bluuebunny/crossref_metadata_2025 huggingface dataset:
Comparison of IAEA 1987 and AAPM 1983 protocols for dosimetry calibration of radiotherapy beams
Authors: Huq M. Saiful , Nath Ravinder
Published: March 2003

The IAEA 1987 protocol is an international protocol which has made a number of improvements over the AAPM 1983 protocol for calibration of high‐energy photon and electron beams. We present a detailed numerical comparison between the two protocols by calculating (i) Ngas and ND for PTW (PMMA wall), Capintec (air‐equivalent plastic wall) and NEL (graphite wall) Farmer type ionization chambers for 60Co γ rays; (ii) dose‐to‐water with chamber in water irradiated by 4‐ or 25‐MV x rays; (iii) dose‐to‐water with chamber in water, PMMA, and polystyrene phantoms irradiated by 5‐ and 10‐MeV electrons; and (iv) dose‐to‐water with chamber in water irradiated by 20‐MeV electrons. For photons, the IAEA protocol gives results which are in good agreement with the AAPM protocol; on average the IAEA results are 0.6% smaller than the AAPM results while discrepancies between the two are in the range of −0.4% to −1.2%. For 10‐MeV electrons also, the IAEA protocol gives results which are in excellent agreement with the AAPM protocol; on average the IAEA results are 0.3% smaller than the AAPM results while discrepancies between the two are in the range of −1.0% to +0.5%. In contrast to the above, for 5‐MeV electrons, the IAEA protocols give results smaller than the AAPM protocol by 2.0% on average with discrepancies between protocols ranging from −4.1% to −0.7% depending upon the ionization chamber and phantom material used. For 5‐MeV electrons, the discrepancies are particularly large for polystyrene phantom; the average discrepancies being −1.4%, −1.1%, and −3.6% for water, PMMA, and polystyrene, respectively. If data for 5‐MeV electrons with polystyrene phantom are excluded, then the overall agreement between the two protocols for photons and electrons is within the range of −1.9% to +0.5%. Principal reasons for the observed discrepancies are (i) IAEA uses the correct expression for ND resulting in up to +0.8% correction; (ii) IAEA uses the most recent stopping power ratio for graphite‐to‐air resulting in up to +0.5% correction; (iii) IAEA uses a correction of up to +0.8% for the central electrode which AAPM ignores; (iv) the present estimates of the percent depth doses which arise from the differences in measurement depths in the two protocols; and (v) IAEA uses measured values of the fluence correction factor while AAPM uses a theoretical estimate resulting in corrections of up to −2.2%.
Metal–polysiloxane shields for radiation therapy of maxillo–facial tumors
Authors: Farahani M. , Eichmiller F. C. , McLaughlin W. L.
Published: March 2003

In the treatment of some head and neck lesions with high‐intensity radiation (teletherapy), an essential procedure is the application of an individually customized shielding appliance, which is designed, modeled, and formed into a working extra‐ or intraoral stent for the purpose of sparing healthy tissues. The present state of the art is slow and technique intensive, which can add to patient discomfort and inconvenience during molding and fabrication. A new formulation is described, which offers speed and ease of forming a moldable composite stent especially for intraoral use. Interleaved stacks of calibrated thin radiochromic film strips and soft‐tissue‐simulating plastic (polystyrene) layers gave a means of mapping one‐ or two‐dimensional profiles of dose distributions adjacent to the high‐density shielding materials using a spectrophotometer equipped with a gel scanner or a scanning laser‐beam microdensitometer. Tests using collimated gamma‐ray beams from a 60Co teletherapy unit were made in order to measure the dose distribution near interfaces of tissue‐simulating polymer and the composite stent material with and without mixtures of metals (Ag–Cu and Sn–Sb). These results show that quickly formed composites made of a flexible resin with high concentrations of powdered spherical metal alloys provide effective custom‐designed shielding, and, with a thin overlayer of the resin without metal, a diminished back‐scattered radiation dose to normal tissues. An example of a successful formulation is a mixture of 90% by weight Ag–Cu alloy powder in a vinyl polysiloxane resin. This material is a moldable putty which, upon polymerization, forms a rigid elastomeric material, providing a half‐value layer of ≊2.5 to 2.8 cm for a gamma‐ray beam from a 60Co source.
Extra lethal damage due to residual incompletely repaired sublethal damage in hyperfractionated and continuous radiation treatment
Authors: Chen Junwen , van de Geijn J. , Goffman T.
Published: March 2003

