bluuebunny/crossref_metadata_2025 huggingface dataset:
Verification and correction of setup deviations in tangential breast irradiation using EPID: Gain versus workload
Authors: Pouliot J. , Lirette A.
Published: March 2003

Purpose: When a deviation in the treatment setup is identified, when or how should it be corrected? With the aim of improving the precision and reducing the systematic errors while maintaining the workload to a minimum, a study was performed to define the proper set of actions for the correction of tangential breast setup deviations during the course of the treatment. Patients and Methods: Clinical data were taken from a prospective study of more than 2200 on‐line Electronic Portal Images (EPI) from 20 patients treated with tangential irradiation following a partial mastectomy for an early stage (I and II) breast cancer. The values of the central lung distance were then entered in the verification/correction procedure and modified to correct only the portion of the deviation most likely attributed to systematic errors according to the maximum likelihood. The verification/correction procedure uses an action level equal to FML⋅α/√N, where α is proportional to the standard deviation (α=nσ) and N is the number of consecutive fractions delivered after the start of the treatment. FML is the fraction of the deviation due to the systematic errors estimated from the maximum likelihood of the two distributions. In addition to n, one needs to specify Nmax, the maximum number of consecutive measurements without correction, to apply the procedure. The combination of these two parameters C(n,Nmax) will determine the reduction of systematic errors (gain) and the number of measurements and corrections (workload) associated to the procedure. Results: The effects of three combinations, C(1,1), C(2,2), and C(3,4) were studied. Also, the analysis of the results after application of the procedure with and without the factor of maximum likelihood made individually for each patient demonstrates the importance of the FML. Conclusions: The verification/correction procedure with the inclusion of the FML can effectively improve the accuracy when applied to clinical data. With the specific workload related to measurements and corrections performed at our institution, we have found that in the case of the tangential breast treatment, the optimum combination uses an action level equal to 2σ and a maximum of two consecutive measurements without correction.
The effect of detector size to the broadening of the penumbra—a computer simulated study
Authors: Chang Kai‐Siung , Yin Fang‐Fang , Nie Kai‐Wen
Published: March 2003

The effect of detector size to the broadening of the measured beam penumbra has been a subject of numerous studies. Based on measured data, linear and quadratic curves have been proposed to describe the relationship between the measured penumbra width, between 10%–90% and 20%–80% intensities, and the detector size. Extrapolations of these curves to zero detector size also suggest that the inherent penumbras can be deduced. However, due to experimental noise, especially when a small ionization chamber is used, and the inherent penumbra is not known, it is difficult to discern the superiority of either model. In this study, one‐dimensional convolution using a thin circular disk shape detector was employed to analyze the effect of detector size to the broadening of the penumbra. A set of beams with different inherent penumbra widths, ranging from 0 to 350 in arbitrary unit, was first generated. Each beam was then convoluted with the response function of the detectors with different sizes from 10 to 280, in arbitrary unit. The result is an output signal with penumbra that is wider than the inherent penumbra. The plots of penumbra widths to detector radii are a family of concave parabolic curves with different inherent penumbra widths. The concave portions of the curve represent results from scanning with detectors equal to or smaller than the penumbra width, and the linear portions represent results from scanning with detectors much larger than the penumbra. The curves are nothing but different scales of one curve. The extrapolations of the curves to the penumbra axis when the detector radius approaches zero give the deduced inherent penumbra widths. The deduced inherent penumbra widths approximate the inherent penumbra widths satisfactorily. From the graphs provided, the inherent penumbra can be deduced using the detector radius and the measured penumbra width.
Analysis of noise in phase contrast MR imaging
Authors: Andersen A. H. , Kirsch J. E.
Published: March 2003

In this work we analyze the effects of inherent random noise on the detectability of low‐contrast vessel structures that possess slow flow. When flow is encoded in more than one direction, the number of independent noise contributions increases in addition to the scan time. In a fast‐flow scenario, only the noise contribution from sampling along the direction of flow is of any significance. At slow flow rates, however, it becomes necessary to account for the noise in each encoded Cartesian direction. The degree to which noise affects low‐contrast detectability also depends on the method of phase contrast image processing employed. A theoretical analysis of the statistical properties of signal and noise in processed phase contrast magnitude images is presented and verified from experimental MR image data. Results show a progressively increased bias in the processed phase contrast image magnitude at slow flow rates due to contributions from inherent random noise. The amount of this bias increases with the number of physical directions in which flow is encoded and is larger for complex difference processed images than for phase difference processing. Correspondingly, the output signal‐to‐noise ratio associated with flow is compromised.
Three‐dimensional multimodality medical image registration using a parameter accumulation approach
Authors: Chen Qin‐sheng , Defrise Michel , Deconinck Frank
Published: March 2003

