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Nov 2010

Volume 37, Issue 11, pp. 5565-6112

Spotlight Figure

Med. Phys. 37, 5787 (2010); http://dx.doi.org/10.1118/1.3491675 (5 pages)

Chunhua Men, H. Edwin Romeijn, Xun Jia, and Steve B. Jiang
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POINT/COUNTERPOINT: The title “radiation oncology physicist” should be changed to “oncologic physicist”

William P. Kowalsky, Ph.D., Martin W. Fraser, M.S., and Colin G. Orton, Ph.D., Moderator

Med. Phys. 37, 5565 (2010); http://dx.doi.org/10.1118/1.3481357 (3 pages)

Online Publication Date: 6 October 2010

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Abstract Unavailable
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87.53.Jw Therapeutic applications, including brachytherapy
87.19.xj Cancer
87.57.U- Nuclear medicine imaging

RADIATION THERAPY PHYSICS: Statistical analysis of dose heterogeneity in circulating blood: Implications for sequential methods of total body irradiation

Janelle A. Molloy

Med. Phys. 37, 5568 (2010); http://dx.doi.org/10.1118/1.3495816 (11 pages) | Cited 1 time

Online Publication Date: 6 October 2010

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Purpose: Improvements in delivery techniques for total body irradiation (TBI) using Tomotherapy® and intensity modulated radiation therapy have been proven feasible. Despite the promise of improved dose conformality, the application of these “sequential” techniques has been hampered by concerns over dose heterogeneity to circulating blood. The present study was conducted to provide quantitative evidence regarding the potential clinical impact of this heterogeneity.
Methods: Blood perfusion was modeled analytically as possessing linear, sinusoidal motion in the craniocaudal dimension. The average perfusion period for human circulation was estimated to be approximately 78 s. Sequential treatment delivery was modeled as a Gaussian-shaped dose cloud with a 10 cm length that traversed a 183 cm patient length at a uniform speed. Total dose to circulating blood voxels was calculated via numerical integration and normalized to 2 Gy per fraction. Dose statistics and equivalent uniform dose (EUD) were calculated for relevant treatment times, radiobiological parameters, blood perfusion rates, and fractionation schemes. The model was then refined to account for random dispersion superimposed onto the underlying periodic blood flow. Finally, a fully stochastic model was developed using binomial and trinomial probability distributions. These models allowed for the analysis of nonlinear sequential treatment modalities and treatment designs that incorporate deliberate organ sparing.
Results: The dose received by individual blood voxels exhibited asymmetric behavior that depended on the coherence among the blood velocity, circulation phase, and the spatiotemporal characteristics of the irradiation beam. Heterogeneity increased with the perfusion period and decreased with the treatment time. Notwithstanding, heterogeneity was less than ±10% for perfusion periods less than 150 s. The EUD was compromised for radiosensitive cells, long perfusion periods, and short treatment times. However, the EUD was unaffected (within 10%) for perfusion periods of less than 150 s or treatment times of 20 min or greater. Treatment over six fractions improved the EUD per fraction such that all parametric combinations resulted in unaffected EUD. The stochastic models confirmed these results.
Conclusions: Dose heterogeneity in circulating blood cells is clinically acceptable for typical treatment times, perfusion rates, and cell types. Development of conformal, sequential TBI treatment techniques should not be withheld based on concerns over circulating blood dose heterogeneity.
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87.53.Bn Dosimetry/exposure assessment
87.53.Jw Therapeutic applications, including brachytherapy
87.19.U- Hemodynamics
87.19.rh Fluid transport and rheology
87.10.Mn Stochastic modeling
02.70.Rr General statistical methods

RADIATION MEASUREMENT PHYSICS: Patient radiation dose in prospectively gated axial CT coronary angiography and retrospectively gated helical technique with a 320-detector row CT scanner

Shigenobu Seguchi, Takahiko Aoyama, Shuji Koyama, Keisuke Fujii, and Chiyo Yamauchi-Kawaura

