bluuebunny/crossref_metadata_2025 huggingface dataset:
On the cause of the variation in tissue‐maximum ratio values with source‐to‐detector distance
Authors: Thomadsen Bruce R. , Kubsad Shrikant S. , Paliwal Bhudatt R. , Shahabi Siamak , Mackie T. Rockwell
Published: March 2003

While tissue‐maximum ratios (TMR) for 60cobalt treatment units have been shown to be independent of source‐to‐axis distance (SAD), high‐energy photon beams demonstrate variations in their TMR as a function of SAD. Some authors have asserted that the distance dependence of the TMR stems from electron contamination in the beams, while others have suggested low‐energy, scattered photons as the cause. Using a magnet to sweep contaminant electrons out of the photon treatment beam eliminates any variation in TMR with distance. Thus, electron contamination accounts for all of the distance dependence, and any low‐energy, scattered photons behave indistinguishably like the high‐energy photons.
An equivalent‐square formula for head‐scatter factors
Authors: Vadash Paul , Bjärngard Bengt
Published: March 2003

A simple formula is evaluated for calculating the equivalent square collimator setting that gives the same headscatter factor as a given rectangular field. The expression requires that one parameter is determined experimentally. It is found that one single value of this parameter can be used for the six x‐ray beams studied (two accelerators, two energies, with and without wedge).
Calculation of beta dosimetry in radiation synovectomy using Monte Carlo simulation (EGS4)
Authors: Johnson L. Scott , Yanch Jacquelyn C.
Published: March 2003

Using the EGS4 Monte Carlo code, absorbed dose rate factors were estimated for four radionuclides of interest in radiation synovectomy, an intra‐articular radiation therapy to treat rheumatoid arthritis. The treatment consists of the injection of a beta‐emitting radionuclide into the joint capsule in order to eliminate diseased synovium through irradiation. The radionuclides investigated are 32P, 90Y, 165Dy, and 198Au. Calculations reveal the absorbed dose factor (cGy cm2/MBq s) as a function of distance (mm) in an EGS4 model of the rheumatic joint. The model incorporates bone, articular cartilage, joint capsule, and tissue (synovium) components found in all synovial joints, with dimensions in the model corresponding to dimensions typically found in larger joints, e.g., the knee, shoulder, or hip. Results are compared with previous, analytical approaches to beta dosimetry in radiation synovectomy. In addition, radiation backscatter due to the presence of bone is investigated and determined to have a negligible enhancement effect on absorbed dose to synovium.
Tumor localization using fundus view photography for episcleral plaque therapy
Authors: Evans Michael D. C. , Astrahan Melvin A. , Bate Rosario
Published: March 2003

The accurate localization of ophthalmic tumors on the scleral surface is important when calculating radiation dose to the tumor and adjacent structures from episcleral I‐125 plaques. This is particularly true for tumors close to the fovea or optic nerve. A fundus view diagram of the eye is often used by ophthalmologists to describe the size and shape of the tumor perimeter as well as its geographical location on the retinal surface. There is, however, an inherent inaccuracy associated with the use of a hand drawn diagram to obtain physical measurements. A computer planning program has been developed which uses CT and a digitized photographic montage of the posterior hemisphere to estimate the size and location of posteriorly located tumors. A simple phantom was constructed to test the software and a direct comparison made between planned versus measured tumor size and location in an excised eye.
A survey of fluoroscopic exposure rates: AAPM Task Group No. 11 Report
Authors: Boone John M. , Pfeiffer Douglas E. , Strauss Keith J. , Rossi Raymond P. , Lin Pei‐Jan Paul , Shepard Jeffrey S. , Conway Burton J.
Published: March 2003

Fluoroscopic procedures, in general, result in much higher exposures to patients than do most types of radiographic procedures [National Council on Radiation Protection, Report 100, p. 31 (1989)]. In spite of this, fluoroscopic exposure rates can vary widely between systems, and often for no apparent reason. The charge of AAPM Task Group No. 11 was to evaluate fluoroscopic exposure rates at the entrant surface of the x‐ray image intensifier, and to disseminate this information so that medical physicists could compare their own exposure rate measurements with typical values. The measurement protocol was defined for various system configurations. Sheets of copper were used to attenuate the x‐ray beam, and the input exposure rate at the image intensifier (at the input mode closest to 23‐cm diameter) in the absence of a scattering medium was determined. With 2 mm of copper as x‐ray beam filtration, the median fluoroscopic exposure rate at the image intensifier was found to be 16.5 nC/kg/s (64.0 μR/s), with an average kV of 77 and mA of 2.0 (n=62).
Optical lumen measurement of coronary artery latex casts
Authors: Zhang Qian , Parker Dennis L. , Wu Jiang
Published: March 2003

