Index of content:
Volume 36, Issue 6, June 2009
- Joint Imaging/Therapy Scientific Session: Ballroom C
- Cone Beam CT
TH‐D‐BRC‐01: Improvement of Megavoltage Cone‐Beam CT Image Quality Using a Low‐Atomic Number X‐Ray Target36(2009); http://dx.doi.org/10.1118/1.3182672View Description Hide Description
Purpose: to investigate the application of an unflattened photonbeam, generated using a low atomic number (Z) x‐ray target, to MV cone‐beam computed tomography(CBCT)imaging. Improvements of imagecontrast and contrast‐to‐noise‐ratio (CNR) versus dose are quantified and compared to the standard 6MV beam. Limitation of the contrast advantage with patient separation is examined. Method and Materials: The experimental beam was generated by a 2100EX linac (Varian Medical, Inc) by placing a 1.0 cm‐thick Al target 9 mm below the primary collimator vacuum window and operating the linac in 6 MeV electron mode. The flattening filtration was removed. Projections were acquired using an AS1000 detector every 2° through 360°. CBCTcontrast was compared for both the low‐Z‐target and 6MV beams.CNR was measured as a function of dose using a bone/lung phantom containing a central ionization chamber. The same phantom was located in cylindrical containers ranging in diameter from 13 cm to 25 cm to measure the rate of reduction of CBCTcontrast with separation. Finally, a pig head was imaged allowing a qualitative comparison. Results:Contrast is improved by a factor ranging from 1.8 to 3.4 (mean 2.3) with the low‐Z‐target beam. Over an imaging dose range from 3 cGy to 23.5 cGy, CNR improves by a consistent factor of 1.7 and 2.4 for bone and lung, respectively. Contrast deteriorates with separation more rapidly for the low‐Z‐target beam than for 6MV; however for the maximum diameter of 25 cm contrast remains superior by a factor of 1.4 and 1.5 for bone and lung, respectively. Images of the pig head demonstrate qualitatively improved CNR and preservation of spatial resolution. Conclusion:Contrast and CNR are improved significantly in CBCTimages using the low‐Z‐target beam, over a clinically‐useful range of patient separation. Conflict of interest: Research sponsored by Varian Medical, Incorporated.
TH‐D‐BRC‐02: Dosimetric Characterization of An Imaging Beam Line with a Carbon Electron Target for Megavoltage Cone Beam Computed Tomography36(2009); http://dx.doi.org/10.1118/1.3182673View Description Hide Description
Purpose: Megavoltage cone beam CTimage quality can be significantly improved with an imaging beam line (IBL) with no flattener and a carbon electron target. The IBL imaging dose is non‐negligible, however, and the high keV‐range x‐ray fluence makes beam modeling nontrivial. An IBL was modeled with the Philips Pinnacle3 treatment planning system, verified experimentally, and applied to clinical cases. Method and Materials: The IBL was modeled and the image acquisition dose was verified in a customized acrylic cylindrical phantom with 196 ion chambermeasurements. Agreement between the measured and modeled IBL dose was quantified with the 3D γ‐index. Representative IBL and TBL imaging dose distributions were calculated for head and neck and prostate patients and included in treatment plans using the imaging dose incorporation (IDI) method. Surface dose was measured for the TBL and IBL for four head and neck cancer patients with metal oxide field effect transistors. Results: The depth dose and profile curves had 97% passing γ‐indices for dosimetric and distance acceptance criteria of 3%, 3 mm, and 100% passed for 5.2%, 5.2 mm. For the ion chambermeasurements of phantom image acquisition dose, the IBL model had 93% passing γ‐ indices for acceptance criteria of 3%, 3 mm, and 100% passed for 4%, 4 mm. Differences between the IBL‐ and TBL‐based IMRT treatment plans created with the IDI method were dosimetrically insignificant. IBL surface dose was greater than TBL by 18% (p = 2x10minus;6). Conclusion: The IBL can be modeled with acceptable accuracy using a standard TPS, and accounting for IBL dose in treatment plans with the IDI‐method is straightforward. The resulting composite dose distributions, assuming similar imaging doses, are negligibly different from those of the TBL. The increased IBL surface dose relative to the TBL is likely clinically insignificant.
Conflict of Interest: Partially funded by Siemens.
