bluuebunny/crossref_metadata_2025 huggingface dataset:
TU‐EE‐A1‐03: Comparison of High Dose Rate (HDR) Vs Intensity Modulated Radiation Therapy (IMRT) for Prostate Boost Treatment
Authors: Bose S , Huq M , Bahri S , Lalonde R , Selvaraj R , Brandner E , Yue N
Published: June 2005

Purpose: The objective of this study is to compare dosimetric characteristics of prostate treatments using HDR brachytherapy and IMRT technique. Method and Materials: Five HDR patients were selected for IMRT planning. Patients underwent ultrasound guided catheter placement for HDR. CT images were obtained and imported into the Nucletron PLATO Brachytherapy system. The prostate, urethra, bladder and rectum were contoured on axial slices. The dose was calculated and optimized by graphical optimization. The CT images of these structures were exported from the PLATO to Eclipse workstation for IMRT planning and comparison. For each patient, the DVH of HDR and IMRT plans were generated, drawn on the same scale and compared. Results: In IMRT plans the DVH curves for PTV dropped sharply and reached to zero volume of the prostate at about 6.4 Gy. In HDR plans the DVH curves for PTV showed a long tail up to a very high dose. About 10% of the PTV for prostate received greater than 12 Gy (200%) of the prescribed dose (6 Gy) in HDR plans. In contrast, the same volume in IMRT plans received less than 6 Gy (100%). Average prostate V90 and V100 dose was about 6.3 Gy and 4.12 Gy respectively for HDR, and 6.09 Gy and 5.74 Gy for IMRT plans, respectively. UrethraV90 dose for IMRT plans showed similar levels (93%), whereas in HDR the dose varied widely (60 to 100%). In all plans, the dose to the bladder and rectum was significantly lower in HDR than in IMRT plans. Conclusions: HDR brachytherapy may reduce normal tissue toxicities in prostate boost treatments, even though the dose homogeneity inside the PTV is far worse than in IMRT treatments. Another advantage of HDR over IMRT is that the organ motion is not a significant concern as in IMRT.
TU‐EE‐A2‐06: Patient Setup Accuracy and Precision of Prostate Radiation Treatment with BrainLab ExacTrac X‐Ray System with Implanted Fiducials
Authors: Yokoyama S , Koeplin D , Stack T , Jackson G , Newland K , Van Trease R , Sawyer T
Published: June 2005

Purpose: To confirm the intra‐radiation treatment motion of the prostate is small enough so that daily positioning of the prostate is a valid effort. To test the automatic target alignment of the ExacTrac X‐ray® system. Method and Materials: The target positioning data of 51 radiation treatment sessions were collected retrospectively from 5 prostate cancer patients treated with IMRT between June and August in 2004. All patients were in supine treatment position and immobilized with HipFix (Med‐Tec, Inc.) without any padding or mold on the couch. Four gold fiducial markers (φ 1.2 mm, 3.0 mm long, Med‐Tec, Inc.) were implanted in the patient's prostate under ultrasonic guidance prior to the planning CT scan. Prior to the treatment, fiducials were localized using stereoscopic kV x‐ray imaging system (ExacTrac X‐ray®, BrainLab) and the couch was automatically shifted to a correct the target position. There are two sets of data collected. 1) pre‐treatment target deviation after the automatic positioning with kV x‐ray images. 2) post‐treatment target deviation. Results: 1) The average pre‐treatment deviations of the target after the automatic positioning were — 0.2 ± 1.3 mm (vertical), −0.1 ± 1.0 mm (longitudinal), and 0.0 ± 0.4 mm (lateral). 2) The average isocenter shifts (difference of the target positions between pre‐ and post‐treatment) were −0.2 ± 1.0 mm (1SD for all errors, vertical), −0.3 ± 1.2 mm (longitudinal), and 0.1 ± 0.5 mm (lateral). Conclusion: Both averages of post‐treatment shift and pre‐treatement deviation of the target were negligible considering the uncertainty. Both results showed lower positioning uncertainty by half in the lateral direction compared with the vertical and longitudinal directions, which suggest the lateral target motion was restricted better than the vertical and longitudinal. Same magnitude of position uncertainty of the pre‐deviation and post‐shift indicates the limit of the precision of daily ExacTrac's automatic positioning.
TU‐FF‐A3‐04: Impact of Room Illuminance On Black Level Luminance and Contrast Detection for Off‐Axis Viewing On High Resolution Normal and High‐Bright Flat Panel Displays
Authors: Thomas J , Romanyukha A , Chakrabarti K , Kaczmarek R
Published: June 2005