To facilitate calibration and testing of quantitative coronary angiography (QCA) systems, an optical technique has been developed to measure the vessel dimensions of complex coronary artery phantoms. The technique provides accurate geometric data such as centerline position and cross‐sectional area of vessel branches. The sequential steps involved in the measurement include taking pictures of different views, tracing magnified images, point‐by‐point digitizing and computing the center positions and radii along each vessel branch. The accuracy of the optical technique is assessed using the measurement of a precisely known aluminum phantom. Measurement repeatability is assessed by multiple measurement of the same branches on a barium impregnated latex cast which provides a realistic coronary artery phantom. The measurement is demonstrated to be accurate to within 0.01 mm and repeatable to within 0.05 mm on the wire phantom measurement and 0.08 mm on the realistic phantom measurement.
A cosine modulation artifact in modulation transfer function computations caused by the misregistration of line spread profiles
Authors: Steckner Michael C. , Drost Dick J. , Prato Frank S.
Published: March 2003

Modulation transfer functions (MTFs) are used to analyze the spatial frequency transfer characteristics of medical imaging systems. By definition, accurate MTFs should not include the effects of image noise and they should not be aliased. Therefore, many techniques used to compute MTFs register and average together multiple profiles to improve both signal to noise and/or eliminate aliasing. It is demonstrated that improper registration of individual profiles can cause errors of up to 100% in the MTF. Computer modelling shows that a maximum allowable error of 2% in the MTF requires a registration precision of ±1/9 of a pixel for each profile, if the profile was sampled at twice the cutoff frequency of the MTF. One suggested registration method, demonstrated with experimental magnetic resonance image data, is 1.5 times more accurate than the minimum requirement.
Rare‐earth scatter fractions in chest radiography
Authors: Petrone Thomas J. , Steidley K. David
Published: March 2003

A beam stop technique was used to measure the densitometric scatter fractions under three regions of a humanoid chest phantom utilizing LaOBr and Gd2O2S screens. For these receptors, the scatter fractions under the lung and retrocardiac areas were 13%–36% lower than published values for CaWO4. In the mediastinal area, there was no significant difference between the CaWO4 and rare‐earth phosphors. The use of grids reduces the scatter fraction in all three regions by 40%–75%. With and without a grid, radiographic contrast with Gd2O2S was measured utilizing simulated lesions placed above the lung and retrocardiac areas.
Selection of radionuclides for radioimmunotherapy
Authors: Mausner Leonard F. , Srivastava Suresh C.
Published: March 2003

Selection criteria for radionuclides to be used in radioimmunotherapy are reviewed. Among the topics covered are half‐life, specific activity, type of emission, and ease of preparation. (AIP)
Radioimmunotherapy dose estimation in patients with B‐cell lymphoma
Authors: Siegel J. A. , Goldenberg D. M. , Badger C. C.
Published: March 2003

Trials of radiolabeled antibody therapy in patients with B‐cell lymphoma have been the most promising of any in radioimmunotherapy. Response rates of greater than 90% with many complete remissions have been reported by several groups using either low (185–370 MBq) or high (8.6–22.5 GBq) doses of I‐131‐labeled antibodies against B‐cell antigens. Estimated doses delivered to normal organs have ranged from 0.2 to 2.2 mGy/MBq and have shown similar interpatient variation in all series, despite differences in antibody specificity and dosimetric techniques. Tumor doses have ranged from 0.5 to 5.4 mGy/MBq. There has been little correlation of tumor response with estimated tumor dose. Toxicity has been limited to bone marrow suppression which has been greater with the higher amounts of I‐131. An advantage for a particular antibody specificity or for high dose compared to multiple low doses has yet to be demonstrated.
Evaluation of inherent gray‐level dynamic range in a digital image using the runs test and join‐count statistics
Authors: Chuang Keh‐Shih , Huang H. K.
Published: March 2003

The dynamic range of the gray level of a digital image is limited by the noise it contains. Two statistical methods called “runs test” and “join‐count statistic” are used to measure the noise level in a digital image. A residual image is formed by subtracting an original image from its smoothed version. Theoretically, the noise level in the residual image should be identical to that in the original image. The noise level is determined by examining each bit plane of the residual image individually starting from the least significant bit up to the bit plane whose statistic does not show a random pattern. Images from three digital modalities: computerized tomography, magnetic resonance, and computed radiography are used to evaluate the gray‐level dynamic range. Both methods are easy to implement and fast to perform.
A technique of scatter‐glare correction using a digital filtration
Authors: Honda Michitaka , Ema Takehiro , Kikuchi Katsuya , Ohe Mitsuo , Komatsu Ken‐ichi
Published: March 2003

A scatter‐glare correction technique for x‐ray images acquired with an antiscatter grid was developed. In the technique, the scatter‐glare image was estimated from exposure conditions and subtracted from the acquired image. The basic procedure in the estimation of the scatter‐glare image is convolution filtering; however, the novel aspects of the technique are as follows: (1) To estimate the scatter‐glare intensity, a formula that does not include the term of object thickness was used. With this formula, the correction can be performed, even for nonuniform phantoms; (2) To estimate the scatter‐glare distribution, the experimental scatter‐glare point spread function (PSF) was directly used as a convolution kernel. Although the shape of the PSF changed slightly for water thicknesses of 5–25 cm, we applied the PSF measured at a water thickness of 15 cm to the correction experiments. For the stepped water phantom (10–20 cm), scatter‐glare estimation produced an average error of 10%, with respect to the lead bar data. Furthermore, the improvement of image quality and quantitative accuracy resulting from the correction was examined.
Automatic segmentation of liver structure in CT images
Authors: Bae Kyongtae T. , Giger Maryellen L. , Chen Chin‐Tu , Kahn Charles E.
Published: March 2003

