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Dec 1999

Volume 26, Issue 12, pp. 2517-2711

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RADIATION PROTECTION: Proposition: A pregnant resident physician should be excused from training rotations such as angiography and nuclear medicine because of the potential exposure of the fetus

Edward L. Nickoloff, Libby Brateman, and William R. Hendee, Moderator

Med. Phys. 26, 2517 (1999); http://dx.doi.org/10.1118/1.598786 (3 pages)

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Abstract Unavailable
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87.55.N- Radiation monitoring, control, and safety
87.59.Dj Angiography
87.57.U- Nuclear medicine imaging

RADIATION PROTECTION: Evaluation of neutron dose in the maze of medical electron accelerators

E. Carinou, V. Kamenopoulou, and I. E. Stamatelatos

Med. Phys. 26, 2520 (1999); http://dx.doi.org/10.1118/1.598787 (6 pages) | Cited 9 times

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MCNP code was used to simulate neutron and prompt gamma ray transport for a range of maze geometrical parameters, wall composition, and wall surface lining. Verification measurements were performed at two medical electron accelerator facilities. A very good agreement was observed between the results of the measurements and the MCNP simulation. MCNP code results were compared with the results of analytical equations used for the calculation of maze effectiveness, derived by Kersey and McCall. A good agreement exists between the simulation results and the results of the analytical methods for maze lengths longer than 8.5 m. However, the results of the present study showed that for shorter maze lengths, Kersey’s method tended to overestimate neutron dose at the door entrance, whereas McCall’s method with the neutron room scattered correction applied, showed an underestimation of neutron dose. Furthermore, according to MCNP simulation results, the use of barytes concrete instead of standard concrete as room shielding material, reduced neutron dose at the door entrance by about 20%. Finally, it was shown that lining with layers of wood and borated polyethylene significantly reduced the neutron dose at the door entrance by 45% and 65%, respectively. © 1999 American Association of Physicists in Medicine.
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87.53.Bn Dosimetry/exposure assessment
29.20.-c Accelerators
87.55.K- Monte Carlo methods

RADIATION PROTECTION: A schema for estimating absorbed dose to organs following the administration of radionuclides with multiple unstable daughters: A matrix approach

K. A. Hamacher and G. Sgouros

Med. Phys. 26, 2526 (1999); http://dx.doi.org/10.1118/1.598788 (3 pages) | Cited 4 times

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The dosimetry of alpha particle emitters requires that all decays, including those of unstable intermediates be included in the calculation. Such calculations are complicated by the potential differential biologic distribution of each of the intermediates. In this work we present a formalism which will account for the known biodistribution factors of the daughters and the resulting effective biodistribution which will depend upon the site at which the parent radionuclide decays. The number of decays or cumulated activity of a daughter radionuclide present in a particular tissue is estimated using a probability matrix which describes the likelihood of daughter decay in a particular tissue as a function of the decay site of the parent. An example of three initial compartments is provided to illustrate the use of this formalism. Such modeling may be used to evaluate the feasibility of using radionuclides whose decay includes alpha-emitting intermediates. Model validation and refinement will require an assessment of the fate of free, alpha-emitting intermediates in various biological milieus. © 1999 American Association of Physicists in Medicine.
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87.53.Bn Dosimetry/exposure assessment
87.57.un Radiopharmaceuticals
87.57.uq Dosimetry

RADIATION TREATMENT PHYSICS: On the use of apparent activity (Aapp) for treatment planning of 125I and 103Pd interstitial brachytherapy sources: Recommendations of the American Association of Physicists in Medicine Radiation Therapy Committee Subcommittee on Low-Energy Brachytherapy Source Dosimetry

Jeffrey F. Williamson, Bert M. Coursey, Larry A. DeWerd, William F. Hanson, Ravinder Nath, Mark J. Rivard, and Geoffrey Ibbott

Med. Phys. 26, 2529 (1999); http://dx.doi.org/10.1118/1.598789 (2 pages) | Cited 16 times

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Abstract Unavailable
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87.53.Jw Therapeutic applications, including brachytherapy
87.53.Bn Dosimetry/exposure assessment
87.57.uq Dosimetry
06.20.F- Units and standards

RADIATION TREATMENT PHYSICS: Radial dose distribution, dose to water and dose rate constant for monoenergetic photon point sources from 10 keV to 2 MeV: EGS4 Monte Carlo model calculation

Gary Luxton and Gabor Jozsef

Med. Phys. 26, 2531 (1999); http://dx.doi.org/10.1118/1.598790 (8 pages) | Cited 23 times

