bluuebunny/crossref_metadata_2025 huggingface dataset:
TU‐A‐I‐617‐02: PET Scanner Performance: Quality Assurance and Acceptance Testing
Authors: Kemp B
Published: June 2005

Quality assurance of PET scanners must be performed on a regular basis to maintain and confirm proper scanner performance. These procedures should track system stability and be sensitive to changes in scanner operation. The quality control and calibration of a PET scanner includes detector and electronic characterizations such as adjustment of PMT gain, definition of crystal and energy maps and coincidence timing calibration. Briefly, the crystal map converts the analog position of the detected event to a specific crystal within the block detector, while the coincidence timing calibration adjusts for timing delays so events from each block are time stamped equivalently. These characterizations are applied to the PET data during acquisition. Daily quality assurance should determine whether the scanner is suitable for clinical use. A uniform cylinder of activity or rod sources are a good source of quality control data. These scans can be used to monitor system stability and to determine which crystals, blocks and modules (buckets) are more (or less) sensitive than the respective system average. The NEMA PET NU2‐2001 standard should be followed for acceptance testing. This standard uses a polyethylene phantom of 700mm axial length with a 3.2 mm line source to measure scatter fraction, count losses and randoms. This phantom approximates the out‐of‐field activity distributions of whole body scans. The measurement of sensitivity is conducted with a line source surrounded by known absorbers, and the sensitivity with no absorbers can be found by extrapolation. The intent of the image quality measurement is mimic a whole body scan using a torso phantom containing hot and cold spheres of various diameters (representing lesions) in a warm background. This presentation will focus on the calibrations and corrections required to maintain proper system performance. The presentation will also describe the rationale and methodology of the NEMA NU2‐2001 performance standards.
TU‐C‐I‐609‐03: Effects of Exposure and Structural Background On the Detection of Computer‐Simulated Nodules in Digital Chest Phantom Images: 2‐AFC Study
Authors: Lai C , Shaw C , Sabloff B , Wang T , Liu X , Chen L , Meng Y , Tu S , Altunbas M , Kappadath S
Published: June 2005

Purpose: To investigate how the exposure level and structural background affect the detection of computer‐simulated nodules in digital chest phantom images. Method and Materials: Images of an anthropomorphic chest phantom were acquired with an a‐Si/CsI flat‐panel based digital radiography system at 120 kVp and a tube current ranging from 0.25 to 32 mAs. The source‐to‐image distance (SID) was increased and the x‐ray beam filtered to further reduce the exposure levels. 1‐cm diameter computer‐simulated nodules with a contrast ratio of 2.68% were computer generated at various locations of the chest images. Two‐alternate forced choice (2‐AFC) experiments were performed to measure the ratio of correct observations as a function of the exposure level for various structural backgrounds present. For each exposure level and location, 30 pairs of 512×512 images were generated and displayed in pairs on a review workstation for reading. The ratio of correct observations versus exposure levels were computed from the reading scores for various locations. The plots along with the exposures corresponding to 90% accuracy were used to study the effects of structural backgrounds. Results: For all regions, the accuracy generally increases from 0.5 to 1.0 with the exposure. However, the threshold exposure for 90% accuracy varies with the location. In the abdomen region, the ratio of correct observations reaches 90% at a higher exposure level (∼ 0.4 mAs). In the lung region, the ratio reaches 90% at a much lower exposure level (∼ 0.0625 mAs). The difference in the threshold exposures may be due to the heavier attenuation in the abdomen region. Complexity of structural backgrounds may affect the threshold exposure and the level and consistency of the ratios measured. This work was supported in part by a research grant EB000117 from the NIBIB and a research grant CA104759 from the NCI.
TU‐C‐I‐609‐04: Optimization of Radiographic Imaging Using Simulated Radiography and Electronic Observers
Authors: Winslow M , George Xu X , Yazici B , Son I
Published: June 2005

Purpose: Software incorporating an anatomical radiograph simulator, effective dose calculator, and image quality analysis has been developed to determine the optimal balance between patient dose and image quality. Method and Materials: Radiograph simulation is divided into the primary and scattered portions of an x‐ray interaction. A tomographic phantom, VIP‐Man, is used to simulate the scattered portion using the ESGnrc Monte Carlo code. The primary portion is simulated using the projection ray‐tracing method through the Visible Human CT data set. To produce a realistic radiograph, the software simulates quantum noise, blurring effects, lesions, detector absorption efficiency, and other imaging artifacts. The primary and scattered portions of the simulated radiograph are combined to form an image for simulated observer studies and image quality analysis. Absorbed doses in the VIP‐Man phantom were also obtained from the Monte Carlo simulations. Once radiographs are produced, they are analyzed using several simulated observer filters including; non‐prewhitening matched filter with eye, Hotelling, and channelized Hotelling. Using the VIP‐Man phantom as a representative patient, a CsI flat panel detector and a small lesion in the left lung was examined as a test case. Energy was varied mono‐energetically while fluence was varied. Approximately 2000 simulated images and 200,000 data files were analyzed. Results: Receiver Operator Characteristic (ROC) curves and the corresponding Area Under Curve (AUC) charts were developed for the 2000 simulated images. For the patient size considered, the energy at which the minimum effective dose required to obtain an AUC of 0.85 was determined to be 70 keV. Conclusion: Coupling an imaging simulator with a computerized simulated observer represents a new avenue for maintaining image quality while minimizing patient dose. Rescaling the VIP‐Man phantom will extend the database of representative patients for a verity of imaging tasks.
TU‐C‐I‐609‐05: Image Contrast and Lesion Detection in Chest Radiography
Authors: Vance A , Huda W , Ogden K , Scalzetti E , Roskopf M
Published: June 2005