In the conventional linear–quadratic model of single‐dose response, the α and β terms reflect lethal damage created during the delivery of a dose, from two different presumed molecular processes, one linear with dose, the other quadratic. With the conventional one‐fraction‐per‐day (or less) regimens, the sublethal damage (SLD), presumably repairing exponentially over time, is essentially completely fixed by the time of the next dose of radiation. If this assumption is true, the effects of subsequent fractions of radiation should be independent, that is, there should be little, if any, reversible damage left from previous fractions, at the time of the next dose. For multiple daily fractions, or for the limiting case, continuous radiation, this simplification may overlook damaged cells that have had insufficient time for repair. A generalized method is presented for accounting for extra lethal damage (ELD) arising from such residual SLD for hyperfractionation and continuous irradiation schemes. It may help to predict differences in toxicity and tumor control, if any, obtained with “unconventional” treatment regimens. A key element in the present model is the finite size and the dynamic character of the pool of sublethal damage. Besides creating the usual linear and quadratic components of lethal damage, each new fraction converts a certain fraction of the existing SLD into ELD, and creates some new SLD. The expressions developed by Thames [Int. J. Radiat. Biol. 47, 319–339 (1987)] for fractionated treatment (the IR model) and by Dale [Br. J. Radiol. 58, 515–528 (1985); 59, 919–927 (1986)] for protracted and fractionated treatment are found to be similar to our results in the limiting case where the pool of SLD is very large (infinite). Calculations with the present model suggest that enhancement of radiation effect with hyperfractionation due to conversion of sublethal damage to lethal damage should be modest within current clinical schedules, because of the judicious use of 4‐h or preferably longer interfraction times, which permit sufficient repair of SLD. Shorter interfraction times would increase conversion of SLD to ELD and might lead to an unacceptable increase in toxicity. In continuous radiation, use of the ELD concept enables fitting computed survival curves to published experimental survival curves demonstrating decrease of effectiveness of a given total dose for dose rates below a certain level: low dose rate effect. Inclusion of proliferation will help in gaining insight into the limiting dose rate at which irradiation becomes ineffective. Key words: time‐dose response, LQ‐model, radiotherapy, radiobiology, incomplete repair, sublethal damage, extra lethal damage
The design of megavoltage projection imaging systems: Some theoretical aspects
Authors: Swindell W. , Morton E. J. , Evans P. M. , Lewis D. G.
Published: March 2003

This study investigates factors associated with the imaging of a patient using a high‐energy radiotherapy treatment beam. Both single‐stage (e.g., solid‐state detector) and two‐stage (e.g., scintillation screen plus TV) systems are considered. First an expression is derived that relates dose at the buildup depth in the object to the structure of the object, the scatter‐to‐primary signal‐variance ratio and the differential‐signal‐to‐noise ratio in the image. Second the number of bits required to digitize the image is derived. Third the effect of scattered radiation is investigated for photon counting, photopeak, and Compton detector types. Fourth the effect of noise in the detection process is considered. Finally, the relationship between x‐ray source size, detector aperture, and image magnification is derived. The optimum magnification for given source size and detector aperture is discussed in terms of the system transfer function. The study indicates that at a primary beam energy of 2 MeV, a dose of 10−3 cGy is required to detect reliably the presence of a bone section of area 10×10 mm and thickness 4 mm in 250 mm of soft tissue. For this example, it is also estimated that a digitization accuracy of 10 bits is required. The calculations indicate that for a Compton detector, the scatter‐to‐primary signal‐variance ratio drops from a value of around 30% at the exit surface of the object to 5% at a distance of 80 cm from the object with a consequent small reduction in the dose required to form the image.
Effects of voltage ripple and current mode on diagnostic x‐ray spectra and exposures
Authors: Matsumoto Masao , Kubota Hideaki , Hayashi Hideki , Kanamori Hitoshi
Published: March 2003

The voltage‐ripple dependence relationship on x‐ray energy‐spectral values (energy fluence per unit interval of photon energy) and exposures at 70‐kV peak were obtained theoretically by using the semiempirical formula of emission spectra given by Birch and Marshall [Phys. Med. Biol. 24, 505 (1979)]. The calculations were performed with and without various thicknesses of aluminum. As the ripple increases, the energy‐spectral values decrease as expected. When the ripple is large, however, energy‐spectral values (per mAs) take the minimum values; therefore, the exposure (per mAs) also reaches the minimum value for the unsaturating current modes, contrary to expectation. The reasons for this phenomenon were clarified. Exposures clearly take the minimum value in 2‐pulse units. This phenomenon was experimentally verified.
Simulation study of cerebral blood flow measurements in xenon‐CT: Evaluation of washin/washout procedures
Authors: Polacin Arkadiusz , Kalender Willi A. , Eidloth Heidemarie
Published: March 2003

Simulation programs have been created that allow one to vary image pixel noise, the number and the distribution of scans with time, cerebral tissue parameters, and the type of xenon CT inhalation procedure in order to investigate CBF measurements with respect to accuracy and signal‐to‐noise ratio (SNR). In particular, standard washin studies were compared to washin/washout studies. Based on the results of these simulations, a new protocol is suggested; it consists of only 3 min of xenon inhalation (washin phase) and 3 min of washout, monitored by one reference and six enhancement scans taken at 1‐min intervals. Compared with a standard 8‐min washin study of equal total dose, flow standard deviation (s.d.) for an unconstrained least‐squares algorithm is reduced by factors of 2.2 and 1.2 for gray and white matter, respectively; for flow distributed uniformly from 20 to 80 ml/min/100 g, an average s.d. reduction factor of 1.7 is achieved. This was confirmed experimentally in a volunteer study using noise power spectrum analysis. In addition, effects of tissue heterogeneity have been investigated; both the bias and s.d. of flow estimates due to varying proportions of white and gray matter in a given volume element are reduced in washin/washout protocols. When compared to a short washin‐only study of 4.5 min, the 3‐min washin/3‐min washout study provides an improvement of flow s.d. by a factor of 1.6 and 1.9 for gray and white matter, respectively. The very short xenon inhalation time and the short total examination time of the suggested new protocol are expected to significantly reduce the patient movement which is considered to be a main source of errors of xenon‐CT in clinical CBF measurements.
Primary dose in photon beams with lateral electron disequilibrium
Authors: Nizin P. S. , Chang X. S.
Published: March 2003