Med. Phys. 37, 5579 (2010); http://dx.doi.org/10.1118/1.3496985 (7 pages) | Cited 1 time

Online Publication Date: 6 October 2010

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Purpose: The aim of this study was to evaluate radiation dose to patients undergoing computed tomography coronary angiography (CTCA) for prospectively gated axial (PGA) technique and retrospectively gated helical (RGH) technique.
Methods: Radiation doses were measured for a 320-detector row CT scanner (Toshiba Aquilion ONE) using small sized silicon-photodiode dosimeters, which were implanted at various tissue and organ positions within an anthropomorphic phantom for a standard Japanese adult male. Output signals from photodiode dosimeters were read out on a personal computer, from which organ and effective doses were computed according to guidelines published in the International Commission on Radiological Protection Publication 103.
Results: Organs that received high doses were breast, followed by lung, esophagus, and liver. Breast doses obtained with PGA technique and a phase window width of 16% at a simulated heart rate of 60 beats per minute were 13 mGy compared to 53 mGy with RGH technique using electrocardiographically dependent dose modulation at the same phase window width as that in PGA technique. Effective doses obtained in this case were 4.7 and 20 mSv for the PGA and RGH techniques, respectively. Conversion factors of dose length product to the effective dose in PGA and RGH were 0.022 and 0.025 mSv mGy−1 cm−1 with a scan length of 140 mm.
Conclusions: CTCA performed with PGA technique provided a substantial effective dose reduction, i.e., 70%–76%, compared to RGH technique using the dose modulation at the same phase windows as those in PGA technique. Though radiation doses in CTCA with RGH technique were the same level as, or some higher than, those in conventional coronary angiography (CCA), the use of PGA technique reduced organ and effective doses to levels less than CCA except for breast dose.
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87.53.Bn Dosimetry/exposure assessment
87.57.Q- Computed tomography
87.59.Dj Angiography
87.85.Pq Biomedical imaging

RADIATION THERAPY PHYSICS: “SABER”: A new software tool for radiotherapy treatment plan evaluation

Bo Zhao, Michael C. Joiner, Colin G. Orton, and Jay Burmeister

Med. Phys. 37, 5586 (2010); http://dx.doi.org/10.1118/1.3497152 (7 pages) | Cited 1 time

Online Publication Date: 6 October 2010

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Purpose: Both spatial and biological information are necessary in order to perform true optimization of a treatment plan and for predicting clinical outcome. The goal of this work is to develop an enhanced treatment plan evaluation tool which incorporates biological parameters and retains spatial dose information.
Methods: A software system is developed which provides biological plan evaluation with a novel combination of features. It incorporates hyper-radiosensitivity using the induced-repair model and applies the new concept of dose convolution filter (DCF) to simulate dose wash-out effects due to cell migration, bystander effect, and/or tissue motion during treatment. Further, the concept of spatial DVH (sDVH) is introduced to evaluate and potentially optimize the spatial dose distribution in the target volume. Finally, generalized equivalent uniform dose is derived from both the physical dose distribution (gEUD) and the distribution of equivalent dose in 2 Gy fractions (gEUD2) and the software provides three separate models for calculation of tumor control probability (TCP), normal tissue complication probability (NTCP), and probability of uncomplicated tumor control (P+). TCP, NTCP, and P+ are provided as a function of prescribed dose and multivariable TCP, NTCP, and P+ plots are provided to illustrate the dependence on individual parameters used to calculate these quantities. Ten plans from two clinical treatment sites are selected to test the three calculation models provided by this software.
Results: By retaining both spatial and biological information about the dose distribution, the software is able to distinguish features of radiotherapy treatment plans not discernible using commercial systems. Plans that have similar DVHs may have different spatial and biological characteristics and the application of novel tools such as sDVH and DCF within the software may substantially change the apparent plan quality or predicted plan metrics such as TCP and NTCP. For the cases examined, both the calculation method and the application of DCF can change the ranking order of competing plans. The voxel-by-voxel TCP model makes it feasible to incorporate spatial variations of clonogen densities (n), radiosensitivities (SF2), and fractionation sensitivities (α/β) as those data become available.
Conclusions: The new software incorporates both spatial and biological information into the treatment planning process. The application of multiple methods for the incorporation of biological and spatial information has demonstrated that the order of application of biological models can change the order of plan ranking. Thus, the results of plan evaluation and optimization are dependent not only on the models used but also on the order in which they are applied. This software can help the planner choose more biologically optimal treatment plans and potentially predict treatment outcome more accurately.
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87.55.dk Dose-volume analysis
87.57.R- Computer-aided diagnosis

