AAPM Banner
  • Volume/Page
  • Keyword
  • DOI
  • Citation
  • Advanced
   
 
 
 
Search Issue | RSS Feeds RSS
Previous Issue Next Issue

May 2011

Volume 38, Issue 5, pp. 2311-2823

Spotlight Figure

Med. Phys. 38, 2515 (2011); http://dx.doi.org/10.1118/1.3574874 (8 pages)

Vorakarn Chanyavanich, Shiva K. Das, William R. Lee, and Joseph Y. Lo
Enlarge Image | Read More
Page 1 of 7 Pages Next Page | Jump to Page
FREE

EDITORIAL: Radiological emergencies and the medical physicist

William Hendee

Med. Phys. 38, 2311 (2011); http://dx.doi.org/10.1118/1.3581376 (2 pages)

Online Publication Date: 21 April 2011

Full Text: Read Online (HTML) | Download PDF

Abstract Unavailable
Show PACS
87.53.-j Effects of ionizing radiation on biological systems
89.60.Gg Impact of natural and man-made disasters
FREE

EDITORIAL: The global outreach of Medical Physics

William Hendee

Med. Phys. 38, 2313 (2011); http://dx.doi.org/10.1118/1.3568928 (2 pages)

Online Publication Date: 22 April 2011

Full Text: Read Online (HTML) | Download PDF

Abstract Unavailable
Show PACS
01.30.Ww Editorials
87.00.00 Biological and medical physics
FREE

POINT/COUNTERPOINT: All graduate medical physics programs should have an original research component

David W. O. Rogers, Janelle A. Molloy, and Colin G. Orton

Med. Phys. 38, 2315 (2011); http://dx.doi.org/10.1118/1.3533902 (3 pages) | Cited 2 times

Online Publication Date: 25 April 2011

Full Text: Read Online (HTML) | Download PDF

Abstract Unavailable
Show PACS
87.85.-d Biomedical engineering
87.55.-x Treatment strategy

RADIATION THERAPY PHYSICS: A distance to dose difference tool for estimating the required spatial accuracy of a displacement vector field

Nahla K. Saleh-Sayah, Elisabeth Weiss, Francisco J. Salguero, and Jeffrey V. Siebers

Med. Phys. 38, 2318 (2011); http://dx.doi.org/10.1118/1.3572228 (6 pages) | Cited 1 time

Online Publication Date: 25 April 2011

Full Text: Read Online (HTML) | Download PDF

Show Abstract
Purpose: To introduce a tool, termed distance to dose difference (DTD), which estimates the required spatial accuracy of displacement vector fields (DVFs) used for mapping four dimensional dose values.
Methods: Dose mapping maps dose values from an irradiated geometry to a reference geometry. DVF errors result in dose being mapped from the wrong spatial location in the irradiated geometry, with a dose error equal to the dose difference between the error-free and sampled spatial locations. The DTD, defined as the distance to observe a given dose difference in the irradiated geometry, quantifies the permitted DVF error to ensure a prespecified desired dose mapping accuracy is achieved. To demonstrate the DTD, a treatment plan is generated with a 5 mm internal target volume-to-planning target volume margin for an intensity modulated radiation therapy lung patient. The DTD is evaluated for mapping dose from the end of inhale image with a dose error tolerance of 3.30 Gy, which equals 5% of the 66 Gy prescription dose. The DTD is loaded into the treatment planning system to visualize positional dependencies of permissible DVF errors overlaid on the patient’s anatomy and DTD-volume-histograms are generated.
Results: DTD values vary with location in the patient anatomy. For the test case, DTD analysis indicates that accurate DVFs (∼1 mm) are required in high dose gradient regions while large DVF errors (>20 mm) are acceptable in low dose gradient regions. Within the clinical target volume (CTV), tolerated DVF uncertainties range from 1 to 12 mm, depending on location. Ninety percent of the CTV volume had DTD values less than 4 mm.
Conclusions: The DVF spatial accuracy required to meet a dose mapping accuracy tolerance depends on the spatial location within the dose distribution. For dose mapping, DVFs accuracy must be highest in dose gradient regions, while less accurate DVFs can be tolerated in uniform dose regions. The DTD tool provides a useful first estimate of DVF required spatial accuracy.
Show PACS
87.55.dk Dose-volume analysis
87.57.Q- Computed tomography
87.19.Wx Pneumodyamics, respiration