Purpose: To assess the affects of varying image display contrast on lesion detection in chest radiography. Method and Materials: We investigated the detection of simulated mass lesions in three anatomical regions of chest radiographs (apex, hilum, and sub‐diaphragm). Three sizes of simulated lesion were investigated (2.5, 5.5 and 10 mm), which were randomly added to each selected anatomical region of adult radiographs with no known pathology. Detection performance was the lesion intensity (I92%) that produced a detection accuracy of 92% in 4 Alternate Forced Choice (4‐AFC) experiments. We investigated how detection performance (I92%) varied as the display window width was varied by a factor of two (500 to 1000). Results: In the apex and hilum, reducing display contrast by a factor of two reduced detection performance by approximately 20%, and this was true for all lesion sizes investigated. In the sub‐diaphragm region, the results obtained depended on the lesion being detected. For the large 10 mm lesion, reducing the display contrast by a factor of two also resulted in a drop in detection performance of ∼20%, whereas for the smallest (2.5 mm) lesion, display contrast had no significant effect on detection performance (<5%). Conclusion: Changing the display contrast in chest radiography only has a modest effect on lesion detection. In the hilum and apex, the effect of image display on detection performance was the same for all lesion sizes. In the sub‐diaphragm region, however, detection of small lesions was independent of image display contrast.
TU‐C‐I‐609‐06: Task‐Based Optimization of Dual‐Energy Imaging Systems Using Generalized NEQ
Authors: Richard S , Siewerdsen J , Jaffray D , Moseley D
Published: June 2005

Purpose: Dual‐energy (DE) imaging is a promising advanced application of flat‐panel detectors (FPDs) for early detection of lung cancer. This paper investigates the performance of FPD‐based DE imaging systems by extending descriptions of generalized detective quantum efficiency (GDQE) and noise‐equivalent quanta (GNEQ) to a task‐based optimization of system design, image acquisition, and reconstruction. Method and Materials: This investigation incorporates anatomical noise into traditional descriptions of DQE and NEQ to yield the GNEQ for DE imaging systems. Imaging performance was calculated using cascaded systems analysis and combined with anatomical noise measured and modeled as 1/f noise. Calculations of GNEQ were combined with Fourier descriptions of imaging task to yield the detectability index, d′, taken as an objective function for optimization of DE image reconstruction (tissue cancellation), kVp, added filtration, and allocation of dose between image pairs. Results: Analysis of GNEQ reveals a tradeoff between increased quantum noise and reduced anatomical noise in DE imaging systems. Optimal DE image reconstruction (i.e., selection of tissue cancellation parameter, wt) was found to exhibit moderate task dependence, ranging from wt=0.49 to 0.44 for gaussian detection and discrimination tasks, respectively. Optimal kVp pairs were identified as a function of imaging dose — e.g., (90 / 150 kVp) at high dose (1 mR), decreasing to (60 / 120 kVp) at low dose (10 uR). The analysis quantifies the potential improvement in performance associated with addition of high‐Z filters (e.g., Cu, Ce, and Ta). For fixed total entrance dose, optimal allocation of dose between high‐ and low‐kVp images depends on the imaging task, highlighting the tradeoff between quantum and anatomical noise. Conclusions: Experimental and theoretical analysis of GNEQ and task‐based detectability index provides a fundamental understanding of the factors governing DE imaging performance and offers a powerful methodology for optimization of system design and image acquisition.
TU‐C‐I‐609‐07: Optimization of a Real Time Dual‐Energy Subtraction Technique Based On a Flat Panel Detector
Authors: Ducote JL , Xu T , Molloi S
Published: June 2005

Purpose: To determine the optimal x‐ray spectra for dual‐energy subtraction with a flat panel detector (FPD), and to quantify the effects of dynamic filtration and FPD gain settings on image quality, tube loading, patient exposure, and overall system performance. Method and Materials: A simulation study was performed using available empirically determined data of x‐ray spectra. The lungs and mediastinum of the chest were modeled with 12.5 cm and 20 cm thick regions of Lucite, respectively. Coronary calcification was modeled with a 1 mm thickness of bone‐equivalent plastic. The FPD was modeled as an ideal detector with a 600 micron thick layer of CsI. Scatter was not considered in this study. Low and high energy images were normalized to a desired energy deposit in the detector dependent on FPD gain. A figure‐of‐merit (FOM) was used t6o quantify the overall system performance. The effects of various silver filter thicknesses (0–1000 microns), high to low energy image signal ratios (1–8), and two dual‐energy noise reduction algorithms were evaluated for their effect on image quality, patient entrance exposure, tube loading, and FOM improvement. Results: As the thickness of the high‐energy filter increased, image contrast, contrast‐to‐noise ratio, FOM and tube loading increased. Patient exposure was reduced by approximately 10% for the range of filter thicknesses studied. The FOM was maximized with a FPD signal ratio of approximately 3 without application of dual‐energy noise reduction. However, dual gain operation did not show any improvement after noise reduction. Conclusion: An optimal dynamic filter combined with dual‐energy noise reduction can improve the system FOM by a factor of 5. There is no further improvement in the system FOM when dual detector gain is used in conjunction with dual‐energy noise reduction.
TU‐C‐J‐6B‐02: Assessment of Tumor Proliferation During Radiotherapy
Authors: Jeraj R , Smith E , Barbee D , Dick D , Nickles J , DeJesus O , Ballegeer E , Forrest L
Published: June 2005