36(2009); http://dx.doi.org/10.1118/1.3182674View Description Hide Description
Purpose: To develop a fast Monte‐Carlo‐based scatter‐correction algorithm for clinical keV cone‐beam CT(CBCT)images.Method and Materials: Estimates of the scatter in the projection‐views of a CBCT scan were obtained by an iterative process, each step consisting of: (1) a coarse CBCT reconstruction; (2) simulation of photon histories for projections using a purpose‐written Monte Carlo code; (3) scoring scatter contributions to fixed points on the detector (a “forced detection” technique); and (4) subtraction of scatter‐estimates from the measured pixel‐values. The scatter signal at each pixel was estimated using linear interpolationspatially between the values calculated at the fixed points and angularly between projection angles. Following convergence to a set of scatter‐corrected profiles, a final full‐resolution scatter‐corrected reconstruction was performed. All CBCT reconstructions were performed using software developed in‐house. The x‐ray tube spectrum and the energy‐response of the detector were both modeled. To validate the technique, projection measurements (120 kV and 0.4 mAs per projection) of a Catphan quality‐assurance phantom (The Phantom Laboratory) were obtained using a Synergy XVI CBCT unit (Elekta Limited). Results: Typically the algorithm took less than 2 min to complete 4 iterations on a desktop PC, after which convergence was obtained. Qualitatively, the algorithm resulted in an improved image with the characteristic ‘cupping’ artifacts, due to scatter, disappearing. Quantitatively, non‐uniformity was decreased after correction from about 15% to 1% or less at a cost of an increase in image noise from 3.7% to 5.1%. CT number accuracy was also markedly improved. Conclusion: It was shown Monte‐Carlo‐based scatter‐correction of clinical keV CBCTimages does not have to be prohibitively slow. Such a scatter‐correction can be successfully performed in a few CPU minutes.
TH‐D‐BRC‐04: Improved Dose‐Image Quality Trade‐Off in Cone‐Beam CT by Optimization of Beam Modulation Filter Shape36(2009); http://dx.doi.org/10.1118/1.3182675View Description Hide Description
Purpose: To introduce a method for optimizing the bow‐tie filter (BTF) shape in CBCT to achieve the maximum imagecontrast and minimum imagenoise relative to the patient dose. Materials and Methods: The proposed method finds the BTF shape that minimizes an objective function that accounts for dose delivered by the CBCT scan, imagenoise and contrast. The BTF shape, constant for all projections, is described by parallel translation of a curve, represented by few (4–8) drive points. The objective function combines dose and contrast‐to‐noise ratio at the central slice, reconstructed by filtered backprojection. Dose and projection data can be computed either by analytical model or by the Monte Carlo method. Nelder‐Mead optimization is used to find the set of drive point coordinates that minimizes the objective function. Results: The method was applied for two cylindrical water phantoms of diameters of 18 cm and 22 cm. The computation time for MC‐based optimization was four hours, mainly spent on MC calculations. We currently investigate how the (time‐dependent) MC statistical fluctuation affects the convergence of the process. The convergence also does not depend on the selection of the drive points that describe the BTF shape, however the convergence is not achieved when the objective function is not sensitive to location of one or more drive points. Conclusions: Our method is an important tool for improving the trade‐off between dose and image quality in CBCT. Describing the BTF shape description by a few drive points simplifies the problem and makes it solvable by a simple optimization method, while coupling the optimization code with the MC simulator gives the possibility of realistic modeling of the CBCT unit, at the expense of increased computation time.
This work was supported by a research grant provided by Varian Medical Systems.
TH‐D‐BRC‐05: Respiratory Motion Correction of Cone‐Beam CT in Abdomen Using a Patient‐Specific Motion Model36(2009); http://dx.doi.org/10.1118/1.3182676View Description Hide Description
Purpose: Respiratory motion reduction methods to improve cone‐beam CT quality (CBCT) have focused on the thorax, but reduced tissuecontrast in abdomen poses additional challenges. We report a method to correct CBCT in abdomen, using a motion model adapted to the patient from a prior respiration‐correlated CT (RCCT) image set. Method and Materials: Model adaptation consists of nonrigid image registration that maps each RCCT image to a reference image in the set, followed by principal component analysis (PCA) to reduce noise in the resultant deformation fields and relate them to diaphragm position and motion (inhalation or exhalation). CBCT projection images are sorted into subsets according to diaphragm position in the images and reconstructed, yielding a set of low‐quality 3‐D images. Model application deforms the CBCTimages to a reference CBCT in the set; combining all images yields a high‐quality CBCTimage with reduced motion artifacts. We also investigate a simpler correction method, which does not use PCA and correlates motion state with respiration phase. Comparison of contrast‐to‐noise ratios of pixel intensities within kidneys relative to surrounding background tissue provides a quantitative assessment of relative organ visibility. Results: Evaluation of CBCT examples in upper abdomen shows that streaking artifacts and blurring of liver, kidneys, spleen, bowel and implanted fiducial markers are visibly reduced with PCA‐model‐based correction. Phase‐based motion correction without PCA reduces blurring less effectively; in addition, implanted markers appear broken up, indicating inconsistencies in the correction. Model‐based motion correction shows the highest contrast‐to‐noise ratios in the cases examined. Conclusion: Motion correction of CBCT in abdomen is feasible and yields observable improvement. The PCA‐based model is an important component: first, by removing noise; second, by relating deformation to diaphragm position rather than phase, thus accommodating breathing pattern changes between imaging sessions.