It is shown that in narrow monoenergetic photon beams under conditions of lateral electron disequilibrium, primary absorbed dose P(r) is a simple function of beam radius r : P(r)=Pλ⋅(1−e−γ⋅r), where Pλ is the primary dose in broad beams for which complete lateral electron equilibrium exists, and γ depends on photon energy and absorbing medium. This formula was evaluated using Monte‐Carlo‐generated data for the primary dose in water from monoenergetic photons in the energy range from 2 to 8 MeV. The primary dose was studied in beams of radii 0.006 cm<r<5.0 cm and within the depth interval 0.5 cm≤d≤24 cm. It was concluded that the saturation equation above provides an accurate description of the primary dose from monoenergetic photon beams, as judged by comparison with Monte Carlo results.
Analytic approximation of the log‐signal and log‐variance functions of x‐ray imaging systems, with application to dual‐energy imaging
Authors: Cardinal H. Neale , Fenster Aaron
Published: March 2003

In the analysis of x‐ray system performance, the log‐signal function, or negative logarithm of the relative detector signal, and the analogously defined log‐variance function, are of central importance. These are smooth, monotonic functions of object thickness, which are nonlinear for nonmonoenergetic x‐ray source spectra. If we assume a dual‐energy decomposition of the object into two basis materials, then they can be written as analytic functions f (x,y) and f *(x,y), respectively, of the component thicknesses (x,y) of the object. In this paper, we analytically develop the Taylor series of these functions, prove that they converge everywhere, and parametrize their coefficients via suitable central spectral moments of the basis‐material attenuation coefficients. We then show how the lower‐order moments can be used to construct, in closed form, smooth, monotonic, second‐order (conic) surface functions which closely approximate f (x,y) and f *(x,y) over the entire feasible domain. A simplified construction, based on using appropriate asymptotic values of the basis‐material attenuation coefficients to match the asymptotic behavior of these functions, is also given. The inclusion of image components with K‐edge absorption spectra, such as iodine, is done without effort. Extension of the results to the construction of similar (virtually exact) third‐order (cubic) surface approximations is straightforward. As an illustration of the broad applicability of this approach, we extend our analysis to the construction of similar approximations to the inverse (decomposition) functions for an arbitrary dual‐energy system, and investigate their numerical accuracy for a model dual‐kVp system. We conclude that this extended analysis provides an accurate description of the system behavior in terms of a small number of physically meaningful parameters. This parametrization permits greater physical insight into the system behavior, while at the same time simplifying its mathematical description, and similarly facilitates the analysis of various measures of imaging performance via either analytic or numerical methods.
Semianalytical expressions for and Prepl for electron beams
Authors: Wielopolski Lucian , Pai Sujatha , Mlyn Marc
Published: March 2003

The tables of the mean restricted collision mass stopping power ratios for water, polystyrene and acrylic relative to air given in the AAPM TG‐21 protocol have been fitted to an expression with 20 coefficients using the depth in the phantom and the mean incident electron energy as two independent variables. Using these expressions, the calculated values agree with the tabulated values within +0.5% in 85% of the cases and within ±1.0% in 95% of the cases. For each of the four cylindrical chamber inner diameters, given in the protocol, the electron fluence correction Prepl has been fitted to an expression with four coefficients using the mean electron energy at depth z as an independent variable.
Comparisons of sectioned micro‐TLD dose measurements with predicted dose from 131I‐labeled antibody
Authors: Langmuir Virginia K. , Wessels Barry W. , Mendonca Holly , Yorke Ellen D. , Montilla Lino
Published: March 2003

The dose distribution from radioimmunotherapy is very heterogeneous because of variability in antigen expression, antibody penetration, and tumor architecture. Many models of dose distribution have been constructed but it has been very difficult to confirm these predictions with actual measured doses. The purpose of this study was to determine what degree of resolution could be obtained using mini‐thermoluminescent dosimeter(s) (TLD) in a micrometastasis model. TLDs were inserted into 1‐mm‐diam multicell tumor spheroids that had been treated with 131I‐labeled antibody. The spheroids were then sectioned at 30‐μm intervals and the TLD sections (measuring 0.14×0.1×0.03 mm) were removed and read. Calibration of the TLDs was done with whole TLDs using external beam radiation and an 131I‐containing gel, and with TLD sections using external beam radiation. Predicted doses were determined by measuring the activity in individual spheroids and the time the TLDs were in the spheroids and incorporating these numbers into a model that assumed either surface binding of 131I or some degree of penetration. There was a correlation between the measured TLD dose and the predicted absorbed dose when comparing the low‐ to high‐dose regions. However, there was considerable variation within any particular dose range, probably due to heterogeneity in TLD grain size. TLD sections of this size, originating from standard chips available from the manufacturer, are useful in measuring cumulative absorbed doses from 131I‐labeled antibody within 4% of the calculated dose for doses in the range of 150 Gy, within 6% to 20% for the 40‐Gy range, and within a factor of two for doses less than 25 Gy.
A technical analysis of an intraoperative radiation detection probe
Authors: Wilson Latresia A. , Kuhn Joseph A. , Corbisiero Raffael M. , Smith Merle , Beatty J. David , Williams Lawrence E. , Rusnak Miro , Kondo Russell L. , Demidecki Andre J.
Published: March 2003