The segmentation and three‐dimensional representation of the liver from a computed tomography (CT) scan is an important step in many medical applications, such as in the surgical planning for a living‐donor liver transplant and in the automatic detection and documentation of pathological states. A method is being developed to automatically extract liver structure from abdominal CT scans using a priori information about liver morphology and digital image‐processing techniques. Segmentation is performed sequentially image‐by‐image (slice‐by‐slice), starting with a reference image in which the liver occupies almost the entire right half of the abdomen cross section. Image processing techniques include gray‐level thresholding, Gaussian smoothing, and eight‐point connectivity tracking. For each case, the shape, size, and pixel density distribution of the liver are recorded for each CT image and used in the processing of other CT images. Extracted boundaries of the liver are smoothed using mathematical morphology techniques and B‐splines. Computer‐determined boundaries were compared with those drawn by a radiologist. The boundary descriptions from the two methods were in agreement, and the calculated areas were within 10%.
Absolute volumetric blood flow measurements using dual‐energy digital subtraction angiography
Authors: Molloi Sabee , Qian Yao‐Jin , Ersahin Atila
Published: March 2003

In recent years, as a solution to the well‐documented problems associated with visual interpretation of coronary arteriograms, more physiologic means of assessing coronary artery stenosis are being investigated. Absolute arterial blood flow assessed as a function of time can be a valuable aid in the analysis of functional significance of arterial lesions and obstructions. An absolute volumetric blood flow measurement technique using a motion immune dual‐energy subtraction technique is being investigated, where the kVp and filtration are switched at 30 Hz. The low‐ and high‐energy images are corrected for scatter and veiling glare before subtraction. In this technique, the absolute arterial blood flow is calculated by combining the videodensitometric analysis of spatial and temporal aspects concerning the contrast propagation through the arterial bed using tissue suppressed energy subtracted images. The blood flow measurement technique was validated using a pulsatile pump and a flow chamber imaged over a Humanoid chest phantom. A 20‐MHz Doppler flow probe was used to validate the measurement of phasic volumetric blood flow. The measured (M) and known (K) mean blood flow for the entrance vessel technique and the videodensitometric calibration technique were related by M=1.14K −0.12 (r2=0.98) and M=1.12K−0.23 (r2=0.90), respectively. The results indicate that phasic volumetric blood flow can be measured using a CCD camera in conjunction with real time dual‐energy subtraction.
The x‐ray centennial—Thompsons and Thomsons
Authors: Almond Peter R.
Published: March 2003

When x rays were discovered by Wilhelm Roentgen in November, 1895, the news spread rapidly through Europe, Great Britain, and the United States and many individuals became involved in their development. Some of the more prominent participants shared the name of Thompson or Thomson, which causes confusion when the history of x rays is discussed because of their similar pronunciation. In Britain they were William Thompson (Lord Kelvin), J. J. Thomson, and Silvanus P. Thompson and in the United States it was Elihu Thomson. In addition, one of the first books on x rays published in the United States was written by Edward Thompson.
A finite element approach for modeling photon transport in tissue
Authors: Arridge S. R. , Schweiger M. , Hiraoka M. , Delpy D. T.
Published: March 2003

The use of optical radiation in medical physics is important in several fields for both treatment and diagnosis. In all cases an analytic and computable model of the propagation of radiation in tissue is essential for a meaningful interpretation of the procedures. A finite element method (FEM) for deriving photon density inside an object, and photon flux at its boundary, assuming that the photon transport model is the diffusion approximation to the radiative transfer equation, is introduced herein. Results from the model for a particular case are given: the calculation of the boundary flux as a function of time resulting from a δ‐function input to a two‐dimensional circle (equivalent to a line source in an infinite cylinder) with homogeneous scattering and absorption properties. This models the temporal point spread function of interest in near infrared spectroscopy and imaging. The convergence of the FEM results are demonstrated, as the resolution of the mesh is increased, to the analytical expression for the Green's function for this system. The diffusion approximation is very commonly adopted as appropriate for cases which are scattering dominated, i.e., where μs≫μa, and results from other workers have compared it to alternative models. In this article a high degree of agreement with a Monte Carlo method is demonstrated. The principle advantage of the FE method is its speed. It is in all ways as flexible as Monte Carlo methods and in addition can produce photon density everywhere, as well as flux on the boundary. One disadvantage is that there is no means of deriving individual photon histories.
Decomposition of pencil beam kernels for fast dose calculations in three‐dimensional treatment planning
Authors: Bortfeld Thomas , Schlegel Wolfgang , Rhein Bernhard
Published: March 2003