Purpose: Flat panel display image quality is known to be highly dependent on viewing angle. The contribution of room illuminance to black level luminance is well understood for CRTs, but, is not clearly understood for flat panel displays. We have investigated the effect of room illuminance and off axis viewing on displayed image quality. Two high resolution 9.2 megapixel flat panels were evaluated under various room illuminance levels and viewing angles — a standard IBM T221. Method and Materials: Black level luminance was measured at 0, ±15, ±30 and ±45 degrees with room illuminance levels of 0, 2, 5, 10 and 50 lux. Luminance was measured with a Minolta LA‐100a spot meter and illuminance levels were measured with an International Light Luminance Meter IL 1400a. The luminance target (3% of total display area) was centered on the display. Reader studies using a computer generated contrast detail phantom were conducted at each viewing angle and room illuminance level. Five readers read the target images. Specular reflections were minimized for all measurements and reader observations. Results: Minimization of specular reflections in the viewing room minimized the change in measured black level luminance. As room illuminance levels increased and viewing angles became more acute black level luminance increased. Reader results are presented as k‐values and a correlation with room illuminance and black level luminance is demonstrated. The optimal viewing condition was shown to be 0 degrees with a room illuminance of 2 lux. Substantial degradation of measured k‐value and black level luminance occurred at other viewing conditions. Conclusion: Visual perception of small targets is substantially impacted by both viewing angle and room illuminance. The results of these measurements help to explain the observed variations in 5 and 50 lux data presented at the Pittsburgh AAPM meeting.
TU‐FF‐A3‐06: A Novel Algorithm for MTF/NPS Measurement for LCD/CRT Characterization and Its Evaluation: A Directional‐Based Approach
Authors: Suri J , Poolla A
Published: June 2005

Purpose: This paper presents an improved strategy for MTF and NPS measurements based on neighborhood approach, unlike the conventional approach. Method and Materials: Since resolution (MTF) and noise (NPS) are important quantitative measures of image quality, our protocol characterizes the displays using these parameters. The novelty of our evaluation strategy lies in the way we compute our MTF. This involves computation of MTF in eight different directions from its central position, averaging them and then replacing it at its central location. Such a method is a directional‐based MTF, and offers advantage of taking neighborhood pixels. Results: It is observed that the directional MTF is higher than the conventional MTF. Hence, directional‐based MTF approach presents a better method of computing the MTF. This also is justified by saying that the noise is being averaged by considering the directional approach. A similar behavior is observed for the NPS of the two displays. The noise observed in the NPS curves of the LCDs can be attributed to the discrete pixel structure of the LCDs. CRTs in this aspect have lower noise compared to the LCD displays. But the higher MTF of the LCDs indicates that LCDs have higher resolution, which is an important physical property for mammography applications. Conclusion: Based on the experimental results we conclude the following: (a) Our algorithm demonstrated the measurement of MTF using directional approach was considerably higher than the conventional approach; (b) LCDs and CRTs have a larger MTF difference using directional‐based approach compared to conventional approach; (c) All the measurements show that MTF and NPS curves obtained using directional‐based approach are consistent and smooth; (d) All the MTF measurements using directional‐based approach are better for LCDs compared to the conventional CRTs.
TU‐FF‐A4‐03: Concept and Evaluation of Averaged 4‐D CT Imaging in Determining the Internal Target Volume for Extracranial Stereotactic Radiotherapy of Lung Nodules
Authors: Drzymala R , Liu J , Hampton C , Bradley J
Published: June 2005