Purpose: To explore the basis for quantitative in‐vivo tumor imaging during radiation therapy with the aim to adaptively modify the treatment according to the response. Method and Materials: Two positron emission tomography (PET) imaging agents were used to assess the response. Fluorodeoxygluocose (18F‐FDG), a metabolic marker, is not ideal agent for monitoring tumor tissue response, because increased metabolism in response to radiotherapy due to inflammatory cells. 3′‐deoxy‐fluorothymidine (18F‐FLT) has recently been proposed as a marker for imaging tumor proliferation. Several canine subjects with recurrent soft tissue sarcomas were repeatedly imaged with PET/CT before, during and after the radiation treatment. The tumors were treated with 60Co with one or two fractions of 8Gy. Approximately 200 MBq of FDG/FLT activity was administered per scan. Standard uptake values (SUV) were calculated to evaluate uptake in the tumor region, as well as in the surrounding organs. The CT data between the imaging sessions was co‐registered and PET data compared. Results: Tumor response to therapy varied significantly between the subjects. High heterogeneity (up to 50%) of the tumor was observed in some of the cases. Early post treatment scans (3 days) showed already significant decrease in FLT uptake, followed by an increase 6 days after the treatment due to accelerated repopulation. Severe redistribution of the tumor proliferation potential was observed during the treatment. In the follow‐up study 6‐weeks post‐treatment both, FDG and FLT PET showed low uptake in the tumor region (SUV=1.9 vs. SUV=1.0) with the slightly elevated FDG uptake on the periphery of the tumor. Conclusion: FLT proved to be a better treatment response monitoring agent than FDG. High heterogeneity of the proliferation activity, as well as redistribution of the proliferation potential indicates strong potential for treatment adaptation as well as potential problems with treatment planning.
TU‐C‐J‐6B‐03: Deformable Fusion of 4D PET Images
Authors: Zhang T , Pyzalski R , Jaskowiak C , Tome W , Mackie T , Paliwal B
Published: June 2005

Purpose: PET images are useful in defining treatment targets. Removing distortion from respiratory motion in the PET images is of importance for gated or 4D radiation treatments. We developed a deformable fusion method to sum up the PET images at different phases. The motion‐blurring‐free PET images are obtained without the sacrifice of signal to noise ratio (SNR) or increase of scanning time. Method and Materials: A GE Discovery CT/PET scanner was interfaced with Varian's RPM breathing tracking system. 4D CT and 4D PET images were correlated according to the breathing phases. Full exhalation phase was used as the planning phase. Deformable image registrations of the 4D CT images yielded the displacement maps of the lungs relative to the planning phase. PET images at each breathing phase were then warped into the planning phase using the displacement maps. Motion‐blurring‐free PET images were obtained by summing the warped PET images. Results: Gated PET images at each individual phase are very noisy. The uncertainty from intensity fluctuations compromises the motion reduction by gating the scan. PET images are too fuzzy to be directly used in elastic registration. Instead, we registered the 4D CT images. With regular patient breathing patterns, 4D CT images correlate with 4D PET images. Consequently, the displacement maps from registration of the 4D CT images can be used in warping the 4D PET. Image warping removed the motion distortion in the PET images. The summed images have reduced motion blurring and the same SNR as that of non‐gated PET images. Conclusion: Deformable fusion of 4D PET images reduce the motion distortion in PET image and retain the SNR without increasing data acquisition time. The resultant PET images can be used in more accurate target definition in gated or 4D radiation treatments.
TU‐C‐J‐6B‐05: A Novel Volume Subdivision‐Based Algorithm to Register Respiration Phase‐Correlated Lung CT Images
Authors: Shekhar R , Lei P , Walimbe V , Yu C , D'Souza W
Published: June 2005

Purpose: To develop a deformable image registration algorithm that will allow for temporal interpolation of phase‐correlated 3D CT data sets and test it on lung CT images. Method and Materials: We have developed a new deformable image registration algorithm that starts with a global rigid‐body matching between two (volumetric) images, progressively subdivides one image, and refines the spatial matching locally for each of the subvolumes. When subvolumes reach a user‐defined size, the algorithm interpolates all local solutions to generate a smooth deformation field. The above algorithm was applied to two lung CT image sets, each with three frames corresponding to end‐inhalation (IN), mid‐exhalation (MID) and end‐exhalation (EX). These images were acquired in conjunction with a spirometer and using a spiral CT scanner. In each case, the IN image was registered to the MID image and the MID image to the EX image. An interactive multi‐planar reformatting tool was used to evaluate the result of image registration quantitatively. For quantitative evaluation, reduction in root‐mean squared (RMS) error was computed for each image pair before and after image registration. Results: Visual inspection indicated an improved matching of anatomical features following registration. For the four image registrations performed (two for each case), the RMS error reduced from 13.8 to 7.2, 19.9 to 12.7, 13.5 to 8.0, and 12.2 to 7.0, indicating improved spatial matching following image registration. Conclusions: We have developed a new deformable image registration and shown its application to gated lung CT images. The algorithm effectively aligned phase‐correlated CT images. Full development of this deformable registration capability will allow tumor trajectory mapping and dose registration.
TU‐C‐J‐6B‐09: Automatic Contour Delineation On Subsequent CT Images Using Deformable Registration
Authors: Wang H , O'Daniel J , Ahamad A , Garden A , Mohan R , Dong L
Published: June 2005