A method for the calculation of three‐dimensional dose distributions for high‐energy photon beams is presented. The main features are (i) the calculation is fast enough to allow interactive three‐dimensional treatment planning, and (ii) irregularly shaped or compensated fields, which are required to fit three‐dimensional dose distributions to target volumes, are adequately taken into consideration. The method is based on the pencil beam convolution technique and shares its features concerning accuracy. A considerable gain in speed is achieved by decomposing the pencil beam kernel into three separated terms, thus reducing the required number of two‐dimensional convolutions. The convolutions are performed in the frequency domain via the fast Hartley transform. Using these techniques, the calculation time for the convolutions is only about 8 s on a DEC VAX station 3100. This is one‐fourth to one‐third of the calculation time for the ray tracing through the three‐dimensional CT data set, which has to be performed in any case. Results of the calculation are compared with measurements in a homogeneous phantom for 15 MV photons. Two irregular fields shaped with a multileaf collimator are considered. The deviations between measured and calculated absolute dose values are smaller than ±2%.
Experimental verification of a three‐dimensional dose calculation algorithm using a specially designed heterogeneous phantom
Authors: Ayyangar K. , Palta J. R. , Sweet J. W. , Suntharalingam N.
Published: March 2003

A solid heterogeneous phantom made up of 25‐ and 50‐mm cubes of materials with different electron densities was used to verify the accuracy of a three‐dimensional (3‐D) dose calculation algorithm. This algorithm uses 3‐D information obtained from contiguous CT (computed tomography) slices, spaced 5 mm apart. Primary and scatter doses at a point are calculated by using information from ray‐tracing CT voxels. The algorithm was developed on a Stardent model 1500 Supergraphic workstation. Cubes of materials with different electron densities were stacked up to simulate finite heterogeneities in three dimensions. This design allows verification of the algorithm for surface contour corrections and finite heterogeneities in the treatment field. Thermoluminescent lithium fluoride chips were placed in grooves milled on the cubes for dose measurement at various points. Different experiments were performed to investigate both the accuracy of the dose calculation algorithm and the utility of the versatile test phantom.
Experimental determination of fluence perturbation factors for five parallel‐plate ionization chambers
Authors: Kuchnir Franca T. , Reft Chester S.
Published: March 2003

The calibration of parallel‐plate chambers for absolute dosimetry is an unsettled matter. The medical physics community has not yet agreed on a practical method of obtaining Ngas, although several researchers are working on this problem. If the photon and electron fluence perturbation factors, KwallKcomp, were known for chambers of standard construction with full buildup provision, then an in‐air Co‐60 calibration could be applied to these, as is done with cylindrical chambers. We have obtained such correction factors for five commercially available chambers based on measurements in air and in homogeneous phantoms relative to matched cylindrical chambers of known dosimetric parameters. For three of the chambers (Markus, Holt and Exradin) we find that KwallKcomp=1.000±0.008, in excellent agreement with available results from Monte Carlo calculations. The values for the other two chambers (NACP and Capintec) are different than 1. Our results are compared to recently published values, both calculated and measured.
Scatter factors for a 25‐MV x‐ray beam
Authors: Bjärngard Bengt E.
Published: March 2003

The scatter factors, i.e., the ratios between the primary and scatter components of absorbed dose, have been determined for a 25‐MV x‐ray beam. The primary dose is described by attenuation and head‐scatter factors. The attenuation coefficient decreased from 0.028 cm−1 at the surface to 0.025 cm−1 at 30 cm depth. The head‐scatter factor varied about 8% for collimator settings from 5×5 to 40×40 cm2. To determine the scatter factors, measured tissue‐output ratios were divided by the attenuation and head‐scatter factors for the same depth and field size. The results were fitted, with agreement within ±1%, to a model which assumes a linear relation between the scatter factor and the field size if the ratio between field size and depth is constant. The model requires transient electron equilibrium, which is present when the depth exceeds 6 cm and the field size 6×6 cm2. The scatter factor increases to 1.33 at 30‐cm depth and 40×40 cm2 field size. The peak scatter factor (PSF) at 3.5‐cm depth reaches 1.06 for the largest field. Both PSF and NPSF, the normalized peak scatter factor, are affected by electron disequilibrium, which causes some conceptual ambiguities and numerical uncertainties.
Dosimetry of large wedged high‐energy photon beams
Authors: Podgorsak Matthew B. , Kubsad Shrikant S. , Paliwal Bhudatt R.
Published: March 2003

The dependence of the wedge factor and central axis depth dose on field size was evaluated for 6‐, 10‐, and 24‐MV wedged photon beams for field sizes up to 40×40 cm2. The wedge factor for 60°, 45°, 30°, and 15° wedges in a 24‐MV beam was found to vary by as much as 25%, 12%, 9%, and 5%, respectively, over a field size range of 5×5 to 40×40 cm2. For 10 and 6 MV wedged beams, the wedge factors varied by up to 17% and 15%, respectively, over the same field size range. The depth dose curves for the wedged beams differed significantly from the open beam profiles. At 6 MV, the wedges caused beam hardening while at 24 MV, with the exception of the 15° wedge, all wedged beams were softer than the open beams, for all field sizes. At 10 MV, wedged fields of size less than 20×20 cm2 were hardened relative to the open beam, whereas larger wedged fields had depth dose values within ±1% of the 10‐MV open‐beam depth dose data. Accurate treatment planning for large wedged fields and high‐energy photon beams thus requires the use of wedged beam depth dose curves and field size specific wedge factors. It was established that an equivalent square field for a rectangular wedged field can be determined using the standard open beam formulation. The largest difference between the wedge factor for a rectangular beam and its equivalent square beam was 2.5% and occurred for 24‐MV elongated fields. The depth dose values for a rectangular wedged field and its equivalent square beam were within ±1% at all energies studied.
Surface dose perturbation due to air gap between patient and bolus for electron beams
Authors: Sharma S. C. , Johnson M. W.
Published: March 2003