Purpose: To evaluate the accuracy and robustness of averaging 4‐D images to determine the internal target volume (ITV) for small lung tumors during breathing. Method and Materials: A Philips Brilliance 16‐slice CT simulator (CT‐sim) obtained axial cinematographic (cine') images (512×512 pixels, 480mm FOV) of a phantom that moved with known distance and velocity in both the longitudinal and axial planes. The CT‐sim recorded 25 cine' images in each of 16 slices, 1.5mm thick to create a 24mm slab of data at different times during a motion cycle. The images at each slice position were averaged to produce composite slices. Composite slices were assembled longitudinally to produce a composite slab. Concatenated composite slabs are called an Averaged 4‐D (4‐DAvg) image set. Patients were scanned using the protocol above. Gross tumor volume (GTV) appearing in the 4‐DAvg image set was contoured to produce an mGTV (ITV minus the margin for sub‐clinical disease) before superposition on beam's‐eye‐view DRRs and transferred to an on‐line electronic portal imager (EPI) for comparison of the mGTV position to that of the GTV as observed during treatment. Results: Phantom measurements showed key factors which govern the ability of the 4‐DAvg image set to accurately represent the location of objects during motion: slice thickness, cine' duration relative to the period of motion and the sampling frequency of the cine' relative to the duration of motion at its extremes. We determined the position of objects undergoing a 0.2Hz movement over 20mm to within approximately 1.0mm. Similar results were obtained for a patient in the stereotactic body frame using an EPI. Conclusion: For tumors with motions having a frequency about 0.2Hz over a 20mm range, one can obtain positional accuracy within approximately 1mm using the 4‐DAvg technique outlined above. Conflict of Interest: Partly funded by Elekta, Inc., Norcross, GA.
TU‐FF‐A4‐06: Dynamic Accuracy of An Implanted Wireless AC Electromagnetic Sensor for Guided Radiation Therapy; Implications for Real‐Time Tumor Position Tracking
Authors: Parikh P , Hubenschmidt J , Vertatschitsch E , Dimmer S , Wright J , Low D
Published: June 2005

Purpose: A wireless tumor localization and tracking system using three implanted AC electromagnetic transponders is in clinical trials for use in prostate cancer (Calypso® Medical). Phantom‐based studies have shown sub‐millimeter spatial localization accuracy in static tests. Accuracy has not been evaluated for dynamic motion found in lung tumors. This study was designed to determine the feasibility of using this patient positioning system for real‐time tumor‐tracking. Materials and Methods: A 4‐dimensional (4D) stage capable of arbitrary multidimensional motion with speeds up to 10 cm/sec was constructed. Two elliptical trajectory paths were created with peak‐to‐peak motion of 1cm × 2cm × 1cm and 2cm × 4cm × 2cm in the x, y and z directions. Each trajectory was operated with periods of 15 – 20 cycles per minute. The Calypso System was operated using one and two transponders with radiofrequency signal integration times of 33 ms 100 ms. The transponders were mounted on the 4D‐stage, with the ellipse centroids positioned 14 cm from the array. The effects of ellipse size, speed, number of transponders and signal integration time on transponder localization accuracy were evaluated by comparing the intended and measured trajectories. Results: The root mean square (RMS) position difference was less than 1 mm for all tested combinations. While small, the RMS error was largest for the large ellipse at 20 cycles per minute compared with the small ellipse at 15 cycles per minute. The single‐transponder system with 67 ms integration time had the smallest overall error, with a maximum single‐point error of 1.3mm. Conclusions Use of a wireless electromagnetic implanted transponder system for real‐time tumor‐tracking is feasible, with RMS errors less than 1mm for high‐speed multidimensional ellipses. This compares favorably with continuous fluoroscopic tracking methods without an ionizing radiation burden. This work is currently being expanded to patient‐derived tumor trajectories.
TU‐FF‐A1‐02: Commissioning Fast Monte Carlo Dose Calculation for Lung Treatment Planning
Authors: Craig J , Wong E , Mulligan M , Gladwish A , Gaede S , Chen J
Published: June 2005

