properties for certain tissues.
Comprehensive analysis of electron beam central axis dose for a radiotherapy linear accelerator
Authors: Shiu Almon S. , Tung Samuel S. , Nyerick Carl E. , Ochran Timothy G. , Otte Victor A. , Boyer Arthur L. , Hogstrom Kenneth R.
Published: March 2003

This work evaluates the application of AAPM task group 25 (TG25) methodology for determination of central axis depth dose for a radiotherapy linear accelerator, whose dual scattering foil system and applicators were recently modified. The percent depth dose (%DD) and the dose output factor have been measured for square and rectangular fields at 100‐ and 110‐cm source‐to‐surface distance (SSDs). At 100‐cm SSD, results showed that %DD for a specific energy and field size can vary with applicator, the largest variation being for the 20‐MeV, 10×10‐cm field where a spread of ±2.5% or ±3 mm about the mean %DD is observed. The square‐root method determines rectangular field %DD within 1%. Output factors for rectangular fields are calculated from square field values more accurately using a square‐root method than the equivalent‐square method recommended by TG25. At 110‐cm SSD, the %DD calculated from that at 100‐cm SSD using an inverse square factor does not agree with measured values for all fields. The maximum difference observed for the 20‐MeV, 6×6‐cm field was 5.5% or 10 mm. Output data at the 110‐cm SSD show that the square‐root method is suitable for determination of the air‐gap correction factors of rectangular fields. In summary, the recommendations of TG25 work reasonably well for central axis electron beam dosimetry for this version of a radiotherapy linear accelerator, except in limited cases where applicator‐scattered electrons apparently cause minor but clinically significant discrepancies.
Evaluation of a dual bias dual metal oxide‐silicon semiconductor field effect transistor detector as radiation dosimeter
Authors: Soubra M. , Cygler J. , Mackay G.
Published: March 2003

A new type of direct reading semiconductor dosimeter has been investigated as a radiation detector for photon and electron therapy beams of various energies. The operation of this device is based on the measurement of the threshold voltage shift in a custom‐built metal oxide‐silicon semiconductor field effect transistor (MOSFET). This voltage is a linear function of absorbed dose. The extent of the linearity region is dependent on the voltage controlled operation during irradiation. Operating two MOSFETS at two different biases simultaneously during irradiation will result in sensitivity (V/Gy) reproducibility better than ±3% over a range in dose of 100 Gy and at a dose per fraction greater than 20×10−2 Gy. The modes of operation give this device many advantages, such as continuous monitoring during irradiation, immediate reading, and permanent storage of total dose after irradiation. The availability and ease of use of these MOSFET detectors make them very promising in clinical dosimetry.
Comparison of radiation dosimetry in water and in solid phantom materials for I‐125 and Pd‐103 brachytherapy sources: EGS4 Monte Carlo study
Authors: Luxton Gary
Published: March 2003

A model based on the EGS4 Monte Carlo system is developed for calculating dose rate to water from an embedded low‐energy brachytherapy source, given measurement data of dose rate to water within a water‐substitute solid phantom for a source of given strength. The EGS4‐based model is used to calculate point source dose rate distributions per unit source strength for water and for several species of solid phantoms. As a model for the dose rate distribution to an infinitesimal water inhomogeneity embedded in the solid phantom, the Monte Carlo is used to calculate dose rate to a thin spherical shell of water contained within the solid phantom at various distances centered on the source. Comparing this distribution to that calculated for a homogeneous water phantom, one can derive correction factors to determine dose rate to homogeneous water medium from measurements with water dose‐calibrated detectors within the solid phantom. These factors are calculated for polymethylmethacrylate (PMMA or acrylic), solid water (WT1), and RW‐1, a material optimized for low‐energy dosimetry, with photon spectra from Pd‐103 and from two commercial models of I‐125 seed used as input. For model 6711 I‐125 seeds at 1 cm in PMMA and WT1, the calculated ratios of dose rate to water to dose rate to water in the solid phantom are 0.893 and 1.038, respectively. Applying these factors to published, apparently divergent measurements of the specific dose rate constant based on measurements in PMMA and WT1 phantoms, the specific dose rate constant inferred for the model 6711 seed are found to be in excellent agreement: 0.878 cGy h−1U−1 (1.115 cGy‐ cm2h−1mCi−1) from the mean of the WT1 measurements and 0.879 cGy h−1U−1 from the PMMA measurements. Among the phantoms studied for Pd‐103 and I‐125, measurements in RW‐1 would most closely approximate those for water.
Semiautomated matching and seed position location for implanted ribbons
Authors: Kassaee Alireza , Altschuler Martin D.
Published: March 2003

