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Dec 2002

Volume 29, Issue 12, pp. 2745-2965

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RADIATION THERAPY PHYSICS: History by history statistical estimators in the BEAM code system

B. R. B. Walters, I. Kawrakow, and D. W. O. Rogers

Med. Phys. 29, 2745 (2002); http://dx.doi.org/10.1118/1.1517611 (8 pages) | Cited 43 times

Online Publication Date: 15 November 2002

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A history by history method for estimating uncertainties has been implemented in the BEAMnrc and DOSXYZnrc codes replacing the method of statistical batches. This method groups scored quantities (e.g., dose) by primary history. When phase-space sources are used, this method groups incident particles according to the primary histories that generated them. This necessitated adding markers (negative energy) to phase-space files to indicate the first particle generated by a new primary history. The new method greatly reduces the uncertainty in the uncertainty estimate. The new method eliminates one dimension (which kept the results for each batch) from all scoring arrays, resulting in memory requirement being decreased by a factor of 2. Correlations between particles in phase-space sources are taken into account. The only correlations with any significant impact on uncertainty are those introduced by particle recycling. Failure to account for these correlations can result in a significant underestimate of the uncertainty. The previous method of accounting for correlations due to recycling by placing all recycled particles in the same batch did work. Neither the new method nor the batch method take into account correlations between incident particles when a phase-space source is restarted so one must avoid restarts.
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87.55.K- Monte Carlo methods
87.55.-x Treatment strategy
87.53.Bn Dosimetry/exposure assessment

RADIATION THERAPY PHYSICS: A personal-computer-based method to obtain “star-shots” of mechanical and optical isocenters for gantry rotation of linear accelerators

M. K. Woo

Med. Phys. 29, 2753 (2002); http://dx.doi.org/10.1118/1.1521937 (3 pages) | Cited 5 times

Online Publication Date: 18 November 2002

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This work describes a method to obtain “star-shots” of the mechanical and optical isocenters of linear accelerators, similar to the star-shots of radiation isocenters normally obtained using films. In this method a digital camera is connected to a personal computer so that multiply exposed images can be taken at a fixed camera position. A mechanical pointer or a wire aligned along the optical axis can then be imaged by the camera. Multiple exposures at varying gantry angles are then superimposed on a digital image which can be analyzed by the computer to give a high-resolution star-shot. The method provides a convenient way for a linear accelerator quality assurance procedure. © 2002 American Association of Physicists in Medicine.
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87.56.Fc Quality assurance equipment
29.20.-c Accelerators
41.85.-p Beam optics
87.55.Qr Quality assurance in radiotherapy

RADIATION THERAPY PHYSICS: Experimental determination of beam quality conversion factors kQ in clinical photon beams using ferrous sulphate (Fricke) dosimetry

Åsa Palm and Olof Mattsson

Med. Phys. 29, 2756 (2002); http://dx.doi.org/10.1118/1.1521941 (7 pages)

Online Publication Date: 20 November 2002

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The implementation of protocols based on absorbed dose to water standards requires beam quality conversion factors, kQ. Calculated values of kQ are available for ionization chambers used for reference dosimetry. Ideally, kQ should be experimentally determined at the same beam qualities as that of the user. In this work we measure kQ factors in clinical photon beams and compare them with calculated and measured values. Beam quality conversion factors are determined for clinical photon beams of nominal energies 4 MV, 6 MV, 15 MV, and 25 MV, for commonly used cylindrical ionization chambers. Twelve chambers of eight different types are used. For three of them, no experimental data have previously been available. The experimental procedure is based on measurements with ionization chambers and Fricke dosimetry in the reference beam (60Co γ radiation) and in clinical linear accelerator beams. The kQ values determined in this work generally agree within 0.5% with previously reported experimental values both when %dd(10)x and TPR20,10 are used for beam quality specification. The agreement with calculated data is generally within 0.5%, except for the 15 MV beam. For this beam the measured values are usually between 0.5% and 1% lower than the data taken from the TG-51 protocol or the TRS-398 code of practice. For three NE2571 chambers and three NE2581 chambers, the maximum observed deviation of individual kQ values is 0.2% and 0.4%, respectively. © 2002 American Association of Physicists in Medicine.
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87.53.Bn Dosimetry/exposure assessment
87.56.Da Ancillary equipment