A parameter accumulation method based on the Hough transformation is proposed to register three‐dimensional (3‐D) multimodality medical images. The estimation of registration parameters is decomposed into separate estimations of rotation, using directional vectors, and translation, using positional vectors. Similarly, the rotation parameters are decomposed into the rotation axis and angle, which are then estimated separately. This kind of decomposition reduces the parametric dimension and improves the computing efficiency which has been a major concern in implementing the Hough transformation. When 3‐D rotation is involved, evaluating registration error is not straightforward. This paper introduces an equivalent error angle as a criterion to evaluate the performance of 3‐D registration methods. Experimental results indicate that a least‐squares fitting is superior to the parameter accumulation with data contaminated by additive noise only. When mismatched feature points (outliers) exist in the data set, however, the parameter accumulation approach is more accurate. The application of the proposed approach to the registration of 3‐D PET and CT images is demonstrated.
A nondivergent specification of the mean treatment dose in interstitial brachytherapy
Authors: Yu Yan
Published: March 2003

The harmonic mean formalism is investigated as a nondivergent specification of the mean treatment dose in three distinct applications of interstitial brachytherapy: (a) permanent 125I prostate implant; (b) double plane 192Ir linear source array; and (c) multiple catheter optimized high dose rate implant. In each case the harmonic treatment dose (HTD) can be calculated in regions that contain radioactive sources or otherwise high dose gradient, to yield objective and consistent results with little computational effort. The need to identify local dose minima, geometric centers, or seed planes is largely avoided, which makes the formalism suitable for automated computation. Dose distribution uniformity is analyzed in terms of the fractional spread for the HTD and, where the periphery of the target volume is delineated, the uniformity number and the peripheral uniformity number. It is shown that the harmonic mean dose formalism is a practical alternative to the ICRU dose specification parameters in certain areas of brachytherapy.
Measurement and calculation of heterogeneity correction factors for an Ir‐192 high dose‐rate brachytherapy source behind tungsten alloy and steel shields
Authors: Kirov A. S. , Williamson J. F. , Meigooni A. S. , Zhu Y.
Published: March 2003

Shields made of high atomic number material are commonly used in vaginal applicators with high dose‐rate (HDR) 192Ir remotely afterloaded brachytherapy sources. However little data is available for the dose distribution around such shields. Heterogeneity correction factors (HCFs) are defined as the ratio of the dose to a point with the heterogeneity (shield) in place, divided by the dose to the same point with no heterogeneity. Using thermoluminescent dosimeters (TLDs) in solid water phantom we have measured the HCFs behind 6 and 20 mm diam tungsten alloy disks, 4 and 2 mm thick and a 4 mm thick steel disk, positioned 15 mm from the source. For each measurement point, the heterogeneity correction factors were also inferred from Monte Carlo simulations, which accurately modeled the experimental geometry. The agreement between measured and calculated HCFs on the average was within 6%. Tungsten alloy disks resulted in about two times greater dose reduction in water (HCF≊0.4, for 20×4 mm disk) than for a steel disk with the same dimensions (HCF≊0.85). Reducing the disk diameter to 6 mm increased the dose transmission up to about 25%. Increasing the source‐to‐detector distance from 4 to 7 cm caused a change in HCF from 2% to more than 20%, depending on disk material and diameter. The detector artifact effects arising from the finite size and different composition of the TLD chips were determined.
A new source localization algorithm with no requirement of one‐to‐one source correspondence between biplane radiographs
Authors: Li S. , Chen G. T. Y. , Pelizzari C. A. , Reft C. , Roeske J. C. , Lu Y.
Published: March 2003

Conventional source localization algorithms require a one‐to‐one source correspondence between films. This requirement makes source localization cumbersome and error prone because multiple sources must be carefully digitized and some sources can be obscured or missed. A new source localization algorithm is described in this paper. The algorithm fits a ribbon or needle image on film to a linear‐quadratic equation, then analytically determines the 3‐D ribbon locus by its image on the other projection, and finally localizes the sources in the ribbon by tracing along the ribbon image. Only three points per ribbon per film are required, and corresponding points need not be identified on the other film. Phantom experiments and tests on clinical cases demonstrate that the source localization algorithm can increase the efficiency by a factor of up to 5, improve accuracy to about 1 mm, and reconstruct obscured or shifted sources without decreased accuracy and efficiency. The simplicity and minimal entry of data make this technique desirable for clinical use.
Direct reading measurement of absorbed dose with plastic scintillators—The general concept and applications to ophthalmic plaque dosimetry
Authors: Flühs D. , Heintz M. , Indenkämpen F. , Wieczorek C. , Kolanoski H. , Quast U.
Published: March 2003

We have developed dosemeters based on plastic scintillators for a variety of applications in radiation therapy. The dosemeters consist basically of a tissue‐substituting scintillator probe, an optical fiber light guide, and a photomultiplier tube. The background light generated in the light guide can be compensated by a simultaneous measurement of the light from a blind fiber. Plastic scintillator dosemeters combine several advantageous properties which render them superior to other dosemeter types for many applications: minimal disturbance of the radiation field because of the homogeneous detector volume and the approximate water equivalence; no dependence on temperature and pressure (under standard clinical conditions) and angle of radiation incidence; no high voltage in the probe; high spatial resolution due to small detector volumes; direct reading of absorbed doses; and a large dynamical range. The high spatial resolution together with direct reading make these detectors suitable for real‐time 3‐D dosimetry using multi‐channel detector systems. Such a system has been developed for eye plaque dosimetry and successfully employed for dosimetric treatment optimization. The plaque optimization can be performed by dosimetric measurements for the individual patient (“dosimetric treatment planning”). The time consumption for this procedure is less than for a physically correct computer‐based therapy planning, e.g., by means of a Monte Carlo simulation.
A grey‐level image alignment algorithm for registration of portal images and digitally reconstructed radiographs
Authors: Hristov D. H. , Fallone B. G.
Published: March 2003