Purpose: To implement a deformable registration algorithm to automatically delineate regions of interest (ROIs) on daily CT images by transforming the corresponding ROIs from a reference CT image. Method and Materials: An intensity‐based ‘Demons’ deformable registration algorithm was used to find the correspondence between planning CT image and daily CT image. After reference ROIs were delineated manually on the reference CT image, the reference ROIs can be mapped onto each daily CT image. We tested this method on one head‐and‐neck patient (tonsil tumor). The patient received 3 CT scans per week prior to treatment for a total of 16 CT scans. The deformed ROIs were visually evaluated by a radiation oncologist. The dose‐volume histograms (DVHs) of the deformed left and right parotids were calculated and compared with the planned DVHs. In addition, a cumulative dose distribution was calculated by mapping the daily dose distribution from each of the daily CT images back to the planning CT using the deformable registration method. Thus, the DVHs from the cumulative dose distribution can be calculated to represent the final “delivered” dose distribution. The volumetric changes during the elapsed treatment days for targets and critical structures were also investigated. Results: The deformed contours reasonably matched with gross anatomy presented by the CT images. The daily DVHs showed a large variation during the course of treatment due to both setup errors and internal organ deformation. In spite of large variation in DVHs of the parotids in daily CT images, the DVHs of the cumulative dose agreed reasonably well with the planned DVHs for both parotids in this case. Conclusion: We demonstrated that an intensity‐based deformable registration algorithm can be used effectively to map the planning ROIs to subsequent CT images acquired during treatment. This allows effortless replanning in an adaptive CT‐guided radiotherapy strategy.
TU‐C‐T‐6C‐02: Simplifying IMRT with Direct Aperture Optimization
Authors: Yu C , Shepard D , Earl M , Yi B
Published: June 2005

Earlier and more widely used method for IMRT planning is beamlet‐based inverse planning. It divides each beam into beamlets or bixels in the same way as planar images are composed of pixels. A computer algorithm is then used to optimize the weights of these beamlets. The optimized weight distribution is also called intensity maps. In order for the desired intensities to be delivered, a leaf‐sequencing algorithm is used to translate the intensity maps into a sequence of deliverable beam shapes called segments. This two‐step process (intensity optimization and leaf sequencing) puts delivery consideration outside the optimization and complicates IMRT. In beamlet based inverse planning, optimizing the intensity distributions is only the first step of the planning process. Leaf sequencing, or creating a desired beam intensity using overlapping MLC shapes, is a mathematically intractable problem. A large number of complex field shapes are often needed. This can lead to a loss in efficiency and an increase in collimator artifacts. Attempts have been made to simplify the delivery by either grouping the intensity levels or smoothing the intensity distributions. However, such attempts are always accompanied by the reduction of plan quality. When a large number of segments are used, there are typically segments requiring a low number of monitor units (MUs) along with small off‐axis fields. These segments introduce new challenges for accurate dose calculation and unrealistic requirements for geometric accuracy of the MLC and dosimetric accuracy of the linear accelerator. As the result, QA efforts must be intensified in order to achieve the well‐established safety and accuracy standards. Realizing that this two‐step process and the large number of beam segments resulting from the poor process are the root causes of the labor intensive nature of IMRT, we directly optimize the beam apertures and their weights. This method is termed “Direct Aperture Optimization (DAO)”. With DAO, only a few segments are needed to generate treatment plans that rival those produced by employing the two‐step process. Most importantly, what you see is what the patient will get, since all the delivery related effects are already included in the dose distribution and the beam segments generated by the plan can be directly transferred to the linac for delivery. In this presentation, we will describe the DAO process and compare it with beamlet‐based inverse planning. We will show that the number of segments can be drastically reduced without affecting plan quality. We will also show that the effects of organ motion are inversely proportional to the segment size and segment MUs. With reasonably sized field segments, the requirement for MLC geometric accuracy and linac's dosimetric accuracy will be significantly lessened and the need for extensive QA will diminish. The delivery time will also be in line with conventional practice allowing more patients to receive the benefits of IMRT. The effects of organ motion during IMRT delivery will also be smaller.
TU‐C‐P‐618‐02: Part II: Practical Implementation Issues: Panel Discussion
Authors: Bice W , Brezovich I , Hevezi J , White G
Published: June 2005

Panelists will discuss the practical issues associated with a change to direct billing by Medical Physicists. Implementation of Legislative, Regulatory and Reimbursement System changes and practical issues associated with the third party payor system and associated professional practice and business issues will be discussed.
TU‐C‐T‐617‐02: Accuracy of GammaKnife Stereotactic Space Localization as a Function of Patient Alignment During Imaging
Authors: Cernica G , Wang Z , Malhotra H , de Boer S , Podgorsak M
Published: June 2005