RADIATION THERAPY PHYSICS: Checking monitor unit calculations using coordinate transformations to calculate off-axis distances in the collimator frame of reference

Paula L. Petti

Med. Phys. 29, 2763 (2002); http://dx.doi.org/10.1118/1.1523410 (4 pages)

Online Publication Date: 20 November 2002

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An important part of treatment-planning QA is to check Monitor Units (MUs) calculated by treatment-planning programs. This is generally straightforward, unless the central axis is blocked. One way to check MUs in this case is to select a reference point in the open portion of the field and use the off-axis distance (OAD), as well as other relevant data, to verify the dose. If wedges are employed in the treatment, the OAD must be specified in the collimator frame-of-reference because one must know where the calculation point is with respect to the wedge to calculate the dose correctly. The purpose of this paper is to describe a method of calculating the OAD in the collimator frame-of-reference using the system of coordinate transformations described by Siddon [Med. Phys. 8, 766–774 (1981)]. © 2002 American Association of Physicists in Medicine.
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87.55.-x Treatment strategy
87.55.Qr Quality assurance in radiotherapy
87.56.-v Radiation therapy equipment
87.55.N- Radiation monitoring, control, and safety

RADIATION IMAGING PHYSICS: A CdZnTe slot-scanned detector for digital mammography

James G. Mainprize, Nancy L. Ford, Shi Yin, Eli E. Gordon, William J. Hamilton, Tümay O. Tümer, and Martin J. Yaffe

Med. Phys. 29, 2767 (2002); http://dx.doi.org/10.1118/1.1523932 (15 pages) | Cited 7 times

Online Publication Date: 25 November 2002

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A new high-resolution detector has been developed for use in a slot-scanned digital mammography system. The detector is a hybrid device that consists of a CCD operating in time-delay integration mode that is bonded to a 150-μm-thick CdZnTe photoconductor array. The CCD was designed with a detector element pitch of 50 μm. Two devices were evaluated with differing crystalline quality. Incomplete charge collection was a source of reduction in DQE. This occurs in both devices due to characteristically low mobility-lifetime products for CdZnTe, with the greatest losses demonstrated by the multicrystalline sample. The mobility-lifetime products for the multicrystalline device were found to be 2.4×10−4 and 4.0×10−7 cm2/V for electrons and holes, respectively. The device constructed with higher quality single crystal CdZnTe demonstrated mobility-lifetime products of 1.0×10−4 and 4.4×10−6 cm2/V for electrons and holes. The MTF and DQE for the device were measured at several exposures and results were compared to predictions from a linear systems model of signal and noise propagation. The MTF at a spatial frequency of 10mm−1 exceeded 0.18 and 0.56 along the scan and slot directions, respectively. Scanning motion and CCD design limited the resolution along the scan direction. For an x-ray beam from a tungsten target tube with 40 μm molybdenum filtration operated at 26 kV, the single crystal device demonstrated a DQE(0) of 0.70±0.02 at 7.1×10−6 C/kg (27 mR) exposure to the detector, despite its relatively poor charge collection efficiency. © 2002 American Association of Physicists in Medicine.
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87.59.E- Mammography
85.60.Dw Photodiodes; phototransistors; photoresistors
42.79.Pw Imaging detectors and sensors
85.60.Gz Photodetectors (including infrared and CCD detectors)
72.20.Fr Low-field transport and mobility; piezoresistance
72.40.+w Photoconduction and photovoltaic effects

RADIATION THERAPY PHYSICS: Modification of prostate implants based on postimplant treatment margin assessment

Amy Mueller, Kent Wallner, Gregory Merrick, Jacques Couriveau, Steven Sutlief, Wayne Butler, Lixin Gong, and Paul Cho

Med. Phys. 29, 2782 (2002); http://dx.doi.org/10.1118/1.1521120 (6 pages) | Cited 7 times