RADIATION THERAPY PHYSICS: GPU-accelerated Monte Carlo convolution/superposition implementation for dose calculation

Bo Zhou, Cedric X. Yu, Danny Z. Chen, and X. Sharon Hu

Med. Phys. 37, 5593 (2010); http://dx.doi.org/10.1118/1.3490083 (11 pages) | Cited 2 times

Online Publication Date: 6 October 2010

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Purpose: Dose calculation is a key component in radiation treatment planning systems. Its performance and accuracy are crucial to the quality of treatment plans as emerging advanced radiation therapy technologies are exerting ever tighter constraints on dose calculation. A common practice is to choose either a deterministic method such as the convolution/superposition (CS) method for speed or a Monte Carlo (MC) method for accuracy. The goal of this work is to boost the performance of a hybrid Monte Carlo convolution/superposition (MCCS) method by devising a graphics processing unit (GPU) implementation so as to make the method practical for day-to-day usage.
Methods: Although the MCCS algorithm combines the merits of MC fluence generation and CS fluence transport, it is still not fast enough to be used as a day-to-day planning tool. To alleviate the speed issue of MC algorithms, the authors adopted MCCS as their target method and implemented a GPU-based version. In order to fully utilize the GPU computing power, the MCCS algorithm is modified to match the GPU hardware architecture. The performance of the authors’ GPU-based implementation on an Nvidia GTX260 card is compared to a multithreaded software implementation on a quad-core system.
Results: A speedup in the range of 6.7–11.4× is observed for the clinical cases used. The less than 2% statistical fluctuation also indicates that the accuracy of the authors’ GPU-based implementation is in good agreement with the results from the quad-core CPU implementation.
Conclusions: This work shows that GPU is a feasible and cost-efficient solution compared to other alternatives such as using cluster machines or field-programmable gate arrays for satisfying the increasing demands on computation speed and accuracy of dose calculation. But there are also inherent limitations of using GPU for accelerating MC-type applications, which are also analyzed in detail in this article.
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87.55.kd Algorithms
87.55.dk Dose-volume analysis
02.70.Uu Applications of Monte Carlo methods

RADIATION IMAGING PHYSICS: Evaluation of an improved algorithm for producing realistic 3D breast software phantoms: Application for mammography

K. Bliznakova, S. Suryanarayanan, A. Karellas, and N. Pallikarakis

Med. Phys. 37, 5604 (2010); http://dx.doi.org/10.1118/1.3491812 (14 pages) | Cited 4 times