Purpose: To establish the accuracy of stereotactic space localization for image sets where there is misalignment of the patient's head (stereotactic frame and fiducial box) relative to the CT‐ or MR‐ scanner axis. Method and Materials: Seven 0.5‐mm diameter steel spheres separated by 2 cm were rigidly mounted to a string tied tightly at each end to diametrically opposite posts attached to a Leksell stereotactic frame. The steel spheres spanned the entire stereotactic space. A standard CT fiducial box was applied to the frame in the usual clinical manner. A baseline CT scan (1 mm slice thickness) was obtained with the fiducial box perfectly aligned with the scanner axis. After localization of the image set, the (x,y,z) coordinate of the center of each steel sphere was recorded. The frame was then repositioned, re‐aligned and re‐scanned and the coordinates re‐obtained to verify reproducibility. Repeat CT scans with varying fiducial box misalignments up to 20 degrees in the sagittal, coronal, and transverse axes, respectively, were subsequently obtained. After localization, the (x,y,z) coordinates of the steel spheres in each misaligned image set were recorded and compared to the baseline coordinates. Results: The mean difference between the baseline and the respective coordinates in misaligned geometries was 0.25 mm (σ=0.16 mm). The largest difference (0.87 mm) was found with the greatest misalignment in the coronal plane. These errors are within the accuracy of the image sets. Conclusion: Accurate localizations were obtained for image sets with misalignments up to 20 degrees relative to the CT‐ or MR‐ scanner axis. Patients with very inferior frame placements or concomitant medical conditions that prevent perfect alignment without significant discomfort can be positioned comfortably but rigidly and, despite misalignment with the CT‐ or MR‐ imaging axis, the fiducial box will nonetheless afford an accurate localization of the stereotactic space.
TU‐C‐T‐617‐05: Effect of MLC Leaf Width and PTV Margin On the Treatment Planning of Intensity‐Modulated Stereotactic Radiosurgery Or Fractionated Stereotactic Radiotherapy
Authors: Chang J , Yenice K , Narayana A
Published: June 2005

Purpose: To investigate the effect of MLC leaf width and PTV margin on intensity modulated radiosurgery (IMSRS) and radiostherapy (IMSRT) dose distributions. Method and Materials: Twelve patients previously treated with IMSRS or IMSRT were retrospectively planned with a 5mm or 3mm MLC leaf width and a 3mm or 2mm PTV margin using the already contoured CTV, critical structures and organs at risk (OARs). The same beam arrangement, planning parameters and plan selection criteria were used in each four plans for a given patient. Same target coverage was achieved by renormalizing each plan so that the prescription dose covered at least 99% of the PTV. Plan indexes — Dmax, Dmin, and Dmean, conformity index (CI), V70, V50, D95 and V95 were calculated from the dose‐volume histograms of PTV, normal tissue, or OARs. Ratios of plan indexes were computed and hypotheses tests were performed on the mean ratios to determine the significance of the relative changes. Results: The PTV was well covered for all plans. The PTV was 25% smaller when 2mm instead of 3mm PTV margin was used; CI of 3mm MLC was 7% lowered than that of 5mm MLC. The decrease of MLC leaf width had a similar effect as that of PTV margin in reducing V70 and V50 of the normal tissue and Dmean of brainstem by ∼10%. However, Dmean of optic nerve and chiasm was more sensitive to the change of PTV margin. Conclusion: For IMSRT, the combination of 5mm MLC and 2mm PTV margin is dosimetrically equal to that of 3mm MLC and 3mm PTV margin for both PTV coverage and normal tissue sparing. The use of 5mm MLC and 2mm PTV margin for IMSRS is problematic because V70 and V5 are ∼10% higher than that of 3mm MLC and 2mm PTV margin.
TU‐C‐T‐617‐07: Optimal Configuration of a Dedicated SBRT Treatment Unit for Radiation Therapy of the Lung
Authors: Palamaru M. , Jaffray D
Published: June 2005

Purpose: In the interest of optimizing lung stereotactic body radiation therapy (SBRT), the operating characteristics of a new dedicated unit with high‐resolution MMLC, tunable energy range, and removable flattening filter were assessed. The system provides cone‐beam CT capabilities for image‐guided placement of the treatment isocentre. Measurements, Monte Carlo (MC) simulations, and treatment planning trials were initiated to allow rational selection of the optimal operating characteristics of this device in the management of lung cancer. Method and Materials: In this study, two theoretical models were created: one using the convolution/superposition (C/S) algorithm and the other using the MC simulation (BEAMnrc05) of the treatment head. The models were validated through comparison with measurements made using water tank (CC08–0.08cm3 volume, Wellhofer ion chamber) for a range of operating points of the accelerator (energy and presence of flattening filter). In addition, normoxic PAG gel measurements with MR‐based readout were performed in order to assess the 3D dose performance. Results: The MC and C/S models reproduced the measured dose distribution in water with an accuracy of 2% or 1mm. Modification of the beam energy decreased the dose rate from 675MU/min (6MV) to 56MU/min (∼3MV). Removal of the flattening filter increased dose rate by a factor of 1.5–2 and a ∼5% reduction in the 80%–20% measured penumbra in water. On‐going planning studies will provide assessment of these changes on realistic clinical scenarios in the context of lung SBRT. Conclusion: A flexible treatment‐imaging platform has been installed in our facility for SBRT. Selection of optimal operating point for this device for lung SBRT is being pursued. The removal of the flattening filter showed some merits for the recovery of dose rate and the decrease in penumbral width. Further studies are needed to identify the best machine configuration for lung SBRT. Conflict of Interest: supported by Elekta.
TU‐C‐T‐617‐09: Role of CT, and MR Angiographies in Accurate Target Volume Definition of Patients with Cerebral Arteriovenous Malformations (AVM) Treated with Stereotactic Radiosurgery
Authors: Rahimian J , Chen J , Girvigian M , Miller M , Lee J , Shieh G , Kerwin T , Volpp B
Published: June 2005