Purpose: To commission a commercial Monte Carlo (MC) simulation package, NXEGS (Numerix LLC), for photon beam dose calculations. We investigated within NXEGS the EGS4 compatibility mode, fast MC, and post processing (PostP). Method and Materials: We commissioned NXEGS and Pinnacle 6.2b with the same set of measured data. We compared its dose calculation accuracy and efficiency with the collapsed cone convolution algorithm in Pinnacle and the National Research Council EGS4. Dose distributions were compared in three phantoms: a water phantom to check the output and beam profiles; a water phantom with a lung slab to test the inhomogeneity correction; and a water phantom with 1‐ 3 cm diameter cylindrical air pockets to test the PostP algorithm. We also compared fast MC using PostP with Pinnacle for a three‐field lung treatment plan. Number of histories is chosen to give +/− 2% dose accuracy at the isocenter. All doses were converted to cGy per MU. Results: Fast MC improves computational speed by a factor of ∼10 from the EGS4 compatibility mode. PostP decreases number of histories required and hence the computation time by another factor of ∼10. PostP adds ∼1 minute per 106 dose voxels. Inside the lung slab, fast MC with PostP differed from Pinnacle by ∼0.03cGy/MU with a misalignment of ∼2mm whereas fast MC with PostP agreed within 0.03cGy/MU of EGS4. PostP did not preserve the dose perturbation from ⩾1cm air inhomogeneities. Conclusion: Without PostP, the accuracy and computational time scaled with number of histories. When we specify ±2% accuracy in the target volume, the dose calculation time using fast MC with PostP is comparable to Pinnacle for a three‐field lung plan. NXEGS fast MC with PostP predicts the dose spread due to electron transport in lung with good accuracy‐to‐speed ratio and is suitable for routine treatment planning.
TU‐FF‐A1‐05: Limitations of the Photon and Electron Transport Algorithms in GEANT4 for Radiotherapy Applications
Authors: Poon E , Verhaegen F
Published: June 2005

Purpose: To examine the limitations in the photon and electron transport algorithms in GEANT4, so as to identify its suitability for radiotherapy applications. Method and Materials: A set of simulations were performed to access the accuracy of GEANT4. We calculated depth dose distributions of photon and electron beams incident on a water phantom, and examined the effects of dose perturbation in the presence of a lead interface. The results are compared to the EGSnrc Monte Carlo code. We attempt to provide explanations for the differences observed. An investigation of the problems in the condensed history algorithm is done through a series of cavity simulations under Fano conditions. Results: In homogeneous water, the depth dose distribution of a 10 MeV incident electron beam depends on the electron step size limitations and secondary particle production thresholds. The disagreement with BEAMnrc is over 4%, which is partly due to differences in cross section data among the EM models. For incident photon beams of 400 keV and 6 MeV, GEANT4 agrees with EGSnrc to within 0.4% and 2% respectively beyond the depth of dose maximum. Larger differences are found in the buildup regions due to problems in electron transport. In a Pb/water geometry, we observe an underdose of up to 84% upstream of the interface for a 100 kVp beam. This is mainly due to the neglect of spin effects in electron elastic scattering. In the Fano cavity simulations, problems in electron step and boundary crossing algorithms result in an underestimation of cavity dose by up to 39%. Accurate cavity response can be obtained using severe step size restrictions. Conclusion: GEANT4 is unsuitable for simulations where electron disequilibrium exists. Improvements in boundary crossing and backscattering are needed, and spin effects should be modeled.
TU‐FF‐A2‐02: Beam‐Weight Optimization for the Field‐In‐Field Treatment‐Planning Technique Using Fuzzy Logic
Authors: Petti P
Published: June 2005