An algorithm for automatic registration of pairs of portal images based on image correlation is presented. It uses a fast‐Fourier‐transform‐based cross‐correlation operator to find the optimal registration, accounting for both in‐plane translations and rotations. Different cross‐correlation operators have been tested: the Pearson linear correlation coefficient has been implemented by fast Fourier transform and its performance has been compared to that of the more conventional normalized cross‐correlation. A sequential approach has been applied to speed up the registration considerably without degrading the performance of the algorithm. The algorithm has also been applied to the automatic registration of portal images to digitally reconstructed radiographs (DRRs), which have been modified to resemble megavoltage images. The results are indicative of the feasibility of this approach to the inspection of patient setup in radiation therapy.
Customization of a radiation management system to support invivo patient dosimetry using diodes
Authors: Fontenla D. P. , Curran J. , Yaparpalvi R. , Vikram B.
Published: March 2003

Considerable effort is generally made in the quality control of radiation therapy units and in patient chart checking to ensure accurate delivery of treatment to the patients. Record and verify (RV) systems have been implemented which will inhibit the beam from being turned ON if the parameters set on the medical linear accelerators do not match a preprogrammed file. It should be emphasized, however, that RV systems are designed only to ensure proper setup of parameters on the therapy units and bear no direct link with proper patient position. The only real link between treatment planning and delivery to the patient is in‐vivo dosimetry. Diodes provide a convenient way of measuring the dose received by the patient in real time and, after the initial tedious calibrations, require very little additional effort. In order to facilitate diode use by the therapists in our institution, the radiation management system (RMS) database in use with our linear accelerators was interfaced with the output obtained from a commercially available diode dosimetry system. The chart printout format was altered to read this value from the RMS database allowing a near real‐time recording of the actual dose received by the patient for up to three sites concurrently. This provides for an immediate additional check to the planned and received doses already recorded by the RMS RV software. This procedure, if carried out during the first treatment and subsequently on a weekly basis, would ensure that differences of the order of 5% or larger between planned and delivered dose would not go undetected.
Unified measurement of observer performance in detecting and localizing target objects on images
Authors: Swensson Richard G.
Published: March 2003

In this paper methods used to measure observer performance are reviewed, and a simple general model for finding and reporting target objects in gray‐scale image backgrounds is presented. That model provides the basis for a combined measurement of detection and localization performance in various image‐interpretation tasks, whether by human observers or by realized computer algorithms. The model assumes that (1) an observer's detection response and first choice of target location both depend on the “maximally suspicious” finding on an image, (2) a correct (first‐choice) localization of the actual target occurs if and only if its location is selected as the most suspicious, and (3) a target's presence does not alter the degree of suspicion engendered by any other (normal) image findings. Formalization of these assumptions relates the ROC curve, which measures the ability to discriminate between images containing targets and images without targets, to the “Localization Response” (LROC) curve, which measures the conjoint ability to detect and correctly localize the actual targets in those images. A maximum‐likelihood statistical procedure, developed for a two‐parameter “binormal” version of this model, concurrently fits both the ROC and LROC curves from an observer's image ratings and target localizations for a set of image interpretations. The model's application is illustrated (and compared to standard ROC analysis) using sets of rating and localization data from radiologists asked to search chest films for pulmonary nodules. This model is then extended to multiple‐report (“free‐response”) interpretations of multiple‐target images, under the stringent requirement that an observer's detection capability and criterion for reporting possible targets both remain stationary across images and across the successive reports made on a given image. That extended model yields formulations and predictions for the so‐called “Free‐Response” (FROC) curve, and for a recently proposed “Alternative FROC” (AFROC) curve. Tests of that model's “stationarity” assumptions are illustrated using radiologists' free‐search interpretations of chest films for pulmonary nodules, and they suggest that human observers may often violate those assumptions when making multiple‐report interpretations of images.
Higher‐order multiple scattering theories for charged particle transport
Authors: Pomraning G. C. , Prinja Anil K.
Published: March 2003

The Fermi pencil beam formula and the higher‐order multiple scattering theory due to Jette are shown to result from a perturbative treatment of the linear Boltzmann equation with Fokker–Planck scattering. Using asymptotic one‐dimensional solutions for the transverse integrated (spherical) fluence as well as its variance, approximate higher‐order pencil beam theories are constructed. These simple and explicit formulae are shown, by comparison with benchmark Monte Carlo results, to be significantly more accurate than the Fermi and Jette equations, particularly at large distances from the beam axis.
1H MRI phase thermometry invivo in canine brain, muscle, and tumor tissue
Authors: MacFall James R. , Prescott Deborah M. , Charles H. Cecil , Samulski Thaddeus V.
Published: March 2003

