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ABSTRACT: Purpose: To present a framework for measurement-guided VMAT dose reconstruction to moving patient voxels from a known motion kernel and the static phantom data, and to validate this perturbation-based approach with the proof-of-principle experiments.Methods: As described previously, the VMAT 3D dose to a static patient can be estimated by applying a phantom measurement-guided perturbation to the treatment planning system (TPS)-calculated dose grid. The fraction dose to any voxel in the presence of motion, assuming the motion kernel is known, can be derived in a similar fashion by applying a measurement-guided motion perturbation. The dose to the diodes in a helical phantom is recorded at 50 ms intervals and is transformed into a series of time-resolved high-density volumetric dose grids. A moving voxel is propagated through this 4D dose space and the fraction dose to that voxel in the phantom is accumulated. The ratio of this motion-perturbed, reconstructed dose to the TPS dose in the phantom serves as a perturbation factor, applied to the TPS fraction dose to the similarly situated voxel in the patient. This approach was validated by the ion chamber and film measurements on four phantoms of different shape and structure: homogeneous and inhomogeneous cylinders, a homogeneous cube, and an anthropomorphic thoracic phantom. A 2D motion stage was used to simulate the motion. The stage position was synchronized with the beam start time with the respiratory gating simulator. The motion patterns were designed such that the motion speed was in the upper range of the expected tumor motion (1-1.4 cm∕s) and the range exceeded the normally observed limits (up to 5.7 cm). The conformal arc plans for X or Y motion (in the IEC 61217 coordinate system) consisted of manually created narrow (3 cm) rectangular strips moving in-phase (tracking) or phase-shifted by 90° (crossing) with respect to the phantom motion. The XY motion was tested with the computer-derived VMAT MLC sequences. For all phantoms and plans, time-resolved (10 Hz) ion chamber dose was collected. In addition, coronal (XY) films were exposed in the cube phantom to a VMAT beam with two different starting phases, and compared to the reconstructed motion-perturbed dose planes.Results: For the X or Y motions with the moving strip and geometrical phantoms, the maximum difference between perturbation-reconstructed and ion chamber doses did not exceed 1.9%, and the average for any motion pattern∕starting phase did not exceed 1.3%. For the VMAT plans on the cubic and thoracic phantoms, one point exhibited a 3.5% error, while the remaining five were all within 1.1%. Across all the measurements (N = 22), the average disagreement was 0.5 ± 1.3% (1 SD). The films exhibited γ(3%∕3 mm) passing rates ≥90%.Conclusions: The dose to an arbitrary moving voxel in a patient can be estimated with acceptable accuracy for a VMAT delivery, by performing a single QA measurement with a cylindrical phantom and applying two consecutive perturbations to the TPS-calculated patient dose. The first one accounts for the differences between the planned and delivered static doses, while the second one corrects for the motion.
Medical Physics 02/2013; 40(2):021708. · 2.83 Impact Factor
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ABSTRACT: The 6 MV flattening filter-free (FFF) beam has been commissioned for use with compensators at our institution. This novel combination promises advantages in mitigating tumor motion due to the reduced treatment time made possible by the greatly increased dose rate of the FFF beam. Given the different energy spectrum of the FFF beam and the beam hardening effect of the compensator, the accuracy of the treatment planning system (TPS) model in the presence of low-density heterogeneities cannot be assumed. Therefore, inhomogeneity correction factors (ICF) for an FFF beam attenuated by brass slabs were measured and compared to the TPS calculations in this work. The ICF is the ratio of the point dose in the presence of inhomogeneity to the dose in the same point in a homogeneous medium. The ICFs were measured with an ion chamber at a number of points in a flat water-equivalent slab phantom containing a 7.5 cm deep heterogeneity (air or 0.27 g/cm3 wood). Comparisons for the FFF beam were carried out for the field sizes from 5× 5 to 20 × 20 cm2 with the brass slabs ranging from 0 to 5 cm in thickness. For a low-density wood heterogeneity in a slab phantom, with the exception of the point 1cm beyond the proximal buildup interface, the TPS handles the inhomogeneity correction with the brass-filtered 6 MV FFF beam at the requisite 2% error level. The combinations of field sizes and compensator thicknesses when the error exceeds 2% (2.6% maximum) are not likely to be experienced in clinical practice. In terms of heterogeneity corrections, the beam model is adequate for clinical use.