Purpose: The field‐in‐field treatment‐planning technique is commonly used in tangential‐field irradiation of breast cancer, but it can also applied to other sites, eliminating the need for wedge filters in the treatment plan. The purpose of this work is to investigate an efficient and reliable method that employs fuzzy logic to optimize the beam weights for treatment plans that utilize the field‐in‐field treatment‐planning strategy. Method and Materials: Three tumor sites were considered, one in the breast, one in the brain, and one at the skull base. Four beam segments were used for the breast plan, and 8 segments were employed in the brain and skull‐base plans. The Pinnacle3 treatment‐planning system was used to design the beam apertures in all cases. The dose per monitor unit (MU) delivered by each beam separately, calculated at a selection of points within the target volume using the Pinnacle 3 planning system, served as input to the optimization procedure. In the optimization process, relative cold‐ and hot‐spot volumes within the target were defined as fuzzy variables. Development of a fuzzy rule set to determine how to adjust the number of MU delivered through each segment was simplified by dividing the target volume into sub‐volumes associated with each beam segment. The change in the number of MU was a fuzzy variable, which was de‐fuzzified using the center‐of‐mass method. Results: The field‐in‐field plans developed using fuzzy logic were comparable to standard plans employing wedges developed by experienced dosimetrists. The fuzzy optimization was also fast, requiring only seconds to complete. For both the fuzzy‐optimized and standard plans, the relative cold‐spot volume within the target was between 0 and about 6%, and the hot‐spot volume was less than 1%. Conclusion: Fuzzy optimization theory can be successfully applied to optimize the beam weights for field‐in‐field treatment planning.
TU‐FF‐A2‐03: Effects of CT Calibration On Tissue Inhomogeneity Correction in Radiotherapy
Authors: Song H , Chen Z , Ahmad M , Deng J , Fan Y , Nath R
Published: June 2005

Purpose: To quantify the effect of CT calibration on tissue inhomogeneity correction in radiotherapy. Method and Materials: Materials with known electron densities relative to water ranging from 0 to 2.34 were scanned with two GE Light Speed CTs at the four available KVs of 80, 100, 120 and 140 KV and a GE 9800 CT. The effect of using mismatched calibration curves on calculation of tissue inhomogeneity corrections were evaluated using the effective TMR method. Results: All CT calibration curves agree with each other between air and water region. Above the density of water, the calibration curves for lower kV were significantly higher than those for higher kV, by up to 560 HU for cortical bone (ρe= 1.69). Thus, using a generic calibration curves would lead to an error in the determination of electron density and hence the tissue inhomogeneity correction. The effect of CT number error (ΔHU) on dose calculation is approximated as ΔD/t [%/cm] = TMR′* ΔHU/2000, for HU > 1000, where t is the physical thickness of the dense material layer and TMR′ is the TMR gradient. For example if a cortical bone is scanned with GE Light Speed at 80 KV but the CT calibration curve of 140 KV is used in dose calculation and the treatment radiation beam is the 6 MV at field size of 1×1 cm2, the dose calculation error down stream of the bone layer will be about 1.2% per cm of bone. The effect is smaller for the 18 MV beam due to the smaller TMR gradient. Conclusion: For best accuracy of tissue inhomogeneity correction, the KV specific, instead of a generic, CT calibration curve should be used.
TU‐FF‐A2‐05: The Dosimetric Stability of the Prostate and Critical Structures in the Presence of Internal Motion for An Adaptive Correction Strategy
Authors: Keall P , Lauve A , Hagan M , Siebers J
Published: June 2005

Purpose: An adaptive correction strategy has been proposed in which internal motion is corrected for by repositioning the MLC aperture as an alternative to the current practice of patient repositioning. The study purpose was to investigate the dosimetric stability of this strategy in the presence of internal motion. Method and Materials: Internal motion shifts were introduced in 25 prostate plans by shifting the PTV, rectum and bladder contours with respect to the bony‐anatomy. Thirty‐six randomly selected isotropic displacements of magnitude 0.5, 1.0, 1.5 and 2.0 cm were sampled for each patient, totaling 3600 errors. The adaptive correction strategy, which shifted the beams‐eye‐view of the MLC aperture to match the contours, was used to correct each of these errors. Recomputed plans were compared to the original treatment plans via dose‐volume histogram analysis. Changes of more than 5% of the prescription dose were deemed clinically significant. Results: The adaptive correction strategy produced small dose discrepancies for all structures considered apart from the femoral heads. Coverage of the PTV was excellent: D5, D95 and Dmean where changes were <5% for all of the 3600 simulated internal motion shifts. D33 of the rectum was increased by >5% in 9/3600 sampled internal motion shifts, while the bladder D20 deviated by >5% in 9/3600 samples. The femoral heads Dmean increased by >5% for 651/3600 (18%) internal motion shifts. However, D2 was not increased by >5% for any of the internal motion shifts. Conclusion: These data demonstrate the robustness of the proposed adaptive correction strategy for correction of internal motion. The corrections can be performed remotely, thus eliminating errors resulting from increases in treatment time or from patient repositioning. This method has the ability to correct for both interfraction displacement of the prostate and for intrafraction prostate motion. Supported by NCI R01CA93626 and R01CA98524.
WE‐C‐I‐609‐03: Study of Radial Gradient Features in LDA Classifier for Automated CT Lung Nodule Detection
Authors: Roy A , Armato S
Published: June 2005