Conventional angiography used to delineate cerebral AVMs is an invasive technique, and associated with significant risks as well as costs. Numerous publications have proposed CTA, and MRA for stereotactic planning of AVM in place of conventional angiograms. The purpose of this study is to evaluate the role of CTA, MRA, and fused CTA and MRA images in accuracy of AVM target volume definition for patients treated with stereotactic radiosurgery. Materials and Methods: 8 patients (12–58 years old) who underwent SRS treatments with a Brainlab Novalis SRS system for cerebral arteriovenous malformations (AVM) were retrospectively studied. The CTA was acquired using a GE dual slice CT, with 90 cc nonionic contrast injected to a peripheral vein at a rate of 3cc/sec, 15 seconds prior to the spiral CT acquisition. The MRA was acquired using a GE MRI system that includes vascular time of flight SPGR pulse sequences. The CTA was used for stereotactic localization as well as target volume definition. The CTA and MRA datasets were co‐registered using the Brainlab's planning software. The AVM target volumes were contoured using the fused images by an expert reference observer. Three independent observers using CTA, MRA, and fused images independently contoured the nidus. The calculated average target volumes for each patient contoured by the 3 observers were plotted against AVM volume outlined by the reference observer. A linear regression analysis was performed on the curves. Results: The linear regression analysis yielded a slope of 0.96, 0.96, and 1.03 using MRA, CTA, and fused images, respectively. The study showed accurate nidus definition is possible by using either CTA, or MRA. However, using the fused images yield more precise AVM target delineation. Conclusion: Precise AVM target volume definition is possible by utilizing CTA, and MRA fused images for stereotactic radiosurgery.
TU‐C‐T‐617‐10: The Use of a Miniature Multileaf Collimator in Stereotactic Proton Therapy
Authors: Slopsema R , Paganetti H , Kooy H , Bussiere M , Sisterson J , Flanz J , Bortfeld T
Published: June 2005

Purpose: To evaluate — in terms of dose conformity, neutron dose, activation, and leakage — the use of a Miniature Multileaf Collimator (MMLC) in stereotactic proton therapy, both as an aperture and for segmented intensity modulated proton therapy (IMPT). Method and Materials: Firstly, we compute the neutron production and activation of the MMLC with Geant4. In a simple geometry — a cylindrical target hit by a pencil proton beam — we determine the neutron distribution as function of position and energy, and compare tungsten, brass, nickel, and lead. Given the point‐spread functions, we assess the secondary‐neutron dose to the patient for the different materials and MMLC distance to patient. Secondly, we model the Radionics ConforMAX™ MMLC as integrated in our stereotactic proton beamline, and compare the dose distributions for the MMLC and an equivalent aperture. The results are validated with radiographic film measurements. Finally, we measure the proton leakage, activation, and neutron dose. Results: The neutron production in tungsten, the leaf material in most MLC's, is more than twice that of nickel. For a 10 cm diameter field, with a range of 15 g/cm2 and modulation of 5 g/cm2, and with the MMLC positioned at 30.0 cm from the patient, the maximum neutron dose, i.e. for a closed MLC, is ∼0.04% of the target dose. The MMLC leaf width at iso center is 3.2 mm; the lateral penumbra in air is 0.6 mm. Conclusion: In beam‐lines with a large SAD, such as our stereotactic beam‐line, the use of the MMLC as aperture is feasible. The device can be positioned far away from the patient, which reduces the neutron dose to acceptable levels, while hardly compromising the lateral penumbra. Given a decrease in proton efficiency of a factor of 3, the MMLC could also be used for IMPT.
TU‐C‐T‐6E‐05: Dosimetry Acccracy Of Delivery Of Gold Beam Data IMRT Plans On Similar Linear Accelerators With Same Vendor
Authors: Selvaraj RN , Myron GP , Lalonde RJ , Yue NJ , Heron DE , Huq MS
Published: June 2005

