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Jul 2010

Volume 37, Issue 7, pp. 3497-3911

Spotlight Figure

Med. Phys. 37, 3715 (2010); doi:10.1118/1.3455702 (10 pages)

Jia Wang, Shudong Jiang, Zhongze Li, Roberta M. diFlorio-Alexander, Richard J. Barth, Peter A. Kaufman, Brian W. Pogue, and Keith D. Paulsen
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POINT/COUNTERPOINT: The use of effective dose for medical procedures is inappropriate

Caridad Borrás, D.Sc., Walter Huda, Ph.D., and Colin G. Orton, Ph.D., Moderator

Med. Phys. 37, 3497 (2010); doi:10.1118/1.3377778 (4 pages) | Cited 4 times

Online Publication Date: 15 June 2010

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Abstract Unavailable
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87.57.uq Dosimetry
87.59.B- Radiography

RADIATION THERAPY PHYSICS: Wide field array calibration dependence on the stability of measured dose distributions

Thomas A. Simon, William E. Simon, Darren Kahler, Jonathan Li, and Chihray Liu

Med. Phys. 37, 3501 (2010); doi:10.1118/1.3442028 (9 pages) | Cited 2 times

Online Publication Date: 15 June 2010

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Purpose: The aim of this work was to simulate the effect of dose distribution changes on detector array calibrations and to explore compensatory methods that are used during calibration measurements.
Methods: The array calibration technique that was investigated is known as wide field (WF) calibration. Using this method, a linear array [y-axis (65 detectors) of the IC PROFILER™ (Sun Nuclear Corporation, Melbourne, FL)] is calibrated with three measurements (α, θ, and λ); each measurement uses the same radiation field, which is larger than the array. For measurement configuration θ, the array is rotated by 180° from its position in α; for λ, the array is shifted by one detector from its position in θ. The relative detector sensitivities are then determined through ratios of detector readings at the same field locations (using θ and λ). This method results in error propagation that is proportional to the number of detectors in the array. During the procedure, the calibration protocol operates under three postulates, which state that (a) the beam shape does not change between measurements; (b) the relative sensitivities of the detectors do not change; and (c) the scatter to the array does not change as the array is moved. The WF calibration’s sensitivity to a postulate (a) violation was quantified by applying a sine shaped perturbation (of up to 0.1%) to α, θ, or λ, and then determining the change relative to a baseline calibration. Postulate (a) violations were minimized by using a continuous beam and mechanized array movement during θ and λ. A continuously on beam demonstrated more stable beam symmetry as compared to cycling the beam on and off between measurements. Additional side-scatter was also used to satisfy postulate (c).
Results: Simulated symmetry perturbations of 0.1% to θ or λ resulted in calibration errors of up to 2%; α was relatively immune to perturbation (<0.1% error). Wide field calibration error on a linear accelerator with similar symmetry variations was ±1.6%. Using a continuous beam during θ and λ with additional side-scatter reduced the calibration error from ±1.6% to ±0.48%.
Conclusions: This work increased the reproducibility of WF calibrations by limiting the effect of measurement perturbations primarily due to linear accelerator symmetry variations. The same technique would work for any array using WF calibration.
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87.55.dk Dose-volume analysis
87.53.Jw Therapeutic applications, including brachytherapy

RADIATION THERAPY PHYSICS: Total skin electron therapy (TSET): A reimplementation using radiochromic films and IAEA TRS-398 code of practice

P. Schiapparelli, D. Zefiro, F. Massone, and G. Taccini

Med. Phys. 37, 3510 (2010); doi:10.1118/1.3442301 (8 pages)