The effect of air gaps under bolus material on the surface dose for electron beams is investigated. Dose measurements were performed for air gaps from 0.0 to 3.0 cm and bolus thicknesses of 0.5 and 1.0 cm using the various electron energies and cone sizes available on an electron linear accelerator. Our results show that the surface dose decreases for lower electron energies, smaller field sizes, thicker boluses, and larger air gaps.
CT‐based simulation with laser patient marking
Authors: Ragan Don P. , He Tongming , Mesina Carmen F. , Ratanatharathorn Vaneerat
Published: March 2003

A CT‐based simulator has been assembled based on a commercial CT scanner, virtual simulation software developed at the University of North Carolina and a laser drawing device to transfer the radiation portals from the virtual simulator to the patient. The simulation process can be completed in approximately 1 h; under most cases, the treatment portals can be designed and the patient marked in one session. The device has an inherent accuracy of ±1 mm. The portal projection accuracy in clinical cases is observed to be better than 2 mm.
Cylindrical dose distributions in pseudodynamic rotation radiosurgery: An experimental study
Authors: Sixel Katharina E. , Podgorsak Ervin B. , Souhami Luis
Published: March 2003

A linac‐based radiosurgical technique is reported which produces cylindrical isodose distributions covering cylindrical targets of arbitrary orientations within the patient's head. The technique uses rectangular collimators and 4 degrees of freedom: gantry and couch rotation, as defined by a previously known dynamic rotation technique, collimator rotation, and collimator length adjustment. The relationship between the four parameters is derived and because of its complexity, the cylindrical dynamic rotation technique is introduced as a pseudodynamic technique. For cylindrical targets, the cylindrical pseudodynamic technique in comparison to the standard spherical technique produces considerable dose savings to healthy tissue surrounding the target and gives a similar or better dose fall‐off outside the target.
Correction for distortion in a beam outline transfer device in radiotherapy CT‐based simulation
Authors: Ragan Don P. , He Tongming , Liu Xunliang
Published: March 2003

In using a CT scanner as a radiation therapy simulator, it would be helpful to be able to transfer the beam outline from the computer plan to the patient's skin. A beam outline transfer device has been constructed and installed on a Siemens' DRH CT scanner gantry. The planned treatment beam geometry from a 3‐D computerized simulation and planning system can be projected onto the patient's skin surface accurately and efficiently. The positioning accuracy achieved is within ±0.1 cm over a 20 cm×20 cm field. Integrating the device into the CT scanner, simplifies the device and reduces the cost over an externally mounted device. Two unsuccessful methods to correct the projection distortions are also mentioned. In order to achieve the reported beam outline transfer accuracy, a system based on our empirically derived calibration procedure is described.
A simple method of producing depth ionization data for electron energy constancy check
Authors: Islam Mohammad K. , Rashid Harunor , Gaballa Hani , Ting Joseph , Rosenow Ulf F.
Published: March 2003

A simple method has been developed to reproduce depth ionization data of electron beams for energy determination. The method utilizes a simple set of equipment, a combination of a specially designed wedge‐shaped polystyrene phantom and a linear array of detectors, to collect the necessary data. The wedge‐shaped phantom provides varying depths to various detectors in the array. The ionization readings received from the detectors were corrected for off‐axis ratio and plotted against corresponding ray‐line depths to produce depth ionization curves. The instrument setup was fast and simple. The relevant data, for a high‐energy linear accelerator with multiple electron energies, were collected in minutes. The depths of 80% and 50% ionization determined by this method were found to differ by 2 and 3 mm, respectively, at the most, with those determined by a conventional method.
Assessment of a linear accelerator for segmented conformal radiation therapy
Authors: Zacarias A. S. , Lane R. G. , Rosen I. I.
Published: March 2003

Segmented conformal radiation therapy is a new computer‐controlled treatment technique under investigation in which the target volume is subdivided into thick transverse segments each of which is then treated individually by rectangular transverse abutting fields. In order to obtain uniform dose at abutments, the machine isocenter remains fixed in the patient and field edges are defined by independently moving focused collimator jaws to give matching geometric divergence. Mechanical variation in jaw and gantry positioning will create some dose variation at field abutments. Film dosimetry was used to study the radiation field positioning accuracy and precision of a commercial linear accelerator. A method of field position calibration was developed using multiple nonabutting fields exposed on the same radiograph. Verification of collimator jaw calibration measurements was performed using multiple abutting fields exposed on a single radiograph. Measurements taken over 5 months of clinical accelerator operation studied the effects of simple jaw motion, simple gantry motion, and combined jaw/gantry motion on jaw position precision and accuracy. The inherent precision and accuracy of radiation field positioning was found to be better than ±0.3 mm for both jaws with all types of motions except for the Y2 jaw under combined jaw/gantry motion. When the ability to deliver abutting beams was verified in clinical mode, the average dose variation at abutments was less than 6% at all gantry angles except for one. However, due to accelerator software limitations in clinical mode, the settings for collimator positions could not take advantage of the maximum accuracy of which the hardware is capable. Improved software should allow positioning accuracy which would significantly reduce the dose variation at abutments. The results demonstrate that the mechanical precision of the accelerator is suitable for delivery of segmented conformal therapy treatments.
Experimental determination of 32P dose backscatter factors at and near soft‐tissue boundaries
Authors: Nunes Josane , Prestwich William V. , Kwok Cheuk S.
Published: March 2003