Purpose: Under certain circumstances, IMRT patients need to be transferred from one Linac to another. Since a same set of commercial Gold Beam Data (GBD) have been used to model the treatment planning systems for multiple similar types of Linacs at our centers, the dosimetric accuracy needs to be investigated to determine the necessity of replanning when the treatment of an IMRT patient is moved from one machine to another. In this study, we investigated the dosimetric accuracies of IMRT plans delivered on different similar Varian accelerators. Method and Materials: In our institution, GBD has been used to model the Eclipse™ treatment planning systems for certain types of Varian machines (23 EX). IMRT treatments were planned using the Eclipse™ systems with 6 and 10 MV photon beams for different anatomical sites. Corresponding QA plans were also generated. The QA plans were used in the validation measurements, and the same measurements (same plans, same MU & dMLC files and phantom) were repeated on five of those Linacs. The measurements were performed using both ion chambers and films. Results: For plans using 6 MV photon beams, the mean ratios of planned to measured dose on the five machines for breast, prostate, tonsil, pelvis and brain plans were 0.984(SD0.0078), 0.998(SD0.0029), 0.979(SD0.0121), 1.006(SD0.0132) and 1.012(SD0.0054), respectively. For plans using 10 MV photon beams, the mean ratios of planned to measured dose on two of the machines for breast and pelvis were 0.979(SD0.00) and 0.949(SD0.0042), respectively. In the film analysis, a good agreement was found between films and plan isodoses on a coronal plane (for high dose gradients ± 2 mm and low dose gradients ± 4 mm). Conclusion: This study demonstrated that the IMRT patients could be transferred from one machine to another similar Varian machine with same energy without replanning.
TU‐C‐T‐6E‐07: Monte Carlo Investigation of Dosimetric Differences Between SMLC and DMLC IMRT Delivery Techniques in Heterogeneous Media
Authors: Tyagi N , Moran J , Litzenberg D , Fraass B , Chetty I
Published: June 2005

Purpose: To investigate dosimetric differences between SMLC and DMLC IMRT delivery techniques in heterogeneous media using MC. We hypothesize that subtle differences in the energy spectra between SMLC and DMLC, which may not be detected in water, will be accentuated in inhomogeneous, patient‐like tissues. With the ability to account for MLC design details and perform accurate transport in heterogeneous media, MC provides a powerful means to evaluate IMRT delivery methods and may be useful as a tool for IMRT QA. Method and Materials: The BEAMnrc code was used to simulate patient independent components of a Varian 21EX linac. The resultant Phase space file was then used as input for the DPM MC code which simulates the jaws, 120‐leaf MLC geometry, and the patient or phantom. Transport through the jaws and MLC is accomplished using multiple scatter photon transport. Sampling algorithms have been developed for SMLC and DMLC by randomly sampling leaf segments based on the MU index. Accuracy of the simulations was assessed in a homogeneous media when compared to measurements in a solid water phantom. To assess the impact of MLC design and delivery technique in heterogeneous media, dose was calculated in a phantom consisting of material slabs ranging in density from lung to cortical bone using SMLC and DMLC leaf sequences. Results: Excellent agreement was seen between the Monte Carlo models for SMLC and DMLC sequencing when compared to the respective film measurements in homogenous media. Significant differences, up to 10%, were seen between the delivery techniques in lung equivalent media when calculated in heterogeneous slab geometry. Conclusion: This study provides evidence that heterogeneous tissue may exacerbate energy differences between SMLC and DMLC delivery methods. Such differences may be clinically important considering that they will not be detected during QA which is typically conducted using homogeneous phantoms.
TU‐C‐T‐6E‐09: The DAVID System — a Device for In‐Vivo Verification of IMRT and Conformal Irradiation Techniques
Authors: Poppe B , Harder D , Kollhoff R , Djouguela A , Rubach A , Schuele E
Published: June 2005

Purpose: While dosimetric plan verification ensures consistency between planned and measured dose distributions in the pretreatment phase, a daily in‐vivo verification of the beam profiles by a radiation detector positioned at the entrance side of the patient has not been clinically available so far. In this work we present the DAVID system which is able to perform a daily in‐vivo verification of IMRT beams in front of the patient during the treatment. Method and Materials: The DAVID system is a flat, translucent multi‐wire ionization chamber. It is placed in the accessory holder of the linear accelerator. Each detection wire of the chamber is positioned exactly in the projection line of two opposing leafs of the MLC. The measurement signal of each detection wire is directly proportional to the opening of the leaf pair. Therefore the number of measurement channels equals the number of leaf pairs. After a successful dosimetric verification of an IMRT plan, the values measured by the DAVID system are stored as reference values. During daily treatment the signals are re‐measured and compared to the reference values. In case of a deviation beyond a threshold a warning occurs. Results: The error detection capability for a 1 cm × 1 cm field is a leaf position error of less than 0.5 mm. The inherent limit due to electronic noise of the chamber is 1mm for a 20 cm × 20 cm field (all values related to the isocenter). Conclusion: Clinical examples demonstrate that the DAVID system is a relevant tool to improve the reliability of IMRT treatments. Because the DAVID system operates as an ionization chamber, disadvantages which might be observed in other devices, such as aging, are not to be expected. Conflict of Interest: This work was performed in collaboration with PTW‐Freiburg Dr. Pychlau GmbH, Freiburg, Germany.
TU‐C‐T‐6E‐10: Using Fluence‐Separation to Account for Energy Spectra Dependence in Computing Dosimetric ASi EPID Images for IMRT Fields
Authors: Li W , Moore J , Siebers J
Published: June 2005

