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Feb 2013

Volume 40, Issue 2, pp. 020401-029901–10

Spotlight Figure

Med. Phys. 40, 022301 (2013); http://dx.doi.org/10.1118/1.4773035 (10 pages)

Min-Ji Kim, Geon-Ho Jahng, Soo-Yeol Lee, and Chang-Woo Ryu
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EDITORIAL: A few changes in Medical Physics

William Hendee, Editor

Med. Phys. 40, 020401 (2013); http://dx.doi.org/10.1118/1.4788656 (1 page)

Online Publication Date: 22 January 2013

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Abstract Unavailable
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01.30.Xx Publications in electronic media
87.00.00 Biological and medical physics
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POINT/COUNTERPOINT: DASSIM-RT is likely to become the method of choice over conventional IMRT and VMAT for delivery of highly conformal radiotherapy

Lei Xing, Ph.D., Mark H. Phillips, Ph.D., and Colin G. Orton, Ph.D., Moderator

Med. Phys. 40, 020601 (2013); http://dx.doi.org/10.1118/1.4773025 (3 pages) | Cited 1 time

Online Publication Date: 7 January 2013

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87.55.dk Dose-volume analysis

MEDICAL PHYSICS LETTER: Development of a graphite probe calorimeter for absolute clinical dosimetry

James Renaud, David Marchington, Jan Seuntjens, and Arman Sarfehnia

Med. Phys. 40, 020701 (2013); http://dx.doi.org/10.1118/1.4773870 (6 pages)

Online Publication Date: 9 January 2013

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The aim of this work is to present the numerical design optimization, construction, and experimental proof of concept of a graphite probe calorimeter (GPC) conceived for dose measurement in the clinical environment (U.S. provisional patent 61/652,540). A finite element method (FEM) based numerical heat transfer study was conducted using a commercial software package to explore the feasibility of the GPC and to optimize the shape, dimensions, and materials used in its design. A functioning prototype was constructed inhouse and used to perform dose to water measurements under a 6 MV photon beam at 400 and 1000 MU/min, in a thermally insulated water phantom. Heat loss correction factors were determined using FEM analysis while the radiation field perturbation and the graphite to water absorbed dose conversion factors were calculated using Monte Carlo simulations. The difference in the average measured dose to water for the 400 and 1000 MU/min runs using the TG-51 protocol and the GPC was 0.2% and 1.2%, respectively. Heat loss correction factors ranged from 1.001 to 1.002, while the product of the perturbation and dose conversion factors was calculated to be 1.130. The combined relative uncertainty was estimated to be 1.4%, with the largest contributors being the specific heat capacity of the graphite (type B, 0.8%) and the reproducibility, defined as the standard deviation of the mean measured dose (type A, 0.6%). By establishing the feasibility of using the GPC as a practical clinical absolute photon dosimeter, this work lays the foundation for further device enhancements, including the development of an isothermal mode of operation and an overall miniaturization, making it potentially suitable for use in small and composite radiation fields. It is anticipated that, through the incorporation of isothermal stabilization provided by temperature controllers, a subpercent overall uncertainty will be achieved.
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87.53.Bn Dosimetry/exposure assessment
87.10.Kn Finite element calculations
87.10.Rt Monte Carlo simulations
07.20.Fw Calorimeters

MEDICAL PHYSICS LETTER: Multifractal analysis of laser Doppler flowmetry signals before and after arm-cranking exercise in an older healthy population

Markos Klonizakis and Anne Humeau-Heurtier

Med. Phys. 40, 020702 (2013); http://dx.doi.org/10.1118/1.4774362 (5 pages)

