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Jan 2013

Volume 40, Issue 1, pp. 010401-017501-15

Spotlight Figure

Med. Phys. 40, 010701 (2013); http://dx.doi.org/10.1118/1.4771935 (5 pages)

Liangzhong Xiang, Bin Han, Colin Carpenter, Guillem Pratx, Yu Kuang, and Lei Xing
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EDITORIAL: Medical Physics becomes a hybrid gold open-access journal

William Hendee, Editor and Sam Armato, Chair

Med. Phys. 40, 010401 (2013); http://dx.doi.org/10.1118/1.4772400 (2 pages)

Online Publication Date: 4 January 2013

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Abstract Unavailable
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99.10.Np Editorial note
43.10.Df Other acoustical societies and their publications, online journals, and other electronic publications
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POINT/COUNTERPOINT: TG-142 is unwarranted for IGRT QA

Scott Dube, M.S., Jennifer O’Daniel, Ph.D., and Colin G. Orton, Ph.D., Moderator

Med. Phys. 40, 010601 (2013); http://dx.doi.org/10.1118/1.4766437 (3 pages)

Online Publication Date: 6 December 2012

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Abstract Unavailable
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01.30.-y Physics literature and publications
87.56.-v Radiation therapy equipment
87.55.-x Treatment strategy
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MEDICAL PHYSICS LETTER: X-ray acoustic computed tomography with pulsed x-ray beam from a medical linear accelerator

Liangzhong Xiang, Bin Han, Colin Carpenter, Guillem Pratx, Yu Kuang, and Lei Xing

Med. Phys. 40, 010701 (2013); http://dx.doi.org/10.1118/1.4771935 (5 pages)

Online Publication Date: 18 December 2012

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Purpose: The feasibility of medical imaging using a medical linear accelerator to generate acoustic waves is investigated. This modality, x-ray acoustic computed tomography (XACT), has the potential to enable deeper tissue penetration in tissue than photoacoustic tomography via laser excitation.
Methods: Short pulsed (μs-range) 10 MV x-ray beams with dose-rate of approximately 30 Gy/min were generated from a medical linear accelerator. The acoustic signals were collected with an ultrasound transducer (500 KHz central frequency) positioned around an object. The transducer, driven by a computer-controlled step motor to scan around the object, detected the resulting acoustic signals in the imaging plane at each scanning position. A pulse preamplifier, with a bandwidth of 20 KHz–2 MHz at −3 dB, and switchable gains of 40 and 60 dB, received the signals from the transducer and delivered the amplified signals to a secondary amplifier. The secondary amplifier had bandwidth of 20 KHz–30 MHz at −3 dB, and a gain range of 10–60 dB. Signals were recorded and averaged 128 times by an oscilloscope. A sampling rate of 100 MHz was used to record 2500 data points at each view angle. One set of data incorporated 200 positions as the receiver moved 360°. The x-ray generated acoustic image was then reconstructed with the filtered back projection algorithm.
Results: The x-ray generated acoustic signals were detected from a lead rod embedded in a chicken breast tissue. The authors found that the acoustic signal was proportional to the x-ray dose deposition, with a correlation of 0.998. The two-dimensional XACT images of the lead rod embedded in chicken breast tissue were found to be in good agreement with the shape of the object.
Conclusions: The first x-ray acoustic computed tomography image is presented. The new modality may be useful for a number of applications, such as providing the location of a fiducial, or monitoring x-ray dose distribution during radiation therapy. Although much work is needed to improve the image quality of XACT and to explore its performance in other irradiation energies, the benefits of this modality, as highlighted in this work, encourage further study.
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87.57.Q- Computed tomography
87.63.dh Ultrasonographic imaging
87.85.Pq Biomedical imaging
87.57.nf Reconstruction
87.85.Ox Biomedical instrumentation and transducers, including micro-electro-mechanical systems (MEMS)

MEDICAL PHYSICS LETTER: A field-shaping multi-well avalanche detector for direct conversion amorphous selenium