Beta‐ray dose backscatter factors with respect to soft tissue were measured using an extrapolation chamber. The beta‐ray dose backscatter factor is a measure of the change effected in absorbed dose to a soft‐tissue medium when part of the medium is replaced by a material other than soft tissue (i.e., a scatterer); the source is located at the boundary between the two media. The dependencies of backscatter factor on scatterer atomic number and on source geometry are investigated, and the variation of backscatter factor with distance from the boundary is determined. For a 32P point source, backscatter factors with respect to Mylar, a soft‐tissue substitute, at 0.55 mg/cm2 from the boundary, are, 29.65[0.12]%, 31.07[0.24]%, 19.30[0.48]%, 16.27[0.35]%, 5.46[0.11]% and −26.44[0.02]% for bismuth, tungsten, cadmium, copper, aluminum, and air scatterers, respectively. Backscatter factors measured for a 32P planar source are generally smaller than those for a point source. The variation of backscatter factor with distance from the boundary is well represented analytically by sums of exponentials. Therefore, the rate of decrease of backscatter factor with distance can be specified by a relaxation length, defined as the depth through which the backscatter factor is reduced by 1/e, where e is the base of the natural logarithm. For example, with a 32P planar source, relaxation lengths in Mylar are 588[7] mg/cm2 and 238[2]mg/cm2 for bismuth and aluminum scatterers, respectively. Qualitative interpretation of backscatter factor depth profiles is presented. In addition, the variations of backscatter factor with scatterer atomic number and with source geometry are discussed with reference to existing experimental findings on beta particle reflection.
Routine verification of strength of 137Cs brachytherapy sources using a NaI detector
Authors: Wierzbicki Jacek G. , Meyer Richard
Published: March 2003

An inexpensive and easy method of calibration of brachytherapy 137Cs tubes is described. Cesium brachytherapy sources are calibrated relative to an NIST‐calibrated source. The detection system is a NaI detector and single channel analyzer with the window set on the photo peak.
A safe method of analysis for mechanical damage in spherical radioactive sources used in remote afterloading brachytherapy devices
Authors: Raaphorst G. P. , Szanto J. , Cygler J. , Laewen A.
Published: March 2003

The examination of brachytherapy sources in a quality assurance program can result in significant exposure to staff. Even though exposure may be within prescribed limits attempts should be made to reduce such exposure to as low as reasonably achievable (ALARA concept). In this report, a system that can be used to examine small spherical radiation sources while minimizing exposure to staff is described.
Acoustic power calibrations of cylindrical intracavitary ultrasound hyperthermia applicators
Authors: Hynynen K.
Published: March 2003

Preliminary clinical results indicate that some tumors can be heated well utilizing cylindrical ultrasound sources placed in body cavities. In this paper a simple method for measuring the acoustic power from cylindrical intracavitary transducers will be described. The radially propagating acoustic field was converted to a beam with a single propagation direction by a brass reflector, and the radiation force generated by this beam on an absorbing target was measured. The power output of several clinical intracavitary arrays varied significantly between identically shaped transducer elements. The results show that it is important to measure the acoustic power output from each element prior to its clinical use. The radiation force technique is simple and sensitive and can be easily adapted to be used as a routine clinical quality assurance method.
Sampling techniques for the evaluation of treatment plans
Authors: Lu X‐Q. , Chin L. M.
Published: March 2003

Sampling techniques using randomly distributed points and regular Cartesian grids were compared for the evaluation of volume, dose‐volume histogram, tumor control, and normal tissue complication probabilities in radiation treatments. Particularly, the uncertainties associated with each sampling technique in estimating the dose‐volume histograms for several dose distributions are analyzed in detail. It is found that the estimation of these parameters using sampling points on a regular Cartesian grid is, in general, significantly more efficient than using random points. This finding is different from other published results. The choice of grid size for sampling was analyzed according to the AAPM recommended uncertainty on the dose delivered to the patient. It was concluded that when grid sampling is used, a grid size of 0.5 cm is adequate for most plans to meet the guidelines.
A simple one‐dimensional model for understanding planar dose‐versus‐depth profiles in mixed photon/electron fields
Authors: Fehl D. L.
Published: March 2003

A one‐dimensional model is described for illustrating the perturbing effects of mixed photon/electron fields on planar dose‐versus‐depth profiles in low Z materials (Z≤20), and at moderate photon energies (0.3–10 MeV). An electron field may arise from photon interactions in filters upstream of a dose‐versus‐depth experiment. The effects of upstream mass, scattering of electrons out of a gap between a filter and the experiment, and differences between the incident electron energy distribution and locally produced KERMA are modeled and compared to Discrete Ordinates calculations. At transitional depths in a dose‐versus‐depth experiment, the distortions can be significant and obscure the profile of the photon component alone.
The response characteristics of a newly designed plane‐parallel ionization chamber in high‐energy photon and electron beams
Authors: Gerbi Bruce J.
Published: March 2003