RADIATION IMAGING PHYSICS: Estimator for photon counting energy selective x-ray imaging with multibin pulse height analysis

Robert E. Alvarez

Med. Phys. 38, 2324 (2011); http://dx.doi.org/10.1118/1.3570658 (11 pages) | Cited 1 time

Online Publication Date: 25 April 2011

Full Text: Read Online (HTML) | Download PDF

Show Abstract
Purpose: This paper describes a noniterative estimator for the energy dependent information from photon counting detectors with multibin pulse height analysis (PHA). The estimator uses the two function decomposition of the attenuation coefficient [R. E. Alvarez and A. Macovski, Phys. Med. Biol. 21, 733–744 (1976)] and its output is the line integrals of the basis set coefficients. The output noise variance and bias is compared to other noniterative estimators and to the Cramèr-Rao lower bound (CRLB).
Methods: The estimator first computes an initial estimate from a linearized maximum likelihood estimator. The errors in the initial estimates are determined at a set of points from measurements on a calibration phantom. The errors at these known points are interpolated to create two-dimensional look up tables of corrections to the initial estimates. During image acquisition, the linearized maximum likelihood estimate for each data point is used as an input to the correction look up tables, and the final output is the sum of the estimate and the correction. The performance of the estimator is compared to generalizations of the polynomial and rational polynomial estimators for multibin data. The estimators are compared by the mean square error (MSE) and its components, the bias, and the variance of the output. The variance is also compared to the CRLB. The performance is simulated with two to five bins PHA data. The CRLB at a fixed object thickness is also computed as a function of the number of bins.
Results: For two bin data, all the estimators’ variances are equal to the CRLB. With three or more bins, only the proposed estimator achieves the CRLB while the others, which were not optimized for noise performance, have much larger output variance. The bias of the proposed estimator is equal to the polynomial estimator for calibration phantoms with 40 or more steps, that is, 1600 combinations of basis materials, but is larger than the rational polynomial bias. In all cases at the photon counts tested, the MSE is essentially equal to the variance, indicating that the bias errors are negligible compared to the variance.
Conclusions: The estimator provides a noniterative method to compute the energy dependent information from multibin PHA data that achieves the CRLB over a wide range of operating conditions and has low output bias. The estimator can be calibrated based on the measurements of a calibration phantom; so, it does not require measurements of the x-ray energy spectrum or the detector response functions.
Show PACS
87.59.B- Radiography
06.20.fb Standards and calibration
02.60.-x Numerical approximation and analysis

RADIATION THERAPY PHYSICS: Initial application of a geometric QA tool for integrated MV and kV imaging systems on three image guided radiotherapy systems

Weihua Mao, Michael Speiser, Paul Medin, Lech Papiez, Timothy Solberg, and Lei Xing

Med. Phys. 38, 2335 (2011); http://dx.doi.org/10.1118/1.3570768 (7 pages) | Cited 1 time

