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Dec 2009

Volume 36, Issue 12, pp. 5377-5722

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POINT/COUNTERPOINT: Radiation departments should be certified to provide certain new technologies such as IGRT

Christopher F. Njeh, Ph.D., Abdul Rashid, Ph.D., and Colin G. Orton, Ph.D., Moderator

Med. Phys. 36, 5377 (2009); http://dx.doi.org/10.1118/1.3223359 (3 pages)

Online Publication Date: 5 November 2009

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Abstract Unavailable
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87.55.-x Treatment strategy

TISSUE MEASUREMENTS: Cumulative sum quality control for calibrated breast density measurements

John J. Heine, Ke Cao, and Craig Beam

Med. Phys. 36, 5380 (2009); http://dx.doi.org/10.1118/1.3250842 (11 pages) | Cited 3 times

Online Publication Date: 5 November 2009

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Purpose: Breast density is a significant breast cancer risk factor. Although various methods are used to estimate breast density, there is no standard measurement for this important factor. The authors are developing a breast density standardization method for use in full field digital mammography (FFDM). The approach calibrates for interpatient acquisition technique differences. The calibration produces a normalized breast density pixel value scale. The method relies on first generating a baseline (BL) calibration dataset, which required extensive phantom imaging. Standardizing prospective mammograms with calibration data generated in the past could introduce unanticipated error in the standardized output if the calibration dataset is no longer valid.
Methods: Sample points from the BL calibration dataset were imaged approximately biweekly over an extended timeframe. These serial samples were used to evaluate the BL dataset reproducibility and quantify the serial calibration accuracy. The cumulative sum (Cusum) quality control method was used to evaluate the serial sampling.
Results: There is considerable drift in the serial sample points from the BL calibration dataset that is x-ray beam dependent. Systematic deviation from the BL dataset caused significant calibration errors. This system drift was not captured with routine system quality control measures. Cusum analysis indicated that the drift is a sign of system wear and eventual x-ray tube failure.
Conclusions: The BL calibration dataset must be monitored and periodically updated, when necessary, to account for sustained system variations to maintain the calibration accuracy.
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87.59.ej Digital mammography
87.19.xj Cancer

RADIATION THERAPY PHYSICS: Evaluation of similarity measures for use in the intensity-based rigid 2D-3D registration for patient positioning in radiotherapy

Jian Wu, Minho Kim, Jorg Peters, Heeteak Chung, and Sanjiv S. Samant

Med. Phys. 36, 5391 (2009); http://dx.doi.org/10.1118/1.3250843 (13 pages) | Cited 7 times

Online Publication Date: 5 November 2009

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Purpose: Rigid 2D-3D registration is an alternative to 3D-3D registration for cases where largely bony anatomy can be used for patient positioning in external beam radiation therapy. In this article, the authors evaluated seven similarity measures for use in the intensity-based rigid 2D-3D registration using a variation in Skerl’s similarity measure evaluation protocol.
Methods: The seven similarity measures are partitioned intensity uniformity, normalized mutual information (NMI), normalized cross correlation (NCC), entropy of the difference image, pattern intensity (PI), gradient correlation (GC), and gradient difference (GD). In contrast to traditional evaluation methods that rely on visual inspection or registration outcomes, the similarity measure evaluation protocol probes the transform parameter space and computes a number of similarity measure properties, which is objective and optimization method independent. The variation in protocol offers an improved property in the quantification of the capture range. The authors used this protocol to investigate the effects of the downsampling ratio, the region of interest, and the method of the digitally reconstructed radiograph (DRR) calculation [i.e., the incremental ray-tracing method implemented on a central processing unit (CPU) or the 3D texture rendering method implemented on a graphics processing unit (GPU)] on the performance of the similarity measures. The studies were carried out using both the kilovoltage (kV) and the megavoltage (MV) images of an anthropomorphic cranial phantom and the MV images of a head-and-neck cancer patient.
Results: Both the phantom and the patient studies showed the 2D-3D registration using the GPU-based DRR calculation yielded better robustness, while providing similar accuracy compared to the CPU-based calculation. The phantom study using kV imaging suggested that NCC has the best accuracy and robustness, but its slow function value change near the global maximum requires a stricter termination condition for an optimization method. The phantom study using MV imaging indicated that PI, GD, and GC have the best accuracy, while NCC and NMI have the best robustness. The clinical study using MV imaging showed that NCC and NMI have the best robustness.
Conclusions: The authors evaluated the performance of seven similarity measures for use in 2D-3D image registration using the variation in Skerl’s similarity measure evaluation protocol. The generalized methodology can be used to select the best similarity measures, determine the optimal or near optimal choice of parameter, and choose the appropriate registration strategy for the end user in his specific registration applications in medical imaging.
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87.53.Jw Therapeutic applications, including brachytherapy
87.57.nj Registration
87.57.nf Reconstruction
87.19.xj Cancer
02.60.Pn Numerical optimization