TH‐D‐BRC‐06: The Investigation and Correction of a Bowtie‐Related Cone‐Beam CT Circular Band Artifact36(2009); http://dx.doi.org/10.1118/1.3182678View Description Hide Description
Purpose: In the course of developing projection‐space preprocessing algorithms for improving on‐board CBCTCT number accuracy and uniformity, a persistent, prominent circular band artifact (CBA) with asymmetric illumination shadows was discovered. The CBA remain unchanged even after applying beam‐hardening, scatter subtraction, and veiling glare corrections to Varian OBI full‐fan projection data but only when a bow‐tie filter is used. This study investigates the causes and correction strategies of the CBA. Method and Materials:CBCTimages were acquired, preprocessed, and reconstructed with an in‐house FDK engine for phantoms of different diameters and locations relative to isocenter, on several OBI systems, to characterize CBA behavior and to form hypotheses as to its origin. Numerical simulations were used to evaluate all hypothesized contributing factors, as assessed by the necessary experimental measurements. A custom calibration was performed to identify the dependence of kV source location and flat‐panel detector pose as a function of gantry angle. Different correction approaches were tried on both synthetic and measured datasets, including gantry‐angle‐dependent normalization, full‐ and partial kV beam geometry calibrations, and empirical cancellation. The interplay between the CBA corrections and scatter and beam hardening corrections was also studied. Results: The CBA had a diameter of about 15 cm, was centered at the isocenter, and had similar asymmetric illumination, for all phantom dimensions, locations, and machines, and was reproducible over time. It appeared only when the bowtie filter was used. Simulations and experimental studies identified that a combination of geometric wobble and the bowtie filter slope caused the artifact. Gantry‐angle dependent calibrations of normalization were sufficient for about 80% CBA mitigation, but that complete elimination required gantry‐angle dependent beam‐hardening corrections. Conclusion:CBCT geometric wobble with the presence of bowtie filter could cause a circular band artifact.
Supported by NIH P01 CA116602 and a grant from Varian Medical Systems.
TH‐D‐BRC‐07: Impact of Respiratory Biofeedback On Adaptively Sampled 4D‐CBCT Image Quality: Initial Experiences36(2009); http://dx.doi.org/10.1118/1.3182680View Description Hide Description
Purpose: Linac‐mounted 4D‐Cone Beam CT(CBCT)imaging is an important tool for IGRT. Our protocol uses an in‐house built audio‐video (AV) biofeedback device to regularize patient respiration during 4D‐CBCT data acquisition. In this study, the impact of the respiratory stabilization on phase‐correlated 4D‐CBCT image quality is evaluated in our full field‐of‐view (FOV) 4D‐CBCT procedure using the Varian OBI and RPM respiratory monitoring system. Materials and methods: We have implemented a 4D‐CBCT imaging procedure with a 450.0 mm diameter FOV using adaptive acquisition time and projection sampling frequency. An AV respiratory biofeedback device consisting of a LCD visual and audio feedback channels was used. It provides the patient with a real‐time visual and auditory cues whenever the RPM deviates from the selected reference breathing trace. During the 8∼10 minute data acquisition, the patient was instructed to duplicate the reference trace as closely as possible both in terms of displacement and time. The acquired projections were sorted into 10 groups by the associated phase recorded by the RPM system. The effects of the AV biofeedback on the respiratory regularity were studied in terms of average cycle length, baseline variation and displacement. Subsequently, the corresponding 4D‐CBCT images were compared with different acquisition AV modes, such as free breathing, audio only and AV regularization. Results: The patient respiratory track presents larger variation in the free‐breathing data acquisition. The auditory instruction to the patient exhibits control of the displacement but a noticeable baseline drift through the time. The AV biofeedback device improves the reproducibility of respiratory pattern. Conclusion: Our results demonstrated that the AV biofeedback device improved the reproducibility of patient respiratory pattern in our 4D‐CBCT data acquisition. The image quality benefits from the improved projection consistency within an extended time span in every phase bin.
Supported by PPG NIH P01 116602.