A technical evaluation was made of a commercial intraoperative radiation probe. This device utilizes a CsI(Tl) scintillation detector and light pipe arrangement to count gamma radiation in vivo. After determining the optimal window and threshold setting, additional evaluations included linearity, distance response function, detector dead time, counter reproducibility, detector sensitivity, angular resolution, and energy resolution. Detector dead time (21.2 μs) was found to be characteristic of a nonparalysable system. Activity response for each radionuclide was linear (R=0.99) both with and without collimation. Energy resolution, 25% at 210 keV, was not sufficient to separate the two photons (172 and 247 keV) emitted by 111In. Detector sensitivity was 1136 and 626 counts per s per microcurie of 111In and 99mTc, respectively. The mean effective distance from the face of the uncollimated probe to the crystal was determined to be 2.03 cm in air.
Missing tissue compensators: Evaluation and optimization of a commercial system
Authors: Ansbacher W. , Robinson D. M. , Scrimger J. W.
Published: March 2003

A commercial system for producing retracted compensators has been adapted to suit local needs, and is evaluated here. It comprises a magnetic field surface digitizer and computer‐driven milling machine. Improvements in dose distributions, resulting in standard deviations of the mean dose between 2% and 3%, have been achieved for treatment fields in wax phantoms simulating the head and neck regions. Optimization of compensator shape to allow for changes in the amount of scattered radiation has resulted in a further improvement in dose uniformity, particularly near the field borders; for these compensators the standard deviation was as low as 1.6%. The system using the basic algorithm has been in clinical use since July.
Gadolinium neutron capture therapy for brain tumors: A computer study
Authors: Masiakowski Jerzy T. , Horton John L. , Peters Lester J.
Published: March 2003

A Monte Carlo computer study of the total dose distribution from neutrons and prompt gamma emissions (but excluding the contribution from conversion and Auger electrons) for gadolinium neutron capture therapy of brain tumors has been carried out in order to test the theoretic feasibility of this modality using commercially available magnetic resonance contrast media. The three‐dimensional dose distribution calculations were performed in a spherical head phantom with a spherical tumor at the center. Potentially achievable gadolinium concentrations of 150 μg/g of tissue in tumor and 3 μg/g in normal tissue were assumed with enrichment to 79.9% gadolinium‐157, as supplied by Oak Ridge National Laboratory. Irradiation was assumed to be with a 2‐keV monoenergetic cylindrical epithermal neutron beam having a radius of 4 cm. The three‐dimensional thermal neutron fluence resulting from the 2‐keV beam propagation through the tissue was modeled. For a single neutron beam, the maximum dose is delivered within the tumor but the dose is very inhomogeneous across the tumor volume due to rapid decrease of thermal neutron fluence with depth. Two parallel opposed neutron beams deliver to the interface of normal and malignant tissue 70%–80% of the maximum dose received at the center of the tumor. To deliver an average tumor dose of 500 cGy in 10 min would require a 2‐keV source neutrons number of 8.0×1011 per s within the geometry of the beam.
Analytic microdosimetry for radioimmunotherapeutic alpha emitters
Authors: Stinchcomb T. G. , Roeske J. C.
Published: March 2003

Analytic microdosimetry using Fourier transform techniques has been applied to internal alpha emitters. These techniques need revision and simplification for use with short‐lived radionuclides such as those which may be useful for radioimmunotherapy. Analytic methods may have advantages over Monte Carlo methods in some cases (e.g., where time is important). Applications to eight different source geometries show the usefulness of these techniques. Comparisons of some of the results to Monte Carlo calculations prove its accuracy. For a uniform source of 5.867‐MeV alphas spread throughout the volume outside a cell surface, the two methods agree well. Results are within 1% both for the average specific energy and for the number of hits. Analytic microdosimetry provides an alternate method to use for the critical evaluation of models that seek to predict the relation between alpha energy deposition and cell survival data. Similarly, it may be helpful to point the way toward the rational interpretation of general biological results for antibodies labeled with alpha emitters.
Microbeam radiation therapy
Authors: Slatkin D. N. , Spanne P. , Dilmanian F. A. , Sandborg M.
Published: March 2003