Journal of Applied Clinical Medical Physics 01/2013; 14(3):3990. · 1.29 Impact Factor
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ABSTRACT: To develop and validate a volume-modulated arc therapy (VMAT) quality assurance (QA) tool that takes as input a time-resolved, low-density (∼10 mm) cylindrical surface dose map from a commercial helical diode array, and outputs a high density, volumetric, time-resolved dose matrix on an arbitrary patient dataset. This first validation study is limited to a homogeneous "patient."
A VMAT treatment is delivered to a diode array phantom (ARCCHECK, Sun Nuclear Corp., Melbourne, FL). 3DVH software (Sun Nuclear) derives the high-density volumetric dose using measurement-guided dose reconstruction (MGDR). MGDR cylindrical phantom results are then used to perturb the three-dimensional (3D) treatment planning dose on the patient dataset, producing a semiempirical volumetric dose grid. Four-dimensional (4D) dose reconstruction on the patient is also possible by morphing individual sub-beam doses instead of the composite. For conventional (3D) dose comparison two methods were developed, using the four plans (Multi-Target, C-shape, Mock Prostate, and Head and Neck), including their structures and objectives, from the AAPM TG-119 report. First, 3DVH and treatment planning system (TPS) cumulative point doses were compared to ion chamber in a cube water-equivalent phantom ("patient"). The shape of the phantom is different from the ARCCHECK and furthermore the targets were placed asymmetrically. Second, coronal and sagittal absolute film dose distributions in the cube were compared with 3DVH and TPS. For time-resolved (4D) comparisons, three tests were performed. First, volumetric dose differences were calculated between the 3D MGDR and cumulative time-resolved patient (4D MGDR) dose at the end of delivery, where they ideally should be identical. Second, time-resolved (10 Hz sampling rate) ion chamber doses were compared to cumulative point dose vs time curves from 4D MGDR. Finally, accelerator output was varied to assess the linearity of the 4D MGDR with global fluence change.
Across four TG-119 plans, the average PTV point dose difference in the cube between 3DVH and ion chamber is 0.1 ± 1.0%. Average film vs TPS γ-analysis passing rates are 83.0%, 91.1%, and 98.4% for 1%∕2 mm, 2%∕2 mm, and 3%∕3 mm threshold combinations, respectively, while average film vs 3DVH γ-analysis passing rates are 88.6%, 96.1%, and 99.5% for the same respective criteria. 4D MGDR was also sufficiently accurate. First, for 99.5% voxels in each case, the doses from 3D and 4D MGDR at the end of delivery agree within 0.5% local dose-error∕1 mm distance. Moreover, all failing voxels are confined to the edge of the cylindrical reconstruction volume. Second, dose vs time curves track between the ion chamber and 4D MGDR within 1%. Finally, 4D MGDR dose changes linearly with the accelerator output: the difference between cumulative ion chamber and MGDR dose changed by no more than 1% (randomly) with the output variation range of 10%.
Even for a well-commissioned TPS, comparison metrics show better agreement on average to MGDR than to TPS on the arbitrary-shaped measurable "patient." The method requires no more accelerator time than standard QA, while producing more clinically relevant information. Validation in a heterogeneous thoracic phantom is under way, as is the ultimate application of 4D MGDR to virtual motion studies.
Medical Physics 07/2012; 39(7):4228-38. · 2.83 Impact Factor
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ABSTRACT: Compensator-based IMRT coupled with the high dose rate flattening filter free (FFF) beams offers an intriguing possibility of delivering an intensity modulated radiation field in just a few seconds. As a first step, the authors evaluate the dosimetric accuracy of the treatment planning system (TPS) FFF beam model with compensators.