Online Publication Date: 15 June 2010

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Purpose: The aim of this work is to present an updated implementation of total skin electron therapy (TSET) using IAEA TRS-398 code of practice for absolute dosimetry and taking advantage of the use of radiochromic films. The optimization of quality control tests is also included.
Methods: A Varian 2100 C/D linear accelerator equipped with the special procedure HDTSe− (high dose rate total skin electron mode, E = 6 MeV) was employed to perform TSET irradiations using the modified Stanford technique. The commissioning was performed following the AAPM report 23 recommendations. In particular, for dual-field beams irradiation, the optimal tilt angle was investigated and the dose distribution in the treatment plane was measured. For a complete six dual-field beams irradiation, the treatment skin dose on the surface of a cylindrical phantom was evaluated by radiochromic films and the B factor which relates the single dual-field skin dose to the six dual-field skin dose was assessed. Since the TRS-398 reference conditions do not meet the requirements of TSET absolute dosimetry, GafChromic EBT films were also employed to check and validate the application of the protocol. Simplified procedures were studied to verify beam constancy in PMMA phantoms without the more difficult setup of total skin irradiation.
Results: The optimized geometrical setup for dual-field beams was: Tilt angle = ±19°, SSD = 353 cm, and the beam degrader (200×100×1 cm3) placed at 320 cm from the source. As regards to dose homogeneity in the treatment plane, for dual-field beams irradiation, the mean relative dose value was 97%±5% (normalizing to 100% at the calibration point level). For six dual-field beams irradiation, the multiplication factor B was 2.63. In addition, beam quality, dose rate, and bremsstrahlung contribution were also suitable for TSET treatments. The TRS-398 code of practice was used for TSET dosimetry, as dose measurements performed by ionization chamber and radiochromic film agreed within 2.5%. Simplified quality control tests and baseline values were presented in order to check flatness, symmetry, and field size with radiochromic films and output and beam quality constancy with ionization chamber. Short-term reproducibility and MU linearity tests were also included.
Conclusions: Commissioning parameters met the requirements of TSET treatments and the matching of AAPM guidelines with the IAEA code of practice was successful. Frequent beam performance controls can be easily performed through the presented quality assurance tests. Radiochromic dosimetry facilitated the TSET commissioning and played a major role to validate the application of TRS-398.
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87.53.Jw Therapeutic applications, including brachytherapy
87.53.Bn Dosimetry/exposure assessment
87.55.N- Radiation monitoring, control, and safety

TISSUE MEASUREMENTS: Contrast detection in fluid-saturated media with magnetic resonance poroelastography

Phillip R. Perriñez, Adam J. Pattison, Francis E. Kennedy, John B. Weaver, and Keith D. Paulsen

Med. Phys. 37, 3518 (2010); doi:10.1118/1.3443563 (9 pages) | Cited 1 time

Online Publication Date: 15 June 2010

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Purpose: Recent interest in the poroelastic behavior of tissues has led to the development of magnetic resonance poroelastography (MRPE) as an alternative to single-phase MR elastographic image reconstruction. In addition to the elastic parameters (i.e., Lamé’s constants) commonly associated with magnetic resonance elastography (MRE), MRPE enables estimation of the time-harmonic pore-pressure field induced by external mechanical vibration.
Methods: This study presents numerical simulations that demonstrate the sensitivity of the computed displacement and pore-pressure fields to a priori estimates of the experimentally derived model parameters. In addition, experimental data collected in three poroelastic phantoms are used to assess the quantitative accuracy of MR poroelastographic imaging through comparisons with both quasistatic and dynamic mechanical tests.
Results: The results indicate hydraulic conductivity to be the dominant parameter influencing the deformation behavior of poroelastic media under conditions applied during MRE. MRPE estimation of the matrix shear modulus was bracketed by the values determined from independent quasistatic and dynamic mechanical measurements as expected, whereas the contrast ratios for embedded inclusions were quantitatively similar (10%–15% difference between the reconstructed images and the mechanical tests).
Conclusions: The findings suggest that the addition of hydraulic conductivity and a viscoelastic solid component as parameters in the reconstruction may be warranted.
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87.61.-c Magnetic resonance imaging
87.19.rd Elastic properties