The temperature sensitivity of the chemical shift of water (approximately 0.01 ppm/°C) provides a potential method to monitor temperature changes in vivo or in vitro through the changes in phase of a gradient‐echo magnetic resonance (MR) image. This relation was studied at 1.5 T in gel materials and in vivo in canine brain and muscle tissue, heated with a radio frequency (rf) annular phased array hyperthermia antenna. The rf fields associated with heating (130 MHz) and imaging (64 MHz) were decoupled using bandpass filters providing isolation in excess of 100 dB, thus allowing simultaneous imaging and rf heating without deterioration of the MR image signal‐to‐noise ratio. In a gel, temperature sensitivity of the MR image phase was observed to be (4.41±0.02) phase degrees/°C for Te=20 ms, which allowed temperature changes of 0.22 °C to be resolved for a 50‐mm3 region in less than 10 s of data acquisition. In vivo, for Te=20 ms, the temperature sensitivity was (3.2±0.1) phase degrees/°C for brain tissue, (3.1±0.1) phase degrees/°C for muscle, and (3.0±0.2) phase degrees/°C for a muscle tumor (sarcoma), allowing temperature changes of 0.6 °C to be resolved in a 16‐mm3 volume in less than 10 s of data acquisition. We conclude that, while the technique is very sensitive to magnetic field inhomogeneity, stability, and subject motion, it appears to be useful for in vivo temperature change measurement.
Frequency‐domain spectroscopy and imaging of tissue and tissue‐simulating media
Authors: Pogue Brian W.
Published: March 2003

The goal of this work was to develop and study the use of a diagnostic in vivo tissue spectroscopy system based upon frequency‐domain light measurements. Intensity‐modulated light which is incident upon a scattering sample creates waves of light intensity which propagate through the medium in a manner which is dependent upon the scattering and absorption characteristics of the tissue. These waves can be used to recover these optical interaction parameters using a diffusion model of light propagation in tissue. This method can be used to quantify chromophores for dosimetry in therapeutic laser treatments or for diagnostic medical applications. The theoretical modeling for diffuse fluorescence signals was also developed and experimentally tested in a tissue‐simulating phantom with excellent agreement, suggesting that fluorescence lifetime or quantum yield can be made from tissue. Preliminary work was done on an optical tomography algorithm using measurements of phase and intensity at multiple points on a tissue surface to reconstruct images of the optical properties of the interior. The development of a tomographic imaging system was examined in the final section of this thesis with data from a tissue simulating phantom.
The effect of x‐ray beam alignment on the performance of antiscatter grids
Authors: Carlin Michael D. , Nishikawa Robert M. , MacMahon Heber , Doi Kunio
Published: March 2003

While the qualitative effects of grid misalignment are known, we have quantified the effect of different degrees of grid misalignment on image contrast and patient exposure. Radiographs were made of a phantom consisting of five lead disks on top of a 15 cm block of lucite. Four 60 lines/cm grids, having grid ratios of 3:1, 4:1, 6:1, and 8:1 were used. When the tube was angled more than three degrees across the grid lines, the contrast improvement factor decreased substantially for all four grids, as much as 46% for an 8:1 grid with a 12° misalignment. There was a concomitant decrease in film optical density, which if compensated for by an increase in patient exposure, would lead to a higher effective bucky factor. With the exception of the 3:1 grid, if the grid is misaligned by more than 6°, higher signal‐to‐noise ratios can be attained by removing the grid and using the increased patient exposure to reduce noise.
kQ factors for ionization chamber dosimetry in clinical proton beams
Authors: Vatnitsky S. M. , Siebers J. V. , Miller D. W.
Published: March 2003

We discuss a formalism for clinical proton beam dosimetry based on the use of ionization chamber absorbed dose‐to‐water calibration and beam quality correction factors. A quantity kQ, the beam quality correction factor, is defined which corrects the absorbed dose‐to‐water calibration factor ND,w in a reference beam of quality Q0 to that in a user's beam of quality Q1. This study of proton beam quality correction factors used 60Co (kQγ) and proton (kQp) reference beams. The kQγ factors were measured using combined water calorimetry and ionometry for PTW and Capintec–Farmer‐type ionization chambers, and were computed from standard dosimetry protocols. Agreement between measured and calculated kQγ values for both chambers was found within 1.2% in the plateau region for a monoenergetic 250‐MeV beam and within 1.8% at the spread‐out Bragg peak for a 155‐MeV range‐modulated beam. Comparison of absorbed doses to water determined in the range‐modulated 155‐MeV beam was performed with the PTW chamber using three calibration methods: Ngas calibration (AAPM Report 16), ND,w,γ calibration in a 60Co beam in conjunction with a kQγ factor, and ND,w,p calibration in a proton beam in conjunction with a kQp factor. Absorbed doses to water obtained with the three methods agreed within 2% when ionization chamber dosimetry data were analyzed using the proton W‐value for air from the AAPM Report 16 and the ICRU 49 proton stopping powers. The use of the proton‐calibrated reference ionization chamber, in conjunction with the beam quality correction factor kQp, significantly reduced the systematic uncertainty of the absorbed dose determination.
An evaluation of the Y2O3:Eu3+ scintillator for application in medical x‐ray detectors and image receptors
Authors: Cavouras D. , Kandarakis I. , Panayiotakis G. S. , Evangelou E. K. , Nomicos C. D.
Published: March 2003