36(2009); http://dx.doi.org/10.1118/1.3182681View Description Hide Description
Purpose: To model and validate the Varian OBI kV CBCTscanner and to assess radiation dose to the anatomical patient phantom. Method and Materials:Monte Carlo code, MCNPX, was used to simulate the x‐ray source including the energy spectrum, filter, and scan trajectory. The scanner with full‐fan and half‐fan scan modes was validated by comparing simulated peripheral doses against measured peripheral doses. The validated scannermodel was then involved with the phantom of an anatomical patient phantom to calculate the organ doses. Results: Comparison between simulated and measured peripheral results performed on OBI kV CBCT with body and head phantoms shows good agreement in terms of the discrepancy between the simulated and measured Dcentral, as well as simulated and measured CBCTDIw. The nuances were ranged 1% – 8%. It was found that, during the IGRTCBCTimaging procedure in low dose mode with total 252 mAs used, that eye lens received the largest dose around 6 cGy for head‐neck scan, and thymus received the largest dose around 7 cGy for lung‐chest scan,. Conclusion: This work demonstrates the ability of modeling and validating kV CBCTscanner by using Monte Carlo technique, as well as rapidly and accurately assessing organ doses by combining the kV CBCTscannermodel and the patient phantom.
TH‐D‐BRC‐09: A Status Update On the Development of Proton CT at Loma Linda University Medical Center36(2009); http://dx.doi.org/10.1118/1.3182682View Description Hide Description
At present, proton therapy treatment plans are carried out using data from X‐ray CT scans. This method relies on the conversion of CT Hounsfield units to relative protonstopping power values. Since these quantities are not related on a one‐to‐one basis, uncertainties in addition to other prevalent X‐ray CT uncertainties due to beam‐hardening and high‐density artifacts are introduced into the treatment planning calculations. Protoncomputed tomography (pCT) is a novel imaging modality that is capable of directly reconstructing relative stopping power of the patient through individual protonenergy loss measurements. The implementation of pCT requires a proton gantry with at least 180 deg rotational capability and sufficient proton energy to penetrate the anatomical proton planning region. Existing proton treatment centers with 230–250 MeV maximum proton energy should be able to use pCT for most targets in the head and neck and thoracic/abdominal regions, while there may be some restrictions in the pelvic region for adult patients. Design considerations of a modern pCT scanner and simulation results have previously been published. This contribution summarizes recent developments in the pCT project at Loma Linda University Medical Center (LLUMC). Both GEANT4‐simulated and experimental pCT imaging data obtained with a small pCT prototype developed in collaboration with the Santa Cruz Institute of Particle Physics and installed on the proton research beam line at LLUMC will be presented. Hardware decisions regarding the next‐generation pCT scanner, which will permit scanning of head‐sized objects, will be presented and discussed. Progress has also been made in the formulation of the most likely path (MLP) of protons through an object, and parallelizable iterative reconstruction algorithms that can be implemented on general‐purpose commodity graphics processing units (GPGPUs).
- Imaging for Therapy Assessment
36(2009); http://dx.doi.org/10.1118/1.3182469View Description Hide Description
Purpose: Uncertainty in the location of the distal dose edge is one of the main concerns about proton therapy. It leads to cautious treatment plans that partially neutralize the dosimetric advantage of protons. Vertebral bone marrow responds to radiation with fatty replacement that is visible on post‐treatment MRI. This presents a unique opportunity to visualize radiation effectsin vivo. We have developed a method that uses spine MRI changes to precisely localize the distal dose edge in spinal protonradiation patients. Method and Materials: We carefully registered treatment planningCT scans and follow‐up T1‐weighted MRI scans from 10 proton spinal radiation patients. A radiationdose‐MRI signal intensity curve was created using the lateral beam penumbra in the sacrum. This curve was then used to quantitatively examine possible systematic or spatially varying proton range errors. Results: In the lateral penumbra there was a gradual increase in signal intensity with higher dose throughout the full dose range of 0–37.5 Gy. In the distal dose fall‐off region, the beam appeared to penetrate farther than planned in the central part of the vertebral bodies. The mean overshoot in five patients was 2.71 mm (95% confidence interval 1.13–4.28 mm). These errors are probably not clinically significant with current treatment planning procedures. Conclusion: We have demonstrated in vivoproton range verification based on post‐treatment spine MRI changes. Our analysis indicates that if range errors occur in spine treatments, their magnitude is at most a few millimeters in the majority of patients. It may be possible to extend our technique to MRI sequences that show early bone marrow changes. It could then be used for adaptive modification of spine radiation plans in order to reduce radiationdose to bone marrow and other normal tissues.
