Volume 37, Issue 3, March 2010
- medical physics letters
- radiation therapy physics
- radiation imaging physics
- radiation measurement physics
- magnetic resonance physics
- nuclear medicine physics
- optical physics
- ultrasound physics
- thermotherapy physics
- tissue measurements
- radiation protection physics
- radiation biology
- books and publications
Index of content:
37(2010); http://dx.doi.org/10.1118/1.3298377View Description Hide Description
- MEDICAL PHYSICS LETTERS
37(2010); http://dx.doi.org/10.1118/1.3314072View Description Hide DescriptionPurpose:
Verify experimentally the theoretical prediction ofF. Tessier and I. Kawrakow [Med. Phys.37, 96–107 (2010)] that it is possible to design a thimble ionization chamber with no shift in its effective point of measurement (EPOM), i.e., a chamber that provides a measure of the dose to the medium at the location of its central axis.Methods:
Measure dose from a 25 MV photon beam incident on water with an Exradin A1SL ionization chamber inside a thin sleeve (as a means of effectively increasing the thimble wall thickness). The depth-dose curve is compared to that obtained using a well-characterized PTW Roos parallel-plate chamber.Results:
With an appropriate increase in thimble wall thickness, the EPOM shift of the Exradin A1SL vanishes. Further increase of the wall thickness yields a chamber with a positive (downstream) shift in its point of measurement.Conclusions:
It is possible to design a thimble ionization chamber with a zero EPOM shift by adjusting the wall thickness.
- RADIATION THERAPY PHYSICS
37(2010); http://dx.doi.org/10.1118/1.3302141View Description Hide Description
Computer aided modeling of anatomic deformation, allowing various techniques and protocols in radiation therapy to be systematically verified and studied, has become increasingly attractive. In this study the potential issues in deformable image registration (DIR) were analyzed based on two numerical phantoms: One, a synthesized, low intensity gradient prostate image, and the other a lung patient’s CTimage data set. Each phantom was modeled with region-specific material parameters with its deformation solved using a finite element method. The resultant displacements were used to construct a benchmark to quantify the displacement errors of the Demons and B-Spline-based registrations. The results show that the accuracy of these registration algorithms depends on the chosen parameters, the selection of which is closely associated with the intensity gradients of the underlying images. For the Demons algorithm, both single resolution (SR) and multiresolution (MR) registrations required approximately 300 iterations to reach an accuracy of 1.4 mm mean error in the lung patient’s CTimage (and 0.7 mm mean error averaged in the lung only). For the low gradient prostate phantom, these algorithms (both SR and MR) required at least 1600 iterations to reduce their mean errors to 2 mm. For the B-Spline algorithms, best performance (mean errors of 1.9 mm for SR and 1.6 mm for MR, respectively) on the low gradient prostate was achieved using five grid nodes in each direction. Adding more grid nodes resulted in larger errors. For the lung patient’s CT data set, the B-Spline registrations required ten grid nodes in each direction for highest accuracy (1.4 mm for SR and 1.5 mm for MR). The numbers of iterations or grid nodes required for optimal registrations depended on the intensity gradients of the underlying images. In summary, the performance of the Demons and B-Spline registrations have been quantitatively evaluated using numerical phantoms. The results show that parameter selection for optimal accuracy is closely related to the intensity gradients of the underlying images. Also, the result that the DIR algorithms produce much lower errors in heterogeneous lung regions relative to homogeneous (low intensity gradient) regions, suggests that feature-based evaluation of deformable image registration accuracy must be viewed cautiously.