It is proposed to carry out radiotherapy and radiosurgery for brain lesions by crossfiring an array of parallel, closely spaced microbeams of synchrotron‐generated x rays several times through an isocentric target, each microbeam in the array having an ≊25‐μm‐wide adjustable‐height rectangular cross section. The following inferences from the known tissue sparing of 22‐MeV deuteron microbeams in the mouse brain and the following exemplary Monte Carlo computations indicate that endothelial cells in the brain that are lethally irradiated by any microbeam in an array of adequately spaced microbeams outside an isocentric target will be replaced by endothelial cells regenerated from microscopically contiguous, minimally irradiated endothelium in intermicrobeam segments of brain vasculature. Endothelial regeneration will prevent necrosis of the nontargeted parenchymal tissue. However, neoplastic and/or nonneoplastic targeted tissues at the isocenter will be so severely depleted of potentially mitotic endothelial and parenchymal cells by multiple overlapping microbeams that necrosis will ensue. The Monte Carlo computations simulate microbeam irradiations of a 16‐cm diameter, 16‐cm‐long cylindrical human head phantom using 50‐, 100‐, and 150‐keV monochromatic x rays.
Analysis of contributing factors to the occurrence of off‐focus radiation (OFR)
Authors: Roeck Werner W. , Martin John T. , Beach Robert
Published: March 2003

Off‐focus radiation (OFR) has previously been described by many investigators [R. Thoraeus, Acta Radiologica 18, 753 (1937); L. Mallet and R. Maurin, Radiology 48, 628–632 (1947); J. F. Timmer, Medicamundi 19(2), 52–54 (1974); G. U. Rao, Appl. Radiol. 3(3), 45–49 (1974); R. Birch, Br. J. Radiol. 49, 951–955 (1976); W. W. Roeck, AAPM Symposium, 217–247 (1991)]. Off‐focus radiation is frequently manifested on radiographs by the appearance of faint images of anatomical structures outside the collimated field of interest, i.e., soft tissues, ear lobes and hair, which we will refer to as “penumbral images.” The observable small detail resolution within these penumbral images led to the assumption that minute sources of increased radiation intensity (secondary microfocal spots) must be present outside the area of the primary focal spot. The existence of these multiple secondary microfocal spots has been established and their locations within the area of the anode surface from which the general OFR originates was identified. The number, size, and distribution of the secondary focal spots vary over time and their magnitudes vary widely. The source of the well‐focused electron beams creating the spots is attributed to the cold cathode emission principle.
Uncertainty analysis of absorbed dose calculations from thermoluminescence dosimeters
Authors: Kirby T. H. , Hanson W. F. , Johnston D. A.
Published: March 2003

Thermoluminescence dosimeters (TLD) are widely used to verify absorbed doses delivered from radiation therapy beams. Specifically, they are used by the Radiological Physics Center for mailed dosimetry for verification of therapy machine output. The effects of the random experimental uncertainties of various factors on dose calculations from TLD signals are examined, including: fading, dose response nonlinearity, and energy response corrections; reproducibility of TL signal measurements and TLD reader calibration. Individual uncertainties are combined to estimate the total uncertainty due to random fluctuations. The Radiological Physics Center's (RPC) mail out TLD system, utilizing throwaway LiF powder to monitor high‐energy photon and electron beam outputs, is analyzed in detail. The technique may also be applicable to other TLD systems. It is shown that statements of ±2% dose uncertainty and ±5% action criterion for TLD dosimetry are reasonable when related to uncertainties in the dose calculations, provided the standard deviation (s.d.) of TL readings is 1.5% or better.
High‐energy x‐ray beam characterization using photonuclear reaction
Authors: Wielopolski Lucian , Ren Xuemin
Published: March 2003

Spectral changes in the primary photon beam of a high‐energy medical electron accelerator have been monitored using photonuclear reactions. The beam across the radiation field was measured using induced activity in Teflon cylinder and copper foil samples at several off‐axis angles, at different SSDs in air and in a solid water phantom. The induced activities of the positron emitters were measured using a coincidence detection system. The experimental results are in agreement with Monte Carlo calculations and Schiff's theory.
Wedge factor dependence on depth and field size for various beam energies using symmetric and half‐collimated asymmetric jaw settings
Authors: Niroomand‐Rad Azam , Haleem Mahmoud , Rodgers James , Obcemea Ceferino
Published: March 2003

The depth‐ and field‐size dependence of the in‐phantom wedge factor have been determined for a Cobalt‐60 (Co‐60) teletherapy unit and four medical linear accelerators with 4‐, 6‐, 10‐, and 18‐MV x‐ray beams containing 15°–60° (nominal) lead, brass, and steel wedge filters. Measurements were made with ionization chambers in solid water or water with a source‐skin distance of 80 or 100 cm. Field sizes varied from 4×4 cm up to a maximum allowable size for each wedge filter. Measurements were performed for symmetric and half‐collimated asymmetric fields at depth of maximum dose, 5‐ and 10‐cm depths. For half‐collimated fields, wedge factor reference points were located at a fixed off‐axis distance from the collimator's rotational axis. These systematic measurements on wedges indicate that the wedge factor dependence on depth and field size is a function of beam energy as well as the design of the treatment head and wedge filters. Significance of the results reported herein are discussed for the most commonly used treatment depths and field sizes with various beam energies and wedge filters.
Head‐scatter factors and effective x‐ray source positions in a 25‐MV linear accelerator
Authors: Tatcher Morris , Bjärngard Bengt
Published: March 2003