WE‐C‐BRC‐02: Combined Dynamic Contrast Enhanced Magnetic Resonance Imaging (DCE‐MRI) and Magnetic Resonance Thermal Imaging (MRTI) for Optimal Hyperthermia Treatment of Advanced Extremity Sarcomas: Fifteen Patients Update36(2009); http://dx.doi.org/10.1118/1.3182470View Description Hide Description
Purpose: To deliver optimal hyperthermia (HT) treatments for patients with advanced extremity sarcomas using DCE‐MRI and MRTI Method and Materials: Patients were treated on a protocol using radiotherapy consisting of 45 Gy and once a week HT for 5 weeks in a setting that allows the use of a 1.5 T GE magnet for imaging. Pre‐ and post first HT treatment DCE‐MR images were acquired and analyzed with software from iCAD Inc. (Nashua, NH) based on a full time point pharmacokinetics (PK) analysis that measures tumor's permeability (Ktrans) and extracellular volume fraction (Ve). Multi‐slice temperature rise images were obtained using the proton resonance frequency shift technique during heating in a 140 MHz phased array HT applicator and compared to invasive temperature measurements. Thermal dose metrics were calculated and correlated with response and with the PK parameters. Results: Fifteen patients were enrolled on this protocol; ten patients were heated. Permeability maps derived from DCE‐MR images were used to guide the placement of the invasive catheter to span regions varying from high to low perfusion. The MRTI images were used to steer the power, with the intent of heating the tumor uniformly while maintaining surrounding normal tissue at lower temperatures. From the evaluable HT treatments, we achieved excellent correlation (ΔT<1°C) between the MRTI and invasive measurements. As a trend, patients that were either pathological complete or partial responders had a decrease in Ktrans. The Ve parameter showed no clear trend with pathological response or % necrosis. As expected, tumors that were more vascularized (higher Ktrans) heated less than tumors with a high degree of necrosis or fluid pockets. Conclusion: DCE‐MRI coupled with MRTI provides information on tumor environment that can improve planning, delivery, and evaluation of HT treatments.
Supported by a grant from the NCI CA42745.
WE‐C‐BRC‐03: Evaluating Extent of Cell Death in 3D Mid‐To‐High Frequency Ultrasound by Registration with Whole‐Mount Tumor Histopathology36(2009); http://dx.doi.org/10.1118/1.3182471View Description Hide Description
Purpose: In this study, we investigate the precision and accuracy of mid‐to‐high frequency ultrasound imaging to assess non‐invasively cell death for incorporation of this method in pre‐clinical and clinical practice to characterize tumor response to radiotherapy.Method and Materials:Tumor xenografts (n=8) of head and neck cancer were exposed to radiationdoses of 2, 4 and 8Gy. Ultrasoundimages were collected with an ultrasoundscanner using frequencies of 15–35MHz before and 24 hours after exposure to radiation. Irradiated tumors exhibited large hyperechoic regions in ultrasoundsimages 24 hours after exposure to radiation that corresponded to areas of cell death in histology. The ultrasoundimages were registered with the histological images of the tumor slices taken at regular intervals. The tumor was contoured on histological slices and ultrasoundimages, the regions of cell death were contoured on histological slices and the hyperechoic regions were contoured on ultrasoundimages. Each set of contours was converted to a surface mesh. The volume and center of mass were calculated for each representation determined by a surface mesh. Results: The average difference between the relative (to histology) volume representations in histology and ultrasound were 10.7%±8.9% for tumor and 21.7%±12.2% for cell death. The average differences between the relative (to the maximum dimension of the tumor) center of mass of volume representations in histology and ultrasoundimages were 2.7%±2.0% for tumor and 15.5%±8.9 % for cell death. Conclusion: The method provides the correspondence between the volumes of cell death assessed from histology and from ultrasound imaging and can be used to assess early tumor response to radiotherapy. Part of the differences associated with cell death representation in histological and ultrasoundimages (21.7%) was caused by the differences in tumor representation (10.7%) in these images. The effect of these uncertainties is the subject of ongoing investigation.