37(2010); http://dx.doi.org/10.1118/1.3301618View Description Hide DescriptionPurpose:
Graphic processing units (GPUs) are increasingly used for scientific applications, where their parallel architecture and unprecedented computing power density can be exploited to accelerate calculations. In this paper, a new GPU implementation of a convolution/superposition (CS) algorithm is presented.Methods:
This new GPU implementation has been designed from the ground-up to use the graphics card’s strengths and to avoid its weaknesses. The CS GPU algorithm takes into account beam hardening, off-axis softening, kernel tilting, and relies heavily on raytracing through patient imaging data. Implementation details are reported as well as a multi-GPU solution.Results:
An overall single-GPU acceleration factor of 908× was achieved when compared to a nonoptimized version of the CS algorithm implemented in PlanUNC in single threaded central processing unit (CPU) mode, resulting in approximatively 2.8 s per beam for a 3D dose computation on a 0.4 cm grid. A comparison to an established commercial system leads to an acceleration factor of approximately 29× or 0.58 versus 16.6 s per beam in single threaded mode. An acceleration factor of 46× has been obtained for the total energy released per mass (TERMA) calculation and a 943× acceleration factor for the CS calculation compared to PlanUNC. Dose distributions also have been obtained for a simple water-lung phantom to verify that the implementation gives accurate results.Conclusions:
These results suggest that GPUs are an attractive solution for radiation therapy applications and that careful design, taking the GPU architecture into account, is critical in obtaining significant acceleration factors. These results potentially can have a significant impact on complex dose delivery techniques requiring intensive dose calculations such as intensity-modulated radiation therapy(IMRT) and arc therapy. They also are relevant for adaptive radiation therapy where dose results must be obtained rapidly.
37(2010); http://dx.doi.org/10.1118/1.3301594View Description Hide DescriptionPurpose:
To present, implement, and test aself-consistent pseudoinverse displacement vector field (PIDVF) generator, which preserves the location of information mapped back-and-forth between image sets.Methods:
The algorithm is an iterative scheme based on nearest neighbor interpolation and a subsequent iterative search. Performance of the algorithm is benchmarked using a lung 4DCT data set with six CTimages from different breathing phases and eight CTimages for a single prostrate patient acquired on different days. A diffeomorphic deformable image registration is used to validate our PIDVFs. Additionally, the PIDVF is used to measure the self-consistency of two nondiffeomorphic algorithms which do not use a self-consistency constraint: The ITK Demons algorithm for the lung patient images and an in-house B-Spline algorithm for the prostate patient images. Both Demons and B-Spline have been QAed through contour comparison. Self-consistency is determined by using a DIR to generate a displacement vector field (DVF) between reference image and study image . The same DIR is used to generate . Additionally, our PIDVF generator is used to create . Back-and-forth mapping of a set of points (used as surrogates of contours) using and is compared to back-and-forth mapping performed with and . The Euclidean distances between the original unmapped points and the mapped points are used as a self-consistency measure.Results:
Test results demonstrate that the consistency error observed in back-and-forth mappings can be reduced two to nine times in point mapping and 1.5 to three times in dose mapping when the PIDVF is used in place of the B-Spline algorithm. These self-consistency improvements are not affected by the exchanging of and . It is also demonstrated that differences between and can be used as a criteria to check the quality of the DVF.Conclusions:
Use of DVF and its PIDVF will improve the self-consistency of points, contour, and dose mappings in image guided adaptive therapy.
37(2010); http://dx.doi.org/10.1118/1.3301607View Description Hide DescriptionPurpose:
The objective was to characterize a new Yb-169 high dose rate source for brachytherapy application.Methods:
Monte Carlo simulations were performed using theMCNP5 F6 energy deposition tallies placed around the Yb-169 source at different radial distances in both air-vacuum and water environments. The calculations were based on a spherical water phantom with a radius of 50 cm. The output from the simulations was converted into radial dose rate distribution in polar coordinates surrounding the brachytherapy source.Results:
The results from Monte Carlo simulations were used to calculate the AAPM Task Group 43 dosimetric parameters: Anisotropy function, radial dose function, air kerma strength, and dose rate constant. The results indicate a dose rate constant of, anisotropy function ranging from 0.44 to 1.00 for radial distances of 0.5–10 cm and polar angles of 0°–180°.Conclusions:
The data from the Yb-169 HDR source, Model M42, presented in this study show that this source compares favorably with another source of Yb-169, Model 4140, already approved for brachytherapy treatment.
Evaluation of delivered monitor unit accuracy of gated step-and-shoot IMRT using a two-dimensional detector array37(2010); http://dx.doi.org/10.1118/1.3310806View Description Hide Description
Purpose: To overcome the problem of organ motion in intensity-modulated radiation therapy(IMRT), gated IMRT is often used for the treatment of lungcancer. In this study, the authors investigated the accuracy of the delivered monitor units (MUs) from each segment during gated IMRT using a two-dimensional detector array for user-specific verification purpose.