A 6 MV FFF beam from a TrueBeam accelerator (Varian Medical Systems, Palo Alto CA) was modeled in PINNACLE TPS (v. 9.0, Philips Radiation Oncology, Fitchburg WI). Flat brass slabs from 0.3 to 7 cm thick and an 18° brass wedge were used to adjust the beam model. A 2D (MAPCHECK) and 3D (ARCCHECK) diode arrays (Sun Nuclear Corp, Melbourne FL), were investigated for use with the compensator FFF beams. Corrections for diode sensitivity caused by the spectral changes in the beam were introduced. Four compensator plans based on the AAPM TG-119 report were developed. A composite ion chamber measurement, beam by beam MAPCHECK measurements, and a composite ARCCHECK measurement were performed. The array results were analyzed with the same thresholds as in TG-119 report-3%/3 mm with global dose normalization-as well as with the more stringent combinations of the gamma analysis criteria.
The FFF beam shows a greater variation of the effective attenuation coefficient with brass thickness due to the prevalence of the low energy photons compared to the conventional 6X beam. As a result, a compromise had to be made while trying to achieve dose agreement for a combination of field sizes, brass thicknesses, and measurement depths (≥5 cm in water). An agreement of measured and calculated dose to within 1% was observed for brass thicknesses up to 2 cm. For the 3 cm slab, an error of up to 2.8% was noted for the field sizes above 10 × 10 cm(2), and up to 3.8% for the 5 × 5 cm(2) field. Both diode arrays exhibit a substantial sensitivity drop as the compensator thickness increases, reaching 10% for a 7 cm brass slab. A simple correction based on the brass thickness along the ray was introduced to counteract this effect. Pooled for five profiles, the average ratio of uncorrected and corrected MAPCHECK to ion chamber readings are 0.966 and 1.008, respectively. With the proper correction, all MAPCHECK measurement to calculation comparisons exhibit 100% γ(3%/3 mm) passing rates with global dose-error normalization. For the TG-119-type plans, the average γ(2%/2 mm) passing rate with local normalization is 94% (range 87.8%-98.3%). The lower ARCCHECK γ-analysis passing rates (corrected for diode sensitivity) are predictable based on the observed PDD discrepancies. However, with the 3%/3 mm thresholds and global normalization, the average γ-analysis passing rate is 96.4% (range 89.9%-100%).
MAPCHECK analysis demonstrates high passing rates with the stringent γ(2%/2 mm) and local normalization criteria combination. The geometry of the ARCCHECK array creates a stress test for the FFF TPS model because of the shallow depth of the entrance diodes and large air cavity. Hence, the ARCCHECK γ-analysis passing rates are lower than with the MAPCHECK, while still on par with TG-119.
Medical Physics 01/2012; 39(1):342-52. · 2.83 Impact Factor
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ABSTRACT: The goal of any dosimeter is to be as accurate as possible when measuring absolute dose to compare with calculated dose. This limits the uncertainties associated with the dosimeter itself and allows the task of dose QA to focus on detecting errors in the treatment planning (TPS) and/or delivery systems. This work introduces enhancements to the measurement accuracy of a 3D dosimeter comprised of a helical plane of diodes in a volumetric phantom.
We describe the methods and derivations of new corrections that account for repetition rate dependence, intrinsic relative sensitivity per diode, field size dependence based on the dynamic field size determination, and positional correction. Required and described is an accurate "virtual inclinometer" algorithm. The system allows for calibrating the array directly against an ion chamber signal collected with high angular resolution. These enhancements are quantitatively validated using several strategies including ion chamber measurements taken using a "blank" plastic shell mimicking the actual phantom, and comparison to high resolution dose calculations for a variety of fields: static, simple arcs, and VMAT. A number of sophisticated treatment planning algorithms were benchmarked against ion chamber measurements for their ability to handle a large air cavity in the phantom.
Each calibration correction is quantified and presented vs its independent variable(s). The virtual inclinometer is validated by direct comparison to the gantry angle vs time data from machine log files. The effects of the calibration are quantified and improvements are seen in the dose agreement with the ion chamber reference measurements and with the TPS calculations. These improved agreements are a result of removing prior limitations and assumptions in the calibration methodology. Average gamma analysis passing rates for VMAT plans based on the AAPM TG-119 report are 98.4 and 93.3% for the 3%/3 mm and 2%/2 mm dose-error/distance to agreement threshold criteria, respectively, with the global dose-error normalization. With the local dose-error normalization, the average passing rates are reduced to 94.6 and 85.7% for the 3%/3 mm and 2%/2 mm criteria, respectively. Some algorithms in the convolution/superposition family are not sufficiently accurate in predicting the exit dose in the presence of a 15 cm diameter air cavity.