36(2009); http://dx.doi.org/10.1118/1.3182472View Description Hide Description
Purpose: Assessment of treatment response in solid tumors is essential for disease management but typical response measures are limited and non‐robust. We compared the predictive power of several PET‐based measures in cancer patients receiving therapy. Method and Materials: Fourteen adult patients with advanced solid malignancies were treated with sunitinib malate, a molecular targeted agent. Using the cellularproliferation marker FLT, whole‐body PET/CT scans were acquired pre‐, mid‐, and post‐treatment. Lesions were segmented on PETimages and treatment response was assessed via percent change of the following measures: SUVmean, SUVmax, SUVpeak, SUVtotal, and PET defined tumor size: unidimensional, bidimensional, and volume. PET response assessment and clinical endpoint of therapy were used independently to classify patients into response categories: partial response (PR), stable disease (SD), or progressive disease (PD). Predictive power and robustness of PET measures were tested by varying response category cutoffs to generate ROC curves and Matthews correlation coefficients (MCC), a classification quality measure. Results: SUV measures demonstrated improved predictive power over PET defined size measures (MCCaverage: 0.27 vs. 0.21). Of all measures, SUVpeak was the most robust response predictor (MCCaverage: 0.30, MCCmax: 1.0), especially for prediction of PR. Of PET defined size measures, bidimensional size was the most robust response predictor. Changes in SUV measures pre‐ to mid‐treatment showed improved predictive power over pre‐ and mid‐ to post‐treatment changes (MCCaverage: 0.32, 0.27, and 0.22 respectively). Prediction of PR was vastly superior to that of SD and PD (MCCaverage: 0.44, 0.16, and 0.14 respectively). ROC analysis supported MCC results. Conclusion: Comparison of PET‐based measures revealed that SUV measures were more predictive of response than PET defined size measures. SUVpeak, a rarely used functional measure, demonstrated the most robust predictive power. Future treatment response studies should test and implement more robust measures, like SUVpeak, for improved response assessment.
36(2009); http://dx.doi.org/10.1118/1.3182473View Description Hide Description
Background: Sunitinib (SU) is a tyrosine kinase inhibitor (TKI) with activity against VEGFR. During VEGFR TKI withdrawal, increased pain at sites of metastasis has been observed, which we hypothesize is due to the proliferative flare. ‐fluoro‐3′‐deoxy‐3′‐L‐fluorothymidine (FLT) PETimaging was used as a marker of treatment response, to assess the proliferative flare and to investigate perfusion and vascular status of the tumor.Methods: 14 patients with advanced solid malignancies have been enrolled. SU was given at the standard dose of 50 mg for 4 weeks, followed by a 2 week break. 90‐minute dynamic FLT PET/CT scans were obtained at the baseline, week 4, and week 6. 8 patients with adequate follow‐up time were classified in two groups: with clinical benefit (CB; progressive disease (PD)>6 mo) or without clinical benefit (noCB; PD<6 mo). Changes in the peak FLT standardized uptake value (SUVpeak) were calculated. In addition, FLT kinetic analysis was performed to extract perfusion and vasculature status of the tumor.Results: The differences in FLT uptake were significant between the two groups (CB: −20%, noCB: ∼0%). Interestingly, both groups exhibited a significant increase in SUVpeak during SU withdrawal, but with a significantly higher increase in the group without clinical benefit (CB: +20%, noCB: +50%). The kinetic analysis revealed significant differences in the perfusion at week 4 between the groups (CB: ∼0%, noCB: −40%), with no significant differences at the end of the treatment cycle. Conclusions: Change in SUVpeak was associated with the degree of clinical response. Proliferative increase in the target lesions during SU withdrawal was observed, and was higher for patients with poor clinical outcome.
Research sponsored in part by Pfizer.
WE‐C‐BRC‐06: Determining Changes to Tissues Irradiated During Proton Treatment Delivery by Measuring Secondary Prompt Gamma‐Ray Spectra36(2009); http://dx.doi.org/10.1118/1.3182474View Description Hide Description
Purpose: Recent studies have investigated the use of secondary ‘prompt’ gamma‐rays emitted from patients during proton therapy as a possible method to verify dose delivery. The purpose of this study was to determine if the prompt gamma‐ray spectra emitted from biological tissues could be used as a method to analyze changes to tissuesirradiated during proton treatment delivery.Method and Materials: We preformed measurements and Monte Carlo calculations of the prompt gamma emission spectra from tissue equivalent phantoms and tissue targets. The emission spectra from several types of tissue where characterized according to the emission lines from the individual elemental constituents of the target tissue. Next, we studied the individual emission lines from the tissue targets as a function of elemental concentration and density. Results: These results show that the prompt gamma emission lines from the major elemental components can be identified in the measured and calculated spectra. The intensities of these emission lines were found to be a function of concentration of each element and the physical density of the tissue.Conclusion: Based on the results of these preliminary studies, we conclude that it may be possible to determine both the atomic composition and physical density of irradiatedtissues by measuring the prompt gamma ray spectra emitted during proton treatment delivery. Development of a technique to measure prompt emission spectra during daily treatment delivery would allow for the direct measurement of changes to irradiatedtissues, such as changes to tumor hypoxia or tissue densities over the course of proton treatment delivery.