The suitability of a Y2O3:Eu3+ scintillator for use in radiation detectors and medical image receptors was studied. Y2O3:Eu3+ was used in the form of laboratory prepared screens of different coating thicknesses. The x‐ray luminescence efficiency of the screens was measured for tube voltages between 50–200 kVp and in both transmission and reflection modes of observation. The intrinsic x ray to light conversion efficiency (nc) and other parameters of the Y2O3:Eu3+ phosphor material related to optical scattering, absorption, and reflection were determined. These were used in the calculation of the image transfer characteristics, MTF and zero frequency DQE, for various screen coating thicknesses and x‐ray tube voltages. The light emission spectrum of Y2O3:Eu3+ was measured (narrow band peak at 613 nm) and its spectral compatibility to the spectral sensitivity of several commonly employed optical photon detectors was determined. The x‐ray luminescence efficiency varied with x‐ray tube voltage, attaining maximum value at about 80 kVp for all screen thicknesses. It also varied with coating thickness reaching 25 μW m−2/mR s−1 and 18 μW m−2/mR s−1 at 175 mg/cm2 for reflection and transmission modes, respectively. The intrinsic x ray to light conversion efficiency and the image transfer characteristics were found to be comparable to several commercially used phosphors: nc=0.095, MTF0.05 ranged between 10 and 25 line pairs per mm and peak values of DQE(0) varied between 0.33 and 0.14 in the coating thickness and kVp ranges useful for x‐ray imaging. Spectral compatibility to some red sensitive optical photon detectors was excellent (0.9 or better). Results indicated that Y2O3:Eu3+ is a medium to high overall performance material that could be used in medical x‐ray detectors and image receptors.
Practical application of a scan‐rotate equalization geometry to mammography
Authors: Sabol John M. , Soutar Ian. C. , Plewes D. B.
Published: March 2003

The presence of dense fibroglandular tissue within the breast is the most significant cause of failure to detect breast cancer with mammography. The dense tissue often produces a range of exposure which exceeds the useful dynamic range of film‐screen mammography. It has been shown that equalization radiography overcomes the latitude limitations of film‐screen imaging. Equalization compensates for regional variations in x‐ray transmission within the patient through spatial modulation of the entrance exposure. We have proposed rotary scanning equalization radiography (RSER), a scan‐rotate geometry for efficient equalization radiography. In RSER the image receptor is exposed by repeated scans of a source‐modulated fan beam. The fan beam is rotated with respect to the patient between scans. Numerical simulations and theoretical analysis have shown that the superposition of exposure from appropriately modulated fan beams at a variety of angles is an entrance exposure that effectively equalizes the film exposure. The design and characteristics of a prototype RSER imaging system are described. Anthropomorphic breast phantom images are used to determine the improvement in image contrast obtained with RSER, the expected tube loading, and the presence of artifacts. RSER increases the fraction of the breast imaged with high contrast (at least 90% of peak gradient) from 46% (conventional mammography) to 80%. Subjective examination of the phantom images show that RSER achieves image quality very similar to that of much less efficient equalization geometries with only 2.7 times greater tube loading than conventional mammography. As predicted by theoretical analysis of exposure artifacts in RSER, the prototype RSER system is relatively immune to artifacts. Exposure artifacts were demonstrated for extreme variations in x‐ray transmission within the patient. These results show that RSER is an efficient, practical means of overcoming the latitude limitations of film‐screen mammography, and improving the detection of breast cancer.
The feasibility of quantitative ferrous sulfate/agarose gel dosimetry: Comparison with ionization chamber measurements for photons and electrons
Authors: Smajo Mary Ellen
Published: March 2003

Dosimetry techniques employing a ferrous sulfate‐doped gel read with MRI have shown promise in the measurement of ionizing radiation, particularly as a quality assurance tool for radiotherapy. This work investigated the feasibility of dose quantitation using one such gel. Maps of the change in the spin‐lattice relaxation rate (1/T1) in large (1.5 liter) phantoms were created by implementing a published T1 extraction algorithm. Keeping a sufficiently short time between pre‐irradiation imaging, irradiation (typically to 40 Gy at dmax), and post‐irradiation imaging minimized diffusion. Irradiations were performed with 6 and 16 MeV electrons and 1.25 MeV (cobalt‐60), 6 MV, and 18 MV photons. Central axis depth doses and profiles at various depths were calculated and compared with doses based on ionization chamber measurements in water or plastic phantoms. These comparisons showed good agreement for electrons beyond dmax, but revealed unexpected discrepancies for photons. These discrepancies are not an artifact of the choice of normalization point and do not appear to be the result of a dose saturation effect. Thus, although the ferrous sulfate/agarose gel dosimeter is well suited to quantitative (relative) measurements of electron dose beyond dmax, it is not consistent with ionization‐based dosimetry for photons, implying that care must be taken in the quantitative interpretation of gel‐based dose‐maps. [Copies of the dissertation are available from University Microfilms International, 300 N. Zeeb Rd., Ann Arbor, MI 48106‐1346 (Phone: 1‐800‐521‐3042).]
Monte Carlo simulation of 252CF dose distribution brachytherapy
Authors: Böhm Radoslav , Nikodemová Denisa , Fülöp Marko , Piňak Miroslav
Published: March 2003

