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Dec 2003

Volume 30, Issue 12, pp. 3049-3265

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POINT/COUNTERPOINT: All medical physicists entering the field should have a specific course on research and practice ethics in their educational background

David Switzer, Nicholas Detorie, and William R. Hendee, Moderator

Med. Phys. 30, 3049 (2003); http://dx.doi.org/10.1118/1.1621868 (3 pages) | Cited 2 times

Online Publication Date: 17 November 2003

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Abstract Unavailable
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87.90.+y Other topics in biological and medical physics (restricted to new topics in section 87)
01.75.+m Science and society
01.40.-d Education

RADIATION THERAPY PHYSICS: A phantom study on the positioning accuracy of the Novalis Body system

Hui Yan, Fang-Fang Yin, and Jae Ho Kim

Med. Phys. 30, 3052 (2003); http://dx.doi.org/10.1118/1.1626122 (9 pages) | Cited 39 times

Online Publication Date: 17 November 2003

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A phantom study was conducted to investigate inherent positioning accuracy of an image-guided patient positioning system—the Novalis Body system for three-dimensional (3-D) conformal radiotherapy. This positioning system consists of two infrared (IR) cameras and one video camera and two kV x-ray imaging devices. The initial patient setup was guided by the IR camera system and the target localization was accomplished using the kV x-ray imaging system. In this study, the IR marker shift and phantom rotation were simulated and their effects on the positioning accuracy were examined by a Rando phantom. The effects of CT slice thickness and treatment sites on the positioning accuracy were tested. In addition, the internal target shift was simulated and its effect on the positioning accuracy was examined by a water tank. With the application of the Novalis Body system, the positioning error of the planned isocenter was significantly reduced. The experimental results with the simulated IR marker shifts indicated that the positioning errors of the planned isocenter were 0.6±0.3, 0.5±0.2, and 0.7±0.2 mm along the lateral, longitudinal, and vertical axes, respectively. The experimental results with the simulated phantom rotations indicated that the positioning errors of the planned isocenter were 0.6±0.3, 0.7±0.2, and 0.5±0.2 mm along the three axes, respectively. The experimental results with the simulated target shifts indicated that the positioning errors of the planned isocenter were 0.6±0.3, 0.7±0.2, and 0.5±0.2 mm along the three axes, respectively. On average, the positioning accuracy of 1 mm for the planned isocenter was achieved using the Novalis Body system. © 2003 American Association of Physicists in Medicine.
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87.56.-v Radiation therapy equipment
87.56.Da Ancillary equipment
87.53.-j Effects of ionizing radiation on biological systems

RADIATION IMAGING PHYSICS: Effects of quantum noise and binocular summation on dose requirements in stereoradiography

Andrew D. A. Maidment, Predrag R. Bakic, and Michael Albert

Med. Phys. 30, 3061 (2003); http://dx.doi.org/10.1118/1.1621869 (11 pages) | Cited 10 times

Online Publication Date: 18 November 2003

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In the case of a quantum-noise limited detector, signal detection theory suggests that stereoradiographic images can be acquired with one half of the per-image dose needed for a standard radiographic projection, as information from the two stereo images can be combined. Previously, film–screen stereoradiography has been performed using the same per-image dose as in projection radiography, i.e., doubling the total dose. In this paper, the assumption of a possible decrease in dose for stereoradiography was tested by a series of contrast-detail experiments, using phantom images acquired over a range of exposures. The number of visible details, the effective reduction of the dose, and the effective decrease in the threshold signal-to-noise ratio were determined using human observers under several display and viewing conditions. These results were averaged over five observers and compared with multiple readings by a single observer and with the results of an additional observer with limited stereoscopic acuity. Experimental results show that the total dose needed to produce a stereoradiographic image pair is approximately 1.1 times the dose needed for a single projection in standard radiography, indicating that under these conditions the human visual system demonstrates almost ideal binocular summation. © 2003 American Association of Physicists in Medicine.
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87.59.B- Radiography
42.66.Si Psychophysics of vision, visual perception; binocular vision
87.19.lt Sensory systems: visual, auditory, tactile, taste, and olfaction
87.57.C- Image quality

RADIATION IMAGING PHYSICS: Adaptive temporal resolution optimization in helical cardiac cone beam CT reconstruction

R. Manzke, M. Grass, T. Nielsen, G. Shechter, and D. Hawkes

Med. Phys. 30, 3072 (2003); http://dx.doi.org/10.1118/1.1624756 (9 pages) | Cited 52 times

Online Publication Date: 18 November 2003

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Cone beam computed tomography scanners in combination with heart rate adaptive reconstruction schemes have the potential to enable cardiac volumetric computed tomography (CT) imaging for a larger number of patients and applications. In this publication, an adaptive scheme for the automatic and patient-specific reconstruction optimization is introduced to improve the temporal resolution and image quality. The optimization method permits the automatic determination of the required amount of gated helical cone beam projection data for the reconstruction volume. It furthermore allows one to optimize subvolume reconstruction yielding an increased temporal resolution. In addition, methods for the assessment of the temporal resolution are given which enable a quantitative documentation of the reconstruction improvements. Results are presented for patient data sets acquired in low pitch helical mode using a 16-slice cone beam CT system with parallel ECG recording. © 2003 American Association of Physicists in Medicine.
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87.57.C- Image quality
87.59.bd Computed radiography
87.57.N- Image analysis
87.19.Hh Cardiac dynamics