Methods: The authors planned a 6 MV photon, seven-port step-and-shoot lungIMRTdelivery. The respiration signals for gated IMRTdelivery were obtained from the one-dimensional moving phantom using the real-time position management (RPM) system (Varian Medical Systems, Palo Alto, CA). The beams were delivered using a Clinac iX (Varian Medical Systems, Palo Alto, CA) with the Millennium 120 MLC. The MatriXX (IBA Dosimetry GmbH, Germany) was validated through consistency and reproducibility tests as well as comparison with measurements from a Farmer-type ion chamber. The authors delivered beams with varying dose rates and duty cycles and analyzed the MatriXX data to evaluate MU delivery accuracy.
Results: There was quite good agreement between the planned segment MUs and the MUs computed from the MatriXX within ±2% error. The beam-on times computed from the MatriXX data were almost identical for all cases, and they matched well with the RPM beam-on and beam-off signals. A slight difference was observed between them, but it was less than 40 ms. The gated IMRTdelivery demonstrated an MU delivery accuracy that was equivalent to ungated IMRT, and the delivered MUs with a gating signal agreed with the planned MUs within ±0.5 MU regardless of dose rate and duty cycle.
Conclusions: The authors can conclude that gated IMRT is able to deliver an accurate dose to a patient during a procedure. The authors believe that the methodology and results can be transferred to other vendors’ devices, particularly those that do not provide MLC log data for a verification purpose.
37(2010); http://dx.doi.org/10.1118/1.3314074View Description Hide DescriptionPurpose:
In recent years, there has been an increasing interest in flattening-filter free (FFF) beams. However, since the removal of the flattening filter will affect both the mean and the variance of the energy spectrum, current beam-quality specifiers may not be adequate for reference dosimetry in such beams. The purpose of this work was to investigate an alternative, more general beam-quality specifier.Methods:
The beam-quality specifier used in this work was a combination of the kerma-weighted mean and the coefficient of variation of the linear attenuation coefficient in water. These parameters can in theory be determined from narrow-beam transmission measurements using a miniphantom “in-air,” which is a measurement condition well suited also to small and nonstandard fields. The relation between the Spencer-Attix stopping-power ratios and this novel beam-quality specifier was described by a simple polynomial. For reference, the authors used Monte Carlo calculated spectra and stopping-power data for nine different beams, with and without flattening filter.Results:
The polynomial coefficients were obtained by least-squares optimization. For all beams included in this investigation, the average of the differences between the predicted and the Monte Carlo calculated stopping-power ratios was (1 SD) (including TomoTherapy and CyberKnife example beams).Conclusions:
An alternative dual-parameter beam-quality specifier was investigated. The evaluation suggests that it can be used successfully to predict stopping-power ratios in FFF as well as conventional beams, regardless of filtration.
Intensity modulated proton therapy treatment planning using single-field optimization: The impact of monitor unit constraints on plan quality37(2010); http://dx.doi.org/10.1118/1.3314073View Description Hide DescriptionPurpose:
To investigate the effect of monitor unit (MU) constraints on the dose distribution created by intensity modulated proton therapy (IMPT) treatment planning using single-field optimization (SFO).Methods:
Ninety-four energies between 72.5 and 221.8 MeV are available for scanning beam IMPT delivery at our institution. The minimum and maximum MUs for delivering each pencil beam (spot) are 0.005 and 0.04, respectively. These MU constraints are not considered during optimization by the treatment planning system; spots are converted to deliverable MUs during postprocessing. Treatment plans for delivering uniform doses to rectangular volumes with and without MU constraints were generated for different target doses, spot spacings, spread-out Bragg peak (SOBP) widths, and ranges in a homogeneous phantom. Four prostate cancer patients were planned with and without MU constraints using different spot spacings. Rounding errors were analyzed using an in-house software tool.Results:
From the phantom study, the authors have found that both the number of spots that have rounding errors and the magnitude of the distortion of the dose distribution from the ideally optimized distribution increases as the field dose, spot spacing, and range decrease and as the SOBP width increases. From our study of patient plans, it is clear that as the spot spacing decreases the rounding error increases, and the dose coverage of the target volume becomes unacceptable for very small spot spacings.Conclusions:
Constraints on deliverable MU for each spot could create a significant distortion from the ideally optimized dose distributions for IMPT fields using SFO. To eliminate this problem, the treatment planning system should incorporate the MU constraints in the optimization process and the delivery system should reliably delivery smaller minimum MUs.