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A comprehensive set of dose distributions from monoenergetic photon-emitting isotropic point sources in a medium can be used as a reference database for the dosimetry of photon emitter sources in that medium. Data of this type for water over the photon energy range from 15 keV to 2 MeV have been published based on calculations using a one-dimensional photon transport model. The present work, based on a previously published EGS4 Monte Carlo code, updates the classic data set of Berger and provides more extensive calculations than previously available. Air kerma strength per unit photon emission rate from an isotropic point emitter is obtained as a function of energy using published data for mass energy absorption coefficients. The TG-43 dose rate constant for water as a function of energy is calculated for monoenergetic photon emitters as the ratio of dose rate to water at 1 cm to air kerma strength for unit photon emission rate. Results for the radial dose distribution agree well with the data of Berger between 40 and 400 keV. For energies ⩾500 keV, a previously undescribed buildup region for the radial dose function is identified. Thickness of the buildup region ranges from 1 mm at 500 keV to 8 mm at 2 MeV. Between 15 and 30 keV, the radial dose function within a few millimeters of the emitter is calculated to be 4%–5% higher than values derived from Berger’s data. The maximum dose rate constant for monoenergetic photon emitters occurs at an energy of 60 keV, and has the value 1.355 cGy h−1 U−1, where U is the unit of air kerma strength, 1 μGy m2 h−1. This would correspond to the maximum hypothetical dose rate constant for a brachytherapy photon source emitting photons of energy ⩽2 MeV. © 1999 American Association of Physicists in Medicine.
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87.53.Bn Dosimetry/exposure assessment
87.55.K- Monte Carlo methods

RADIATION TREATMENT PHYSICS: Preprocessing of control portal images for patient setup verification during the treatments in external radiotherapy

Gisèle Hilt, Didier Wolf, and Pierre Aletti

Med. Phys. 26, 2539 (1999); http://dx.doi.org/10.1118/1.598791 (11 pages) | Cited 2 times

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The main problem in processing the control portal images is their poor quality. We have developed a way of improving the image quality to allow a segmentation stage. Three items were studied for this purpose the background gradient of intensity correction, the noise reduction, and the image restoration. The background was corrected by subtracting a smoothed version of the image from the original. We tested 15 noise reduction methods. The most appropriate for control portal images was found to be the truncated average. Finally, four restoration techniques were compared. The maximum a posteriori (MAP) algorithm was the most efficient. The algorithms were tested over a wide range of conditions (image quality). They produced a great improvement in anatomic detail for all the imaging systems, energies, and anatomical zones tested. For example, the signal-to-noise ratio of a SRI-100 pelvis image, acquired with 4 monitor units (MU) at 10 MV (very low quality image), increased from 0.97 to 42.84 after preprocessing. We found that the improvement in image quality facilitated or even enabled segmentation of the control portal images. The percentage of segments belonging to a structure increased from 30% to 65% in the example cited. The preprocessing of control portal images is the first step in checking the patient setup. © 1999 American Association of Physicists in Medicine.
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87.55.km Verification
87.57.N- Image analysis

RADIATION TREATMENT PHYSICS: Distance–dose curve for a miniature x-ray tube for stereotactic radiosurgery using an optimized aperture with a parallel-plate ionization chamber

P. J. Biggs, J. Beatty, and T. Yasuda

Med. Phys. 26, 2550 (1999); http://dx.doi.org/10.1118/1.598792 (5 pages) | Cited 1 time

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A recently developed miniature x-ray tube operating at 40 kV has been used in a randomized trial for the treatment of small intracranial lesions. The diameter of these lesions ranges from 10 to 30 mm. A thin window parallel-plate ionization chamber was used to calibrate the output of the x-ray tube, modified by the addition of a thin platinum aperture to reduce the charge collecting area of the chamber. The effect of such an aperture on the measurement of dose versus distance from the x-ray tube in a phantom has been examined as a function of aperture diameter. Aperture diameters were varied between 0 and 5 mm and dose measurements were made for distances between the x-ray source and the front surface of the chamber of 5–30 mm in water. The ratio of doses measured with and without an aperture, when normalized to unity at a distance of 10 mm, differs significantly from unity, for distances between 7.5 and 15 mm, for aperture diameters <1.5 mm and differs from unity, but less significantly, for apertures ⩾3 mm. For intermediate diameters, however, this dose dependence is minimized, indicating an aperture diameter that provides a similar distance–dose curve as the measurement taken without an aperture over this range of distances. This diameter was found to be between 2 and 2.5 mm with a dose variation of less than ±1%. For distances <7.5 mm, measurements made with a 1.5-mm-diam aperture agree better with those taken with a 1.7-mm-diam chamber compared with a 5.2-mm-diam chamber. © 1999 American Association of Physicists in Medicine.
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07.85.-m X- and γ-ray instruments
87.53.Bn Dosimetry/exposure assessment
29.40.Cs Gas-filled counters: ionization chambers, proportional, and avalanche counters
87.19.L- Neuroscience

RADIATION TREATMENT PHYSICS: Miniature scintillating detector for small field radiation therapy

D. Létourneau, J. Pouliot, and R. Roy

Med. Phys. 26, 2555 (1999); http://dx.doi.org/10.1118/1.598793 (7 pages) | Cited 36 times