36(2009); http://dx.doi.org/10.1118/1.3182475View Description Hide Description
Purpose: With the increased presence of volumetric radiotherapy cone beam CT (RT‐CBCT) systems, the importance of high grade image quality is crucial to achieve daily image‐guided adaptive radiotherapy (IGART) capabilities. We subjected three commercially available RT‐CBCT systems to a battery of standard diagnostic image quality tests acquired under clinical conditions. This study reports on the evaluation of image qualities for RT‐CBCT's and a diagnostic CT.Methods and Materials: RT‐CBCT scans were performed on Elekta XVI, Nucletron Simulix, and Varian OBI Advanced Imaging using clinical pelvis scan settings on a CATPHAN Model 600. They were then compared to results from a GE Lightspeed CT scanner. The phantom contained modules allowing measurement of low contrast resolution, slice thickness, Hounsfield Unit (HU) sensitivity, spatial resolution, and image uniformity. Results: No CBCT system was able to detect any targets in the low contrast module. Six targets could be seen on diagnostic CT's. All RT‐CBCT systems had greater than 33% variations in slice thickness reconstruction. The HU sensitometry test showed absolute differences between accepted HU values and measured values for XVI and Simulix systems to be on average 6 times and 3 times the error seen in a diagnostic CT respectively. HU sensitivity for OBI is within 15% of a diagnostic CT. Both OBI and Simulix had spatial resolution twice that of XVI but were 3 lp/cm worse than a diagnostic CT. Measurements of image uniformity showed that all three RT‐CBCT systems have a standard deviation of HU values on the order of 10 times that of a diagnostic CT.Conclusions: The image qualities of three evaluated RT‐CBCT systems are relatively comparable, yet still inferior to those from helical CT. It is important to note that dosimetric settings can have a clear impact on image quality, and dose measurements were not done for this work.
WE‐C‐BRC‐08: A Method to Evaluate Region‐Specific Pulmonary Function Using 4D CT Images for Lung Cancer Patients Undergoing Radiation Therapy36(2009); http://dx.doi.org/10.1118/1.3182476View Description Hide Description
Purpose: Collateral radiation exposure to healthy lungtissue during radiation therapy can result in changes in structural and biomechanical properties of the lung. These changes may cause various clinical symptoms. The purpose of this study was to develop a functional imaging technique to assess the lung's region‐specific ventilation and pressure during or after radiation treatment.
Method and Materials: With an in‐house developed finite element framework, a heterogeneous elastic model was developed for a lung patient and its Young's moduli were derived from a set of 4D CTimages, acquired during radiation treatment. Each phase of the 4D dataset was registered, using deformable image registration (ITK demons algorithm) with the end‐inhale reference dataset. The resultant deformation matrix was used first to calculate the volumetric variation of each image voxel to generate a 3D ventilation image, and then to compute its corresponding transpulmonary pressure with the mechanical model. Results:Lung volumes on each phase of the acquired 4D dataset were compared with those derived from the deformed model, and were found to be within 1% of each other. The maximum ventilation occurs from phase 1 to phase 2, the earliest expiration phase. The average ventilation increased from 20.2% in phase 2 to 30.8% in phase 5 and their correspondent pressures increased from 1.57 Kpa to 2.25 Kpa. This result is generally consistent with published measurements. Conclusion: This study describes a theoretical approach to calculate the region‐specific ventilation and mechanical functions using deformable image registration. The method may be applied toward understanding how the mechanical properties of damaged lung differ from that of healthy lungtissue, and therefore it has potential applicability as a diagnostic indicator, as well as a tool for predicting radiation‐induced lung damages. Work is underway to correlate this approach with other traditional functional‐imaging modalities used to assess lung function.
WE‐C‐BRC‐09: Targeting Accuracy in Real‐Time Tumor Tracking Via External Surrogates: A Retrospective Clinical Study36(2009); http://dx.doi.org/10.1118/1.3182477View Description Hide Description
Purpose: To quantify the accuracy of real‐time tumor tracking via external surrogates in a large patient population. Method and Materials: A database of 339 fractions in 86 patients, who received hypofractionated stereotactic body radiotherapy with a roboticlinear accelerator (CyberKnife®) between July 2005 and June 2007, was analyzed. The system tracks the tumor in real‐time by using a correspondence model between external marker motion and implanted fiducials. Such model is updated during treatment by intermittent stereoscopic X‐ray imaging, providing ground truth tumor position. The corresponding model includes prediction in the future in order to account for the time lag due to data processing and robot motion (∼115 ms). Predicted tumor positions were compared to ground truth data acquired during treatment to quantify the achieved targeting accuracy. Results: Intermittent imaging provided a statistically robust description of tumor motion 89% of the fractions. Clinical data show that the mean error contribution due to the need for prediction is approximately 0.5 mm. The overall average accuracy in tumor tracking, expressed as the 95% confidence interval of targeting errors, is ∼3.4 mm. We proved that the targeting error increases as a function of the tumor range of motion and treatment site. Larger effects were measured in the superior‐inferior direction, with a 3 mm error increase per 1 cm range of motion variation. Also, unevenly spaced model updating was found to significantly reduce the accuracy in tumor tracking. Conclusion: The accuracy of a state of the art solution for tumor tracking, developed for high precision radiotherapy with a roboticlinear accelerator, has been analyzed. Adequate accuracy can be achieved, with caveats when the range of motion is large and the model updating scheme is irregular. Future work will include detailed analysis on progressive upgrades in the correlation model.