Analysis of dose to patient, spouse/caretaker, and staff, from an implanted trackable radioactive fiducial for use in the radiation treatment of prostate cancera)37(2010); http://dx.doi.org/10.1118/1.3317436View Description Hide DescriptionPurpose:
A fiducial tracking system based on a novel radioactive tracking technology is being developed for real-time target tracking in radiation therapy. In this study, the authors calculate the radiationdose to the patient, the spouse/caretaker, and the medical staff that would result from a Ir192 radioactive fiducial marker permanently implanted in the prostate of a radiation therapy patient.Methods:
Local tissuedose was calculated by Monte Carlo simulation. The patient’s whole body effective dose equivalent was calculated by summing the doses to the sensitive organs. Exposure of the spouse/caretaker was calculated from the NRC guidelines. Exposure of the medical staff was based on estimates of proximity to and time spent with the patient.Results:
The local dose is below 40 Gy at 5 mm from the marker and below 10 Gy at 10 mm from the marker. The whole body effective dose equivalent to the patient is 64 mSv. The dose to the spouse/caretaker is 0.25 mSv. The annual exposures of the medical staff are 0.2 mSv for a doctor performing implantations and 0.34 mSv for a radiation therapist positioning patients for therapy.Conclusions:
The local dose is not expected to have any clinically significant effect on the surrounding tissue which is irradiated during therapy. The dose to the patient is small in comparison to the whole body dose received from the therapy itself. The exposure of all other people is well below the recommended limits. The authors conclude that there is no radiation exposure related contraindication for use of this technology in the radiation treatment of prostate cancer.
37(2010); http://dx.doi.org/10.1118/1.3312276View Description Hide Description
Digital tomosynthesis (DTS) with a linear accelerator-mounted imaging system provides a means of reconstructing tomographic images from radiographic projections over a limited gantry arc, thus requiring only a few seconds to acquire. Its application in the thorax, however, often results in blurred images from respiration-induced motion. This work evaluates the feasibility of respiration-correlated (RC) DTS for soft-tissue visualization and patient positioning. Image data acquired with a gantry-mounted kilovoltage imaging system while recording respiration were retrospectively analyzed from patients receiving radiotherapy for non-small-cell lungcarcinoma. Projection images spanning an approximately 30° gantry arc were sorted into four respiration phase bins prior to DTS reconstruction, which uses a backprojection, followed by a procedure to suppress structures above and below the reconstruction plane of interest. The DTS images were reconstructed in planes at different depths through the patient and normal to a user-selected angle close to the center of the arc. The localization accuracy of RC-DTS was assessed via a comparison with CBCT. Evaluation of RC-DTS in eight tumors shows visible reduction in image blur caused by the respiratory motion. It also allows the visualization of tumor motion extent. The best image quality is achieved at the end-exhalation phase of the respiratory motion. Comparison of RC-DTS with respiration-correlated cone-beam CT in determining tumor position, motion extent and displacement between treatment sessions shows agreement in most cases within 2–3 mm, comparable in magnitude to the intraobserver repeatability of the measurement. These results suggest the method’s applicability for soft-tissue image guidance in lung, but must be confirmed with further studies in larger numbers of patients.