The calculation of absorbed dose rate of photons and neutrons for the linear source of 252Cf with the active length of 2 cm is presented. Monte Carlo code and neutron cross‐section data processed from ENDF/B‐6 library were used. The achieved values of the absorbed dose rates are in good agreement with those published by other authors. The results of the calculation are applied to the gynecological configuration used in brachytherapy.
Electron dose profile shaping by modulation of a scanning elementary beam
Authors: Lief Eugene P. , Larsson Anders , Humm John L.
Published: March 2003

The use of multiple high energy electron beams has been limited in the treatment of deep seated tumors. This is principally because of the rapid increase in the physical electron beam penumbra as a result of the rise in large angle scattering with depth in the patient. This decreases the transverse dose gradient between the target volume and sensitive dose limiting structures and diminishes the ability to conform electron isodose lines to the target volume. If the beam is flat in air, then its profile will become progressively more rounded with depth, due to the increase of scatter out of the beam edges. With a scanning elementary electron beam, such as produced by the Microtron MM50, the characteristics of a broad beam profile are determined by the scan pattern. Using an appropriate scan pattern one can create, at any depth within the range of electrons, various dose profiles with the sharpness not exceeding that of the elementary beam. The objective of this work was to study methods that produce the desired electron beam profile at the depth of the target volume, and to derive the surface fluence profile required. Two approaches were explored to modulate the elementary beam distribution: “amplitude modulation” (AM) and “frequency modulation” (FM). We calculated coordinate and intensity distributions of the 25, 40, and 50 MeV elementary beam pulses at the surface that would yield a flat field at various prescribed depths. The results are in good qualitative agreement with iterative deconvolution calculations by Brahme et al. [Acta Radiol. Oncol. 19, 305–319 (1980)]. The scattering penumbra between the 50%–90% isodose lines can be reduced by up to 40% by beam modulation. The modulation should also enable the combination of multiple electron beams so as to achieve the desired conformal isodose profile as is customarily seen with photon beam planning, but with greater normal tissue sparing due to higher electron longitudinal depth—dose gradients. The results can be also used for electron accelerators that do not use a scanning elementary beam.
A new miniature x‐ray source for interstitial radiosurgery: Device description
Authors: Dinsmore M. , Harte K. J. , Sliski A. P. , Smith D. O. , Nomikos P. M. , Dalterio M. J. , Boom A. J. , Leonard W. F. , Oettinger P. E. , Yanch J. C.
Published: March 2003

A device that generates low‐energy x rays at the tip of a needle‐like probe was developed for stereotactic interstitial radiosurgery. Electrons from a small thermionic gun are accelerated to a final energy of up to 40 keV and directed along a 3 mm outside diameter drift tube to a thin Au target, where the beam size is approximately 0.3 mm. All high‐voltage electronics are in the probe housing, connected by low‐voltage cable to a battery‐operated control box. X‐ray output, which is nearly isotropic, consists of a bremsstrahlung spectrum and several lines between 7 and 14 keV, with characteristic radiation contributing 15% of the total energy output. To date, 14 patients with metastatic brain tumors have been treated with this device.
A new miniature x‐ray device for interstitial radiosurgery: Dosimetry
Authors: Beatty J. , Biggs P. J. , Gall Kenneth , Okunieff P. , Pardo F. S. , Harte K. J. , Dalterio M. J. , Sliski A. P.
Published: March 2003

A miniature, battery operated 40 kV x‐ray device has been developed for the interstitial treatment of small tumors (<3 cm diam) in humans. X rays are emitted from the tip of a 10 cm long, 3 mm diameter probe that is stereotactically inserted into the tumor. The beam, characterized by half‐value layer (HVL), spectrum analysis, and isodose contours, behaves essentially as a point isotropic source with an effective energy of 20 keV at a depth of 10 mm in water. The absolute output from the device was measured using a parallel plate ionization chamber, modified with a platinum aperture. The dose rate in water determined from these chamber measurements was found to be nominally 150 cGy/min at a distance of 10 mm for a beam current of 40 μA and voltage of 40 kV. The dose in water falls off approximately as the third power of the distance. To date, 14 patients have been treated with this device in a phase I clinical trial.
Scattered radiation in portal images: A Monte Carlo simulation and a simple physical model
Authors: Swindell William , Evans Philip M.
Published: March 2003