A. H. Goldan and W. Zhao

Med. Phys. 40, 010702 (2013); http://dx.doi.org/10.1118/1.4769110 (3 pages)

Online Publication Date: 26 December 2012

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Purpose: A practical detector structure is proposed to achieve stable avalanche multiplication gain in direct-conversion amorphous selenium radiation detectors.
Methods: The detector structure is referred to as a field-shaping multi-well avalanche detector. Stable avalanche multiplication gain is achieved by eliminating field hot spots using high-density avalanche wells with insulated walls and field-shaping inside each well.
Results: The authors demonstrate the impact of high-density insulated wells and field-shaping to eliminate the formation of both field hot spots in the avalanche region and high fields at the metal–semiconductor interface. Results show a semi-Gaussian field distribution inside each well using the field-shaping electrodes, and the electric field at the metal–semiconductor interface can be one order-of-magnitude lower than the peak value where avalanche occurs.
Conclusions: This is the first attempt to design a practical direct-conversion amorphous selenium detector with avalanche gain.
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87.55.N- Radiation monitoring, control, and safety
85.30.Kk Junction diodes
87.56.-v Radiation therapy equipment

RADIATION THERAPY PHYSICS: Absorbed doses behind bones with MR image-based dose calculations for radiotherapy treatment planning

Juha Korhonen, Mika Kapanen, Jani Keyriläinen, Tiina Seppälä, Laura Tuomikoski, and Mikko Tenhunen

Med. Phys. 40, 011701 (2013); http://dx.doi.org/10.1118/1.4769407 (9 pages)

Online Publication Date: 7 December 2012

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Purpose: Magnetic resonance (MR) images are used increasingly in external radiotherapy target delineation because of their superior soft tissue contrast compared to computed tomography (CT) images. Nevertheless, radiotherapy treatment planning has traditionally been based on the use of CT images, due to the restrictive features of MR images such as lack of electron density information. This research aimed to measure absorbed radiation doses in material behind different bone parts, and to evaluate dose calculation errors in two pseudo-CT images; first, by assuming a single electron density value for the bones, and second, by converting the electron density values inside bones from T1/T2*-weighted MR image intensity values.
Methods: A dedicated phantom was constructed using fresh deer bones and gelatine. The effect of different bone parts to the absorbed dose behind them was investigated with a single open field at 6 and 15 MV, and measuring clinically detectable dose deviations by an ionization chamber matrix. Dose calculation deviations in a conversion-based pseudo-CT image and in a bulk density pseudo-CT image, where the relative electron density to water for the bones was set as 1.3, were quantified by comparing the calculation results with those obtained in a standard CT image by superposition and Monte Carlo algorithms.
Results: The calculations revealed that the applied bulk density pseudo-CT image causes deviations up to 2.7% (6 MV) and 2.0% (15 MV) to the dose behind the examined bones. The corresponding values in the conversion-based pseudo-CT image were 1.3% (6 MV) and 1.0% (15 MV). The examinations illustrated that the representation of the heterogeneous femoral bone (cortex denser compared to core) by using a bulk density for the whole bone causes dose deviations up to 2% both behind the bone edge and the middle part of the bone (diameter <2.5 cm), but in the opposite directions. The measured doses and the calculated ones in the standard CT image were within 0.4% (through gelatine only) and 0.9% (behind bones).
Conclusions: This study indicates that the decrease in absorbed dose is not dependent on the bone diameter with all types of bones. Thus, performing dose calculation in a pseudo-CT image by assuming a single electron density value for the bones can lead to a substantial misrepresentation of the dose distribution profile. This work showed that dose calculation accuracy can be improved by using a pseudo-CT image in which the electron density values have been converted from the MR image intensity values inside bones.
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87.61.Tg Clinical applications
87.50.cm Dosimetry/exposure assessment
87.55.dk Dose-volume analysis
87.55.kh Applications
87.57.nf Reconstruction
87.57.qh Single-slice
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RADIATION THERAPY PHYSICS: The need for application-based adaptation of deformable image registration