NUCLEAR MEDICINE PHYSICS: Design and construction of a quality control phantom for SPECT and PET imaging

Dylan Christopher Hunt, Harry Easton, and Curtis B. Caldwell

Med. Phys. 36, 5404 (2009); http://dx.doi.org/10.1118/1.3250855 (8 pages)

Online Publication Date: 5 November 2009

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In this article, the authors present a method for quickly and easily constructing test phantoms for PET and SPECT quality assurance. As a demonstration, they constructed a complex prototype test phantom, showing the strengths of the construction method. Images taken using a PET/CT and a SPECT scanner are presented, along with a qualitative evaluation of PET/CT using the test phantom. The construction technique provides a quick, easy, and cost effective means of constructing a phantom for use in nuclear medicine imaging.
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87.57.uh Single photon emission computed tomography (SPECT)
87.57.uk Positron emission tomography (PET)
87.85.Pq Biomedical imaging

RADIATION THERAPY PHYSICS: Tissue equivalency of phantom materials for neutron dosimetry in proton therapy

Stephen Dowdell, Ben Clasie, Andrew Wroe, Susanna Guatelli, Peter Metcalfe, Reinhard Schulte, and Anatoly Rosenfeld

Med. Phys. 36, 5412 (2009); http://dx.doi.org/10.1118/1.3250857 (8 pages) | Cited 2 times

Online Publication Date: 5 November 2009

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Purpose: Previous Monte Carlo and experimental studies involving secondary neutrons in proton therapy have employed a number of phantom materials that are designed to represent human tissue. In this study, the authors determined the suitability of common phantom materials for dosimetry of secondary neutrons, specifically for pediatric and intracranial proton therapy treatments.
Methods: This was achieved through comparison of the absorbed dose and dose equivalent from neutrons generated within the phantom materials and various ICRP tissues. The phantom materials chosen for comparison were Lucite, liquid water, solid water, and A150 tissue equivalent plastic. These phantom materials were compared to brain, muscle, and adipose tissues.
Results: The magnitude of the doses observed were smaller than those reported in previous experimental and Monte Carlo studies, which incorporated neutrons generated in the treatment head. The results show that for both neutron absorbed dose and dose equivalent, no single phantom material gives agreement with tissue within 5% at all the points considered. Solid water gave the smallest mean variation with the tissues out of field where neutrons are the primary contributor to the total dose.
Conclusions: Of the phantom materials considered, solid water shows best agreement with tissues out of field.
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87.53.Jw Therapeutic applications, including brachytherapy
87.53.Bn Dosimetry/exposure assessment
87.19.Ff Muscles
87.55.kh Applications

RADIATION THERAPY PHYSICS: An integral quality monitoring system for real-time verification of intensity modulated radiation therapy

Mohammad K. Islam, Bernhard D. Norrlinger, Jason R. Smale, Robert K. Heaton, Duncan Galbraith, Cary Fan, and David A. Jaffray

Med. Phys. 36, 5420 (2009); http://dx.doi.org/10.1118/1.3250859 (9 pages) | Cited 9 times