Introduction of the improved calibration methodology, enabled by a robust virtual inclinometer algorithm, improves the accuracy of the dosimeter's absolute dose measurements. With our treatment planning and delivery chain, gamma analysis passing rates for the VMAT plans based on the AAPM TG-119 report are expected to be above 91% and average at about 95% level for γ(3%/3 mm) with the local dose-error normalization. This stringent comparison methodology is more indicative of the true VMAT system commissioning accuracy compared to the often quoted dose-error normalization to a single high value.
Medical Physics 09/2011; 38(9):5021-32. · 2.83 Impact Factor
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Physics in Medicine and Biology 06/2011; 56(11):3445-6. · 2.83 Impact Factor
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ABSTRACT: In ITV-based 3D-planning, the information of volume occupancy versus respiratory phase is not utilized. We propose a motion-weighted CTV (mwCTV) delineation method, which carries some 4D-information into planning. This method allows plan optimization based on occupancy-weighting and generation of motion-weighted DVH (mwDVH) that approximate the DVHs of full 4D-dose accumulation.
Occupancy information from contours in 4D-CT is incorporated in the mwCTV generation. Higher-occupancy volumes receive higher dosimetric priority in planning. The temporally-weighted mwCTV is converted to a spatially-weighted mwCTV incorporating the temporal-weighting in mwDVH generation using the 3D-dose distribution. The mwDVHs were compared with DVHs of deformable-image-registration (DIR)-based 4D-dose accumulation and 3D-method for 10 cases.
For all the cases, the mwDVH curves are closer to the 4D-calculated DVH than the 3D-DVHs are, indicating a better approximation of the 4D-DVH. The 70 Gy-covered percentage-CTV volume differed by -2.8% ± 0.8% between 3D and 4D, and 0.3% ± 0.7% between mwDVH and 4D-methods. The mean RMS values of the percentage-volume differences for the 4D-3D is 1.7 ± 1.1, while for the 4D-mwDVH is 0.4 ± 0.3.
The mwCTV and mwDVH method, which is simple in implementation and does not require DIR, is a practical approximation of DIR-based 4D-planning and evaluation.
Radiotherapy and Oncology 03/2011; 99(1):67-72. · 5.58 Impact Factor
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ABSTRACT: To reduce positron emission tomography (PET) and computed tomography (CT) misalignments and standardized uptake value (SUV) errors, cine average CT (CACT) has been proposed to replace helical CT (HCT) for attenuation correction (AC). A new method using interpolated average CT (IACT) for AC is introduced to further reduce radiation dose with similar image quality. Six patients were recruited in this study. The end-inspiration and -expiration phases from cine CT were used as the two original phases. Deformable image registration was used to generate the interpolated phases. The IACT was calculated by averaging the original and interpolated phases. The PET images were then reconstructed with AC using CACT, HCT and IACT, respectively. Their misalignments were compared by visual assessment, mutual information, correlation coefficient and SUV. The doses from different CT maps were analyzed. The misalignments were reduced for CACT and IACT as compared to HCT. The maximum SUV difference between the use of IACT and CACT was ∼3%, and it was ∼20% between the use of HCT and CACT. The estimated dose for IACT was 0.38 mSv. The radiation dose using IACT could be reduced by 85% compared to the use of CACT. IACT is a good low-dose approximation of CACT for AC.