Online Publication Date: 11 January 2013

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Purpose: There is a lot of speculation about the role of nitric-oxide (NO) in the improvement usually noticed in microcirculatory function, following exercise. The knowledge of the underlying mechanisms leading to such an improvement is important as it may help in targeting and implementing therapies for microcirculatory diseases. Through a laser Doppler flowmetry (LDF) signal processing study, the authors’ goal is to compare multifractal spectra of LDF data recorded in both lower leg and forearm, during different exercise conditions, in an older, untrained but healthy population.
Methods: Using the method suggested by Halsey et al. [Phys. Rev. A 33, 1141–1151 (1986)10.1103/PhysRevA.33.1141], multifractal spectra of LDF signals recorded on lower leg and forearm before and after exercise (arm-cranking), before and after acetylcholine (ACh) iontophoresis, were determined on scales in relation with the NO-dependent endothelial activity. The width of each multifractal spectrum was then computed through the maximum and minimum Hölder exponent values for which the multifractal spectrum reaches its minimal values. The results were then compared.
Results: Following exercise and on the scales studied, the average width of the multifractal spectra in both lower leg and forearm does not vary significantly before and after ACh iontophoresis. Similarly, following ACh iontophoresis and exercise, the average width of multifractal spectra remains statistically unchanged, when compared to that measured prior to exercise, in both upper and lower body, although negative trends can be observed.
Conclusions: For the authors’ population and for the type of exercise that the authors have chosen, the authors showed that the width of the multifractal spectra of LDF signals does not change significantly on scales in relation with the NO-dependent endothelial activity. Future studies may involve comparisons with signals obtained in patient populations.
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87.85.Ng Biological signal processing
02.50.-r Probability theory, stochastic processes, and statistics
87.53.Jw Therapeutic applications, including brachytherapy
87.85.gf Fluid mechanics and rheology
87.85.gj Movement and locomotion
87.85.J- Biomaterials

RADIATION THERAPY PHYSICS: Interfractional dose variations in the stomach and the bowels during breathhold intensity-modulated radiotherapy for pancreatic cancer: Implications for a dose-escalation strategy

Akira Nakamura, Keiko Shibuya, Mitsuhiro Nakamura, Yukinori Matsuo, Takehiro Shiinoki, Manabu Nakata, Takashi Mizowaki, and Masahiro Hiraoka

Med. Phys. 40, 021701 (2013); http://dx.doi.org/10.1118/1.4773033 (9 pages)

Online Publication Date: 7 January 2013

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Purpose: This study aims to evaluate the interfractional dose variations in the organs-at-risk (OARs) during pancreatic breathhold intensity-modulated radiotherapy (IMRT) and to assess the impacts of “planning organs-at-risk volume” (POV) structures generated by isotropically expanding the dose-limiting OARs, based on the comparison of the interfractional doses to the OARs between IMRT plans and conventional three-dimensional-conformal radiotherapy (3D-CRT) plans.
Methods: Thirty repeat CT scans were acquired from ten consecutive patients who were receiving chemoradiotherapy for pancreatic cancer. Six IMRT plans for each patient with two levels of prescription (45 and 51 Gy in 15 fractions) and 3 POV margin sizes (5, 7, and 10 mm) were generated based on the initial CT scan under predetermined constraints. Two 3D-CRT plans (39 and 42 Gy in 15 fractions) were simultaneously generated. The dose distribution of all of the treatment plans was recalculated with the repeat CT scans. The interfractional dose variations in the three OARs (stomach, duodenum, and small intestine) were evaluated, and the absolute volumes ≥39 Gy (V39Gy) of the OARs in the IMRT plans were compared to those in the 3D-CRT plans. Regression analyses were performed to assess the relative impact of the factors of interest on the interfractional dose variations of the OARs.
Results: Substantial dose excesses to the three OARs were observed at all of the prescription dose levels and the POV margin sizes on the repeat CT scans. The safety threshold based on the mean stomach V39Gy on the recalculated 39 Gy-3D-CRT plans was 1.9 ml. Statistically significant and marginally insignificant mean V39Gy values above the safety thresholds were observed in the stomach in the 51 Gy-IMRT plans (2.6 and 2.1 ml with the 5- and 7-mm PRV margins, respectively (P = 0.015 and 0.085)). Only in the case of the 10-mm POV margin did the metric fall below the safety threshold to 1.5 ml (P = 0.634). The duodenum and the small intestine did not violate the safety thresholds (1.4 and 3.8 ml, respectively). From the multiple regression analyses, only the margin size (P < 0.001) and the POV V39Gy (P < 0.001) were significantly associated with the distribution of recalculated V39Gy for the stomach. Multiple factors, including the margin size (P = 0.020) and the POV V39Gy (P < 0.001) were associated with the recalculated V39Gy for the duodenum. However, none of the POV parameters for the small intestine were associated with the recalculated V39Gy.
Conclusions: Considerable interfractional dose variation was observed in three critical OARs. At the escalated prescription dose of breathhold IMRT, the dose variations could exceed the dose variations using 3D-CRT at the safe prescription dose level, indicating that a dose-escalation strategy based solely on the initial advantageous dose distribution in a breathhold IMRT can be problematic. Given the current limitations for predicting or coping with variation throughout the treatment course, the use of POV should be considered for safely delivering escalated doses to patients with pancreatic cancer.
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87.55.dk Dose-volume analysis
87.57.Q- Computed tomography
02.50.-r Probability theory, stochastic processes, and statistics
87.19.Wx Pneumodyamics, respiration