Purpose: To study the use of radial gradient index features by an LDA classifier for false positive reduction in automated CT lung nodule detection. Method and Materials: Our database contains 38 diagnostic CT scans, with a total of 82 lung nodules. A radial gradient index (RGI)‐based approach is used to reduce false positives detected by our automated method. For each CT section a complementary image (an “RGI map”) is generated in which the pixel intensity is proportional to the RGI computed along a circle of chosen diameter d, centered at that pixel. As the RGI is maximum for a perfect circle, an RGI map enhances the intensity of nodules relative to neighboring anatomic structures. For every candidate we calculate a set of three RGI features, for each of five different values of the RGI diameter. We evaluate the performance of the classifier by introducing in turn RGI features corresponding to a particular diameter, together with an optimal set of 9 non‐RGI features determined previously. The results are compared with the performance of the LDA without RGI features. Finally, we use stepwise LDA in order to identify optimal features. Results: The performance for d = [12, 16, 20] is optimal (the sensitivity increases from 70 % to 79 % at 0.5 false detections/section) while for d ⩾ 24 performance decreases. A stepwise LDA was performed for 10 random partitions of the database in order to evaluate the relative weight of different features for classification. This revealed that 8 out of 15 RGI features were included 9 or more times within the optimal feature set. Conclusion: Inclusion of RGI features results in a substantially improved FROC performance, which is consistent with results of stepwise linear discriminant analysis. Conflict of Interest: SGA is a shareholder in R2 Technology, Inc.
WE‐C‐I‐609‐04: Observer Evaluation of Semi‐Automated Mesothelioma Measurements
Authors: Armato S , Oxnard G , Kocherginsky M , Vogelzang N , Kindler H , MacMahon H
Published: June 2005

Purpose: Accurate quantification of malignant pleural mesothelioma tumor burden is essential for proper patient management and for the conduct of clinical trials. Although manual measurement of tumor thickness on computed tomography (CT) scans is the current clinical standard for assessing response to therapy, this approach is tedious and time‐consuming. We have developed semi‐automated methods to quantify the extent of mesothelioma on CT scans, and we have assessed the performance of these methods through an observer evaluation study. Method and Materials: Given a user‐specified point along the outer margin of the tumor in a CT section, the computer automatically identifies a corresponding point along the inner margin of the tumor and displays a line segment between the user‐specified outer tumor margin point and the computer‐identified inner tumor margin point. The length of this line segment represents tumor thickness. Three observers independently reviewed semi‐automated measurements (i.e., line segments and their corresponding lengths) generated by three different algorithms at a fixed set of 134 measurement sites in the CT scans of 22 mesothelioma patients. These algorithms are based on morphological characteristics of the automatically segmented lung regions. Observers had the opportunity to accept a measurement or to modify it through a computer interface. Differences between the initial semi‐automated measurements and the measurements as modified by the observers were analyzed. Results: The frequency with which observers accepted the semi‐automated measurements without modification was as high as 86%. Of all measurements across all observers and methods (1206 measurements), 89% were changed by less than 2 mm. Conclusion: We expect these promising computerized methods to become important components of clinical protocols for mesothelioma by making the quantification of tumor extent more efficient and consistent. Conflict of Interest: SGA and HM hold warrants to shares of R2 Technology, Inc. (Sunnyvale, CA).
WE‐C‐I‐609‐10: A Feasibility Study of Atlas‐Based Image Segmentation in 3D Treatment Planning
Authors: Zhang T , Shah H , Nelson I , Tome W , Paliwal B
Published: June 2005