The behavior of the effective source position and the correction factor associated with the collimator opening (head‐scatter factor) were investigated for the 6‐ and 25‐MV x‐ray beams of a linear accelerator. The primary photon fluence was measured in air for square field sizes from 5×5 cm to 40×40 cm at distances from the nominal source of 80 to 140 cm, for open and wedged fields (wedge angle 60 deg). An inverse‐square analysis shows that, for open fields, the effective source position of the accelerator is about the same (approximately 1 cm downstream) at 6 and 25 MV, for all field sizes. For the wedged fields, the effective source position depends on field size and ranges from about 2 to 4 cm. The head‐scatter correction factors for given collimator settings were found to be essentially independent of distance at both energies.
Ytterbium‐169: Calculated physical properties of a new radiation source for brachytherapy
Authors: Mason D. L. D. , Battista J. J. , Barnett R. B. , Porter A. T.
Published: March 2003

Seeds containing radioactive Ytterbium‐169 (169Yb) have recently been manufactured for possible application to brachytherapy. Ytterbium‐169 emits photons with an average energy of 93 keV (excluding energies less than 10 keV), and decays with a half‐life of 32 days. Analytic and Monte Carlo computations have been used to predict physical quantities useful in treatment planning and radiation protection. Analytic calculations based on the primary photon spectrum of 169Yb (excluding energies less than 10 keV) yield an air‐kerma rate constant of 0.0427 cGy cm2 h−1 MBq−1, and an exposure rate constant of 1.80 R cm2 mCi−1 h−1 for this radionuclide. Calculated fmed factors are 0.922 cGy/R for soft tissue and 2.12 cGy/R for bone. The first half‐value layer in lead is 0.2 mm; the first tenth‐value layer is 1.6 mm. Using Monte Carlo simulations, the relative dose distributions around 169Yb seeds (Amersham, prototypes 4 and 5) are provided, and are then compared with those around an 125I seed (3M model 6702). The 169Yb seeds produce more isotropic dose distributions, and for permanent implants, can deliver it at a greater initial dose rate. A value of 1.19 cm−2 was also calculated for the specific dose constant D0, a value which is applicable to both seed types. Radiation protection is not as easily achieved for permanent implants with 169Yb because of the higher energy emissions (vs 125I). However, for temporary implants, Ytterbium‐169 may prove to be a useful substitute for 192Ir or 137Cs because of its relatively lower energy emissions. It is concluded that 169Yb merits further investigation, including dosimetry, radiobiological, and clinical studies.
Neutron capture therapy with 235U seeds
Authors: Liu Hungyuan B. , Brugger Robert M. , Shih Jing‐Luen A.
Published: March 2003

A combination of brachytherapy and neutron capture therapy has been evaluated using 235U metal seeds and external neutron beam irradiation. When thermal neutrons are absorbed by 235U, high‐energy neutrons and gamma rays are produced and some of these deposit energy in surrounding tissue. A Monte Carlo program, using the code MCNP, has been used to evaluate two sizes of 235U seeds in a water phantom. The results of flux suppression around the seeds and dose distributions are illustrated and discussed. The results show that high doses can be delivered in a relatively short time by using 235U seeds with neutron capture therapy. This therapy with multiple needles or seeds can be envisioned as a substitute for traditional brachytherapy to give an effective killing dose.
Macroscopic geometric heterogeneity effects in radiation dose distribution analysis for boron neutron capture therapy
Authors: Moran Jean M. , Nigg David W. , Wheeler Floyd J. , Bauer William F.
Published: March 2003

Calculations of radiation flux and dose distributions for boron neutron capture therapy (BNCT) of brain tumors are typically performed using sophisticated three‐dimensional analytical models based on either a homogeneous approximation or a simplified few‐region approximation to the actual highly heterogeneous geometry of the irradiation volume. Such models should be validated by comparison with calculations using detailed models in which all significant macroscopic tissue heterogeneities and geometric structures are explicitly represented as faithfully as possible. This paper describes such a validation exercise for BNCT of canine brain tumors. Geometric measurements of the canine anatomical structures of interest for this work were performed by dissecting and examining two essentially identical Labrador retriever heads. Chemical analyses of various tissue samples taken during the dissections were conducted to obtain measurements of elemental compositions for the tissues of interest. The resulting geometry and tissue composition data were then used to construct a detailed heterogeneous calculational model of the Labrador head. Calculations of three‐dimensional radiation flux distributions pertinent to BNCT were performed for this model using the TORT discrete‐ordinates radiation transport code. The calculations were repeated for a corresponding volume‐weighted homogeneous‐tissue model. Comparison of the results showed that peak neutron and photon flux magnitudes were quite similar for the two models (within 5%), but that the spatial flux profiles were shifted in the heterogeneous model such that the fluxes in some locations away from the peak differed from the corresponding fluxes in the homogeneous model by as much as 10%–20%. Differences of this magnitude can be therapeutically significant, emphasizing the need for proper validation of simplified treatment planning models.
High‐contrast resolution of CT images for bone structure analysis
Authors: Durand Eric P. , Rüegsegger Peter
Published: March 2003