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In planning stereotactic radiosurgery treatments, depth dose curves, profiles, and dose rate of treatment beams are difficult to obtain with conventional detectors because of loss of lateral electronic equilibrium and volume averaging. A scintillating detector with high spatial resolution and good reliability has been developed to overcome this problem. The miniature dosimeter consists of two identical radiation-resistant 10 m long silica optical fibers, each connected to an independent silicon photodiode. A small cylindrical polystyrene scintillator (3.9 mm3) is optically glued to the detection fiber. The light seen by the photodiode connected to this fiber arises from fluorescence of the scintillator and from the Cerenkov effect produced in silica. The reference signal produced by the fiber without scintillator is used to subtract the Cerenkov light contribution from the raw detector response. The sensitive volume of the scintillating detector is nearly water-equivalent and thus minimizes dose distribution perturbation in water. The miniature dosimeter has a spatial resolution comparable to the film-densitometer system. Profiles of 1 cm diam, 6 MV photon beam measured with both systems show very similar shapes. Furthermore, the use of photodiodes instead of photomultiplier tubes gives a better stability response and offers the possibility to perform absolute dosimetry. © 1999 American Association of Physicists in Medicine.
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87.56.Da Ancillary equipment
87.56.-v Radiation therapy equipment
87.55.-x Treatment strategy
29.40.Mc Scintillation detectors

RADIATION TREATMENT PHYSICS: Intensity-modulation radiotherapy using independent collimators: An algorithm study

Jian-Rong Dai and Yi-Min Hu

Med. Phys. 26, 2562 (1999); http://dx.doi.org/10.1118/1.598794 (9 pages) | Cited 24 times

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The purpose of this work is to investigate algorithms for the delivery of intensity-modulated fields using independent collimators (IC). Two heuristic algorithms are proposed to calculate jaw-setting sequences for arbitrary 2D intensity distributions. The first algorithm is based on searching the whole intensity matrix to find the largest nonzero rectangular area as a segment while the second algorithm is to find a nonzero rectangular area as a segment which makes the complexity of the remaining intensity matrix minimum. After a sequence is obtained, the delivery order of all its segments is optimized with the technique of simulated annealing to minimize the total jaw-moving time. To evaluate these two algorithms, randomly generated intensity matrices and three clinical cases of different complexity have been tested, and the results have been compared with one algorithm proposed for MLC technique. It is shown that the efficiency of IC technique becomes increasingly lower than that of MLC technique, and the relative efficiency of two algorithms proposed here is related to machine dose rate and jaw speed. Assuming the prescribed dose is 200 cGY per fraction, machine dose rate is 250 MU/min, and jaw speed is 1.5 cm/s, the treatment can be delivered within about 20 min for all three cases with the first algorithm. The second algorithm requires longer delivery time under such assumptions. The delivery time can be further reduced through increasing machine dose rate and jaw speed, and developing more efficient algorithms. The use of IC for intensity-modulation radiotherapy has some potential advantages over other techniques. © 1999 American Association of Physicists in Medicine.
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87.53.Bn Dosimetry/exposure assessment
87.56.-v Radiation therapy equipment
87.56.ng Wedges and compensators
29.20.-c Accelerators

RADIATION TREATMENT PHYSICS: Evaluation of a commercial three-dimensional electron beam treatment planning system

G. X. Ding, J. E. Cygler, G. G. Zhang, and M. K. Yu

Med. Phys. 26, 2571 (1999); http://dx.doi.org/10.1118/1.598795 (10 pages) | Cited 9 times

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We evaluated a commercial three-dimensional (3D) electron beam treatment planning system (CADPLAN V.2.7.9) using both experimentally measured and Monte Carlo calculated dose distributions to compare with those predicted by CADPLAN calculations. Tests were carried out at various field sizes and electron beam energies from 6 to 20 MeV. For a homogeneous water phantom the agreement between measured and CADPLAN calculated dose distributions is very good except at the phantom surface. CADPLAN is able to predict hot and cold spots caused by a simple 3D inhomogeneity but unable to predict dose distributions for a more complex geometry where CADPLAN underestimates dose changes caused by inhomogeneity. We discussed possible causes for the inaccuracy in the CADPLAN dose calculations. In addition, we have tested CADPLAN treatment monitor unit and electron cut-out factor calculations and found that CADPLAN predictions generally agree with manual calculations. © 1999 American Association of Physicists in Medicine.
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87.53.Bn Dosimetry/exposure assessment
87.55.-x Treatment strategy
87.55.K- Monte Carlo methods
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Medical Physics is an official science journal of the AAPM and of the COMP/CCPM/IOMP
William R. Hendee, Editor
Departments of Radiology, Radiation Oncology, Biophysics, Community and Public Health, Medical College of Wisconsin
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College Park, Maryland 20740
301-209-3352

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