A difficult, error‐prone, and time consuming problem in brachytherapy is the determination of the three‐dimensional (3‐D) positions of implanted seeds. To triangulate 3‐D seed positions, the corresponding seed images in radiographs taken at different parallax (gantry) angles must be identified (matched). With only two radiographs, there is an inherent problem of ambiguity in matching individual seed images. The use of the assignment algorithm cannot bypass this ambiguity. However, when seeds are implanted within ribbons, their positions in 3‐D space can be determined accurately and (almost always) without ambiguity. This follows because the matching of ribbon images is much less ambiguous than the matching of individual seed images. We describe a semiautomated procedure to match the ribbon images on two radiographs, determine the 3‐D loci of the ribbons, and locate the 3‐D position of each seed on each ribbon. The ribbon matching problem involves a double application of the assignment algorithm. After image matching, the 3‐D locus of each ribbon is found from the intersection of the ray sets from its radiographic images and the analysis of a 3‐D spanning tree through the derived ray set intersection points. The seed positions on a ribbon locus are determined by interpolation of equally spaced seed intervals along the 3‐D ribbon locus. Experiments with a phantom show that for two radiographs separated by any parallax between 20° and 90°, our technique provides a systematic error in 3‐D seed location of 3.0 mm, and a random error of less than 2 mm.
Feasibility of dual radionuclide brain imaging with I‐123 and Tc‐99m
Authors: Ivanovic M. , Weber D. A. , Loncaric S. , Franceschi D.
Published: March 2003

A study was conducted to evaluate the feasibility of simultaneous dual radionuclide brain imaging with 123I and 99mTc using photopeak image subtraction techniques or offset photopeak image acquisition. The contribution of the photons from one radionuclide to a second radionuclide's photopeak energy window (crosstalk) was evaluated for SPECT and planar imaging of a brain phantom containing 123I and 99mTc for a range of activity levels and distribution properties approximating those in rCBF images of the adult human brain. Crosstalk was evaluated for 10% symmetrical energy windows centered on the 123I and 99mTc photopeaks and for 10% energy windows asymmetrically placed to the left and right of the center of the respective photopeaks. Major observations include: (1) in the centered photopeak windows, 99mTc crosstalk in the 123I window is 8.9% of the 99mTc seen in the 99mTc window and ranges from 37.5% to 75.0% of the 123I in the 123I window. 123I crosstalk is 37.8% of the 123I seen in the 123I window and ranges from 4.4% to 8.9% of the 99mTc seen in the 99mTc window; (2) the spatial distribution of a radionuclide's crosstalk photons differs from that observed in the radionuclide's photopeak window; (3) a 99mTc photopeak window offset to the left does not decrease 123I crosstalk, and the percentage of 99mTc scattered photons is significantly increased in the window. Offsetting the 123I window to the right decreases 99mTc crosstalk to 9.0% to 17.9% of the 123I counts, but decreases 123I sensitivity by 39.9%; and (4) offsetting both photopeak windows to the right decreases the 99mTc scattered photons in the 99mTc window, but increases 123I crosstalk to 17.0% to 33.8% of the 99mTc counts. The findings show that image quality, spatial resolution, and quantitative accuracy are degraded to unacceptable levels with the combinations of energy windows tested for dual radionuclide imaging of 99mTc and 123I. This indicates that dual radionuclide imaging must be thoroughly tested and validated before use in clinical studies.
Concentration recovery for SPECT CNS dopamine‐receptor studies for monkeys
Authors: Shao Lingxiong , Billings Jeffrey , Pfeiffer Nuala , Kung Hank F.
Published: March 2003