The modulation transfer function (MTF) of an imaging system may be used as a figure of merit to assess its geometrical resolution. As such it is often derived from the point spread function (PSF) through a Fourier transformation, or through a FFT in case of digital images. Using the special features of the convolution‐backprojection algorithm and making some simplifying assumptions about the number of rays, it is shown that the desired MTF is also obtainable without a Fourier transformation of the PSF, different shapes and sizes of the beam‐profile are then analyzed, and finally it is shown that calculated and measured MTF functions are in good agreement. The effects of aliasing on the cutoff frequency are also discussed.
Distributed self‐capacitance of magnetic resonance imaging birdcage coils
Authors: Harpen Michael D.
Published: March 2003

Presented is a theoretical description of the self‐capacitance of a low‐pass birdcage coil. The problem considered is that the modification of the distributed self‐capacitance produced by the electric interaction between the coil and a cylindrical Faraday screen and illustrates the phenomenon of coil self‐capacitance as well as the influence of Faraday screens. Changes in coil self‐capacitance produced by the presence of the screens are shown to be consistent with measured shifts in coil resonant frequencies.
Quantitative phase‐velocity MR imaging of in‐plane laminar flow: Effect of fluid velocity, vessel diameter, and slice thickness
Authors: Kraft Kenneth A. , Fei Ding‐Yu , Fatouros Panos P.
Published: March 2003

Quantitative MR phase imaging is frequently used to measure spin velocities. A potential difficulty may arise, however, when in‐plane phase images are acquired of a vessel carrying laminar flow, for which the fluid velocity profile is parabolic. In that case, depending on the flow velocity (v), the vessel diameter (D), and the chosen MR slice thickness (ST), a spin velocity gradient will be present to some extent within each intraluminal voxel. The resulting intravoxel phase dispersion may be expected to affect the net pixel phase value, and hence compromise the assumed linear correlation between phase shift and velocity. In this study, the effects of alterations of v, D, and ST on the apparent image phase are investigated for the case of laminar flow directed parallel to the sequence read gradient. A theoretical model is developed and the conclusions experimentally tested using a flow phantom. The data demonstrate that when quantitating in‐plane phase‐flow images, significant velocity underestimations may occur when the net flow‐induced phase shifts are small and the MR slice thickness is an appreciable fraction of the vessel diameter.
Generation and use of measurement‐based 3‐D dose distributions for 3‐D dose calculation verification
Authors: Stern Robin L. , Fraass Benedick A. , Gerhardsson Ansi , McShan Daniel L. , Lam Kwok L.
Published: March 2003

A 3‐D radiation therapy treatment planning system calculates dose to an entire volume of points and therefore requires a 3‐D distribution of measured dose values for quality assurance and dose calculation verification. To measure such a volumetric distribution with a scanning ion chamber is prohibitively time consuming. A method is presented for the generation of a 3‐D grid of dose values based on beam's‐eye‐view (BEV) film dosimetry. For each field configuration of interest, a set of BEV films at different depths is obtained and digitized, and the optical densities are converted to dose. To reduce inaccuracies associated with film measurement of megavoltage photon depth doses, doses on the different planes are normalized using an ion‐chamber measurement of the depth dose. A 3‐D grid of dose values is created by interpolation between BEV planes along divergent beam rays. This matrix of measurement‐based dose values can then be compared to calculations over the entire volume of interest. This method is demonstrated for three different field configurations. Accuracy of the film‐measured dose values is determined by 1‐D and 2‐D comparisons with ion chamber measurements. Film and ion chamber measurements agree within 2% in the central field regions and within 2.0 mm in the penumbral regions.
Coordinate transformation as a primary representation of radiotherapy beam geometry
Authors: Sherouse George W.
Published: March 2003

An approach to both geometric specification of radiotherapy beams and computerized solution of geometric treatment planning problems using coordinate transformations is presented. It is demonstrated that the specification of the geometric relationship of a treatment beam to a patient can be uniquely given by a 4×4 coordinate transformation matrix, and that the matrix representation can be translated from (and to) the more conventional machine‐based specification of geometry. This approach enables a compact representation of the patient/beam geometry which is independent of the specific labeling conventions of the treatment machine and which can be directly exploited in the solution of treatment planning problems. Beam geometry can be easily described either in terms of the natural degrees of freedom of a therapy machine or in terms of alternative, problem‐specific frames of reference. The ability to use these various frames of reference interchangeably allows the designer of treatment design software to present appropriate task‐specific user interfaces for arbitrarily complex tasks, and thus reduce the cognitive burden on users of the software.
Investigation of the nonlinear aspects of imaging through a highly scattering medium
Authors: Haselgrove John C. , Wang Nai Guang , Chance Britton
Published: March 2003

The edge‐spread function that is expected when imaging a sharp edge in a highly scattering medium using a time‐of‐flight imaging system has been investigated. Experimental results have been compared with computer simulations. The effect of scattering is to broaden the image of the edge, accompanied by an offset of the midpoint (50% transmission) of the edge. Small random errors in the measurement of the time of flight results in an increase in the offset of the edge position at very short times of flight. The offset in the midpoint of the edge is evidence of the nonlinear nature imaging process, in view of which we conclude that nonlinear algorithms will be necessary to maximize the image information available from measured intensities.
High‐energy x‐ray beam calibrations using scaling
Authors: Bjärngard Bengt E.
Published: March 2003