Online Publication Date: 6 October 2010

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Purpose: This work presents an improved algorithm for the generation of 3D breast software phantoms and its evaluation for mammography.
Methods: The improved methodology has evolved from a previously presented 3D noncompressed breast modeling method used for the creation of breast models of different size, shape, and composition. The breast phantom is composed of breast surface, duct system and terminal ductal lobular units, Cooper’s ligaments, lymphatic and blood vessel systems, pectoral muscle, skin, 3D mammographic background texture, and breast abnormalities. The key improvement is the development of a new algorithm for 3D mammographic texture generation. Simulated images of the enhanced 3D breast model without lesions were produced by simulating mammographic image acquisition and were evaluated subjectively and quantitatively. For evaluation purposes, a database with regions of interest taken from simulated and real mammograms was created. Four experienced radiologists participated in a visual subjective evaluation trial, as they judged the quality of the simulated mammograms, using the new algorithm compared to mammograms, obtained with the old modeling approach. In addition, extensive quantitative evaluation included power spectral analysis and calculation of fractal dimension, skewness, and kurtosis of simulated and real mammograms from the database.
Results: The results from the subjective evaluation strongly suggest that the new methodology for mammographic breast texture creates improved breast models compared to the old approach. Calculated parameters on simulated images such as β exponent deducted from the power law spectral analysis and fractal dimension are similar to those calculated on real mammograms. The results for the kurtosis and skewness are also in good coincidence with those calculated from clinical images. Comparison with similar calculations published in the literature showed good agreement in the majority of cases.
Conclusions: The improved methodology generated breast models with increased realism compared to the older model as shown in evaluations of simulated images by experienced radiologists. It is anticipated that the realism will be further improved using an advanced image simulator so that simulated images may be used in feasibility studies in mammography.
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87.59.ej Digital mammography
87.57.N- Image analysis

RADIATION IMAGING PHYSICS: In-plane visibility of lesions using breast tomosynthesis and digital mammography

P. Timberg, M. Båth, I. Andersson, S. Mattsson, A. Tingberg, and M. Ruschin

Med. Phys. 37, 5618 (2010); http://dx.doi.org/10.1118/1.3488899 (9 pages) | Cited 2 times

Online Publication Date: 7 October 2010

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Purpose: The purpose of this work was to evaluate the visibility of simulated lesions in 2D digital mammography (DM) and breast tomosynthesis (BT) images of patients.
Methods: Images of the same women were acquired on both a DM system (Mammomat Novation, Siemens Healthcare, Erlangen, Germany) and a BT prototype system adapted from the same type of DM system. Using the geometrical properties of the two systems, simulated lesions were projected and added to each DM image as well as to each BT projection image prior to 3D reconstruction. The same beam quality and approximately the same total absorbed dose to the glandular tissue were used for each breast image acquisition on the two systems. A series of four-alternative forced choice human observer experiments was conducted for each of five simulated lesion diameters: 0.2, 1, 3, 8, and 25 mm. An additional experiment was conducted for the 0.2 mm lesion in BT only at twice the dose level (BT2x). Threshold signal was defined as the lesion signal intensity required for a detectability index (d′) of 2.5. Four medical physicists participated in all experiments. One experiment, consisting of 60 cases, was conducted per test condition (i.e., lesion size and signal combination).
Results: For the smallest lesions (0.2 mm), the threshold signal for DM was 21% lower than for BT at equivalent dose levels, and BT2x was 26% lower than DM. For the lesions larger than 1 mm, the threshold signal increased linearly (in log space) with the lesion diameter for both DM and BT, with DM requiring around twice the signal as BT. The difference in the threshold signal between BT and DM at each lesion size was statistically significant, except for the 0.2 mm lesion between BT2x and DM.
Conclusions: The results of this study indicate that low-signal lesions larger than 1.0 mm may be more visible in BT compared to DM, whereas 0.2 mm lesions may be better visualized with DM compared to BT, when compared at equal dose.
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87.59.ej Digital mammography
87.57.Q- Computed tomography
87.57.rh Mammography
87.57.nf Reconstruction
87.53.Bn Dosimetry/exposure assessment
87.19.xj Cancer

RADIATION THERAPY PHYSICS: Inverse planning for four-dimensional (4D) volumetric modulated arc therapy

Yunzhi Ma, Daniel Chang, Paul Keall, Yiaoqin Xie, Jae-yoon Park, Tae-Suk Suh, and Lei Xing

Med. Phys. 37, 5627 (2010); http://dx.doi.org/10.1118/1.3497271 (7 pages) | Cited 3 times