A 3D global-to-local deformable mesh model based registration and anatomy-constrained segmentation method for image guided prostate radiotherapy37(2010); http://dx.doi.org/10.1118/1.3298374View Description Hide DescriptionPurpose:
In the external beam radiationtreatment of prostate cancers, successful implementation of adaptive radiotherapy and conformal radiationdose delivery is highly dependent on precise and expeditious segmentation and registration of the prostate volume between the simulation and the treatmentimages. The purpose of this study is to develop a novel, fast, and accurate segmentation and registration method to increase the computational efficiency to meet the restricted clinical treatment time requirement in image guided radiotherapy.Methods:
The method developed in this study used soft tissues to capture the transformation between the 3D planning CT (pCT) images and 3D cone-beam CT(CBCT)treatmentimages. The method incorporated a global-to-local deformable mesh model based registration framework as well as an automatic anatomy-constrained robust active shape model (ACRASM) based segmentation algorithm in the 3D CBCTimages. The global registration was based on the mutual information method, and the local registration was to minimize the Euclidian distance of the corresponding nodal points from the global transformation of deformable mesh models, which implicitly used the information of the segmented target volume. The method was applied on six data sets of prostate cancer patients. Target volumes delineated by the same radiationoncologist on the pCT and CBCT were chosen as the benchmarks and were compared to the segmented and registered results. The distance-based and the volume-based estimators were used to quantitatively evaluate the results of segmentation and registration.Results:
The ACRASM segmentation algorithm was compared to the original active shape model (ASM) algorithm by evaluating the values of the distance-based estimators. With respect to the corresponding benchmarks, the mean distance ranged from −0.85 to 0.84 mm for ACRASM and from −1.44 to 1.17 mm for ASM. The mean absolute distance ranged from 1.77 to 3.07 mm for ACRASM and from 2.45 to 6.54 mm for ASM. The volume overlap ratio ranged from 79% to 91% for ACRASM and from 44% to 80% for ASM. These data demonstrated that the segmentation results of ACRASM were in better agreement with the corresponding benchmarks than those of ASM. The developed registration algorithm was quantitatively evaluated by comparing the registered target volumes from the pCT to the benchmarks on the CBCT. The mean distance and the root mean square error ranged from 0.38 to 2.2 mm and from 0.45 to 2.36 mm, respectively, between the CBCTimages and the registered pCT. The mean overlap ratio of the prostate volumes ranged from 85.2% to 95% after registration. The average time of the ACRASM-based segmentation was under 1 min. The average time of the global transformation was from 2 to 4 min on two 3D volumes and the average time of the local transformation was from 20 to 34 s on two deformable superquadrics mesh models.Conclusions:
A novel and fast segmentation and deformable registration method was developed to capture the transformation between the planning and treatmentimages for external beam radiotherapy of prostate cancers. This method increases the computational efficiency and may provide foundation to achieve real time adaptive radiotherapy.
Comparison of Elekta VMAT with helical tomotherapy and fixed field IMRT: Plan quality, delivery efficiency and accuracy37(2010); http://dx.doi.org/10.1118/1.3326965View Description Hide DescriptionPurpose:
Helical tomotherapy (HT) and volumetric modulated arc therapy (VMAT) are arc-based approaches to IMRTdelivery. The objective of this study is to compare VMAT to both HT and fixed field IMRT in terms of plan quality, delivery efficiency, and accuracy.Methods:
Eighteen cases including six prostate, six head-and-neck, and six lung cases were selected for this study. IMRT plans were developed using direct machine parameter optimization in thetreatment planning system. HT plans were developed using a Hi-Art II planning station. VMAT plans were generated using both the SmartArc IMRT module and a home-grown arc sequencing algorithm. VMAT and HT plans were delivered using Elekta’s PreciseBeam VMAT®linaccontrol system (Elekta AB, Stockholm, Sweden) and a TomoTherapy Hi-Art II system (TomoTherapy Inc., Madison, WI), respectively. Treatment plan quality assurance (QA) for VMAT was performed using the IBA MatriXX™ system while an ion chamber and films were used for HT plan QA.Results:
The results demonstrate that both VMAT and HT are capable of providing more uniform target doses and improved normal tissue sparing as compared with fixed field IMRT. In terms of delivery efficiency, VMAT plan deliveries on average took 2.2 min for prostate and lung cases and 4.6 min for head-and-neck cases. These values increased to 4.7 and 7.0 min for HT plans.Conclusions:
Both VMAT and HT plans can be delivered accurately based on their own QA standards. Overall, VMAT was able to provide approximately a 40% reduction in treatment time while maintaining comparable plan quality to that of HT.