Online Publication Date: 5 November 2009

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Purpose: To develop an independent and on-line beam monitoring system, which can validate the accuracy of segment-by-segment energy fluence delivery for each treatment field. The system is also intended to be utilized for pretreatment dosimetric quality assurance of intensity modulated radiation therapy (IMRT), on-line image-guided adaptive radiation therapy, and volumetric modulated arc therapy.
Methods: The system, referred to as the integral quality monitor (IQM), utilizes an area integrating energy fluence monitoring sensor (AIMS) positioned between the final beam shaping device [i.e., multileaf collimator (MLC)] and the patient. The prototype AIMS consists of a novel spatially sensitive large area ionization chamber with a gradient along the direction of the MLC motion. The signal from the AIMS provides a simple output for each beam segment, which is compared in real time to the expected value. The prototype ionization chamber, with a physical area of 22×22 cm2, has been constructed out of aluminum with the electrode separations varying linearly from 2 to 20 mm. A calculation method has been developed to predict AIMS signals based on an elementwise integration technique, which takes into account various predetermined factors, including the spatial response function of the chamber, MLC characteristics, beam transmission through the secondary jaws, and field size factors. The influence of the ionization chamber on the beam has been evaluated in terms of transmission, surface dose, beam profiles, and depth dose. The sensitivity of the system was tested by introducing small deviations in leaf positions. A small set of IMRT fields for prostate and head and neck plans was used to evaluate the system. The ionization chamber and the data acquisition software systems were interfaced to two different types of linear accelerators: Elekta Synergy and Varian iX.
Results: For a 10×10 cm2 field, the chamber attenuates the beam intensity by 7% and 5% for 6 and 18 MV beams, respectively, without significantly changing the depth dose, surface dose, and dose profile characteristics. An MLC bank calibration error of 1 mm causes the IQM signal of a 3×3 cm2 aperture to change by 3%. A positioning error in a single 5 mm wide leaf by 3 mm in 3×3 cm2 aperture causes a signal difference of 2%. Initial results for prostate and head and neck IMRT fields show an average agreement between calculation and measurement to within 1%, with a maximum deviation for each of the smallest beam segments to within 5%. When the beam segments of a prostate IMRT field were shifted by 3 mm from their original position, along the direction of the MLC motion, the IQM signals varied, on average, by 2.5%.
Conclusions: The prototype IQM system can validate the accuracy of beam delivery in real time by comparing precalculated and measured AIMS signals. The system is capable of capturing errors in MLC leaf calibration or malfunctions in the positioning of an individual leaf. The AIMS does not significantly alter the beam quality and therefore could be implemented without requiring recommissioning measurements.
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87.55.ne Therapeutic applications
87.55.D- Treatment planning
87.53.Bn Dosimetry/exposure assessment
87.53.Jw Therapeutic applications, including brachytherapy
87.56.nk Collimators
87.56.bd Accelerators

MAGNETIC RESONANCE PHYSICS: Identification of breast calcification using magnetic resonance imaging

Ali Fatemi-Ardekani, Colm Boylan, and Michael D. Noseworthy

Med. Phys. 36, 5429 (2009); http://dx.doi.org/10.1118/1.3250860 (8 pages) | Cited 2 times