RADIATION THERAPY PHYSICS: Control over structure-specific flexibility improves anatomical accuracy for point-based deformable registration in bladder cancer radiotherapy

S. Wognum, L. Bondar, A. G. Zolnay, X. Chai, M. C. C. M. Hulshof, M. S. Hoogeman, and A. Bel

Med. Phys. 40, 021702 (2013); http://dx.doi.org/10.1118/1.4773040 (15 pages)

Online Publication Date: 7 January 2013

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Purpose: Future developments in image guided adaptive radiotherapy (IGART) for bladder cancer require accurate deformable image registration techniques for the precise assessment of tumor and bladder motion and deformation that occur as a result of large bladder volume changes during the course of radiotherapy treatment. The aim was to employ an extended version of a point-based deformable registration algorithm that allows control over tissue-specific flexibility in combination with the authors’ unique patient dataset, in order to overcome two major challenges of bladder cancer registration, i.e., the difficulty in accounting for the difference in flexibility between the bladder wall and tumor and the lack of visible anatomical landmarks for validation.
Methods: The registration algorithm used in the current study is an extension of the symmetric-thin plate splines-robust point matching (S-TPS-RPM) algorithm, a symmetric feature-based registration method. The S-TPS-RPM algorithm has been previously extended to allow control over the degree of flexibility of different structures via a weight parameter. The extended weighted S-TPS-RPM algorithm was tested and validated on CT data (planning- and four to five repeat-CTs) of five urinary bladder cancer patients who received lipiodol injections before radiotherapy. The performance of the weighted S-TPS-RPM method, applied to bladder and tumor structures simultaneously, was compared with a previous version of the S-TPS-RPM algorithm applied to bladder wall structure alone and with a simultaneous nonweighted S-TPS-RPM registration of the bladder and tumor structures. Performance was assessed in terms of anatomical and geometric accuracy. The anatomical accuracy was calculated as the residual distance error (RDE) of the lipiodol markers and the geometric accuracy was determined by the surface distance, surface coverage, and inverse consistency errors. Optimal parameter values for the flexibility and bladder weight parameters were determined for the weighted S-TPS-RPM.
Results: The weighted S-TPS-RPM registration algorithm with optimal parameters significantly improved the anatomical accuracy as compared to S-TPS-RPM registration of the bladder alone and reduced the range of the anatomical errors by half as compared with the simultaneous nonweighted S-TPS-RPM registration of the bladder and tumor structures. The weighted algorithm reduced the RDE range of lipiodol markers from 0.9–14 mm after rigid bone match to 0.9–4.0 mm, compared to a range of 1.1–9.1 mm with S-TPS-RPM of bladder alone and 0.9–9.4 mm for simultaneous nonweighted registration. All registration methods resulted in good geometric accuracy on the bladder; average error values were all below 1.2 mm.
Conclusions: The weighted S-TPS-RPM registration algorithm with additional weight parameter allowed indirect control over structure-specific flexibility in multistructure registrations of bladder and bladder tumor, enabling anatomically coherent registrations. The availability of an anatomically validated deformable registration method opens up the horizon for improvements in IGART for bladder cancer.
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87.57.Q- Computed tomography
87.53.Jw Therapeutic applications, including brachytherapy
87.57.nj Registration

RADIATION THERAPY PHYSICS: Use of treatment log files in spot scanning proton therapy as part of patient-specific quality assurance

Heng Li, Narayan Sahoo, Falk Poenisch, Kazumichi Suzuki, Yupeng Li, Xiaoqiang Li, Xiaodong Zhang, Andrew K. Lee, Michael T. Gillin, and X. Ronald Zhu

Med. Phys. 40, 021703 (2013); http://dx.doi.org/10.1118/1.4773312 (11 pages)