RADIATION IMAGING PHYSICS: Noise variance analysis using a flat panel x-ray detector: A method for additive noise assessment with application to breast CT applications

Kai Yang, Shih-Ying Huang, Nathan J. Packard, and John M. Boone

Med. Phys. 37, 3527 (2010); doi:10.1118/1.3447720 (11 pages) | Cited 1 time

Online Publication Date: 15 June 2010

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Purpose: A simplified linear model approach was proposed to accurately model the response of a flat panel detector used for breast CT (bCT).
Methods: Individual detector pixel mean and variance were measured from bCT projection images acquired both in air and with a polyethylene cylinder, with the detector operating in both fixed low gain and dynamic gain mode. Once the coefficients of the linear model are determined, the fractional additive noise can be used as a quantitative metric to evaluate the system’s efficiency in utilizing x-ray photons, including the performance of different gain modes of the detector.
Results: Fractional additive noise increases as the object thickness increases or as the radiation dose to the detector decreases. For bCT scan techniques on the UC Davis prototype scanner (80 kVp, 500 views total, 30 frames/s), in the low gain mode, additive noise contributes 21% of the total pixel noise variance for a 10 cm object and 44% for a 17 cm object. With the dynamic gain mode, additive noise only represents approximately 2.6% of the total pixel noise variance for a 10 cm object and 7.3% for a 17 cm object.
Conclusions: The existence of the signal-independent additive noise is the primary cause for a quadratic relationship between bCT noise variance and the inverse of radiation dose at the detector. With the knowledge of the additive noise contribution to experimentally acquired images, system modifications can be made to reduce the impact of additive noise and improve the quantum noise efficiency of the bCT system.
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87.59.E- Mammography
87.57.rh Mammography

MEDICAL PHYSICS LETTERS: Efficient generation of a magnetic field-free line

Tobias Knopp, Marlitt Erbe, Sven Biederer, Timo F. Sattel, and Thorsten M. Buzug

Med. Phys. 37, 3538 (2010); doi:10.1118/1.3447726 (3 pages) | Cited 5 times

Online Publication Date: 15 June 2010

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Purpose: Signal encoding in magnetic particle imaging (MPI) is achieved by moving a field-free point (FFP) through the region of interest. One way to increase the sensitivity of the method is to scan the region of interest with a field-free line (FFL) instead of the FFP. Recently, the first feasible FFL coil setup was introduced. The purpose of this article is to improve the efficiency of the FFL coil geometry even further.
Methods: In order to reduce the electrical power loss of the setup, an additional Maxwell coil pair is introduced that is tailored to generate the static part of the FFL field.
Results: Using the proposed coil assembly, the electrical power loss for the generation of a rotating FFL is considerably reduced compared to previously known coil setups. Furthermore, the quality of the generated FFL is significantly increased.
Conclusions: The proposed coil assembly is almost as efficient as an equivalent FFP scanner. Furthermore, the assembly cannot only be used for FFL imaging but for FFP imaging as well. Hence, the findings of this article denote an important step toward the first practical implementation of the FFL coil geometry.
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87.85.Pq Biomedical imaging
87.50.C- Static and low-frequency electric and magnetic fields effects
87.85.Rs Nanotechnologies-applications

RADIATION MEASUREMENT PHYSICS: Direct absorbed dose to water determination based on water calorimetry in scanning proton beam delivery