Purpose: To develop and evaluate a fully automatic atlas‐based image segmentation method for radiotherapy treatment planning. Method and Materials: A very important part of 3‐dimensional radiotherapy treatment planning is the contouring of normal regions of interests (ROIs). Currently, automatic image segmentation tools provided in treatment planning systems are usually based on edge detection and may not always be satisfactory. Atlas‐based image segmentation is a promising method that uses deformable image registration to register the target images with reference images containing previously contoured ROIs. Atlas‐based image segmentation utilizes all of the information in the images and has the potential to perform better than edge‐detection based algorithms, in which only the edge information is used. The performance of atlas‐based image segmentation relies on the deformable image registration algorithm. We implemented a fast variational deformable image registration algorithm. The algorithm was tested at three common radiotherapy sites: head/neck, chest and pelvis. Physicians manually contoured the ROIs in the reference CT images. The new, randomly selected target images were automatically contoured. The results were evaluated by the physicians. Results: The entire process was completed in less than 15 minutes using a single CPU computer. The ROI masks were transformed into the new images using the displacement maps from deformable image registration. Contours were regenerated from the ROI masks and imported into the Pinnacle™ treatment planning system. In all three clinical sites, the majority of generated ROI's required only minor further modifications. Conclusions: Atlas‐based image segmentation is a powerful automatic contouring tool. Our algorithm provides excellent results in a majority of clinical cases. Implementation of atlas‐based image segmentation may reduce the time required by physicians to contour normal structure ROI's in treatment planning. Additionally, it opens up the possibility of standardizing ROI contours, such as the parotid, in head and neck IMRT.
WE‐C‐J‐6C‐06: Assessing Prostate Volume Changes During Conformal Radiotherapy Using Implanted Fiducial Markers
Authors: Moseley D , White E , Wiltshire K , Haycocks T , Keller H , Siewerdsen J , Sharpe M , Bissonnette J , Catton C , Jaffray D
Published: June 2005

Purpose: Evaluate changes in the volume of the prostate gland during radiation therapy using implanted fiducial markers. Method and Materials: The study includes 15 patients being treated for prostate cancer with 84Gy in 42 fractions. Prior to treatment, gold fiducial markers are implanted in the apex, base and posterior aspect of the prostate under ultrasound guidance. Daily on‐line image‐guidance is achieved using an orthogonal pair of MV portal images. Under research protocol, daily volumetric images are captured using a kV cone‐beam CT enabled linac. These images, reconstructed on a fine voxel grid (0.25mm3), allow precise 3‐D localization of the implanted markers using a threshold based auto‐segmentation technique. Acquisitions that exhibited marker movement due to patient motion were excluded from the analysis. The contoured prostate volumes were between 31.9 and 77.2 cc with a median value of 41.8cc. The triangular area spanned by the three implanted markers was measured each day and normalized by the triangular area in the planning CT to allow comparisons between patients. Results: The correlation between the area of the seed triangle and the contoured prostate volume was measured to be roughly linear with R2 = 0.63. Based on this patient population, the prostate grows in size during the first two weeks of treatment by approximately 4% and then subsequently shrinks by 7%. The crossover point occurs roughly half way through the treatment course. Conclusion: The triangular area spanned by three implanted fiducial markers was shown to be a reasonable surrogate to the prostate volume. Based on this surrogate, the prostate volume in 15 patients was measured to increase in the first two weeks of treatment, followed by a subsequent decrease in volume by approximately 7% over the duration of the treatment course. Conflict of Interest: This work is sponsored by the Elekta Synergy Research Consortium.
WE‐C‐J‐6B‐01: Fast 4D Imaging: Breaking the Speed Limits in MR and CT
Authors: Mistretta C
Published: June 2005