37(2010); http://dx.doi.org/10.1118/1.3326969View Description Hide DescriptionPurpose:
To quantitatively test a breathing motion model using the continuity equation and clinical data.Methods:
The continuity equation was applied to a lungtissue and lungtumor free breathing motion model to quantitatively test the model performance. The model used tidal volume and airflow as the independent variables and the ratio of motion to tidal volume and motion to airflow were defined as and vector fields, respectively. The continuity equation resulted in a prediction that the volume integral of the divergence of the vector field was 1.11 for all patients. The integral of the divergence of the vector field was expected to be zero.Results:
For 35 patients, thevector field prediction was , encompassing the expected value. For the vector field prediction, the average value was .Conclusions:
These results provide quantitative evidence that the breathing motion model yields accurate predictions of breathing dynamics.
37(2010); http://dx.doi.org/10.1118/1.3326948View Description Hide DescriptionPurpose:
The purpose of this work is to calculate two-dimensional (2D) dose rate distributions around the BEBIG (Eckert & Ziegler, BEBIG GmbH, Germany) models GK60M21 (old) and Co0.A86 (new) high dose rate brachytherapysources in an unbounded liquid water phantom. The study includes calculation of absorbed dose to water-kerma ratio around the BEBIG sources and a point source in water. A comparison is made with previously published data.Methods:
The EGSnrcMP Monte Carlo code system is used to calculate the absorbed dose and water-kerma in water and air-kerma strength in vacuum. EGSnrcMP-based user codes such as EDKnrc, FLURZnrc, and DOSRZnrc are employed in the work.Results:
The value of reaches a maximum of for the point source (constant between 3.6 and 4.5 mm from the source) and for the BEBIG sources (constant between 2.6 and 3.2 mm along the transverse axis of the sources).Dose rate data for the new and old sources are comparable to published data for radial distances . Differences up to 9% are observed at points close to the source. In addition for the new source, compared to previously published data, dose rate data are higher by 14% along the longitudinal axis where the source cable is connected. Dose rate differences on the longitudinal axis of this source are explained by varying the length of the simulated source cable.Conclusions:
The 2D rectangular data set calculated in the present work could be considered for quality control on radiotherapytreatment planning systems.
- RADIATION IMAGING PHYSICS
37(2010); http://dx.doi.org/10.1118/1.3276732View Description Hide Description
Digital mammography requires revisiting techniques that have been optimized for prior screen/film mammography systems. The objective of the study was to determine optimized radiographic technique for a digital mammographysystem and demonstrate the potential for dose reduction in comparison to the clinically established techniques based on screen- film. An objective figure of merit (FOM) was employed to evaluate a direct-conversion amorphous selenium FFDM system (Siemens Mammomat , Siemens AG Medical Solutions, Erlangen, Germany) and was derived from the quotient of the squared signal-difference-to-noise ratio to mean glandular dose, for various combinations of technique factors and breast phantom configurations including kilovoltage settings , target/filter combinations (Mo–Mo and W–Rh), breast-equivalent plastic in various thicknesses and densities (100% adipose, 50% adipose/50% glandular, and 100% glandular), and simulated mass and calcification lesions. When using a W–Rh spectrum, the optimized FOM results for the simulated mass and calcification lesions showed highly consistent trends with kVp for each combination of breast density and thickness. The optimized kVp ranged from for 100% adipose breasts to for 100% glandular breasts. The use of the optimized W–Rh technique compared to standard Mo–Mo techniques provided dose savings ranging from 9% for thick, 100% adipose breasts, to 63% for thick, 100% glandular breasts, and for breasts with a 50% adipose/50% glandular composition, from 12% for thick breasts up to 57% for thick breasts.