Most commercial SPECT scanners do not properly compensate for scattering and partial volume effects, which severely impacts the quantitation of clinical and research SPECT studies, especially in CNS dopamine studies. Hence, in this paper, a brain phantom containing a monkey skull was used to calibrate a Picker Prism 3000 SPECT camera for quantitative studies of 123I and 99mTc‐labeled radiopharmaceuticals. In the calibration study, two small vials representing basal ganglia were inserted into a skull phantom. A series of images were taken with different concentration ratios between the basal ganglia and the other brain compartments to simulate the dynamic imaging study of the dopamine D2 receptor in a monkey brain. Based on this calibration, a simple method is proposed for count correction in the region of interest (ROI). This technique is superior to using a single calibration factor from a cylindrical uniform activity phantom since it can compensate for both the partial volume effect and the dynamically changing effect of scattering. A significant difference between the true and measured values was observed in the correction factors (by a factor as much as 3). Furthermore, a cylindrical phantom, approximately the size of a monkey's head and containing the same basal ganglia inserts, was used to investigate the feasibility of using a cylindrical phantom with the proposed correction method. The results indicate that it is feasible to use a cylindrical phantom for the case where the activity inside the brain cavity and the activity in the surrounding tissue are similar.
On‐axis and off‐axis primary dose component in high energy photon beams
Authors: Zefkili Sofia , Kappas Constantin , Rosenwald Jean‐Claude
Published: March 2003

The depth dose of the primary dose component, on axis and off axis of six different x‐ray beams, has been determined from transmission measurements in narrow beam geometry with and without flattening filter using a Perspex column of a cross section large enough to ensure electronic equilibrium. In order to derive the primary photon fluence, a correction for the scatter from the column has been applied according to the following method: A number of spectra taken from the literature have been used for computing a scatter coefficient Sc at different depths by convolution of dose spread arrays. Using the relationship between Sc and the single attenuation coefficient μi to represent each entire spectrum, it has been possible to correct the experimental transmission curves iteratively, until the corresponding values of μ were stabilized and representative of the primary. The measured attenuation coefficients were found to have a linear increase as a function of the distance from the central axis for all the energies and types of linear accelerators. For the same nominal energy, this increase is different from one accelerator to another. The same phenomenon was observed for the attenuation coefficients obtained without the flattening filter in the same experimental conditions. The results are tentatively interpreted considering the angular variation of bremsstrahlung energy spectra with and without a flattening filter as calculated by a Monte Carlo method and they are consistent and useful to take accurately into account the softening of the beam as the off‐axis distance increases.
Evaluation of new algorithms for the interactive measurement of surface area and volume
Authors: Alyassin Abdalmajeid M. , Lancaster Jack L. , Downs J. Hunter , Fox Peter T.
Published: March 2003

The maximum unit normal component (MUNC) method used for surface area measurement and the divergence theorem algorithm (DTA) used for volume measurement were evaluated. The accuracy and precision of these methods were investigated at varying signal‐to‐noise ratios (SNRs), sampling, spatial averaging, and orientation. The accuracy of the MUNC measured surface area, as indicated by the mean error, was 2.0% for seven spherical samples, with SNRs ranging from 5:1 to 39:1. The precision, as indicated by the percent coefficient of variation (% CV) for these samples, was less than 3.0%. Likewise, the accuracy and precision of the DTA measured volume for these samples were both less than 1.0%. MUNC surface area measurement from 23 samples of a computed tomography (CT) image of a wooden sphere (51.44‐mm diameter) with x,y voxel size ranging from 1 to 10 mm and z voxel size ranging from 2 to 14 mm yielded an accuracy of 1.3% and a precision of 2.2%. The DTA volume measurements from 18 samples of the wooden sphere with x,y vowel size ranging from 1 to 8 mm and z size ranging from 2 to 14 mm provided an accuracy of 1.2% and a precision of 1.8%. Measurement of surface area for a cylindrical rod scanned by CT in five different orientations, ranging from along each axis to between all three axes, yielded an accuracy of 3.7% and a precision of 2.0%. The volume of the cylindrical rod measured by the DTA method for these orientations produced an accuracy of 4.0% and a precision of 3.7%. The volume measured by DTA compared well with the volume measured by a modified voxel counting method. The MUNC surface area method was superior to counting surface voxels. The accuracy and precision for five interactive surface area and volume measurements, using paired cut planes to select subsets of a computer‐generated sphere with radius 25 pixels, were both less than 1.0%.
A superconducting cyclotron for neutron radiation therapy
Authors: Maughan Richard L. , Powers William E. , Blosser Henry G.
Published: March 2003