Online Publication Date: 26 April 2011

Full Text: Read Online (HTML) | Download PDF

Show Abstract
Purpose: Several linacs with integrated kilovoltage (kV) imaging have been developed for delivery of image guided radiation therapy (IGRT). High geometric accuracy and coincidence of kV imaging systems and megavoltage (MV) beam delivery are essential for successful image guidance. A geometric QA tool has been adapted for routine QA for evaluating and characterizing the geometric accuracy of kV and MV cone-beam imaging systems. The purpose of this work is to demonstrate the application of methodology to routine QA across three IGRT-dedicated linac platforms.
Methods: It has been applied to a Varian Trilogy (Varian Medical Systems, Palo Alto, CA), an Elekta SynergyS (Elekta, Stockholm, Sweden), and a Brainlab Vero (Brainlab AG, Feldkirchen, Germany). Both the Trilogy and SynergyS linacs are equipped with a retractable kV x-ray tube and a flat panel detector. The Vero utilizes a rotating, rigid ring structure integrating a MV x-ray head mounted on orthogonal gimbals, an electronic portal imaging device (EPID), two kV x-ray tubes, and two fixed flat panel detectors. This dual kV imaging system provides orthogonal radiographs, CBCT images, and real-time fluoroscopic monitoring. Two QA phantoms were built to suit different field sizes. Projection images of a QA phantom were acquired using MV and kV imaging systems at a series of gantry angles. Software developed for this study was used to analyze the projection images and calculate nine geometric parameters for each projection. The Trilogy was characterized five times over one year, while the SynergyS was characterized four times and the Vero once. Over 6500 individual projections were acquired and analyzed. Quantitative geometric parameters of both MV and kV imaging systems, as well as the isocenter consistency of the imaging systems, were successfully evaluated.
Results: A geometric tool has been successfully implemented for calibration and QA of integrated kV and MV across a variety of radiotherapy platforms. X-ray source angle deviations up to 0.8°, and detector center offsets up to 3 mm, were observed for three linacs, with the exception of the Vero, for which a significant center offset of one kV detector (prior to machine commissioning) was observed. In contrast, the gimbal-based MV source positioning of the Vero demonstrated differences between observed and expected source positions of less than 0.2 mm, both with and without gimbal rotation.
Conclusions: This initial application of this geometric QA tool shows promise as a universal, independent tool for quantitative evaluation of geometric accuracies of both MV and integrated kV imaging systems across a range of platforms. It provides nine geometric parameters of any imaging system at every gantry angle as well as the isocenter coincidence of the MV and kV image systems.
Show PACS
87.56.bd Accelerators

RADIATION THERAPY PHYSICS: A modification of flattening filter free linac for IMRT

P. Tsiamas, J. Seco, Z. Han, M. Bhagwat, J. Maddox, C. Kappas, K. Theodorou, M. Makrigiorgos, K. Marcus, and P. Zygmanski

Med. Phys. 38, 2342 (2011); http://dx.doi.org/10.1118/1.3571419 (11 pages)

Online Publication Date: 26 April 2011

Full Text: Read Online (HTML) | Download PDF

Show Abstract
Purpose: This study investigates the benefits of a modified flattening filter free (FFF) linac over the standard (STD) linac equipped with the flattening filter. Energy and angular spread of the electron beam of the FFF linac were modified. Modification of FFF beam parameters is explored to maximize the monitor unit efficiency and to minimize the head scatter in IMRT delivery for large target volumes or targets lying away from the central axis.
Methods: The EGSnrc code is used to model FFF and STD linacs and study basic beam properties for both linac types in various beam configurations. Increasing energy of FFF linac results in similar beam attenuation properties and maximized dose rate compared to STD linac. Matching beam attenuation properties allows a more direct exploration of beam flatness of FFF linac in regard to IMRT delivery, especially away from the central axis where the effective dose rate is considerably smaller than the one at the central axis. Flatness of open beam dose profile of FFF linac is improved by increasing the angular spread of the electron beam. The resulting dose rate within the treatment field and outside of the field (peripheral dose) are characterized and compared to the unmodified FFF and STD linacs.
Results: In order to match beam penetration properties, the energy of FFF is adjusted from 6.5 to 8.0 MeV for small to medium field sizes and from 6.5 to 8.5 MeV for larger ones. Dose rate of FFF vs STD linac increased by a factor of 1.9 (6.5 MeV) and 3.4–4.1 (8.0–8.5 MeV). Adjusting the mean angular spread of the electron beam from 0° to 5°–10° resulted in complete flattening of photon beam for field sizes between 10 × 10 cm2 and 15 × 15 cm2 and partial flattening for field sizes from 15 × 15 cm2 to 30 × 30 cm2. Values of angular spread ≥14° are not recommended as they exceed the opening of the primary collimator, affecting the area at the edges of the field. FFF fields of sizes smaller than 6 × 6 cm2 are already flat and beam flattening is not necessary. Overall, the angular spread of 5°–10° is sufficient and can satisfactorily flatten open beam dose profiles even for larger field sizes. Increasing the electron beam angular spread amounts to a slight decrease of dose rate of FFF linac. However, for angular spread, 5°–10° dose rate factor of FFF vs STD is still about 1.6–2.6, depending on the field size (and the adjusted energy). Similarly, in case of peripheral dose, a moderate increase in dose can be observed for angular spread of 5°–10° and for field sizes 10 × 10 cm2 to 30 × 30 cm2. Lastly, beam flatness of not modified FFF linac can be conveniently described by an analytical function representing a ratio of STD vs FFF doses: 1 + b|r|n.
Conclusions: A modified FFF beamline with increased energy and electron beam angular spread results in satisfactory flattened beam and high dose rate within the field. Peripheral dose remaining at similar (or smaller) level than that of STD linac for the same delivered dose within the treatment field.
Show PACS
87.56.bd Accelerators
87.53.Jw Therapeutic applications, including brachytherapy
29.20.Ej Linear accelerators
29.27.Fh Beam characteristics
FREE