A. Sarfehnia, B. Clasie, E. Chung, H. M. Lu, J. Flanz, E. Cascio, M. Engelsman, H. Paganetti, and J. Seuntjens

Med. Phys. 37, 3541 (2010); doi:10.1118/1.3427317 (10 pages) | Cited 1 time

Online Publication Date: 15 June 2010

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Purpose: The aim of this manuscript is to describe the direct measurement of absolute absorbed dose to water in a scanned proton radiotherapy beam using a water calorimeter primary standard.
Methods: The McGill water calorimeter, which has been validated in photon and electron beams as well as in HDR 192Ir brachytherapy, was used to measure the absorbed dose to water in double scattering and scanning proton irradiations. The measurements were made at the Massachusetts General Hospital proton radiotherapy facility. The correction factors in water calorimetry were numerically calculated and various parameters affecting their magnitude and uncertainty were studied. The absorbed dose to water was compared to that obtained using an Exradin T1 Chamber based on the IAEA TRS-398 protocol.
Results: The overall 1-sigma uncertainty on absorbed dose to water amounts to 0.4% and 0.6% in scattered and scanned proton water calorimetry, respectively. This compares to an overall uncertainty of 1.9% for currently accepted IAEA TRS-398 reference absorbed dose measurement protocol. The absorbed dose from water calorimetry agrees with the results from TRS-398 well to within 1-sigma uncertainty.
Conclusions: This work demonstrates that a primary absorbed dose standard based on water calorimetry is feasible in scattered and scanned proton beams.
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87.55.dk Dose-volume analysis
87.53.Jw Therapeutic applications, including brachytherapy
87.53.Bn Dosimetry/exposure assessment

RADIATION BIOLOGY: Evaluating 99mTc Auger electrons for targeted tumor radiotherapy by computational methods

Adriana Alexandre S. Tavares and João Manuel R. S. Tavares

Med. Phys. 37, 3551 (2010); doi:10.1118/1.3451117 (9 pages)

Online Publication Date: 15 June 2010

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Purpose: Technetium-99m (99mTc) has been widely used as an imaging agent but only recently has been considered for therapeutic applications. This study aims to analyze the potential use of 99mTc Auger electrons for targeted tumor radiotherapy by evaluating the DNA damage and its probability of correct repair and by studying the cellular kinetics, following 99mTc Auger electron irradiation in comparison to iodine-131 (131I) beta minus particles and astatine-211 (211At) alpha particle irradiation.
Methods: Computational models were used to estimate the yield of DNA damage (fast Monte Carlo damage algorithm), the probability of correct repair (Monte Carlo excision repair algorithm), and cell kinetic effects (virtual cell radiobiology algorithm) after irradiation with the selected particles.
Results: The results obtained with the algorithms used suggested that 99mTc CKMMX (all M-shell Coster–Kroning—CK—and super-CK transitions) electrons and Auger MXY (all M-shell Auger transitions) have a therapeutic potential comparable to high linear energy transfer 211At alpha particles and higher than 131I beta minus particles. All the other 99mTc electrons had a therapeutic potential similar to 131I beta minus particles.
Conclusions: 99mTc CKMMX electrons and Auger MXY presented a higher probability to induce apoptosis than 131I beta minus particles and a probability similar to 211At alpha particles. Based on the results here, 99mTc CKMMX electrons and Auger MXY are useful electrons for targeted tumor radiotherapy.
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87.53.Jw Therapeutic applications, including brachytherapy
87.55.K- Monte Carlo methods
87.14.gk DNA

RADIATION MEASUREMENT PHYSICS: Determination of multislice computed tomography dose index (CTDI) using optically stimulated luminescence technology

Chun Ruan, Eduardo G. Yukihara, William J. Clouse, Patricia B. R. Gasparian, and Salahuddin Ahmad