RADIATION IMAGING PHYSICS: Computer-aided diagnosis in high resolution CT of the lungs

Ingrid C. Sluimer, Paul F. van Waes, Max A. Viergever, and Bram van Ginneken

Med. Phys. 30, 3081 (2003); http://dx.doi.org/10.1118/1.1624771 (10 pages) | Cited 29 times

Online Publication Date: 18 November 2003

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A computer-aided diagnosis (CAD) system is presented to automatically distinguish normal from abnormal tissue in high-resolution CT chest scans acquired during daily clinical practice. From high-resolution computed tomography scans of 116 patients, 657 regions of interest are extracted that are to be classified as displaying either normal or abnormal lung tissue. A principled texture analysis approach is used, extracting features to describe local image structure by means of a multi-scale filter bank. The use of various classifiers and feature subsets is compared and results are evaluated with ROC analysis. Performance of the system is shown to approach that of two expert radiologists in diagnosing the local regions of interest, with an area under the ROC curve of 0.862 for the CAD scheme versus 0.877 and 0.893 for the radiologists. © 2003 American Association of Physicists in Medicine.
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87.57.N- Image analysis
87.59.bd Computed radiography

RADIATION THERAPY PHYSICS: Surface and build-up region dosimetry for obliquely incident intensity modulated radiotherapy 6 MV x rays

Nesrin Dogan and Glenn P. Glasgow

Med. Phys. 30, 3091 (2003); http://dx.doi.org/10.1118/1.1625116 (6 pages) | Cited 29 times

Online Publication Date: 18 November 2003

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This study investigates the surface dose and build-up region dosimetry for oblique IMRT beams. The dependence of surface and build-up region doses of 0° (perpendicular incidence) and 75° (oblique incidence) IMRT fields on field size was measured and compared with open field dosimetry. Measurements were performed using a parallel-plate chamber and KODAK EDR2 films in a polystyrene phantom for a 6 cm×6 cm and a 12 cm×12 cm, 6 MV photon beam at depths of 0 mm (surface) through dmax. Data were normalized to the dmax value of each field. Four intensity modulated delivery patterns were created and delivered using step-and-shoot IMRT: (1) six static 1 cm×6 cm strips (IMRTstrip), (2) 12 static 1 cm×12 cm strips (IMRTstrip), (3) intensity modulated beam patterns created by using the inverse planning optimization software (IMRTopt) for 6 cm×6 cm, and (4) IMRTopt for 12 cm×12 cm field sizes. The percent depth doses (PDDs) of 0°, 6 cm×6 cm IMRTstrip beam at the surface and 5 mm were lower by 8.8% and 1.6%, respectively, compared to the open field. The PDDs of 75°, 6 cm×6 cm IMRTstrip beam at the surface and 5 mm were lower by 6.7% and 2.4%, respectively, compared to the open field. This study showed that IMRT itself is not contributing to greater skin doses. © 2003 American Association of Physicists in Medicine.
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87.53.Bn Dosimetry/exposure assessment
87.56.Da Ancillary equipment

RADIATION THERAPY PHYSICS: Clinical helical tomotherapy commissioning dosimetry

John Balog, Gustavo Olivera, and Jeff Kapatoes

Med. Phys. 30, 3097 (2003); http://dx.doi.org/10.1118/1.1625444 (10 pages) | Cited 36 times

Online Publication Date: 18 November 2003

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Helical tomotherapy presented many unique dosimetric challenges and solutions during the initial commissioning process, and some of them are presented. The dose calculation algorithm is convolution/superposition based. This requires that the energy fluence spectrum and magnitude be quantified. The methodology for doing so is described. Aspects of the energy fluence characterization that are unique to tomotherapy are highlighted. Many beam characteristics can be measured automatically by an included megavoltage computed tomography imaging system. This greatly improves data collection efficiency. © 2003 American Association of Physicists in Medicine.
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87.55.-x Treatment strategy

RADIATION THERAPY PHYSICS: Monte Carlo model of the Studsvik BNCT clinical beam: Description and validation

Valerio Giusti, Per M. Munck af Rosenschöld, Kurt Sköld, Bruno Montagnini, and Jacek Capala

Med. Phys. 30, 3107 (2003); http://dx.doi.org/10.1118/1.1626120 (11 pages) | Cited 7 times