RADIATION IMAGING PHYSICS: Phase-contrast digital tomosynthesis

Jeffrey C. Hammonds, Ronald R. Price, Edwin F. Donnelly, and David R. Pickens

Med. Phys. 38, 2353 (2011); http://dx.doi.org/10.1118/1.3574871 (6 pages) | Cited 1 time

Online Publication Date: 26 April 2011

Full Text: Read Online (HTML) | Download PDF

Show Abstract
Purpose: Phase-contrast (PC) edge enhancement occurs at the boundary between different tissues and is an interference effect that results from the differential phase-shifts that the x-rays acquire while traversing the two tissues. While observable in planar phase-contrast radiographs, the impact of digital tomosynthesis on this edge enhancement effect has not been previously reported. The purpose of this work is to demonstrate: (1) that phase-contrast digital tomosynthesis (PC-DTS) is possible with a conventional x-ray source, (2) that the reconstructed tomosynthesis images demonstrate and retain edge enhancement as compared to planar phase-contrast radiographs and (3) tomosynthesis improves object contrast by reducing the effects of superimposed structures.
Methods: An unmodified, commercially available cabinet x-ray system (Faxitron LX-60) was used. The system contains a tungsten anode x-ray tube that was operated at 24 kVp and 3 mAs for each PC radiographic image taken, with a nominal focal spot size of 0.010 mm. The digital detector uses CsI/CMOS with a pixel size of 0.054 mm × 0.054 mm. Objects to be imaged were attached to a computer-controlled rotating motor and are rotated ±25° about a central position in one degree increments. At each increment, three phase-contrast radiographs are taken and then averaged to reduce the effect of noise. These planar images are then used to reconstruct a series of 56 longitudinal tomographic images with an image offset increment of about 0.7 mm.
Results: Tomographic z-plane resolution was measured to be approximately 4 mm. When compared to planar PC images, the tomosynthesis images were shown to retain the PC boundary edge enhancement in addition to an improvement in object contrast.
Conclusions: Our work demonstrates that PC digital tomosynthesis retains the edge-enhancement observed in planar PC radiograph and further improves soft-tissue conspicuity by reducing the effects of superimposed tissue structure.
Show PACS
87.59.bf Digital radiography
87.57.Q- Computed tomography
87.57.nt Edge enhancement
87.57.nf Reconstruction
87.57.cf Spatial resolution
07.85.-m X- and γ-ray instruments
29.40.Mc Scintillation detectors

RADIATION THERAPY PHYSICS: Technical Note: An algorithm to calculate the tissue phantom ratio from depth dose in radiosurgery

Luis Isaac Ramos Garcia and Julio F Almansa

Med. Phys. 38, 2359 (2011); http://dx.doi.org/10.1118/1.3570575 (7 pages)