Online Publication Date: 5 November 2009

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MRI phase and magnitude images provide information about local magnetic field variation (ΔB0), which can consequently be used to understand tissue properties. Often, phase information is discarded. However, corrected phase images are able to produce contrast as a result of magnetic susceptibility differences and local field inhomogeneities due to the presence of diamagnetic and paramagnetic substances. Three-dimensional (3D) susceptibility weighted imaging (SWI) can be used to probe changes in MRI phase evolution and, subsequently, result in an alternate form of contrast between tissues. For example, SWI has been useful in the assessment of negative phase induced ΔB0 modulation due to the presence of paramagnetic substances such as iron. Very little, however, has been done to assess positive phase induced contrast changes resulting from the presence of diamagnetic substances such as precipitated calcium. As ductal carcinoma in situ, which is the precursor of invasive ductal cancer, is often associated with breast microcalcification, the authors proposed using SWI as a possible visualization technique. In this study, breast phantoms containing calcifications (0.4–1.5 mm) were imaged using mammography, computed tomography (CT), and SWI. Corrected phase and magnitude images acquired using SWI allowed identification and correlation of all calcifications seen on CT. As the approach is a 3D technique, it could potentially allow for more accurate localization and biopsy and maybe even reduce the use of gadolinium contrast. Furthermore, the approach may be beneficial to women with dense breast tissue where the ability to detect microcalcification with mammography is reduced.
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87.61.-c Magnetic resonance imaging
87.19.xj Cancer
87.59.E- Mammography
87.57.Q- Computed tomography

RADIATION IMAGING PHYSICS: The myth of the 50-50 breast

M. J. Yaffe, J. M. Boone, N. Packard, O. Alonzo-Proulx, S.-Y. Huang, C. L. Peressotti, A. Al-Mayah, and K. Brock

Med. Phys. 36, 5437 (2009); http://dx.doi.org/10.1118/1.3250863 (7 pages) | Cited 31 times

Online Publication Date: 5 November 2009

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Purpose: For dosimetry and for work in optimization of x-ray imaging of the breast, it is commonly assumed that the breast is composed of 50% fibroglandular tissue and 50% fat. The purpose of this study was to assess whether this assumption was realistic.
Methods: First, data obtained from an experimental breast CT scanner were used to validate an algorithm that measures breast density from digitized film mammograms. Density results obtained from a total of 2831 women, including 191 women receiving CT and from mammograms of 2640 women from three other groups, were then used to estimate breast compositions.
Results: Mean compositions, expressed as percent fibroglandular tissue (including the skin), varied from 13.7% to 25.6% among the groups with an overall mean of 19.3%. The mean compressed breast thickness for the mammograms was 5.9 cm (σ=1.6 cm). 80% of the women in our study had volumetric breast density less than 27% and 95% were below 45%.
Conclusions: Based on the results obtained from the four groups of women in our study, the “50-50” breast is not a representative model of the breast composition.
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87.59.ej Digital mammography
87.57.Q- Computed tomography
87.53.Bn Dosimetry/exposure assessment
87.59.eg Film mammography

RADIATION THERAPY PHYSICS: Functional representation of tissue phantom ratios for photon fields

Otto A. Sauer and J. Wilbert

Med. Phys. 36, 5444 (2009); http://dx.doi.org/10.1118/1.3250867 (7 pages) | Cited 1 time

Online Publication Date: 5 November 2009

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Purpose: The tissue phantom ratio (TPR) is a common dosimetric quantity used in photon dose calculations. For small photon fields with side lengths less than 4 cm, TPR data hardly exist in literature. In this work, a self-contained functional representation of TPR is proposed, valid for the whole range of clinically relevant depth and field sizes. This is especially useful for small fields shaped by multileaf collimators.
Methods: TPRs were measured for quadratic fields with side lengths between 0.4 and 18 cm. The measured data were fitted to a physically meaningful function taking electron buildup, buildup of scattered photons, beam attenuation, and beam hardening into account. The achievable accuracy was tested against measurement and data from the literature.
Results: A set of parameters for the proposed function was derived for 6 and 10 MV beams. The comparison of the calculated and the measured data generally yielded a difference of less than 1%. For field sizes below 2 cm, a systematic discrepancy between the author’s data and those from Cheng et al. [Med. Phys 34, 3149–3157 (2007)] was found.
Conclusions: With the proposed model, TPRs can be calculated for the full range of field sizes and depths required by treatment planning system algorithms and monitor unit check programs with very high accuracy. The method is also useful in detecting and reducing errors in measurement.
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87.55.dk Dose-volume analysis
87.53.Bn Dosimetry/exposure assessment
87.53.Jw Therapeutic applications, including brachytherapy