A new plane‐parallel ionization chamber has been designed by Attix to overcome the shortcoming of previous commercially available parallel‐plate ionization chambers for dosimetry in high‐energy photon and electron beams in radiation oncology. This investigation details the performance characteristics of this new, commercially available plane‐parallel chamber. The magnitude of the polarity effect in high‐energy electron beams is shown to be less than 1% while the polarity effect in high‐energy photon beams is lower than several other plane‐parallel ionization chambers. The over response of the chamber in the buildup region of normally incident high‐energy photon beams is less than 1% for 6‐ and 24‐MV x rays while the response of the new chamber to obliquely incident x‐ray beams was affected much less by the angle of beam incidence than the other chambers tested. These superior response characteristics are primarily due to the construction characteristics of the collecting electrode arrangement. The Attix chamber, with a wall diameter (w) of 40 mm and a plate separation (s) of 1 mm, has an aspect ratio, (w/s), of 40. This exceeds the previously reported design criterion of w/s≥25 required to properly measure surface and buildup dose in either conventional therapy beams or in beams that are highly contaminated.
Dosimetry of wedged fields with asymmetric collimation
Authors: Khan Faiz M.
Published: March 2003

Shaping of wedged fields with asymmetric collimators (independent jaws) produces dosimetric effects which need to be taken into account when calculating isodose distributions and accelerator monitor units. These effects were studied using ion chamber dosimetry in a water phantom. Wedged fields of 30° to 60° wedge angles were shaped by an asymmetric collimator. A system of dose calculation was developed to take into account the changes in dose in the open portion of the field. The formalism utilizes regular symmetric field parameters (e.g., Sc, Sp, and TPR). Calculated point doses and profiles agreed well with the measured data.
The influence of arc weights on the dose distribution for single target radiosurgery
Authors: Plazas Maria Cristina , Lefkopoulos Dimitri , Schlienger Michel
Published: March 2003

To stereotactically irradiate ellipsoidal inclined arteriovenous malformations (AVMs) with a single isocenter one can use an adjusted inclined irradiation space with a reduced angular range of arcs. If one uses the maximum available irradiation space it is necessary to weight some of the arcs in order to closely encompass the lesion. It has been found that there is a relationship between the lesion inclination angle β and the weights of arcs. Conversely when the β angle is known, the weights of the arcs can be determined in advance by means of weighting vectors. The dose volume analysis shows that the isodose volumes remain constant for the different weighting vectors except for the 10% isodose volume which increases slightly as the linear weighting vector's slope α increases. Moreover, this procedure permits the optimization of the superimposition between the lesion and the reference isodose. These results enable the construction of an “a priori knowledge” database that can be included in an optimization algorithm procedure.
Engineering modifications for dynamic stereotactically assisted radiotherapy
Authors: Gillies B. A. , O'Brien P. F. , McVittie R. , McParland C. , Easton H.
Published: March 2003

To perform dynamic stereotactic radiotherapy, a number of modifications to the commercial linear accelerator were required and these are described in this report. A couch drive system was added to rotate the couch in synchrony with the gantry. A high dose rate beam for this technique is acquired through a head modification, which allows easy removal of the radiation field flattener. Because of these modifications, a number of changes and additions were required to the linac interlock system, including a complete new chain of interlocks for use with the stereotactic technique.
Mammographic scanning equalization radiography
Authors: Sabol John M. , Soutar Ian C. , Plewes Donald B.
Published: March 2003

It is well recognized that variations in breast thickness and parenchymal composition can produce a range of exposure which exceeds the latitude of high contrast mammographic film/screen combinations. Optimal imaging of the dense breast is desired since 30%–60% of women present with dense breasts, and they are believed to be at the highest relative risk of developing breast cancer. The application of scanning equalization radiography to mammography has been investigated through the construction and characterization of a prototype mammographic scanning equalization radiography (MSER) system, designed to image mammographic phantoms. The MSER system exposes a Min‐R/MRH cassette by raster scanning a 2.0×1.6 cm beam of pulsed x‐rays across the cassette. A scanning detector behind the cassette measures the local x‐ray transmission of the breast. Feedback of the transmission information is used to modulate the duration of each x‐ray pulse, to equalize the film exposure. The effective dynamic range of the MSER system is 25 times greater than that of conventional mammography. Artifact‐free images of mammographic phantoms show that MSER effectively overcomes the latitude limitations of film/screen mammography, enabling high contrast imaging over a wide range of object x‐ray transmission. Anthropomorphic phantom images show that MSER offers up to a sixfold increase in film contrast in the normally underexposed regions of conventional mammograms. Characterization of the entrance exposure shows that there is not a significant difference in exposure between MSER and conventional mammographic techniques, suggesting that both would pose comparable risk to the patient. Calculations show that the construction of a clinical multiple beam MSER system is feasible with minor changes to existing technology.
Noninvasive digital energy subtraction angiography with a channeling‐radiation x‐ray source
Authors: Gary C. K. , Piestrup M. A. , Boyers D. G. , Pincus C. I. , Pantell R. H. , Rothbart G. B.
Published: March 2003