Online Publication Date: 26 April 2011

Full Text: Read Online (HTML) | Download PDF

Show Abstract
Purpose: To propose a method to calculate the tissue phantom ratio (TPR) using the depth dose and to compare the proposed method with two other methods.Methods: An analytical dose model from Bjärngard was used to describe the depth dose and the TPR. The parameters of the model were derived from depth dose measurements, which were then used to calculate the TPR. The calculated TPR values were compared with actual measurements as well as with TPR values predicted from two methods that also use depth dose, namely, the method proposed by BrainLAB and the conventional method that sets the quotients of the scatter phantom ratios (Sp) to 1.Results: TPR values calculated from the proposed algorithm deviated by −0.2 ± 0.1% (mean deviation) from the experimental measurements, over a range of field sizes and depths.Conclusions: The results of the proposed method were in better agreement with the experimental measurements than were results using the other two methods. Furthermore, the differences between the proposed method and the other methods are statistically significant.
Show PACS
87.55.dk Dose-volume analysis
87.53.Ly Stereotactic radiosurgery

RADIATION THERAPY PHYSICS: Monte Carlo electron source model validation for an Elekta Precise linac

O. A. Ali, C. A. Willemse, W. Shaw, F. H. J. O’Reilly, and F. C. P. du Plessis

Med. Phys. 38, 2366 (2011); http://dx.doi.org/10.1118/1.3570579 (8 pages)

Online Publication Date: 26 April 2011

Full Text: Read Online (HTML) | Download PDF

Show Abstract
Purpose: Electron radiation therapy is used frequently for the treatment of skin cancers and superficial tumors especially in the absence of kilovoltage treatment units. Head-and-neck treatment sites require accurate dose distribution calculation to minimize dose to critical structures, e.g., the eye, optic chiasm, nerves, and parotid gland. Monte Carlo simulations can be regarded as the dose calculation method of choice because it can simulate electron transport through any tissue and geometry. In order to use this technique, an accurate electron beam model should be used.Methods: In this study, a two point-source electron beam model developed for an Elekta Precise linear accelerator was validated. Monte Carlo data were benchmarked against measured water tank data for a set of regular and circular fields and at 95, 100, and 110 cm source-to-skin-distance. EDR2 Film dose distribution data were also obtained for a paranasal sinus treatment case using a Rando phantom and compared with corresponding dose distribution data obtained from Monte Carlo simulations and a CMS XiO treatment planning system. A partially shielded electron field was also evaluated using a solid water phantom and EDR2 film measurements against Monte Carlo simulations using the developed source model.Results: The major findings were that it could accurately replicate percentage depth dose and beam profile data for water measurements at source-to-skin-distances ranging between 95 and 110 cm over beam energies ranging from 4 to 15 MeV. This represents a stand-off between 0 and 15 cm. Most percentage depth dose and beam profile data (better than 95%) agreed within 2%/2 mm and nearly 100% of the data compared within 3%/3 mm. Calculated penumbra data were within 2 mm for the 20 × 20 cm2 field compared to water tank data at 95 cm source-to-skin-distance over the above energy range. Film data for the Rando phantom case showed gamma index map data that is similar in comparison with the treatment planning system and the Monte Carlo source model. The gamma index showed good agreement (2%/2 mm) between the Monte Carlo source model and the film data.Conclusions: Percentage depth dose and beam profile data were in most cases within a tolerance of 2%/2 mm. The biggest discrepancies were in most cases recorded in the first 6 mm of the water phantom. Circular fields showed local dose agreement within 3%/3mm. Good agreement was found between calculated dose distributions for a paranasal sinus case between Monte Carlo, film measurements and a CMS XiO treatment planning system. The electron beam model can be easily implemented in the BEAMnrc or DOSXYZnrc Monte Carlo codes enabling quick calculation of electron dose distributions in complex geometries.
Show PACS
87.53.Bn Dosimetry/exposure assessment
87.17.-d Cell processes
05.10.Ln Monte Carlo methods
Page 1 of 7 Pages Next Page | Jump to Page
Close

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

© 2012, American Association of Physicists in Medicine. Individual readers of this journal, and nonprofit libraries acting for them, are freely permited to make fair use of the material in it, such as to copy an article for use in teaching or research. (For other kinds of copying see "Copying Fees.") Permission is granted to quote from this journal in scientific works with the customary acknowledgment of the source. To reprint a figure, table, or other excerpt requires, in addition, AAPM may require that permission also be obtained from one of the authors. Address inquiries and notices to Penny Slattery, Journal Manager, Medical Physics Journal, AAPM, One Physics Ellipse, College Park, MD 20740-3846; email: journal@aapm.org.

close