Online Publication Date: 18 November 2003

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The neutron beam at the Studsvik facility for boron neutron capture therapy (BNCT) and the validation of the related computational model developed for the MCNP-4B Monte Carlo code are presented. Several measurements performed at the epithermal neutron port used for clinical trials have been made in order to validate the Monte Carlo computational model. The good general agreement between the MCNP calculations and the experimental results has provided an adequate check of the calculation procedure. In particular, at the nominal reactor power of 1 MW, the calculated in-air epithermal neutron flux in the energy interval between 0.4 eV–10 keV is 3.24×109 n cm−2 s−1 (±1.2% 1 std. dev.) while the measured value is 3.30×109 n cm−2 s−1 (±5.0% 1 std. dev.). Furthermore, the calculated in-phantom thermal neutron flux, equal to 6.43×109 n cm−2 s−1 (±1.0% 1 std. dev.), and the corresponding measured value of 6.33×109 n cm−2 s−1 (±5.3% 1 std. dev.) agree within their respective uncertainties. The only statistically significant disagreement is a discrepancy of 39% between the MCNP calculations of the in-air photon kerma and the corresponding experimental value. Despite this, a quite acceptable overall in-phantom beam performance was obtained, with a maximum value of the therapeutic ratio (the ratio between the local tumor dose and the maximum healthy tissue dose) equal to 6.7. The described MCNP model of the Studsvik facility has been deemed adequate to evaluate further improvements in the beam design as well as to plan experimental work. © 2003 American Association of Physicists in Medicine.
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87.55.K- Monte Carlo methods
87.53.Bn Dosimetry/exposure assessment

RADIATION THERAPY PHYSICS: In vivo diode dosimetry for routine quality assurance in IMRT

P. D. Higgins, P. Alaei, B. J. Gerbi, and K. E. Dusenbery

Med. Phys. 30, 3118 (2003); http://dx.doi.org/10.1118/1.1626989 (6 pages) | Cited 23 times

Online Publication Date: 18 November 2003

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Due to the complexity of IMRT dosimetry, dose delivery evaluation is generally done using a treatment plan in which the optimized fluence distribution has been transferred to a test phantom for accessibility and simplicity of measurement. The actual patient doses may be reconstructed in vivo through the use of electronic portal imaging devices or films, but the assessment of absolute dose from these measurements is time-consuming and complicated. In our clinic we have instituted the use of routine diode dosimetry for IMRT patients following the same procedure used for standard radiation therapy patients in which each new treatment field is checked at the start of treatment. For standard cases the dose at dmax is calculated as part of the monitor unit calculation. For the IMRT cases, the dose contribution to the dmax depth for each field is taken from the treatment plan. We found that about 90% of the diode measurements agreed to within ±10% of the planned doses (45/51 fields) and 63% (32/51 fields) achieved ±5% agreement. By using this direct in vivo method to verify the clinical doses delivered, we have been able to make a uniform startup procedure for all patients while simplifying our IMRT QA process. © 2003 American Association of Physicists in Medicine.
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87.55.Qr Quality assurance in radiotherapy
87.56.Da Ancillary equipment

RADIATION THERAPY PHYSICS: Commissioning 6 MV photon beams of a stereotactic radiosurgery system for Monte Carlo treatment planning

Jun Deng, C.-M. Ma, Jenny Hai, and Ravinder Nath

Med. Phys. 30, 3124 (2003); http://dx.doi.org/10.1118/1.1624753 (11 pages) | Cited 14 times

Online Publication Date: 19 November 2003

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The goal of this work is to implement a beam commissioning procedure to generate a multiple source model using a set of standard measurement data for possible Monte Carlo treatment planning in the clinic for a Cyberknife stereotactic radiosurgery system. The required measurement data include the central axis depth dose curve (PDD), the dose profile at dmax(=1.5 cm) of 60 mm cone at 80 cm source-to-surface distance (SSD), and the cone output factors for cones of 5 mm to 60 mm at 80 cm source-to-axis distance (SAD). The employed dual source model has the same structure as the one that has been studied in our previous work while most of the parameters of each source are extracted from the measurement data rather than the beam phase space. The energy spectra will be extracted from the central axis PDD, the fluence distributions will be deconvoluted from the dose profile at dmax, and the source distributions will be determined from the measured cone output factors. Monte Carlo dose calculations in various water phantoms have been performed to verify the beam commissioning procedure. The agreement between the measurements and the commissioning results was within 2%/1 mm for the central axis PDDs and the dose profiles at various depths when an IC-3 chamber was used and within 2% for the cone output factors for various collimator sizes of 5 to 60 mm. Largest difference (9.5%) was observed for the 7.5 mm cone when an IC-10 chamber was used. The large differences can be attributed to the volumetric averaging effect of the IC-10 chamber, whose dimension is comparable to the field of the small cones. The overall agreement between the measurements and the commissioning results is clinically acceptable, which implies that our commissioning tool is adequate for clinical applications of Monte Carlo dose calculations for the Cyberknife stereotactic radiosurgery system. © 2003 American Association of Physicists in Medicine.
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87.55.K- Monte Carlo methods
87.55.-x Treatment strategy
87.53.Bn Dosimetry/exposure assessment
87.53.Ly Stereotactic radiosurgery
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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
One Physics Ellipse
College Park, Maryland 20740
301-209-3352

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