Med. Phys. 37, 3560 (2010); doi:10.1118/1.3455285 (9 pages) | Cited 5 times

Online Publication Date: 15 June 2010

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Purpose: The extensive use of multislice computed tomography (MSCT) and the associated increase in patient dose calls for an accurate dose evaluation technique. Optically stimulated luminescence (OSL) dosimetry provides a potential solution to the arising concerns over patient dose. This study was intended to evaluate the feasibility and accuracy of OSL dosimeter systems in the diagnostic CT x-ray beam energy range.
Methods: MSCT dose profiles were measured by irradiating OSL strips placed inside the extended PMMA head and body phantoms at different scan conditions by varying kVp settings (100, 120, and 140 kVp) and collimated beam widths (5, 10, 20, and 40 mm). All scans in this study were performed using a GE Lightspeed VCT scanner in axial mode. The exposed strips were then read out using a custom-made OSL strip reader and corrected with field-specific conversion factors. Based on the corrected OSL dose profile, the CTDI450-OSL and CTDI100-OSL were evaluated. CTDI100-IC was also obtained using a 100 mm long pencil ionization chamber for accuracy verification. CTDI100-efficiency can be further evaluated by calculating the ratio of CTDI100-OSL and CTDI450-OSL, which was compared to results from previous studies as well.
Results: The OSL detectors were found to have good sensitivity and dose response over a wide range of diagnostic CT x-ray beam energy viz. the primary beam and the scatter tail section of the dose profile. The differences between CTDI100 values obtained using the OSL strips and those obtained with 100 mm long pencil ionization chamber were <±5% for all scan conditions, indicating good accuracy of the OSL system. It was also found that the CTDI100-efficiency did not significantly change as the beam width increased and tube voltage changed. The average CTDI100-efficiency at the center of the head and body phantoms were 72.6% and 56.2%, respectively. The corresponding values for the periphery of the head and body phantoms were 85.0% and 81.7%. These results agreed very well with previous results from the literature using other detection techniques or Monte Carlo simulations.
Conclusions: The LED-based OSL system can be an accurate alternative device for CT dose evaluations. CTDI100 measurement with the use of a 100 mm pencil ionization chamber substantially underestimates the CTDI∞ value even with 5 mm collimated beam width. The established complete set of CTDI100-efficiency correction factors for various scan parameters allows for accurately estimating CTDI∞ with the current use of pencil chamber and dose phantoms. Combined with the simple calibration, it gives this work great potential to be used not only in routine clinical quality assurance checks but also as a promising tool for patient organ dose assessment.
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87.57.qp Multislice
87.57.R- Computer-aided diagnosis

RADIATION MEASUREMENT PHYSICS: The energy dependence of lithium formate and alanine EPR dosimeters for medium energy x rays

Einar Waldeland, Eli Olaug Hole, Einar Sagstuen, and Eirik Malinen

Med. Phys. 37, 3569 (2010); doi:10.1118/1.3432567 (7 pages) | Cited 5 times

Online Publication Date: 15 June 2010

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Purpose: To perform a systematic investigation of the energy dependence of alanine and lilthium formate EPR dosimeters for medium energy x rays.
Methods: Lithium formate and alanine EPR dosimeters were exposed to eight different x-ray beam qualities, with nominal potentials ranging from 50 to 200 kV. Following ionometry based on standards of absorbed dose to water, the dosimeters were given two different doses of approximately 3 and 6 Gy for each radiation quality, with three dosimeters for each dose. A reference series was also irradiated to three different dose levels at a 60Co unit. The dose to water energy response, that is, the dosimeter reading per absorbed dose to water relative to that for 60Co γ-rays, was estimated for each beam quality. In addition, the energy response was calculated by Monte Carlo simulations and compared to the experimental energy response.
Results: The experimental energy response estimates ranged from 0.89 to 0.94 and from 0.68 to 0.90 for lithium formate and alanine, respectively. The uncertainties in the experimental energy response estimates were typically 3%. The relative effectiveness, that is, the ratio of the experimental energy response to that following Monte Carlo simulations was, on average, 0.96 and 0.94 for lithium formate and alanine, respectively.
Conclusions: This work shows that lithium formate dosimeters are less dependent on x-ray energy than alanine. Furthermore, as the relative effectiveness for both lithium formate and alanine were systematically less than unity, the yield of radiation-induced radicals is decreased following x-irradiation compared to irradiation with 60Co γ-rays.
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87.55.dk Dose-volume analysis
87.53.Jw Therapeutic applications, including brachytherapy
87.14.E- Proteins
87.55.K- Monte Carlo methods
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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
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