37(2010); http://dx.doi.org/10.1118/1.3285038View Description Hide DescriptionPurpose:
Due to its limited angular scan range, breast tomosynthesis has lower resolution in the depth direction, which may limit its accuracy in quantifying tissue density. This study assesses the quantitative potential of breast tomosynthesis using relatively simple reconstruction and image processing algorithms. This quantitation could allow improved characterization of lesions as well as image processing to present tomosynthesisimages with the familiar appearance of mammography by preserving more low-frequency information.Methods:
All studies were based on a Siemens prototype MAMMOMAT Novation TOMO breast tomo system with a 45° total angular span. This investigation was performed using both simulations and empirical measurements. Monte Carlo simulations were conducted using the breast tomosynthesis geometry and tissue-equivalent, uniform, voxelized phantoms with cuboid lesions of varying density embedded within. Empirical studies were then performed using tissue-equivalent plastic phantoms which were imaged on the actual prototype system. The material surrounding the lesions was set to either fat-equivalent or glandular-equivalent plastic. From the simulation experiments, the effects of scatter, lesion depth, and background material density were studied. The empirical experiments studied the effects of lesion depth, background material density, x-ray tube energy, and exposure level. Additionally, the proposed analysis methods were independently evaluated using a commercially available QA breast phantom (CIRS Model 11A). All image reconstruction was performed with a filtered backprojection algorithm. Reconstructed voxel values within each slice were corrected to reduce background nonuniformities.Results:
The resulting lesion voxel values varied linearly with known glandular fraction (correlation coefficient) under all simulated and empirical conditions, including for the independent tests with the QA phantom. Analysis of variance performed on the fit line parameters revealed statistically significant differences between the two different background materials and between 28 kVp and the remaining energies (26, 30, and 32 kVp) for the dense experimental phantom. However, no significant differences arose between different energies for the fatty phantom, nor for any of the many other combinations of parameters.Conclusions:
These strong linear relationships suggest that breast tomosynthesisimage voxel values, after being corrected by our outlined methods, are highly positively correlated with true tissue density. This consistent linearity implies that breast tomosynthesisimaging indeed has potential to be quantitative.
Reconstruction of 3D lung models from 2D planning data sets for Hodgkin’s lymphoma patients using combined deformable image registration and navigator channels37(2010); http://dx.doi.org/10.1118/1.3284368View Description Hide DescriptionPurpose:
Late complications (cardiac toxicities, secondary lung, and breast cancer) remain a significant concern in the radiation treatment of Hodgkin’s lymphoma (HL). To address this issue, predictive dose-risk models could potentially be used to estimate radiotherapy-related late toxicities. This study investigates the use of deformable image registration (DIR) and navigator channels (NCs) to reconstruct 3D lung models from 2D radiographic planning images, in order to retrospectively calculate the treatment dose exposure to HL patients treated with 2D planning, which are now experiencing late effects.Methods:
Three-dimensional planning CTimages of 52 current HL patients were acquired. 12 image sets were used to construct a male and a female population lung model. 23 “Reference” images were used to generate lung deformation adaptation templates, constructed by deforming the population model into each patient-specific lung geometry using a biomechanical-based DIR algorithm, MORFEUS. 17 “Test” patients were used to test the accuracy of the reconstruction technique by adapting existing templates using 2D digitally reconstructedradiographs. The adaptation process included three steps. First, a Reference patient was matched to a Test patient by thorax measurements. Second, four NCs (small regions of interest) were placed on the lung boundary to calculate 1D differences in lung edges. Third, the Reference lung model was adapted to the Test patient’s lung using the 1D edge differences. The Reference-adapted Test model was then compared to the 3D lung contours of the actual Test patient by computing their percentage volume overlap (POL) and Dice coefficient.Results:
The average percentage overlapping volumes and Dice coefficient expressed as a percentage between the adapted and actual Test models were found to be and , respectively. Paired -tests demonstrated that the volumetric reconstruction method made a statistically significant improvement to the population lung model shape . The error in the results were also comparable to the volume overlap difference observed between inhale and exhale lung volumes during free-breathing respiratory motion , which implies that the accuracies of the reconstruction method are within breathing constraints and would not be the confining factor in estimating normal tissuedose exposure.Conclusions:
The result findings show that the DIR-NC technique can achieve a high degree of reconstruction accuracy, and could be useful in approximating 3D dosimetric representations of historical 2D treatment. In turn, this could provide a better understanding of the biophysical relationship between dose-volume exposure and late term radiotherapy effects.