When calibrating high‐energy x‐ray beams, it is convenient to measure in a plastic phantom and convert the result to absorbed dose in water at a specified depth, field size, and distance from the source. By combining the scaling theorem with the observation that TAR is independent of the distance to the source, it is demonstrated that the measurement in plastic can be made at the same source–detector distance but scaling depth and field size. Experiments to support this were made in acrylic and water with 6‐ and 25‐MV x rays and at different distances from the source. The agreement between the calibrations in acrylic and water was comparable to that using the current national protocol, which recommends the scaling of depth only while using the same field size and source–detector distance in the two media and correcting for the difference in scatter dose. It was confirmed that the scaling factor may be the ratio between the electron densities of acrylic and water, or the ratio between the linear attenuation coefficients. A method to determine this factor directly is also described.
Long‐term use of an isotope check source for verification of ion chamber calibration
Authors: Barish R. J. , Lerch I. A.
Published: March 2003

A radioactive check source is recommended for operational and constancy checks of dosimetry systems used for the calibration of therapeutic x‐ray generators, including linear accelerators. Eight years of data have been analyzed for two ion chambers (and their associated electrometers) irradiated at fixed geometry in such a device. These dosimetry systems have also been calibrated every 2 years at a single Accredited Dosimetry Calibration Laboratory. Our analysis suggests that when a check source is used, and the results are consistent, the interval between formal calibrations can be lengthened.
Chamber replacement correction in absorbed dose calibrations
Authors: Burns J. E.
Published: March 2003

The corrections in absorbed dose calibration for the replacement of the medium by an ionization chamber shows some discrepancies. The calibration procedures is discussed. (AIP)
The spatial and energy dependence of bremsstrahlung production about beta point sources in H2O
Authors: Simpkin Douglas J. , Cullom S. James , Mackie T. Rockwell
Published: March 2003

A Monte Carlo simulation was performed to characterize the spatial and energy distribution of bremsstrahlung radiation from β point sources important to radioimmunotherapy (RIT). Using the EGS4 Monte Carlo code, the isotropic emission and transport of monoenergetic 0.1‐, 0.5‐, 1.0‐, 2.0‐, and 3.0‐MeV electrons and 32P and 90Y β particles was simulated in an infinite, homogeneous H2O phantom. The probability of bremsstrahlung production for each Monte Carlo‐simulated electron step was accumulated in energy intervals not exceeding 5 keV and stored as a function of radial position. To validate this scheme, the EGS4 code was tested in the continuous slowing down approximation (csda) mode, with resulting radiation yields seen to agree with values in ICRU Report No. 37 (ICRU, Bethesda, MD, 1984) to better than 1.6%. The radiation yield calculated with the simulation of secondary particles is seen to be 3%–5% greater than the csda yield. The photon energy distributions are characterized by a typically broad bremsstrahlung spectrum with the probability of photon generation decreasing with radial distance. In the energy range 0.05–0.511 MeV, the probability for bremsstrahlung production from 90Y (2.76×10−2 decay−1) is twice that from 32P (1.35×10−2 decay−1). When passed through 10 cm of H2O and put upon a standard NaI scintillation camera, count rates of 2.3×10−6 and 1.2×10−6 counts s−1 Bq−1 are estimated from point sources of 90Y and 32P. These results predict the inherent spatial resolution limitation and provide the initial data required for modeling and analyzing the scatter, attenuation, and image formation processes in quantitative imaging of bremsstrahlung for RIT dosimetry.
Some considerations regarding w values for heavy charged‐particle radiotherapy
Authors: Verhey Lynn J. , Lyman John T.
Published: March 2003

The AAPM recommendations regarding the w values for heavy charged‐particles radiotherapy are discussed. (AIP)
Evaluation of a new set of calibration standards for the measurement of fat content via DPA and DXA
Authors: Goodsitt Mitchell M.
Published: March 2003

A simulation study was performed to evaluate a new set of calibration standards for estimating the fat content of the body via dual‐photon absorptiometry (DPA) and dual‐energy x‐ray absorptiometry (DXA). The standards, proposed by Nord and Payne [presented at the 2nd meeting of The Bath Conference on Bone Mineral Measurement (1990)] consist of stearic acid (100% fat) and 0.6% NaCl in water (100% lean). They were compared with other standards consisting of average composition adipose/muscle tissues and fatty adipose / lean muscle tissues. Source and detector properties of a Gd‐153 DPA system and three commercial DXA systems were modeled. For each system and calibration set, rms errors in the calculated fat contents of simulated tissues having fat mass percentages that ranged from about 4%–44% and thicknesses that ranged from 5–20 cm were determined. Beam hardening errors for the systems were evaluated as was a calibration technique employed by one of the manufacturers to correct for such errors. In general, the smallest rms errors (2% or less when the calibration standards and tissues were of equal thickness) were obtained with the average adipose/muscle standards. Equivalent results were obtained with standards consisting of stearic acid and 0.8% NaCl. The latter is a higher salt content than proposed by Nord and Payne and results from differences in the x‐ray attenuation coefficients that were employed in calculating the fat equivalence of water. Other, more convenient standards, such as lucite and water may be employed by using appropriate fat equivalences (∼69% for lucite and ∼10% for water). Beam hardening errors for the DXA systems are considerable, and the simulated correction technique was shown to be effective.