Purpose: Dosimetric aSi EPID images are typically computed using a convolution of energy fluence with an invariant energy deposition kernel. However, Monte Carlo (MC) studies show a strong dependence of the EPID imager response to energy spectra, which, for highly modulated IMRT fields, severely affects the dosimetric accuracy. To account for this, a method is developed that accounts for the radial‐dependence of the energy deposition kernel and considers open field and MLC hardened components of the energy fluence. Method and Materials: Dosimetric EPID images are created by convolving energy fluence with a radially dependent kernel. The energy fluence at the EPID surface was determined by extracting the terma in water from Pinnacle. For each fluence element, the energy fluence (Ψ) was divided into two parts — open field energy fluence Ψo, and MLC blocked field energy fluence Ψc. Ψo and Ψc were convolved separately with their respective energy‐deposition kernels and the results summed. Calculations were compared with measurements for, 3×3–20×20 cm2 fields, rectangular fields, a 10×10 cm2 field centered at (5cm,−5cm)), 3×3–12×12 cm2 MLC‐blocked fields, and dynamic MLC sliding window fields which generate 10×10 cm2 fields with window gaps ranging from 1‐ to 50‐mm. Test cases were compared utilizing profiles and using gamma‐analysis for pixels receiving doses >50 % Dmax. A 3 mm, 3% criteria was used in the gamma analysis. Results: Measured and computed dose profiles agreed for both in‐field and out‐of‐field regions. For the open field test cases, all points evaluated had γ < 1.0. For MLC blocked fields ⩽10×10 cm2, >98.5% of points had γ 1.0 Over 98% of points passed the gamma‐test for most sliding window field. Conclusion: Accurate aSi dosimetric EPID images can be computed when energy spectra hardening is accounted for during the image calculation. Conflict of Interest: This work supported in‐part by Varian Medical Systems.
TU‐D‐I‐611‐03: Cone‐Beam Image Reconstruction for Circular‐Arc Trajectories Via Shift‐Invariant Horizontal and Non‐Horizontal Filtering
Authors: Nett B , Zhuang T , Chen G
Published: June 2005

Purpose: To propose and validate a novel shift‐invariant filtered backprojection (FBP) type cone‐beam reconstruction algorithm for circular‐arc scanning trajectories. This algorithm has been proposed to address two shortcomings in the FDK algorithm; namely, limited visibility of low contrast objects in the off‐center planes and the inability to provide clinically acceptable reconstruction when the scanning path is shorter than the short‐scan condition. Method and Materials: Based upon the application of an exact cone‐beam reconstruction algorithm to the case of a circular trajectory, which does not fulfill the Tuy data sufficiency condition, a new algorithm has been developed. The new algorithm differs from the heuristic FDK algorithm in that multiple sets of filtered data are utilized in reconstructing the value of a single image point. The new algorithm introduces non‐horizontal filtering which is not achievable by heuristic extension fan‐beam FBP reconstruction. In this manner more of the cone‐beam data is utilized in the filtering operation. The new algorithm also enables region‐of‐interest (ROI) reconstruction in a super‐short scan mode (less than the short‐scan angular range). Results: Computer simulations were performed using analytical data generated for a standard low contrast phantom. The new algorithm provided improved visualization of structures that lie in off‐center planes when compared with the FDK algorithm. In addition simulations were performed to validate the ROI reconstruction using a super‐short scan mode. Conclusion: The circular‐arc scanning trajectories are used frequently in cone‐beam CT on interventional C‐Arm systems, micro CT systems and cone‐beam CT guided radiation therapy. A new shift‐invariant FBP type cone‐beam reconstruction algorithm has been proposed and validated for this common source trajectory. In comparison with the standard FDK algorithm the new algorithm provides improved visualization of structures that lie in off‐center planes, and it enables a super‐short scan mode for ROI reconstruction.
TU‐D‐I‐611‐04: Intensity‐Modulated Cone‐Beam CT for Patient‐Specific Distribution of SNR
Authors: Graham S , Siewerdsen J , Keller H , Moseley D , Jaffray D
Published: June 2005

Purpose: In order to maximize task‐specific image quality for cone‐beam CT systems used in image‐guided radiation therapy, investigations are being performed to establish compensating filters capable of modulating the fluence pattern applied during image acquisition for patient and task‐specific distributions of signal‐to‐noise (SNR). The repetitive image‐guidance problem allows optimization at the patient‐specific level by making use of previously acquired patient images to define regions of interest (ROIs) or a distribution of desired image quality. These ROIs or distributions can be employed to generate intensity modulated fluence patterns for application during acquisition. Method and Materials: Investigations into the development of compensation systems for creating SNR patterns are performed through simulations and experimental studies on a bench‐top cone‐beam CT system (Varian 4030A flat‐panel, Rad‐92 X‐ray tube, motorized rotation stage). Numerical simulations (Matlab™) on a Shepp‐Logan phantom are employed to generate binarized masks according to defined ROIs in the object space. Modulation of noise containing sinogram data followed by filtered backprojection permits SNR distribution recovery. Experimental studies using modulating filters were performed to evaluate the robustness of the technique in realistic systems. Results: Simulations into the feasibility of tailor made SNR patterns created by advanced compensation schemes show that it is possible to design a gantry angle dependent compensation system capable of achieving desired SNR in defined ROIs. Experimental results reveal the ability to modify SNR across images providing specific regions with improved SNR while reducing local and/or integral dose. Conclusion: Cone‐beam CT compensators have been explored to provide modulated, patient‐specific, fluence patterns optimizing the distribution of SNR within the resulting image. This technique opens the opportunity to optimize image quality in the most clinically relevant regions of an image, while reducing dose to the patient by reducing the fluence intensity outside defined ROIs.