Neil Kirby, Cynthia Chuang, Utako Ueda, and Jean Pouliot

Med. Phys. 40, 011702 (2013); http://dx.doi.org/10.1118/1.4769114 (10 pages) | Cited 1 time

Online Publication Date: 12 December 2012

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Purpose: To utilize a deformable phantom to objectively evaluate the accuracy of 11 different deformable image registration (DIR) algorithms.
Methods: The phantom represents an axial plane of the pelvic anatomy. Urethane plastic serves as the bony anatomy and urethane rubber with three levels of Hounsfield units (HU) is used to represent fat and organs, including the prostate. A plastic insert is placed into the phantom to simulate bladder filling. Nonradiopaque markers reside on the phantom surface. Optical camera images of these markers are used to measure the positions and determine the deformation from the bladder insert. Eleven different DIR algorithms are applied to the full and empty-bladder computed tomography images of the phantom (fixed and moving volumes, respectively) to calculate the deformation. The algorithms include those from MIM Software (MIM) and Velocity Medical Solutions (VEL) and nine different implementations from the deformable image registration and adaptive radiotherapy toolbox for Matlab. These algorithms warp one image to make it similar to another, but must utilize a method for regularization to avoid physically unrealistic deformation scenarios. The mean absolute difference (MAD) between the HUs at the marker locations on one image and the calculated location on the other serves as a metric to evaluate the balance between image similarity and regularization. To demonstrate the effect of regularization on registration accuracy, an additional beta version of MIM was created with a variable smoothness factor that controls the emphasis of the algorithm on regularization. The distance to agreement between the measured and calculated marker deformations is used to compare the overall spatial accuracy of the DIR algorithms. This overall spatial accuracy is also utilized to evaluate the phantom geometry and the ability of the phantom soft-tissue heterogeneity to represent patient data. To evaluate the ability of the DIR algorithms to accurately transfer anatomical contours, the rectum is delineated on both the fixed and moving images. A Dice similarity coefficient is then calculated between the contour on the fixed image and that transferred, via the calculated deformation, from the moving to the fixed image.
Results: The phantom possesses sufficient soft-tissue heterogeneity to act as a proxy for patient data. Large discrepancies appear between the algorithms and the measured ground-truth deformation. VEL yields the smallest mean spatial error and a Dice coefficient of 0.90. MIM produces the lowest MAD value and the highest Dice coefficient of 0.96, but creates the largest spatial errors. Increasing the MIM smoothness factor above the default value improves the overall spatial accuracy, but the factor associated with the lowest mean error decreases the Dice coefficient to 0.85.
Conclusions: Different applications of DIR require disparate balances between image similarity and regularization. A DIR algorithm that is optimized only for its ability to transfer anatomical contours will yield large deformation errors in homogeneous regions, which is problematic for dose mapping. For this reason, these algorithms must be tested for their overall spatial accuracy. The developed phantom is an objective tool for this purpose.
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87.57.Q- Computed tomography
87.57.nj Registration
42.30.Va Image forming and processing
87.85.G- Biomechanics
87.53.Bn Dosimetry/exposure assessment

RADIATION THERAPY PHYSICS: Technical Note: Contrast solution density and cross section errors in inhomogeneity-corrected dose calculation for breast balloon brachytherapy

Leonard H. Kim, Miao Zhang, Roger W. Howell, Ning J. Yue, and Atif J. Khan

Med. Phys. 40, 011703 (2013); http://dx.doi.org/10.1118/1.4769420 (3 pages)