The scattered radiation in 6 MV radiotherapy portal images is analyzed. First, a quantity SPR * is studied, by means of Monte Carlo (MC) modeling. SPR * is defined as the ratio, on the central axis, of the signal due to scattered radiation to that due to the primary radiation. The detector model mimics a high‐energy photon detector in the context of transit dosimetry. Second, a physical model of SPR * has been developed from first principles. For a cylindrical phantom, placed symmetrically about the isocenter, it predicts that SPR * depends on the area A at the isocenter of the circular field and the phantom thicknessd as follows. SPR *=k0Ad(1+k1d)(1+k2A), where k0=0.0266(L1+L2)2/(L1L2)2, k2=−[L−21+L−22+(L−11+L−12)2 ((2/3 +(3κ/2))]/2π,L1 is the source‐to‐isocenter distance, L2 is the isocenter‐to‐detector distance, and κ is the mean energy of the radiation beam (MeV/0.511). Constant k1, for which there is no simple expression, depends on L2. Comparison to the MC data shows that for 60≤L2≤100 cm the dependence is weak and k1≅2×10−3 cm−1. The root mean square (rms) agreement between the MC‐derived values of SPR * and the physical model is better than 0.001 over a wide range of A and d values likely to be encountered in clinical practice for L2≥50 cm. Third, experimental measurements of the scatter‐to‐primary ratio were obtained using our custom built imaging system mounted on a Philips SL25 linear accelerator. In the first experiment, A was varied from 40 to 400 cm2 with L1=L2=100 cm with d=20 cm. In the second experiment water depth d was varied from 0 to 28 cm with L1=L2=100 cm and A=200 cm2. The rms agreements between the MC data and the experiments were 0.0015 and 0.0045, respectively.
Dosimetry of [15O]water: A physiologic approach
Authors: Narayana Shalini , Hichwa Richard D. , Boles Ponto Laura L. , Ponto James A. , Watkins G. Leonard
Published: March 2003

Earlier dosimetry estimates for [15O]water assumed its instantaneous equilibrium with total body water. This assumption leads to an underestimation of the absorbed doses to organs with high blood flows, since the biodistribution of this short‐lived radiopharmaceutical is dependent upon blood flow to organs. We have developed a physiologically based whole body blood flow model (WBBFM) using a commercially available icon‐driven mathematical simulation software package and applied it to the reevaluation of [15O]water dosimetry in humans. The WBBFM uses multiple parallel compartments to represent organs, heart chambers, the injection site for [15O]water, and blood sampling sites (arterial and venous). Input values to the WBBFM include organ blood flows, organ masses, organ water volumes, organ:blood partition coefficients, injected activity and S‐values of [15O]. The WBBFM is based on the same assumptions that are used in calculating regional blood flow using [15O]water and simulates the human body closely in its physiologic response. The activity in each organ is derived from the simulation and is used to calculate absorbed doses. The WBBFM calculated absorbed doses in μGy/MBq (mrad/mCi) to various organs are as follows: heart—2.66 (9.84), kidneys—2.20 (8.15), thyroid—1.83 (6.78), brain—1.66 (6.13), ovaries—1.25 (4.61), breast—1.24 (4.59), and small intestine—1.03 (3.83). These values are approximately two‐ to threefold higher than the earlier estimates of Kearfott [J. Nucl. Med. 23, 1031–1037 (1982)] and similar to the recent findings of Herscovitch et al. [J. Nucl. Med. 34, 155P (1983)]. We believe this approach yields more realistic dosimetry estimates for [15O]water. Accordingly, we have revised the amount of [15O]water administered during regional blood flow studies at our institution. The relative ease and accuracy of this approach suggests its usefulness in dosimetry estimation for other freely diffusible radiopharmaceuticals.
Measurement of quantum noise in fluoroscopic systems for portal imaging
Authors: Mah Dennis W. , Rowlands J. A. , Rawlinson J. Alan
Published: March 2003

In fluoroscopic portal imaging systems, a metal plate is bonded to a phosphor screen and together these act as the primary x‐ray sensor. The light from the screen is collected and imaged by a lens on the target of a video camera. The demagnification (M) between the large area of the phosphor being imaged and the small active area of the video camera results in poor optical coupling between the screen and the video camera. Consequently x‐ray quantum noise is small compared to other noise sources. By reducing the demagnification, the light from the screen is collected more efficiently, so we were able to increase the x‐ray quantum noise relative to other noise sources and thus unambiguously identify it. The noise power spectrum was measured as a function of M to determine the relationship between the x‐ray quantum noise, shot noise, and amplifier noise. It was found by extrapolation to clinical demagnifications that the amplifier noise dominates x‐ray quantum noise at all spatial frequencies, but the shot noise was less than the x‐ray quantum noise at low spatial frequencies. For low spatial frequencies, this implies that a secondary quantum sink can be avoided. If amplifier noise could be sufficiently reduced, x‐ray quantum limited images could be obtained in clinical systems at low spatial frequencies.
Verification and implementation of dynamic wedge calculations in a treatment planning system based on a dose‐to‐energy‐fluence formalism
Authors: Weber Lars , Ahnesjö Anders , Nilsson Per , Saxner Mikael , Knöös Tommy
Published: March 2003