Online Publication Date: 7 January 2013

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Purpose: The purpose of this work was to assess the monitor unit (MU) values and position accuracy of spot scanning proton beams as recorded by the daily treatment logs of the treatment control system, and furthermore establish the feasibility of using the delivered spot positions and MU values to calculate and evaluate delivered doses to patients.
Methods: To validate the accuracy of the recorded spot positions, the authors generated and executed a test treatment plan containing nine spot positions, to which the authors delivered ten MU each. The spot positions were measured with radiographic films and Matrixx 2D ion-chambers array placed at the isocenter plane and compared for displacements from the planned and recorded positions. Treatment logs for 14 patients were then used to determine the spot MU values and position accuracy of the scanning proton beam delivery system. Univariate analysis was used to detect any systematic error or large variation between patients, treatment dates, proton energies, gantry angles, and planned spot positions. The recorded patient spot positions and MU values were then used to replace the spot positions and MU values in the plan, and the treatment planning system was used to calculate the delivered doses to patients. The results were compared with the treatment plan.
Results: Within a treatment session, spot positions were reproducible within ±0.2 mm. The spot positions measured by film agreed with the planned positions within ±1 mm and with the recorded positions within ±0.5 mm. The maximum day-to-day variation for any given spot position was within ±1 mm. For all 14 patients, with ∼1 500 000 spots recorded, the total MU accuracy was within 0.1% of the planned MU values, the mean (x, y) spot displacement from the planned value was (−0.03 mm, −0.01 mm), the maximum (x, y) displacement was (1.68 mm, 2.27 mm), and the (x, y) standard deviation was (0.26 mm, 0.42 mm). The maximum dose difference between calculated dose to the patient based on the plan and recorded data was within 2%.
Conclusions: The authors have shown that the treatment log file in a spot scanning proton beam delivery system is precise enough to serve as a quality assurance tool to monitor variation in spot position and MU value, as well as the delivered dose uncertainty from the treatment delivery system. The analysis tool developed here could be useful for assessing spot position uncertainty and thus dose uncertainty for any patient receiving spot scanning proton beam therapy.
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87.55.Qr Quality assurance in radiotherapy
87.59.B- Radiography
06.20.Dk Measurement and error theory
87.53.Bn Dosimetry/exposure assessment
87.53.Jw Therapeutic applications, including brachytherapy
87.55.dk Dose-volume analysis

RADIATION THERAPY PHYSICS: Accuracy of volume measurement using 3D ultrasound and development of CT-3D US image fusion algorithm for prostate cancer radiotherapy

Jihye Baek, Jangyoung Huh, Myungsoo Kim, So Hyun An, Yoonjin Oh, DongYoung Kim, Kwangzoo Chung, Sungho Cho, and Rena Lee

Med. Phys. 40, 021704 (2013); http://dx.doi.org/10.1118/1.4767753 (13 pages)

Online Publication Date: 8 January 2013

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Purpose: To evaluate the accuracy of measuring volumes using three-dimensional ultrasound (3D US), and to verify the feasibility of the replacement of CT-MR fusion images with CT-3D US in radiotherapy treatment planning.
Methods: Phantoms, consisting of water, contrast agent, and agarose, were manufactured. The volume was measured using 3D US, CT, and MR devices. A CT-3D US and MR-3D US image fusion software was developed using the Insight Toolkit library in order to acquire three-dimensional fusion images. The quality of the image fusion was evaluated using metric value and fusion images.
Results: Volume measurement, using 3D US, shows a 2.8 ± 1.5% error, 4.4 ± 3.0% error for CT, and 3.1 ± 2.0% error for MR. The results imply that volume measurement using the 3D US devices has a similar accuracy level to that of CT and MR. Three-dimensional image fusion of CT-3D US and MR-3D US was successfully performed using phantom images. Moreover, MR-3D US image fusion was performed using human bladder images.
Conclusions: 3D US could be used in the volume measurement of human bladders and prostates. CT-3D US image fusion could be used in monitoring the target position in each fraction of external beam radiation therapy. Moreover, the feasibility of replacing the CT-MR image fusion to the CT-3D US in radiotherapy treatment planning was verified.
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87.63.D- Ultrasonography
06.30.Bp Spatial dimensions (e.g., position, lengths, volume, angles, and displacements)
43.80.Qf Medical diagnosis with acoustics
87.61.-c Magnetic resonance imaging
87.55.D- Treatment planning
87.57.Q- Computed tomography

RADIATION THERAPY PHYSICS: Monte Carlo simulation based study of a proposed multileaf collimator for a telecobalt machine