Physics in Medicine and Biology 03/2011; 56(8):2559-67. · 2.83 Impact Factor
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ABSTRACT: We introduce a logical process of three distinct phases to begin the evaluation of a new 3D dosimetry array. The array under investigation is a hollow cylinder phantom with diode detectors fixed in a helical shell forming an "O" axial detector cross section (ArcCHECK), with comparisons drawn to a previously studied 3D array with diodes fixed in two crossing planes forming an "X" axial cross section (Delta⁴). Phase I testing of the ArcCHECK establishes: robust relative calibration (response equalization) of the individual detectors, minor field size dependency of response not present in a 2D predecessor, and uncorrected angular response dependence in the axial plane. Phase II testing reveals vast differences between the two devices when studying fixed-width full circle arcs. These differences are primarily due to arc discretization by the TPS that produces low passing rates for the peripheral detectors of the ArcCHECK, but high passing rates for the Delta⁴. Similar, although less pronounced, effects are seen for the test VMAT plans modeled after the AAPM TG119 report. The very different 3D detector locations of the two devices, along with the knock-on effect of different percent normalization strategies, prove that the analysis results from the devices are distinct and noninterchangeable; they are truly measuring different things. The value of what each device measures, namely their correlation with--or ability to predict--clinically relevant errors in calculation and/or delivery of dose is the subject of future Phase III work.
Journal of Applied Clinical Medical Physics 01/2011; 12(2):3346. · 1.29 Impact Factor
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ABSTRACT: Cutaneous red blood cell velocity in vivo can be measured by using capillaroscopy with image processing techniques. However, unlike simulated blood flow images, there is no standard to determine the accuracy of the techniques for computing blood flow velocities. In this paper, we quantitatively evaluated the accuracy of previously proposed optical flow method for measuring red blood cell velocity in nail-fold capillaries. Blood flow images of subjects under normal and occlusion-release conditions were examined by a capillaroscope. To obtain velocity values, the images were further analyzed by using optical flow, cross-correlation and visual inspection methods, respectively. Visual inspection method was taken as the golden standard to determine the accuracy of blood flow velocity measurement using optical flow and cross-correlation techniques. Results showed that optical flow estimation provided superior accuracy to cross-correlation when assessing real blood flow velocity in nail-fold capillaries. Optical flow estimation is able to measure red blood cell velocity with a high accuracy of 91% and 86% when the observed velocity is less than 0.5mm/s under normal and occlusion-release conditions, respectively. In addition, optical flow method showed good agreement with visual inspection in determining blood flow velocity in both normal and occlusion-release conditions when the high-velocity zone is excluded.
Microvascular Research 01/2011; 81(3):252-60. · 2.83 Impact Factor
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ABSTRACT: Quantization of red blood cell (RBC) velocity in micro-vessel is one of the techniques for dynamic observation of microvascular mechanisms. The flow measurement of RBC in micro-vessels is still a challenge nowadays. Image processing for velocity measurement using a frame by frame analysis is a common approach. The accuracy of the calculations, which is algorithm dependant, has rarely been examined. In this paper, we evaluated the accuracy of the existing methods, which includes cross correlation method, Hough transform method, and optical flow method, by applying these methods to simulated micro-vessel image sequences. Simulated experiments in various micro-vessels with random RBC motion were applied in the evaluation. The blood flow variation in the same micro-vessels with different RBC densities and velocities was considered in the simulations. The calculation accuracy of different flow patterns and vessel shapes were also examined, respectively. Based on the comparison, the use of an optical flow method, which is superior to a cross-correlation method or a Hough transform method, is proposed for measuring RBC velocity. The study indicated that the optical flow method is suitable for accurately measuring the velocity of the RBCs in small or large micro-vessels.
Microvascular Research 12/2010; 80(3):477-83. · 2.83 Impact Factor
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ABSTRACT: Image registration is often a required and a time-consuming step in blood flow analysis of large microscopic video sequences in vivo. In order to obtain stable images for blood flow analysis, frame-to-frame image matching as a preprocessing step is a solution to the problem of movement during image acquisition. In this paper, microscopic system analysis without fluorescent labelling is performed to provide precise and continuous quantitative data of blood flow rate in individual microvessels of nude mice. The performance properties of several matching metrics are evaluated through simulated image registrations. An automatic image registration programme based on Powell's optimisation search method with low calculation redundancy was implemented. The matching method by variance of ratio is computationally efficient and improves the registration robustness and accuracy in practical application of microcirculation registration. The presented registration method shows acceptable results in close requisition to analyse red blood cell velocities, confirming the scientific potential of the system in blood flow analysis.