WE‐C‐BRC‐10: The Utility of Surrogates of the Distribution of Pulmonary Function in Individualizing Thoracic Radiotherapy36(2009); http://dx.doi.org/10.1118/1.3182478View Description Hide Description
Purpose: Heterogeneity of lungtissue density and function is a hallmark of pathologic changes in chronic obstructive pulmonary disease (COPD). However, the standard risk assessment approaches, based on dose‐volume histograms (DVH), do not currently use the functional information in evaluating pulmonary tolerance to radiation therapy. We investigated the utility of weighting of dose distributions by surrogate metrics of the lung function (parenchyma density, ventilation distribution) as complementary sources of information for assessing the risk of complications, and for further individualizing of thoracic radiation therapy.Methods: The tissue density and the distribution of ventilatory activity were estimated from respiration‐correlated CT scans of 19 patients. Dose distributions from clinical IMRT (N=17) and proton (N=2) therapy plans were reevaluated by applying importance weighting to separate 1‐cc volume elements in the lung, accordingly with their estimated relative functional load. The histogram metrics: mean lungdose (MLD), share of the 5‐Gy isodose (V5), etc., were compared between various dose‐weighting methods. Regions characterized by high ventilatory activity were segmented, and research IMRT plans were optimized with objectives aiming to maximize sparing of lung function. Results: Substantial differences between the metrics of DVH and ventilation‐weighted histograms were observed, e.g., change of over 5% in MLD in 8 patients, relative increase of over 10% in V20 in 3 patients out of 19. The correlation between the density‐ and ventilation‐weighted metrics was insignificant. The use of additional optimization constraints for highly‐ventilated lung subvolumes allowed for a reduction in the ventilation‐weighted MLD, without compromising the target coverage, or causing deterioration in the standard DVH. Conclusions: Elevated values of ventilation‐weighted MLD may indicate the increased risk of complications in patients with COPD. The use of ventilation distribution maps for selection of beam angles, and definition of optimization constraints for functional avoidance, can potentially reduce the risk, without compromising the therapy efficiency.
- Interfraction Motion and Margin Assessment
TU‐D‐BRC‐01: Repeated Measures Analysis of Variance of Patient Specific Daily Margins to Assess Interfraction Motion for Cervical Cancer Patients Undergoing IMRT Using Daily CBCT Imaging36(2009); http://dx.doi.org/10.1118/1.3182376View Description Hide Description
Purpose: To analyze interfraction motion in cervical cancer patients based on repeated measures analysis of variance of patient‐specific daily uniform margins obtained using daily CBCTimaging.Methods and Materials: Ten cervical cancer patients (stage IB‐IVA) treated with IMRT were analyzed for this study. A CTV consisting of the regional lymph nodes, upper vagina, parametria, cervix and uterus was contoured on the planning CT and daily CBCTimages. Twenty‐five CBCT scans, corresponding to each fraction were planned and attempted for each patient. The acquired CBCTimages for all the fractions were rigidly registered to the planning CT with respect to bony anatomy. The initial planning CTV was then cast onto the registered daily CBCT and were modified to reflect changes due to organ motion and deformation. Patient‐specific daily margins covering 100% of daily CTV volumes were calculated. Repeated measures analysis of variance (ANOVA) was performed on the patient specific daily margins to quantify inter‐patient and intra‐patient variations in estimated margins. Mean values within the subjects were imputed for the missing fractions. F statistics was used to test the null hypothesis that day of the treatment or fraction number had no effect on daily margins. Results: Patient specific daily uniform margins were used to generate an ANOVA table. The data shows a grand mean of 15.3 mm among all patients and fractions which represents the estimate of the average population margin. In addition, the inter‐patient standard deviation in margins is 5.1 mm and intra‐patient standard deviation is 35.4 mm indicating that the variation between patients is significant. The F statistics (F = 0.654) was significant indicating that the day of the treatment or fraction number had no influence on margin. Conclusion: Margins varied substantially between patients indicating adaptive RT approaches are needed for the treatment of cervical cancer patients.