G. Sahani, Sunil Dutt Sharma, P. K. Dash Sharma, D. N. Sharma, and S. A. Hussain

Med. Phys. 40, 021705 (2013); http://dx.doi.org/10.1118/1.4773308 (11 pages)

Online Publication Date: 9 January 2013

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Purpose: The objective of the present work was to propose a design of a secondary multileaf collimator (MLC) for a telecobalt machine and optimize its design features through Monte Carlo simulation.
Methods: The proposed MLC design consists of 72 leaves (36 leaf pairs) with additional jaws perpendicular to leaf motion having the capability of shaping a maximum square field size of 35 × 35 cm2. The projected widths at isocenter of each of the central 34 leaf pairs and 2 peripheral leaf pairs are 10 and 5 mm, respectively. The ends of the leaves and the x-jaws were optimized to obtain acceptable values of dosimetric and leakage parameters. Monte Carlo N-Particle code was used for generating beam profiles and depth dose curves and estimating the leakage radiation through the MLC. A water phantom of dimension 50 × 50 × 40 cm3 with an array of voxels (4 × 0.3 × 0.6 cm3 = 0.72 cm3) was used for the study of dosimetric and leakage characteristics of the MLC. Output files generated for beam profiles were exported to the PTW radiation field analyzer software through locally developed software for analysis of beam profiles in order to evaluate radiation field width, beam flatness, symmetry, and beam penumbra.
Results: The optimized version of the MLC can define radiation fields of up to 35 × 35 cm2 within the prescribed tolerance values of 2 mm. The flatness and symmetry were found to be well within the acceptable tolerance value of 3%. The penumbra for a 10 × 10 cm2 field size is 10.7 mm which is less than the generally acceptable value of 12 mm for a telecobalt machine. The maximum and average radiation leakage through the MLC were found to be 0.74% and 0.41% which are well below the International Electrotechnical Commission recommended tolerance values of 2% and 0.75%, respectively. The maximum leakage through the leaf ends in closed condition was observed to be 8.6% which is less than the values reported for other MLCs designed for medical linear accelerators.
Conclusions: It is concluded that dosimetric parameters and the leakage radiation of the optimized secondary MLC design are well below their recommended tolerance values. The optimized design of the proposed MLC can be integrated into a telecobalt machine by replacing the existing adjustable secondary collimator for conformal radiotherapy treatment of cancer patients.
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87.56.nk Collimators
87.10.Rt Monte Carlo simulations
87.19.xj Cancer
87.55.de Optimization
87.55.Gh Simulation
87.55.kh Applications

RADIATION THERAPY PHYSICS: A method for accurate zero calibration of asymmetric jaws in single-isocenter half-beam techniques

V. Hernandez, J. Sempau, R. Abella, M. Lopez, M. Perez, M. Artigues, and M. Arenas

Med. Phys. 40, 021706 (2013); http://dx.doi.org/10.1118/1.4773314 (6 pages)

Online Publication Date: 10 January 2013

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Purpose: To present a practical method for calibrating the zero position of asymmetric jaws that provides higher accuracy at the central axis and improves dose homogeneity in the abutting region of half-beams.
Methods: Junction doses were measured for each asymmetric jaw using the double-exposure technique and electronic portal imaging devices. The junction dose was determined as a function of jaw position. The shift in the zero jaw position (or in its corresponding potentiometer readout) required to correct for the measured junction dose could thus be obtained. The jaw calibration was then modified to introduce the calculated shift and therefore achieve an accurate zero position in order to provide a relative junction dose that was as close to zero as possible.
Results: All the asymmetric jaws from four medical linear accelerators were calibrated with the new calibration procedure. Measured relative junction doses at gantry 0° were reduced from a maximum of ±40% to a maximum of ±8% for all the jaws in the four considered accelerators. These results were valid for 6 MV and 18 MV photon beams and for any combination of asymmetric jaws set to zero. The calibration was stable over a long period of time; therefore, the need for recalibrating is seldom necessary.
Conclusions: Accurate calibration of the zero position of the jaws is feasible in current medical linear accelerators. The proposed procedure is fast and it improves dose homogeneity at the junction of half-beams, thus, allowing a more accurate and safer use of these techniques.
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87.55.dk Dose-volume analysis
06.20.fb Standards and calibration
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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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