Computer Methods in Biomechanics and Biomedical Engineering 11/2010; 14(4):319-30. · 0.85 Impact Factor
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ABSTRACT: Red blood cell (RBC) dynamics in capillaries is a useful diagnostic tool for many diseases. Previous study showed that optical flow estimation (OFE) is capable of accurately calculating RBC velocities using image registration technique. The computational fluid dynamics (CFD) method is explored in this study to calculate the RBC velocity in capillaries of finger nail-fold for six cases. The two-dimensional capillary images were reconstructed to three-dimensional, assuming circular cross sections. The no-slip boundary conditions were applied on the vessel walls. The initial velocity of the RBC going into each capillary was calculated by OFE. The velocities of multiple points along each capillary calculated by CFD, V(CFD), were compared with OFE calculations, V(OFE). The calculated RBC velocity was in the range of 56-685μm/s. The average difference (V(CFD) - V(OFE)) with one standard deviation is -2.66±18.61μm/s for all the 48 calculation points, and 0.03±0.12μm/s for all except one points (47 points), indicating that CFD can provide a reasonable accuracy in RBC velocity calculation in finger nail-fold capillaries.
Microvascular Research 10/2010; 81(1):68-72. · 2.83 Impact Factor
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ABSTRACT: Compared with multileaf collimator (MLC)-based intensity-modulated radiotherapy (IMRT) for moving targets, compensator-based IMRT has advantages such as shorter beam-on time, fewer monitor units with potentially decreased secondary carcinogenesis risk, better optimization-to-deliverable dose conversion, and often better dose conformity. Some of the disadvantages include additional time for the compensators to be built and delivered, as well as extra cost. Patients undergoing treatment of abdominal cancers often experience weight loss. It would be necessary to account for this change in weight with a new plan and a second set of compensators. However, this would result in treatment delays and added costs. We have developed a method to re-plan the patient using the same set of compensators. Because the weight changes seen with the treatment of abdominal cancers are usually relatively small, a new 4D computed tomography (CT) acquired in the treatment position with markers on the original isocenter tattoos can be registered to the original planning scan. The contours of target volumes from the original scans are copied to the new scan after fusion. The original compensator set can be used together with a few field-in-field (FiF) beams defined by the MLC (or beams with cerrobend blocks for accelerators not equipped with a MLC). The weights of the beams with compensators are reduced so that the FiF or blocked beams can be optimized to mirror the original plan and dose distribution. Seven abdominal cancer cases are presented using this technique. The new plan on the new planning CT images usually has the same dosimetric quality as the original. The target coverage and dose uniformity are improved compared with the plan without FiF/block modification. Techniques combining additional FiF or blocked beams with the original compensators optimize the treatment plans when patients lose weight and save time and cost compared with generating plans with a new set of compensators.
Medical dosimetry: official journal of the American Association of Medical Dosimetrists 03/2010; 36(1):102-8. · 1.26 Impact Factor
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ABSTRACT: For an institution that already owns the licenses, it is economically advantageous and technically feasible to use Pinnacle TPS (Philips Radiation Oncology Systems, Fitchburg, WI) with the BrainLab Novalis delivery system (BrainLAB A.G., Heimstetten, Germany). This takes advantage of the improved accuracy of the convolution algorithm in the presence of heterogeneities compared with the pencil beam calculation, which is particularly significant for lung SBRT treatments. The reference patient positioning DRRs still have to be generated by the BrainLab software from the CT images and isocenter coordinates transferred from Pinnacle. We validated this process with the end-to-end hidden target test, which showed an isocenter positioning error within one standard deviation from the previously established mean value. The Novalis treatment table attenuation is substantial (up to 6.2% for a beam directed straight up and up to 8.4% for oblique incidence) and has to be accounted for in calculations. A simple single-contour treatment table model was developed, resulting in mean differences between the measured and calculated attenuation factors of 0.0%-0.2%, depending on the field size. The maximum difference for a single incidence angle is 1.1%. The BrainLab micro-MLC (mMLC) leaf tip, although not geometrically round, can be represented in Pinnacle by an arch with satisfactory dosimetric accuracy. Subsequently, step-and-shoot (direct machine parameter optimization) IMRT dosimetric agreement is excellent. VMAT (called "SmartArc" in Pinnacle) treatments with constant gantry speed and dose rate are feasible without any modifications to the accelerator. Due to the 3 mm-wide mMLC leaves, the use of a 2 mm calculation grid is recommended. When dual arcs are used for the more complex cases, the overall dosimetric agreement for the SmartArc plans compares favorably with the previously reported results for other implementations of VMAT: gamma(3%,3mm) for absolute dose obtained with the biplanar diode array passing rates above 97% with the mean of 98.6%. However, a larger than expected dose error with the single-arc plans, confined predominantly to the isocenter region, requires further investigation.