The physical and clinical specifications of a neutron therapy facility utilizing a superconducting cyclotron are presented. The cyclotron and its support system are described. Details of the operation of the cyclotron in a hospital environment are given; the requirements of the helium liquifier and cryogenic system are described together with a summary of its mode of operation. The simplicity of the cyclotron control system is discussed. The physical characteristics of the neutron beam are described. The central axis percent depth dose curve is equivalent to that of a 4 MV x‐ray beam. The depth of maximum dose occurs at approximately 9 mm depth and the surface dose is between 40% and 45%. The multirod collimator allows for the production of irregularly shaped fields of size up to 26.5×30 cm, without excessive exposure to operating personnel.
Computer‐aided geometric reconstruction of Fletcher–Suit source positions
Authors: Li Shidong , Pelizzari Charles A. , Reft Chester , Sutton Harold G. , Chen George T. Y.
Published: March 2003

A computer‐aided method for reconstruction of the source positions of a Fletcher–Suit applicator has been developed. The tandem source positions are determined by digitizing of the tip and two arbitrary points on the shaft from each of two orthogonal simulator films. A colpostat source position is reconstructed by digitizing of a single point on the endcap and three arbitrary points on the cylindrical sidewalls of the colpostat on the films. This computer‐aided method considers the true projection geometry and applicator shape and permits localization of the source positions to within a mean error of less than 1 mm. Compared with the conventional method, the new approach (1) is more time efficient because only a few easily identified points are digitized, (2) allows localization when the tandem sources are shifted by a spacer or when colpostat sources are difficult to visualize on the lateral film, and (3) is more accurate than the conventional technique because no manual drawing of source positions on films is involved.
Anisotropy measurements of a high dose rate Ir‐192 source in air and in polystyrene
Authors: Muller‐Runkel R. , Cho S. H.
Published: March 2003

Anisotropy measurements in air and in polystyrene were performed for a Nucletron microSelectron 10 Ci Ir‐192 source. TLD rods of similar sensitivity (±3%) were placed on a semicircle around the source center which had been precisely localized by either radiographic means or direct measurement. The anisotropy, expressed as a ratio of TLD reading at a given angle relative to the longitudinal source axis, over the reading at 90°, was measured for distances between 1 and 10 cm from the source center. For both media, a marked anisotropy of the high dose rate source was found. In air, there is no detectable variation in anisotropy with distance up to 10 cm from the source. In polystyrene, however, the anisotropy becomes less pronounced with increasing distance due to scatter. For both media, no difference, within the estimated experimental accuracy of ±5%, was found between three consecutive sources installed by the vendor.
Automatic three‐dimensional correlation of CT‐CT, CT‐MRI, and CT‐SPECT using chamfer matching
Authors: van Herk Marcel , Kooy Hanne M.
Published: March 2003

Image correlation is often required to utilize the complementary information in CT, MRI, and SPECT. A practical method for automatic image correlation in three‐dimensions (3D) based on chamfer matching is described. The method starts with automatic extraction of contour points in one modality and automatic segmentation of the corresponding feature in the other modality. A distance transform is applied to the segmented volume and a cost function is defined that operates between the contour points and the distance transform. Matching is performed by iteratively optimizing the cost function for 3D translation, rotation, and scaling of the contour points. The complete matching process including segmentation requires no user interaction and takes about 100 s on an HP715/50 workstation. Perturbation tests on clinical data with cost functions based on mean, rms, and maximum distances in combination with two general purpose optimization procedures have been performed. The performance of the methods has been quantified in terms of accuracy, capture range, and reliability. The best results on clinical data are obtained with the cost function based on the mean distance and the simplex optimization method. The accuracy is 0.3 mm for CT‐CT, 1.0 mm for CT‐MRI, and 0.7 mm for CT‐SPECT correlation of the head. The accuracy is usually at subpixel level but is limited by global geometric distortions, e.g., for CT‐MRI correlation. Both for CT‐CT and CT‐MRI correlation the capture range is about 6 cm, which is higher than normal differences in patient setup found on the scanners (less than 4 cm). This means that the correlation procedure seldom fails (better than 98% reliability) and user interaction is unnecessary. For CT‐SPECT matching the capture range is about 3 cm (80% reliability), and must be further improved. The method has already been introduced in clinical practice.
Attenuation in high‐energy x‐ray beams
Authors: Bjärngard Bengt E. , Shackford Hobart
Published: March 2003