RADIATION THERAPY PHYSICS: Fast, accurate photon beam accelerator modeling using BEAMnrc: A systematic investigation of efficiency enhancing methods and cross-section data

Margarida Fragoso, Iwan Kawrakow, Bruce A. Faddegon, Timothy D. Solberg, and Indrin J. Chetty

Med. Phys. 36, 5451 (2009); http://dx.doi.org/10.1118/1.3253300 (16 pages) | Cited 1 time

Online Publication Date: 5 November 2009

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In this work, an investigation of efficiency enhancing methods and cross-section data in the BEAMnrc Monte Carlo (MC) code system is presented. Additionally, BEAMnrc was compared with VMC++, another special-purpose MC code system that has recently been enhanced for the simulation of the entire treatment head. BEAMnrc and VMC++ were used to simulate a 6 MV photon beam from a Siemens Primus linear accelerator (linac) and phase space (PHSP) files were generated at 100 cm source-to-surface distance for the 10×10 and 40×40 cm2 field sizes. The BEAMnrc parameters/techniques under investigation were grouped by (i) photon and bremsstrahlung cross sections, (ii) approximate efficiency improving techniques (AEITs), (iii) variance reduction techniques (VRTs), and (iv) a VRT (bremsstrahlung photon splitting) in combination with an AEIT (charged particle range rejection). The BEAMnrc PHSP file obtained without the efficiency enhancing techniques under study or, when not possible, with their default values (e.g., EXACT algorithm for the boundary crossing algorithm) and with the default cross-section data (PEGS4 and Bethe–Heitler) was used as the “base line” for accuracy verification of the PHSP files generated from the different groups described previously. Subsequently, a selection of the PHSP files was used as input for DOSXYZnrc-based water phantom dose calculations, which were verified against measurements. The performance of the different VRTs and AEITs available in BEAMnrc and of VMC++ was specified by the relative efficiency, i.e., by the efficiency of the MC simulation relative to that of the BEAMnrc base-line calculation. The highest relative efficiencies were ∼935 (∼111 min on a single 2.6 GHz processor) and ∼200 (∼45 min on a single processor) for the 10×10 field size with 50 million histories and 40×40 cm2 field size with 100 million histories, respectively, using the VRT directional bremsstrahlung splitting (DBS) with no electron splitting. When DBS was used with electron splitting and combined with augmented charged particle range rejection, a technique recently introduced in BEAMnrc, relative efficiencies were ∼420 (∼253 min on a single processor) and ∼175 (∼58 min on a single processor) for the 10×10 and 40×40 cm2 field sizes, respectively. Calculations of the Siemens Primus treatment head with VMC++ produced relative efficiencies of ∼1400 (∼6 min on a single processor) and ∼60 (∼4 min on a single processor) for the 10×10 and 40×40 cm2 field sizes, respectively. BEAMnrc PHSP calculations with DBS alone or DBS in combination with charged particle range rejection were more efficient than the other efficiency enhancing techniques used. Using VMC++, accurate simulations of the entire linac treatment head were performed within minutes on a single processor. Noteworthy differences (±1%–3%) in the mean energy, planar fluence, and angular and spectral distributions were observed with the NIST bremsstrahlung cross sections compared with those of Bethe–Heitler (BEAMnrc default bremsstrahlung cross section). However, MC calculated dose distributions in water phantoms (using combinations of VRTs/AEITs and cross-section data) agreed within 2% of measurements. Furthermore, MC calculated dose distributions in a simulated water/air/water phantom, using NIST cross sections, were within 2% agreement with the BEAMnrc Bethe–Heitler default case.
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87.53.Jw Therapeutic applications, including brachytherapy
87.55.K- Monte Carlo methods
87.55.dk Dose-volume analysis
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Medical Physics is an official science journal of the AAPM and of the COMP/CCPM/IOMP
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Departments of Radiology, Radiation Oncology, Biophysics, Community and Public Health, Medical College of Wisconsin
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