Online Publication Date: 27 November 2002

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Purpose: To quantify the extent of additional source placement needed to perfect an implant after execution by standard techniques, assuming that uniform 5 mm treatment margins (TMs) is the criteria for perfection. Materials and Methods: Ten consecutive, unselected patients treated with I-125 brachytherapy were studied. Source placement is planned just inside or outside of the prostatic margin, to achieve a minimum 5 mm TM and a central dose of 150%–200% of the prescription dose. The preimplant prostate volumes ranged from 24 to 85 cc (median: 35 cc). The number of sources implanted ranged from 48 to 102 (median: 63). Axial CT images were acquired within 2 h postoperatively for postimplant dosimetry. After completion of standard dosimetric calculations, the TMs were measured and tabulated at 45° intervals around the prostate periphery at 0.0, 1.0, 2.0, and 3.0 cm planes. Sources were then added to the periphery to bring the TMs to a minimum of 5 mm at each measured TM, resulting in a modified implant. All margin modifications were done manually, without the aid of automated software. Results: Patients’ original (unmodified) D90s ranged from 111% to 154%, with a median of 116%. The original V100s ranged from 94% to 99%, with a median of 96%. No patient required placement of additional sources to meet a minimum D90 of 90% or a minimum V100 of 80%. In contrast, patients required from 7 to 17 additional sources (median: 11) to achieve minimum 5 mm TMs around the entire prostatic periphery. Additional sources equaled from 12% to 24% of the initial number of sources placed (median: 17%). By adding sufficient peripheral sources to bring the TMs to a minimum 5 mm, patients’ average V100 increased from 96% to 100%, and the average D90 increased from 124% to 160% of prescription dose. In the course of achieving a minimum 5 mm TM, the average treatment margin for all patients combined increased from 5.5 to 9.9 mm. The number of sources needed to bring the TMs to a minimum 5 mm was loosely correlated with the preimplant prostate volume and the change in prostate volume from implant-related swelling. Adding sufficient sources to achieve minimum 5 mm TMs increased the prostate volume receiving greater than 200% of the prescription dose (V200) from 39% to 58%, and increased the average urethral point dose (2.00 cm inferior to the bladder) from 154% to 171% of the 144 Gy prescription isodose. Conclusions: Minimum 5 mm TMs are not uniformly achieved with current implant techniques. It seems that doing so, even in experienced hands, will require a reappraisal of our implant techniques, or the addition of intraoperative dosimetric analysis with the capacity to substantially modify the implant with extra sources. © 2002 American Association of Physicists in Medicine.
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87.53.Jw Therapeutic applications, including brachytherapy
87.55.-x Treatment strategy
87.59.bd Computed radiography
87.53.Bn Dosimetry/exposure assessment
87.19.X- Diseases

RADIATION THERAPY PHYSICS: Particle in cell simulation of laser-accelerated proton beams for radiation therapy

E. Fourkal, B. Shahine, M. Ding, J. S. Li, T. Tajima, and C.-M. Ma

Med. Phys. 29, 2788 (2002); http://dx.doi.org/10.1118/1.1521122 (11 pages) | Cited 59 times

Online Publication Date: 27 November 2002

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In this article we present the results of particle in cell (PIC) simulations of laser plasma interaction for proton acceleration for radiation therapy treatments. We show that under optimal interaction conditions protons can be accelerated up to relativistic energies of 300 MeV by a petawatt laser field. The proton acceleration is due to the dragging Coulomb force arising from charge separation induced by the ponderomotive pressure (light pressure) of high-intensity laser. The proton energy and phase space distribution functions obtained from the PIC simulations are used in the calculations of dose distributions using the GEANT Monte Carlo simulation code. Because of the broad energy and angular spectra of the protons, a compact particle selection and beam collimation system will be needed to generate small beams of polyenergetic protons for intensity modulated proton therapy. © 2002 American Association of Physicists in Medicine.
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87.55.K- Monte Carlo methods
87.53.Bn Dosimetry/exposure assessment
87.56.-v Radiation therapy equipment
87.17.-d Cell processes
29.20.-c Accelerators

OPTICAL MEASUREMENT PHYSICS: Photoacoustic tomography of biological tissues with high cross-section resolution: Reconstruction and experiment

Xueding Wang, Yuan Xu, Minghua Xu, Seiichirou Yokoo, Edward S. Fry, and Lihong V. Wang

Med. Phys. 29, 2799 (2002); http://dx.doi.org/10.1118/1.1521720 (7 pages) | Cited 42 times