Attenuation factors in water have been measured by a narrow‐beam technique in various portions of x‐ray beams with nominal energies of 6 and 25 MV, with and without a wedge in the beam. The results were analyzed in terms of an attenuation coefficient μ for small water thicknesses and a beam‐hardening coefficient η that describes the change in attenuation per unit depth. The variation of μ within the field was significant, about 0.5% per centimeter at 6 MV and 0.8% per centimeter at 25 MV for open beams. The heavy wedge used in these experiments caused significant (about 10%) beam hardening at 6 MV, softened the beam somewhat at 25 MV, and increased the variation of μ within the field to 3%–5%. These effects should be taken into account in dose calculations, and correction factors can be designed based on the variation of μ with off‐axis radius for open beams and with off‐axis position for wedged beams. The experimental technique, based on two measurements with the beam going through a water tank with either 26‐ or 50‐cm path length, was simple and highly reproducible. The beam hardening with depth in water, i.e., the value of η, was readily determined but found to be clinically insignificant.
Absorbed dose perturbation caused by diodes for small field photon dosimetry
Authors: Beddar A. S. , Mason D. J. , O'Brien P. F.
Published: March 2003

Uncertainties in dose measurements which may be considered acceptable for standard radiotherapy techniques may not be acceptable for radiosurgery or other precision radiotherapy techniques. One example of this type of uncertainty are the small perturbations to measured dose distributions which are caused by the use of diode detectors. In this article the effect of using diodes to measure small field penumbra is assessed and comments are made about diode orientation and diode construction.
Optimized radial and angular positions in Monte Carlo modeling
Authors: Wang Lihong , Jacques Steven L.
Published: March 2003

In Monte Carlo simulations of light transport in tissues, a grid system is set up to score physical quantities. This study of cylindrically symmetrical problems found that the optimized radial and angular positions for the averaged physical quantities in each grid element are off‐center. The error of the extrapolated physical quantities at the light‐incidence point using the centered radial positions is up to 14.3%.
Methods to calculate the lens efficiency in optically coupled CCD x‐ray imaging systems
Authors: Liu Hong , Karellas Andrew , Harris Lisa J. , D'Orsi Carl J.
Published: March 2003

In lens‐coupled CCD x‐ray imaging, the lens efficiency is of critical importance for attaining an x‐ray quantum‐noise‐limited system. However, the equations and the associated parameters of this process have not always been used properly in some of the literature. This can result in the incorrect estimation of the signal‐to‐noise ratio (SNR). In practice, it is found more convenient to use lens‐coupling efficiency equations which assume that the scintillating screen is an extended Lambertian source, since the number of light photons emerging from various scintillating screens has been measured and reported by several investigators. This communication examines several of the most common types of lens‐coupling efficiency equations and explains how to use them correctly with other parameters to calculate the SNR of a lens‐coupled CCD x‐ray imaging system.
Acceptance testing prone stereotactic breast biopsy units
Authors: Kimme‐Smith Carolyn , Solberg Timothy
Published: March 2003

When the Mammography Quality Standards Act becomes law in October, 1994, stereotactic breast biopsy units may require yearly physicist calibration. Upright stereotactic units can be easily tested using conventional mammography procedures and a gelatin phantom containing simulated calcifications, but prone units are difficult to assess because of the under‐table tube configuration. The two current manufacturers of these units have made different design decisions which affect each unit's calibration. There are a number of important distinctions between screening and prone biopsy units. For the two currently available prone units, a pronounced heel effect makes ion chamber position critical. Focal spot measurements are particularly difficult on one unit because there is no light field. The fixed grid on the other unit must be tested with a flood film. Physicists who inspect these units before their clinical use should be aware of variations needed by this equipment for specific acceptance tests.
The calibration and use of plane‐parallel ionization chambers for dosimetry of electron beams: An extension of the 1983 AAPM protocol report of AAPM Radiation Therapy Committee Task Group No. 39
Authors: Almond Peter R. , Attix Frank H. , Humphries Leroy J. , Kubo Hideo , Nath Ravinder , Goetsch Steve , Rogers David W. O.
Published: March 2003