Channeling radiation could provide a viable source for digital energy subtraction angiography (DESA). A signal to noise ratio (SNR) of 6.2 for a resolution of 0.5 mm×0.5 mm could be achieved using a 6‐mA 100‐ms 20‐MeV electron‐beam pulse and a diamond channeling crystal as the x‐ray source. This article investigates the choice of a DESA contrast agent and the parameters of a channeling‐radiation x‐ray source to develop a channeling‐radiation DESA imaging system. The production of dual‐energy peaks, the maximum available x‐ray flux, the advantages of an area exposure, the necessity of a mosaic Bragg‐crystal filter to reduce patient dose, the optimal energy separation of the peaks for a quasimonochromatic x‐ray source, and the reduction of the signal from bone are discussed, leading to estimated SNRs and image resolution for a channeling‐radiation imaging system. The computer analysis developed to calculate the image quality is also discussed.
Determination of x‐ray tube focal spot position
Authors: Kubota Hideaki , Ozaki Yoshiaki , Matsumoto Masao , Kanamori Hitoshi
Published: March 2003

In x‐ray spectral measurement using a semiconductor detector system, accurate positioning of the detector in front of the anode is very important. This needs accurate determination of the focal spot position on the target in an x‐ray tube. We present a simple technique to determine the focal spot position accurately. Using a pinhole technique, the focal image made by x rays is overlapped on the target image made by visible rays emitted when filament heating takes places. Both images are made in the same geometry with the tube diaphragm and filter removed.
Mammography focal spot measurement with a star pattern: Techniques to avoid inaccuracies
Authors: Kimme‐Smith Carolyn , Chatziioannou Arion
Published: March 2003

The increased emphasis on mammography acceptance testing and quality control make the star pattern test an essential component of the procedure for diagnosing poor resolution due to large focal spots. In this paper procedures for calculating magnification and the correct angle of the star pattern to use are described. The preferred placement of the star pattern in the field and proper exposure technique is also discussed.
Considerations on the calibration of small thermoluminescent dosimeters used for measurement of beta particle absorbed doses in liquid environments
Authors: Demidecki Andrzej J. , Williams Lawrence E. , Wong Jeffrey Y. C. , Wessels Barry W. , Yorke Ellen D. , Strandh Margareta , Strand Sven‐Erik
Published: March 2003

An investigation has been carried out on the factors which affect the absolute calibration of thermoluminescent dosimeters (TLDs) used in beta particle absorbed dose evaluations. Four effects on light output (LO) were considered: decay of detector sensitivity with time, finite TLD volume, dose linearity, and energy dependence. Most important of these was the decay of LO with time in culture medium, muscle tissue, and gels. This permanent loss of sensitivity was as large as an order of magnitude over a 21‐day interval for the nominally 20‐μm‐thick disc‐shaped CaSO4(Dy) TLDs in gel. Associated leaching of the dosimeter crystals out of the Teflon matrix was observed using scanning electron microscopy. Large channels leading from the outside environment into the TLDs were identified using SEM images. A possibility of batch dependence of fading was indicated. The second most important effect was the apparent reduction of light output due to finite size and increased specific gravity of the dosimeter (volume effect). We estimated this term by calculations as 10% in standard “mini” rods for beta particles from 90Y, but nearly a factor of 3 for 131I beta particles in the same geometry. No significant nonlinearity of the log (light output) with log (absorbed dose) over the range 0.05–20.00 Gy was discovered. Energy dependence of the LO was found to be not detectable, within measurement errors, over the range of 0.60–6.0 MeV mean energy electrons. With careful understanding of these effects, calibration via gel phantom would appear to be an acceptable strategy for mini TLDs used in beta absorbed dose evaluations in media. Application of these TLDs to monoclonal antibody therapy was of particular interest in these studies.
Improving agreement between radiation‐delineated field edges on simulation and portal films: The edge tolerance test tool
Authors: McCullough Edwin C. , McCollough Kevin P. , Simon Scott P.
Published: March 2003

The anatomy at the edge of a treatment machine portal film may differ from that shown by the delineator lines on a simulator film by up to 10 mm, even if both the simulator and the treatment machine meet accepted criteria for mechanical tolerances. To assure that this possibility is minimized requires some form of overall alignment check between the simulator and the treatment machine. A new test device, the edge tolerance test tool (ET3), has been designed to permit a quick and accurate check on whether portal film disagreement with simulator films is due to an accumulation of tolerances. Its use should eliminate one source of this common problem.
Energy response of agar‐alanine free radical dosimetry to therapeutic electron beams
Authors: Ciesielski B. , Reinstein L. E. , Meek A. G. , Wielopolski L.
Published: March 2003

Calculations of the energy response of an agar‐alanine phantom dosimeter (AAPD) to electrons in the energy range of 0.15–20 MeV together with experimental results at 16 MeV are presented. It is shown that the sensitivity of the EPR dosimeter (EPR signal/Gray) is independent of alanine crystal size and varies less than 2%, in the electron energy range indicated. Thus, the measured free radical density distribution may be used directly as an indication of the absorbed dose distribution in an irradiated phantom.