Online Publication Date: 27 November 2002

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A modified back-projection approach deduced from an exact reconstruction solution was applied to our photoacoustic tomography of the optical absorption in biological tissues. Pulses from a Ti:sapphire laser (4.7 ns FWHM at 789.2 nm) were employed to generate a distribution of photoacoustic sources in a sample. The sources were detected by a wide-band nonfocused ultrasonic transducer at different positions around the imaging cross section perpendicular to the axis of the laser irradiation. Reconstructed images of phantoms made from chicken breast tissue agreed well with the structures of the samples. The resolution in the imaging cross section was experimentally demonstrated to be better than 60 μm when a 10 MHz transducer (140% bandwidth at −60 dB) was employed, which was nearly diffraction limited by the detectable photoacoustic waves of the highest frequency. © 2002 American Association of Physicists in Medicine.
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87.80.-y Biophysical techniques (research methods)
43.35.Ud Thermoacoustics, high temperature acoustics, photoacoustic effect
87.57.N- Image analysis
42.25.Bs Wave propagation, transmission and absorption
43.80.Vj Acoustical medical instrumentation and measurement techniques
43.35.Wa Biological effects of ultrasound, ultrasonic tomography

THERMOTHERAPY PHYSICS: Using multiple-electrode impedance measurements to monitor cryosurgery

Alex Hartov, Patrick LePivert, Nirmal Soni, and Keith Paulsen

Med. Phys. 29, 2806 (2002); http://dx.doi.org/10.1118/1.1521721 (9 pages) | Cited 7 times

Online Publication Date: 27 November 2002

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We outfitted cryoprobes with electrodes and used them in conjunction with a multiple channel electrical impedance tomography (EIT) system to record data during freezing experiments in a shallow saline tank. We made measurements using electrodes mounted on the probes and the tank’s periphery. Reconstructed images based on both sets of electrodes indicate a significant improvement in the appearance of the ice ball over using tank electrodes alone. The size of the ice balls was varied by deliberately altering the cooling rate. We found a positive correlation between the measured size of the ice ball and the sizes of isocontour lines in the reconstructed impedance maps. Similarly, the shape of the ice balls was altered by circulating the saline about the probe. Two-dimensional reconstructed impedance contours indicated a deformation in agreement with the shape of the ice ball during the experiments. These findings suggest that using multielectrode impedance sensing may constitute a means for monitoring cryosurgery. © 2002 American Association of Physicists in Medicine.
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87.50.wp Therapeutic applications
87.63.Pn Electrical impedance tomography (EIT)
87.57.N- Image analysis

RADIATION THERAPY PHYSICS: A technique of intensity-modulated radiosurgery (IMRS) for spinal tumors

Fang-Fang Yin, Samuel Ryu, Munther Ajlouni, Jingeng Zhu, Hui Yan, Harrison Guan, Kathleen Faber, Jack Rock, Muwaffak Abdalhak, Lisa Rogers, Mark Rosenblum, and Jae Ho Kim

Med. Phys. 29, 2815 (2002); http://dx.doi.org/10.1118/1.1521722 (8 pages) | Cited 38 times

Online Publication Date: 27 November 2002

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This study is to demonstrate the feasibility of spinal radiosurgery using an image-guided intensity-modulated radiosurgical (IMRS) procedure. A dedicated Novalis® shaped beam surgery unit equipped with a built-in micro-multileaf collimator (mMLC) with a single 6 MV photon beam was used. Each patient was simulated in the supine position using an AcQsim CT simulator with infrared sensitive markers for localization. A variety of different treatment plans were developed, but the most common plan was the use of seven coplanar intensity-modulated beams to minimize radiation to critical organs such as the spinal cord and kidneys. An automatic localization device based on infrared and video cameras was used to guide the initial patient setup. Two keV x-ray imaging systems were used to identify potential deviations from the planned isocenter. A total of 25 patients with spinal tumors have been treated using this procedure with a single prescription dose ranging from 6 to 12 Gy. The final verification images indicated that the average isocenter deviation from the planned isocenter was within 2 mm. The phantom verification of isocenter doses indicated that the average deviation of measured isocenter doses from the planned isocenter doses for all patients treated with intensity-modulated beams was less than 2%. Film dose measurement in a phantom study demonstrated good agreement of above 50% isodose lines between the planned and measured results. Preliminary experience shows that precision delivery of high dose radiation could be administered to the planned target volume while the dose to the critical organs is kept within tolerable limits. © 2002 American Association of Physicists in Medicine.
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87.55.-x Treatment strategy
87.59.bd Computed radiography
87.56.ng Wedges and compensators
87.53.Bn Dosimetry/exposure assessment
87.19.L- Neuroscience
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