This report is an extension of the 1983 AAPM protocol, popularly known as the TG‐21 Protocol. It deals with the calibration of plane‐parallel ionization chambers and their use in calibrating therapy electron beams. A hierarchy of methods is presented. The first is to calibrate the plane‐parallel chamber in a high energy electron beam against a cylindrical chamber which has an value that has been obtained from a NIST traceable60Co beam calibration. The second method, which is recommended for implementation by the ADCLs is an in‐air calibration against a NIST‐traceable calibrated cylindrical chamber in a Cobalt‐60 beam to obtain a plane‐parallel‐chamber calibration factor in terms of exposure or air kerma. The third method places the two chambers in a phantom in a Cobalt‐60 beam, and leads to an value for the plane‐parallel chamber. This report also gives and values for five commonly used commercially available plane‐parallel chambers: the Capintec PS‐033, the Exradin P‐11, the Holt, the NACP and the PTW‐Markus. The calculation of these Ngas ratios introduces aKcomp factor which is also calculated for the five parallel plate chambers. The use of the plane‐parallel chambers follows the 1983 AAPM protocol for absorbed dose calibrations of electrons, except that new energy‐dependent Prepl values are given for the Capintec PS‐033 and PTW‐Markus chambers consistent with the consensus of reports in the literature. For all the chambers, however,Prepl is unity for 20 MeV electrons. This report does not address the issue of the use of plane‐parallel chambers in calibrating photon beams.
Dose–volume distributions: A new approach to dose–volume histograms in three‐dimensional treatment planning
Authors: Niemierko Andrzej , Goitein Michael
Published: March 2003

A new approach to calculating and displaying dose–volume relationships in 3D radiation therapy is presented. We have developed a concept of a dose–volume distribution (DVD) and its corresponding differential dose–volume distribution (DDVD), based on organization of the data in the volume rather than in the dose domain. The new concepts make full use of the information that can be obtained from the dose calculation points and the sampling pattern and are designed to overcome shortcomings of the classical concepts of dose–volume histograms (DVH) and differential dose–volume histograms (DDVH). The new concepts can be applied to any number of dose calculation points, but they are especially advantageous when a small number of points is used. DVDs are particularly well suited to pseudo‐ and quasirandom sampling of dose distributions. We have developed an error analysis for DVDs and DDVDs in the case of pseudorandom sampling. We also describe an adaptive technique for minimizing the amount of data needed for purposes of display.
Dosimetric verification of a 3‐D electron pencil beam dose calculation algorithm
Authors: McShan D. L. , Fraass B. A. , Ten Haken R. K.
Published: March 2003

A three‐dimensional electron beam dose calculational algorithm implemented for use in a three‐dimensional treatment planning system is described. The 3‐D electron beam calculations have been in use clinically for more than seven years. The algorithm uses a pencil beam model based on small angle multiple Coulomb scattering theory. Our implementation allows volumetric CT‐based inhomogeneity corrections and provides for irregular field shapes (fields shaped with cerrobend cutouts) and the use of bolus. As part of NCI‐funded work evaluating the state of the art in electron beam treatment planning, extensive algorithm verification was undertaken and results of these tests for our implementation are presented.
Characterization of gypsum attenuators for radiotherapy dose modification
Authors: Arora V. R. , Weeks K. J.
Published: March 2003

Gypsum has significant value as a compensator material for use with high energy x rays in radiation therapy. Compensator thickness may be calculated by using an effective attenuation coefficient (μeff). Detailed measurements using narrow and broad beam geometry were collected to determine this effective attenuation coefficient as a function of energy (4–15 MV), field size, depth in tissue, and thickness of the compensator. An effective attenuation coefficient relation was defined using a least‐square method. It was then determined that extrapolating the broad beam data to a 0×0 cm2 field size resulted in a good approximation to the measured narrow beam attenuation coefficient. The variations in surface dose produced by gypsum attenuators were compared to open beam results. For the energies studied, it was determined that the increase in surface dose was acceptable for clinical application.
A method including edge effects for the estimation of radioimmunotherapy absorbed doses in the tumor xenograft model
Authors: Buras Robert R. , Williams Lawrence E. , Beatty Barbara G. , Wong Jeffrey Y. C. , Beatty J. David , Wanek Philip M.
Published: March 2003