Journal of Applied Clinical Medical Physics 01/2010; 11(3):3240. · 1.29 Impact Factor
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ABSTRACT: Thoracic cancer treatment presents dosimetric difficulties due to respiratory motion and lung inhomogeneity. Monte Carlo and deformable image registration techniques have been proposed to be used in four-dimensional (4D) dose calculations to overcome the difficulties. This study validates the 4D Monte Carlo dosimetry with measurement, compares 4D dosimetry of different tumor sizes and tumor motion ranges, and demonstrates differences of dose-volume histograms (DVH) with the number of respiratory phases that are included in 4D dosimetry. BEAMnrc was used in dose calculations while an optical flow algorithm was used in deformable image registration and dose mapping. Calculated and measured doses of a moving phantom agreed within 3% at the center of the moving gross tumor volumes (GTV). 4D CT image sets of lung cancer cases were used in the analysis of 4D dosimetry. For a small tumor (12.5 cm3) with motion range of 1.5 cm, reduced tumor volume coverage was observed in the 4D dose with a beam margin of 1 cm. For large tumors and tumors with small motion range (around 1 cm), the 4D dosimetry did not differ appreciably from the static plans. The dose-volume histogram (DVH) analysis shows that the inclusion of only extreme respiratory phases in 4D dosimetry is a reasonable approximation of all-phase inclusion for lung cancer cases similar to the ones studied, which reduces the calculation in 4D dosimetry.
Radiation Oncology 01/2010; 5:45. · 2.32 Impact Factor
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ABSTRACT: The tangential-beam technique frequently presents challenges in homogeneity of radiation dose to the target. To ensure an adequate dose to the skin, a bolus is often used. Tomotherapy has already been shown to improve target conformity and homogeneity in other disease sites. Because of the tangential delivery technique and lack of flattening filter in TomoTherapy accelerators, we hypothesize that during chest wall irradiation using tomotherapy, the skin dose will be adequate without bolus.
This study compares the dosimetric differences between tomotherapy chest wall irradiation and traditional linear accelerator-based tangential-beam technique. Tomotherapy treatment plans with and without bolus were compared with tangentialbeam plans. Plans were also generated for phantom studies, and point doses were measured using MOSFET dosimetry to verify the adequate skin dose. Monte Carlo simulations of static beams of both techniques were performed, and dosimetry was compared.
Monte Carlo simulations and measurements confirmed that beams from tomotherapy deliver a higher skin dose than a standard linear accelerator. Skin dose also increases with the incident angle of the beams.
Because of the characteristics of the tomotherapy beam and delivery technique, chest wall treatment plans from tomotherapy showed adequate skin dose [more than 75% of prescribed planning target volume (PTV) dose] even without bolus.