The use of dynamic movements on linear accelerators during irradiation has found a revised interest lately due to the integration of computers to control the accelerator. In this paper, dynamic wedge fields that are produced by moving one of the collimator blocks during irradiation are studied. Since these wedge fields differ from those of mechanical wedges, certain requirements are to be met on the treatment planning system. A pencil‐beam‐based treatment planning system that uses the resultant energy fluence distribution from the dynamic collimator movement has been extensively reviewed. In calculations, the system treats the dynamic collimated field as a single, modulated field that yields calculation times close to those for open beams. Details are given on the theoretical model used for the calculation of dynamically generated dose distributions. Measurements of depth doses, profiles, and output factors in dynamic wedge fields indicate that calculations accurately predict the outcome from dynamic wedges without any additional measurements other than those used for characterization of static open beams.
Verification of absorbed dose determined with plane‐parallel chambers in clinical electron beams following AAPM Task Group 39 protocol using ferrous sulphate dosimetry
Authors: Xu Zhigang , Li Hui , Almond P. R. , Guan T. Y.
Published: March 2003

The absorbed dose values determined with the Exradin and PTW‐Markus plane‐parallel chambers were compared to the values obtained with the ferrous sulphate dosimetry for a number of the Philips SL25 and the Therac 20 electron beams. For the plane‐parallel chambers, the cavity‐gas calibration factor , was derived by a direct comparison with a calibrated cylindrical chamber using the three different calibration methods as proposed by the newly published AAPM TG 39 protocol. For the ferrous sulphate dosimetry, an value of 352×10−6 m−2 kg−1 Gy−1 was adopted from ICRU Report No. 35. The average ratio of the dose values determined with the plane‐parallel chambers and the dose values determined with the Fricke dosimetry system was 1.001±1.4%. These measurements are consistent with the AAPM TG 39 protocol.
Automatic three‐dimensional inspection of patient setup in radiation therapy using portal images, simulator images, and computed tomography data
Authors: Gilhuijs K. G. A. , van de Ven P. J. H. , van Herk M.
Published: March 2003

In external beam radiotherapy, conventional analysis of portal images in two dimensions (2D) is limited to verification of in‐plane rotations and translations of the patient. We developed and clinically tested a new method for automatic quantification of the patient setup in three dimensions (3D) using one set of computed tomography (CT) data and two transmission images. These transmission images can be either a pair of simulator images or a pair of portal images. Our procedure adjusts the position and orientation of the CT data in order to maximize the distance through bone in the CT data along lines between the focus of the irradiation unit and bony structures in the transmission images. For this purpose, bony features are either automatically detected or manually delineated in the transmission images. The performance of the method was quantified by aligning randomly displaced CT data with transmission images simulated from digitally reconstructed radiographs. In addition, the clinical performance was assessed in a limited number of images of prostate cancer and parotid gland tumor treatments. The complete procedure takes less than 2 min on a 90‐MHz Pentium PC. The alignment time is 50 s for portal images and 80 s for simulator images. The accuracy is about 1 mm and 1°. Application to clinical cases demonstrated that the procedure provides essential information for the correction of setup errors in case of large rotations (typically larger than 2°) in the setup. The 3D procedure was found to be robust for imperfections in the delineation of bony structures in the transmission images. Visual verification of the results remains, however, necessary. It can be concluded that our strategy for automatic analysis of patient setup in 3D is accurate and robust. The procedure is relatively fast and reduces the human workload compared with existing techniques for the quantification of patient setup in 3D. In addition, the procedure improves the accuracy of treatment verification in 2D in some cases where rotational deviations in the setup occur.
Analysis of factors affecting the therapeutic gain in high dose rate gynecological implants relative to low dose rate implants
Authors: Sahoo N. , Ling C. C. , Anderson L. L.
Published: March 2003

The radiobiological efficacy of high dose rate (HDR) intracavitary implant relative to that of low dose rate (LDR) implants has been represented quantitatively by the therapeutic factor Γ defined as [EDRtum/ERDorgan]HDR/[ERDtum/ERDorgan]LDR where ERD is the extrapolated response dose. The ERDs for the tumor and a critical organ are calculated using the linear‐quadratic model and depend on the values of α/β ratio, the tumor and normal tissue repair time constants (μt and μl), number (N) of HDR fractions, the dose per fraction (d), and the fractional tumor and organ dose (ft andfl) relative to the dose at the reference point. We have studied the variation of Γ with d, μt, μl, ft, and fl and have derived specific conditions for which Γ can have a value equal to or greater than unity leading to a therapeutic gain in HDR. It is found that Γ is less than unity for commonly used parameter values, specifically when μt is assumed to be equal to μl. However, if μt is greater than μl, Γ can have a value greater than unity for many possible values of ft and fl.
Use of simple quality assurance procedures in the analysis of beam asymmetries on cobalt‐60 treatment units
Authors: Davis M. G. , Horton J. L. , Bencomo J. A.
Published: March 2003

Recently, the mechanical failure of one of the upper collimator mechanical trimmers on a cobalt‐60 unit resulted in large beam asymmetries and unacceptable flatness characteristics. This malfunction was not detected using currently accepted schedules for quality assurance tests. The incident suggests that the frequency of routine beam profile constancy checks should be increased to weekly for cobalt‐60 units.