The temporal relationship of radiolabeled monoclonal antibody (Mab) uptake to tumor size in a nude mouse human colon cancer xenograft model (LS174T) was evaluated as an aid to developing a method for estimation of radioimmunotherapy absorbed dose. Tumors of heterogeneous size were treated with 4.4 MBq (120 μCi) of 90Y‐labeled anti‐Carcinoembryonic Antigen Mab (90Y‐ZCE025). Regression analysis demonstrated an inverse log–log relationship of antibody uptake (%ID/g) to tumor mass in four time intervals investigated (N>10 points/interval):12–24 h, 2–3 d, 5–7 d, and 10–14 d. Curves of predicted radionuclide concentration vs time were then constructed for a range of constant tumor sizes. Xenograft radiation dose was obtained by temporal integration of each curve and application of appropriate dose estimation formulas. For each assumed tumor mass, an edge correction for loss of beta energy outside the target volume was applied assuming a spherical tumor shape. Estimated average absorbed doses were found to vary only from 13.8–10.3 Gy for a 20‐fold change in tumor sizes (0.1–2.0 g, respectively). Such constancy of dose may explain xenograft stasis observed by our group in earlier experiments at this level of administered 90Y activity.
A method for the calculation of electron energy‐straggling spectra
Authors: McLellan J. , Sawchuk S. , Battista J. J. , Sandison G. A. , Papiez L. S.
Published: March 2003

To calculate electron beam dose distributions accurately, numerical methods of electron transport calculations must account for the statistical variation (or “straggling”) in electron energy loss. This paper shows that the various energy straggling theories that are applicable to short path lengths all derive from a single statistical model, known as the compound Poisson process. This model in turn relies on three assumptions: (1) the number of energy‐loss events in a given path length is Poisson distributed; (2) events are mutually independent; and (3) each event has the same probability distribution for energy loss (i.e., the same energy‐loss cross section). Applying the principles of the compound Poisson process and using fast Fourier transforms, a new method for calculating energy‐loss spectra is developed. The spectra calculated using this method for 10, 20, and 30 MeV electrons incident on graphite and aluminum absorbers agreed with Monte Carlo simulations (egs4) within 1% in the spectral peak. Also, stopping powers derived from the calculated spectra agreed within 1.2%, with stopping powers tabulated by the International Commission on Radiation Units and Measurements. Several numerical transport methods “propagate” the electron distribution (in position, direction, and energy) over small discrete increments of path length. Thus the propagation of our calculated spectra over multiple path length increments is investigated. For a low atomic number absorber (graphite in this case), calculated spectra agreed with egs4 Monte Carlo simulations over the full electron range, provided the path length increments were sufficiently small (less than 0.5 g/cm2). It is concluded from these results that numerical methods of electron transport should restrict the size of path length increments to less than 0.5 g/cm2 if energy straggling is to be modeled accurately.
The use of a radiochromic detector for the determination of stereotactic radiosurgery dose characteristicsa)
Authors: McLaughlin W. L. , Soares C. G. , Sayeg J. A. , McCullough E. C. , Kline R. W. , Wu Andrew , Maitz A. H.
Published: March 2003

The measurement of absorbed dose as well as dose distributions (profiles and isodose curves) for small radiation fields (as encountered in stereotactic surgery) has been difficult due to the usual large detector size or densitometer aperture (≳1 mm) relative to the radiation field (as small as 4 mm). The radiochromic direct‐imaging film, when read with a scanning laser microdensitometer (laser beam diameter 0.1 mm), overcomes this difficulty and has advantages over conventional film in providing improved precision, better tissue equivalence, greater dynamic range, higher spatial resolution, and room light handling. As a demonstration of suitability, the calibrated radiochromic film has been used to measure the dose characteristics for the 18‐, 14‐, 8‐, and 4‐mm fields from the gamma‐ray stereotactic surgery units at Mayo Clinic and the University of Pittsburgh. Intercomparisons of radiochromic film with conventional methods of dosimetry and vendor‐supplied computational dose planning system values indicate agreement to within ±2%. The dose, dose profiles, and isodose curves obtained with radiochromic film can provide high‐spatial‐resolution information of value for acceptance testing and quality control of dose measurement and/or calculation.