Online Publication Date: 12 December 2012

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Purpose: Recent recommendations by the American Association of Physicists in Medicine Task Group 186 emphasize the importance of understanding material properties and their effect on inhomogeneity-corrected dose calculation for brachytherapy. Radiographic contrast is normally injected into breast brachytherapy balloons. In this study, the authors independently estimate properties of contrast solution that were expected to be incorrectly specified in a commercial brachytherapy dose calculation algorithm.
Methods: The mass density and atomic weight fractions of a clinical formulation of radiographic contrast solution were determined using manufacturers’ data. The mass density was verified through measurement and compared with the density obtained by the treatment planning system's CT calibration. The atomic weight fractions were used to determine the photon interaction cross section of the contrast solution for a commercial high-dose-rate (HDR) brachytherapy source and compared with that of muscle.
Results: The density of contrast solution was 10% less than that obtained from the CT calibration. The cross section of the contrast solution for the HDR source was 1.2% greater than that of muscle. Both errors could be addressed by overriding the density of the contrast solution in the treatment planning system.
Conclusions: The authors estimate the error in mass density and cross section parameters used by a commercial brachytherapy dose calculation algorithm for radiographic contrast used in a clinical breast brachytherapy practice. This approach is adaptable to other clinics seeking to evaluate dose calculation errors and determine appropriate density override values if desired.
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87.53.Jw Therapeutic applications, including brachytherapy
87.57.Q- Computed tomography
06.20.fb Standards and calibration
87.53.Bn Dosimetry/exposure assessment

RADIATION THERAPY PHYSICS: External beam pulsed low dose radiotherapy using volumetric modulated arc therapy: Planning and delivery

Neelam Tyagi, Kai Yang, Raminder Sandhu, Di Yan, Sean S. Park, Peter Y. Chen, and Brian Marples

Med. Phys. 40, 011704 (2013); http://dx.doi.org/10.1118/1.4769119 (9 pages)

Online Publication Date: 13 December 2012

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Purpose: To evaluate the feasibility of planning and delivering pulsed low dose radiotherapy (PLRT) using volumetric modulated arc therapy (VMAT) on Elektalinacs.
Methods: Ten patients previously treated for glioblastomamultiforme (GBM) were replanned using PLRT VMAT to deliver ten 0.2 Gy pulses separated by 3 min intervals with an effective dose rate of 0.067 Gy/min. VMAT parameters such as leaf speed and arc length were investigated to deliver 2 Gy/fraction to a total of 60 Gy to the target volume in ten subfractions or pulses. Plan quality was assessed using conformity and homogeneity indices. Absolute dose distribution for individual pulses was measured using ArcCHECK diode array. Individual pulses were analyzed for reproducibility and stability using machine log files. Machine characteristics at low monitor units and low dose rate were also investigated.
Results: An optimal arc length of 140°–160° and a leaf speed of 0.18–0.25 cm/° were sufficient to provide equivalent plan coverage and stable delivery. The average time and dose rate required to deliver a single 0.2 Gy pulse was 39.5 ± 2.3 s and 49 ± 32.3 cGy/min. Average reductions in MUs for the VMAT PLRT plan compared to IMRT for PTV was 16% (Range: −5.5%–36.1%) and 10.9% (Range: −18.4%–32.3%) for the initial and boost plan. A significant improvement was seen in maximum doses to all sensitive structures when planned with VMAT PLRT. The average absolute dose gamma passing rate for the 10 pulses combined and 2 Gy plan were 91.6 ± 2.5% and 97.3 ± 1.2%, respectively. Cumulative monitor units, dose rate, gantry angles, and leaf positions evaluated using machine log files were within 2% for all pulses.
Conclusions: Elekta linacs are capable of delivering reproducible and stable PLRT plans. A prospective clinical study employing PLRT is currently in development.
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87.55.D- Treatment planning
87.55.Qr Quality assurance in radiotherapy
87.19.L- Neuroscience

RADIATION THERAPY PHYSICS: Impact of variable RBE on proton fractionation

Alexandru Dasu and Iuliana Toma-Dasu

Med. Phys. 40, 011705 (2013); http://dx.doi.org/10.1118/1.4769417 (9 pages)