Online Publication Date: 7 October 2010

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Purpose: To develop a 4D volumetric modulated arc therapy (VMAT) inverse planning framework.
Methods: 4D VMAT inverse planning aims to derive an aperture and weight modulated arc therapy treatment plan that optimizes the accumulated dose distribution from all gantry angles and breathing phases. Under an assumption that the gantry rotation and patient breathing are synchronized (i.e., there is a functional relationship between the phase of the patient breathing cycle and the beam angle), the authors compute the contribution from different respiration phases through the registration of the phased CT images. The accumulative dose distribution is optimized by iteratively adjusting the aperture shape and weight of each beam through the minimization of the planning objective function. For comparison, traditional 3D VMAT plans are also performed for the two cases and the performance of the proposed technique is demonstrated.
Results: A framework for 4D VMAT inverse planning has been proposed. With the consideration of the extra dimension of time in VMAT, a tighter target margin can be achieved with a full duty cycle, which is otherwise not achievable simultaneously by either 3D VMAT optimization or gated VMAT.
Conclusions: The 4D VMAT planning formulism proposed here provides useful insight on how the “time” dimension can be exploited in rotational arc therapy to maximally compensate for the intrafraction organ motion.
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87.53.Jw Therapeutic applications, including brachytherapy
87.55.de Optimization
87.59.-e X-ray imaging
87.57.Q- Computed tomography
87.57.nj Registration
87.57.cp Artifacts and distortion

RADIATION IMAGING PHYSICS: Combining scatter reduction and correction to improve image quality in cone-beam computed tomography (CBCT)

Jian-Yue Jin, Lei Ren, Qiang Liu, Jinkoo Kim, Ning Wen, Huaiqun Guan, Benjamin Movsas, and Indrin J. Chetty

Med. Phys. 37, 5634 (2010); http://dx.doi.org/10.1118/1.3497272 (11 pages) | Cited 6 times

Online Publication Date: 7 October 2010

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Purpose: The authors propose a combined scatter reduction and correction method to improve image quality in cone-beam computed tomography (CBCT). Although using a beam-block approach similar to previous techniques to measure the scatter, this method differs in that the authors utilize partially blocked projection data obtained during scatter measurement for CBCT image reconstruction. This study aims to evaluate the feasibility of the proposed approach.
Methods: A 1D grid, composed of lead septa, was placed between the radiation source and the imaging object for scatter measurement. Image data were collected from the grid interspace regions while the scatter distribution was measured in the blocked regions under the grid. Scatter correction was performed by subtracting the measured scatter from the imaging data. Image information in the penumbral regions of the grid was derived. Three imaging modes were developed to reconstruct full CBCT images from partial projection data. The single-rotation half-fan mode uses interpolation to fill the missing data. The dual-rotation half-fan mode uses two rotations, with the grid offset by half a grid cycle, to acquire two complementary sets of projections, which are then merged to form complete projections for reconstruction. The single-rotation full-fan mode was designed for imaging a small object or a region of interest. Full-fan projection images were acquired over a 360° scan angle with the grid shifting a distance during the scan. An enlarged Catphan phantom was used to evaluate potential improvement in image quality with the proposed technique. An anthropomorphic pelvis phantom was used to validate the feasibility of reconstructing a complete set of CBCT images from the partially blocked projections using three imaging modes. Rigid-body image registration was performed between the CBCT images from the single-rotation half-fan mode and the simulation CT and the results were compared to that for the CBCT images from dual-rotation mode and conventional CBCT images.
Results: The proposed technique reduced the streak artifact index from 58% to 1% in comparison with the conventional CBCT. It also improved CT number linearity from 0.880 to 0.998 and the contrast-to-noise ratio (CNR) from 4.29 to 6.42. Complete sets of CBCT images with overall improved image quality were achieved for all three image modes. The longitudinal resolution was slightly compromised for the single-rotation half-fan mode. High resolution was retained for the dual-rotation half-fan and single-rotation full-fan modes in the longitudinal direction. The registration error for the CBCT images from the single-rotation half-fan mode was 0.8±0.3 mm in the longitudinal direction and negligible in the other directions.
Conclusions: The proposed method provides combined scatter correction and direct scatter reduction. Scatter correction may eliminate scatter artifacts, while direct scatter reduction may improve the CNR to compensate the CNR degradation due to scatter correction. Complete sets of CBCT images are reconstructed in all three imaging modes. The single-rotation mode can be used for rigid-body patient alignment despite degradation in longitudinal resolution. The dual-rotation mode may be used to improve CBCT image quality for soft tissue delineation in adaptive radiation therapy.
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87.57.Q- Computed tomography
87.57.nj Registration
87.57.nf Reconstruction
87.85.Pq Biomedical imaging