Japanese journal of radiology 11/2009; 27(9):355-62. · 0.65 Impact Factor
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ABSTRACT: A new approach for the measurement of the red blood cell (RBC) velocity from capillary video by using optical flow estimation has been developed. An image registration function based on mutual information was used for stabilizing images in order to cope with slight finger movement during video acquisition. After image alignment, a skeleton extraction algorithm implemented by thinning was followed which enabled tracking blood flow entirely in arteriolar and venular limbs, and the curved segment as well. Optical flow and cross-correlation approaches were applied individually for velocity estimation of twelve microcirculation videos acquired independently from three healthy volunteers. The RBC velocity of 12 vessels at three given measurement sites (arteriolar, curve and venular sites) in a 45-second period of occlusion-release condition of vessel were examined. There were four stages of flow conditions: resting (T(1)), pre-occlusion (T(2)), post-occlusion (T(3)) and release (T(4)). The results from both approaches revealed that the velocity difference among the three sites were not significant. The pattern of distribution of RBC velocity was also reported. The correlation coefficient (r) of the velocity calculated using optical flow and cross-correlation in four stages of blood flow conditions and the overall correlation were: 1-window: r(T1)=0.68, r(T2)=0.67, r(T3)=0.92, r(T4)=0.88 and r(All)=0.79; 2-window: r(T1)=0.84, r(T2)=0.88, r(T3)=0.87, r(T4)=0.93 and r(All)=0.88. The averaged velocity results showed no significant differences between optical flow and 2-window cross-correlation in all flow conditions. Optical flow estimation is not only independent to the direction of flow, but also able to calculate the intensity displacement of all pixels. The proposed velocity measurement system has been shown to provide complete velocity information for the whole vessel limb which demonstrates the advantage of measuring blood flow at the level of microcirculation more accurately.
Microvascular Research 08/2009; 78(3):319-24. · 2.83 Impact Factor
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ABSTRACT: Currently there are no commercially available tools to generate composite plans across different treatment modalities and/or different planning image sets. Without a composite plan, it may be difficult to perform a meaningful dosimetric evaluation of the overall treatment course. In this paper, we introduce a method to generate composite biological effective dose (BED) plans over multiple radiotherapy treatment modalities and/or multistage plans, using deformable image registration. Two cases were used to demonstrate the method. Case I was prostate cancer treated with intensity-modulated radiation therapy (IMRT) and a permanent seed implant. Case II involved lung cancer treated with two treatment plans generated on two separate computed tomography image sets. Thin-plate spline or optical flow methods were used as appropriate to generate deformation matrices. The deformation matrices were then applied to the dose matrices and the resulting physical doses were converted to BED and added to yield the composite plan. Cell proliferation and sublethal repair were considered in the BED calculations. The difference in BED between normal tissues and tumor volumes was accounted for by using different BED models, alpha/beta values, and cell potential doubling times. The method to generate composite BED plans presented in this paper provides information not available with the traditional simple dose summation or physical dose summation. With the understanding of limitations and uncertainties of the algorithms involved, it may be valuable for the overall treatment plan evaluation.
Medical dosimetry: official journal of the American Association of Medical Dosimetrists 06/2009; 35(2):143-50. · 1.26 Impact Factor
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ABSTRACT: To investigate the effectiveness of different abdominal compression levels on tumor and organ motion during stereotactic body radiotherapy of lower lobe lung and liver tumors using four-dimensional (4D)-CT scan analysis.
Three 4D-CT scans were acquired for 10 patients first using with no compression and then compared with two different levels of abdominal compression. The position of the tumor and various organs were defined at the peak inspiratory and expiratory phases and compared to determine the maximum motion.
Mean (+/-SD) medium compression force (MC) and high compression force (HC) were 47.6 +/- 16.0 N and 90.7 +/- 27.1 N, respectively. Mean overall tumor motion was 13.6 mm (2sigma [2 sigma] 11.5-15.6), 8.3 mm (2sigma 6.0-10.5), and 7.2 mm (2sigma 5.4-9.0) for no compression, MC, and HC, respectively. A significant difference in the control of both superior-inferior (SI) and overall motion of tumors was seen with the application of MC and HC when compared with no compression (p < 0.0001 for both). High compression force improved SI and overall tumor motion compared with MC, but this was only significant for SI motion (p = 0.04 and p = 0.06). Significant control of organ motion was only seen in the pancreas (p = 0.01).
Four-dimensional CT shows significant control of both lower lobe lung and liver tumors using abdominal compression. High levels of compression improve SI tumor motion when compared with MC.
International Journal of Radiation OncologyBiologyPhysics 05/2008; 70(5):1571-8. · 4.11 Impact Factor