Online Publication Date: 13 December 2012

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Purpose: To explore the impact of variable proton relative biological effectiveness (RBE) on dose fractionation for clinically relevant situations. A generic RBE = 1.1 is generally used for isoeffect calculations, while experimental studies showed that proton RBE varies with tissue type, dose, and linear energy transfer (LET).
Methods: An analytical expression for the LET and α/β dependence of the linear-quadratic (LQ) model has been used for proton simulations in parallel with the assumption of a generic RBE = 1.1. Calculations have been performed for ranges of LET values and fractionation sensitivities to describe clinically relevant cases, such as the treatment of head and neck and prostate tumors. Isoeffect calculations were compared with predictions from a generic RBE value and reported clinical results.
Results: The generic RBE = 1.1 appears to be a reasonable estimate for the proton RBE of rapidly growing tissues irradiated with low LET radiation. However, the use of a variable RBE predicts larger differences for tissues with low α/β (both tumor and normal) and at low doses per fraction. In some situations these differences may appear in contrast to the findings from photon studies highlighting the importance of accurate accounting for the radiobiological effectiveness of protons. Furthermore, the use of variable RBE leads to closer predictions to clinical results.
Conclusions: The LET dependence of the RBE has a strong impact on the predicted effectiveness of fractionated proton radiotherapy. The magnitude of the effect is modulated by the fractionation sensitivity and the fractional dose indicating the need for accurate analyses both in the target and around it. Care should therefore be employed for changing clinical fractionation patterns or when analyzing results from clinical studies for this type of radiation.
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87.53.Jw Therapeutic applications, including brachytherapy
87.53.Bn Dosimetry/exposure assessment

RADIATION THERAPY PHYSICS: Four-dimensional dose evaluation using deformable image registration in radiotherapy for liver cancer

Sang Hoon Jung, Sang Min Yoon, Sung Ho Park, Byungchul Cho, Jae Won Park, Jinhong Jung, Jin-hong Park, Jong Hoon Kim, and Seung Do Ahn

Med. Phys. 40, 011706 (2013); http://dx.doi.org/10.1118/1.4769427 (8 pages)

Online Publication Date: 13 December 2012

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Purpose: In order to evaluate the dosimetric impact of respiratory motion on the dose delivered to the target volume and critical organs during free-breathing radiotherapy, a four-dimensional dose was evaluated using deformable image registration (DIR).
Methods: Four-dimensional computed tomography (4DCT) images were acquired for 11 patients who were treated for liver cancer. Internal target volume-based treatment planning and dose calculation (3D dose) were performed using the end-exhalation phase images. The four-dimensional dose (4D dose) was calculated based on DIR of all phase images from 4DCT to the planned image. Dosimetric parameters from the 4D dose, were calculated and compared with those from the 3D dose.
Results: There was no significant change of the dosimetric parameters for gross tumor volume (p > 0.05). The increase Dmean and generalized equivalent uniform dose (gEUD) for liver were by 3.1% ± 3.3% (p = 0.003) and 2.8% ± 3.3% (p = 0.008), respectively, and for duodenum, they were decreased by 15.7% ± 11.2% (p = 0.003) and 15.1% ± 11.0% (p = 0.003), respectively. The Dmax and gEUD for stomach was decreased by 5.3% ± 5.8% (p = 0.003) and 9.7% ± 8.7% (p = 0.003), respectively. The Dmax and gEUD for right kidney was decreased by 11.2% ± 16.2% (p = 0.003) and 14.9% ± 16.8% (p = 0.005), respectively. For left kidney, Dmax and gEUD were decreased by 11.4% ± 11.0% (p = 0.003) and 12.8% ± 12.1% (p = 0.005), respectively. The NTCP values for duodenum and stomach were decreased by 8.4% ± 5.8% (p = 0.003) and 17.2% ± 13.7% (p = 0.003), respectively.
Conclusions: The four-dimensional dose with a more realistic dose calculation accounting for respiratory motion revealed no significant difference in target coverage and potentially significant change in the physical and biological dosimetric parameters in normal organs during free-breathing treatment.
Show PACS
87.55.dk Dose-volume analysis
87.57.nj Registration
87.57.Q- Computed tomography
87.53.Bn Dosimetry/exposure assessment
87.19.Wx Pneumodyamics, respiration
87.19.xj Cancer
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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

© 2013, 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.

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