ULTRASOUND PHYSICS: Intensity inhomogeneity correction for the breast sonogram: Constrained fuzzy cell-based bipartitioning and polynomial surface modeling

Chia-Yen Lee, Yi-Hong Chou, Chiun-Sheng Huang, Yeun-Chung Chang, Chui-Mei Tiu, and Chung-Ming Chen

Med. Phys. 37, 5645 (2010); http://dx.doi.org/10.1118/1.3488944 (10 pages)

Online Publication Date: 7 October 2010

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Purpose: To develop an intensity inhomogeneity algorithm for breast sonograms in order to assist visual identification and automatic delineation of lesion boundaries.
Methods: The proposed algorithm was composed of two essential ideas. One was decomposing the region of interest (ROI) into foreground and background regions by a cell-based segmentation algorithm, called constrained fuzzy cell-based bipartition-EM (CFCB-EM) algorithm. The CFCB-EM algorithm deformed the contour in a fuzzy cell-based deformation fashion with the cell structures derived by the fuzzy cell competition (FCC) algorithm as the deformation unit and the boundary estimated by the normalized cut (NC) algorithm as the reference contour. The other was modeling the intensity inhomogeneity in an ROI as a spatially variant normal distribution with a constant variance and spatially variant means, which formed a polynomial surface of order n. The proposed algorithm was formulated as a nested EM algorithm comprising the outer-layer EM algorithm, i.e., the intensity inhomogeneity correction-EM (IIC-EM) algorithm, and the inner-layer EM algorithm, i.e., the CFCB-EM algorithm. The E step of the IIC-EM algorithm was to provide a reasonably good bipartition separating the ROI into foreground and background regions, which included three major component algorithms, namely, the FCC, the NC, and the CFCB-EM. The M step of the IIC-EM algorithm was to estimate and correct the intensity inhomogeneity field by least-squared fitting the intensity inhomogeneity to an nth order polynomial surface. Forty-nine breast sonograms with intensity inhomogeneity, each from a different subject, were randomly selected for performance analysis. Three assessments were carried out to evaluate the effectiveness of the proposed algorithm.
Results: Based on the visual evaluation of two experienced radiologists, in the first assessment, 46 out of 49 breast lesions were considered to have better contrasts on the inhomogeneity-corrected images by both radiologists. The interrater reliability for the radiologists was found to be kappa = 0.479 (p = 0.001). In the second assessment, the mean gradients of the low-gradient boundary points before and after correction of the intensity inhomogeneity were compared by the paired t-test, yielding a p-value of 0.000, which suggested the proposed intensity inhomogeneity algorithm may enhance the mean gradient of the low-gradient boundary points. By using the paired t-test, the third assessment further showed that the Chan and Vese level set method could derive a much better lesion boundary on the inhomogeneity-corrected image than on the original image (p = 0.000).
Conclusions: The proposed intensity inhomogeneity correction algorithm could not only augment the visibility of lesion boundary but also improve the segmentation result on a breast sonogram.
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87.63.D- Ultrasonography
43.80.Vj Acoustical medical instrumentation and measurement techniques
87.57.nm Segmentation
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