Advances in MRI and CT have made it possible to produce time‐resolved series of 3D volume images. In CT the increase in detector rows to the present value of 64 ‐ or even 256 ‐ has brought a significant increase in image quality and reduced acquisition times. The emergence of volumetric cone beam systems employing flat panel detectors also offers the possibility of rapid 4D acquisition. In MRI, 4D sequences have been facilitated by the introduction of multi‐coil parallel imaging methods and several acquisition schemes using novel k‐space sampling schemes. The vast majority of MRI applications use Cartesian acquisitions employing phase encoding. This line‐by‐line scan through k‐pace produces spatial resolution that is linearly related to acquisition time. This is basically a k‐space speed limit imposed by the Nyquist theorem. For dynamic applications this tradeoff between temporal and spatial resolution is unacceptable. When highly undersampled radial acquisitions are used, spatial resolution is determined by readout resolution rather than the number of k‐space lines acquired. The price paid for violating the Nyquist theorem is streak artifacts. However, when the data acquisition is done using a time‐resolved 3D radial acquisition called VIPR(Vastly undersampled Isotropic PRojection imaging), the streaks are often tolerable and have permitted acceleration factors up to 60 relative to Cartesian acquisition for applications such as phase contrast imaging where background signals are cancelled and streaks can only be generated by the vessels of interest. Undersampled cone beam acquisition using an array of x‐ray sources combined with large area detectors has been proposed as a means of increasing the temporal resolution of CT and appears to have similar properties to 3D undersampled MRI acquisition.
WE‐C‐T‐617‐01: A First Step Toward a New National Standard for Direct Calibrations in Clinical High Energy Photon Beams
Authors: Chen‐Mayer H , Tosh R , Bateman F , Speicher P
Published: June 2005

Purpose: Clinical reference dosimetry for high‐energy photon and electron beams is based on ion‐chamber calibrations in a Co‐60 beam for absorbed dose to water, as prescribed in the TG‐51 protocol. A complex formalism is required when converting the calibration to the beam quality of end users. Direct calibrations in clinical high‐energy x‐ray beams will eliminate the complexity and associated uncertainties, resulting in greater accuracy and measurement simplicity. Method and Materials: The current primary standard for absorbed dose is transferred through ionization chamber measurements in a Co‐60 beam and is ultimately traceable to the first‐generation water calorimeter developed at NIST by Domen. We are evaluating a new calorimeter with modern data acquisition in a Clinac 2100C photon beam at 6 MV and 18 MV. Results: To address the convection/conduction problems inherent to water calorimetry, we have devised a novel data‐collection scheme with multiple cycles of radiation‐induced temperature increase as a function of time; this has greatly improved the efficiency of measurements, thereby achieving greater precision. The temperature rise can be extracted by three approaches that yield consistent results; these will be described in detail in the presentation. The absorbed dose determined at 18 MV and 6 MV using the water calorimeter at selected depth along the beam axis are compared to the depth‐dose curves measured with a calibrated ion chamber following the TG‐51 protocol. In addition, the dependence of bridge‐excitation voltage at the higher dose rate (4 Gy/min) behaves differently from that at a lower rate (1 Gy/min) in a Co‐60 beam; this has been investigated in order to assess the influence of the thermistor power on measured results. Conclusion: We have obtained preliminary results and are moving toward a new national standard for direct calibrations in clinical beams.
WE‐C‐T‐617‐05: The Effects of Aperture Size On Low‐Energy Brachytherapy Sk Measurements
Authors: Culberson W , DeWerd L
Published: June 2005

Purpose: To experimentally determine the angular dependence of and interstitial brachytherapy seed air‐kerma strength measurements with a large‐volume ionization chamber and compare with Monte Carlo calculations. Method and Materials: A Variable Aperture Free Air Chamber (VAFAC) has been constructed for making air‐kerma rate measurements of low‐energy photon emitting sources with photon energies up to 70 keV. Its underlying principle is that the air‐kerma rate at a given point is proportional to the increment of ionization per increment of chamber volume at chamber depths greater than the range of secondary electrons originating in the entrance electrode. A unique feature of the VAFAC is its variable aperture stand, which accommodates five apertures with half angles ranging from 4 to 18 degrees. The US primary air‐kerma strength standard is the National Institute of Standards and Technology Wide Angle Free Air Chamber (NIST WAFAC), which has a single fixed aperture size corresponding to a half‐angle of 7.6 degrees. Relative air‐kerma rate measurements have been completed for one seed type with all five aperture sizes. Additionally, MCNP5 particle transport has been used to theoretically determine the expected relative ionization with these aperture sizes. Results: It is shown that the new VAFAC is capable of making air‐kerma rate measurements for low‐energy brachytherapy sources with a noise floor of 1–2 fA. The results from the first seed type indicate that varying the angle from 4 to 18 degrees yields up to a 5% change in the final air‐kerma strength determination. Conclusion: This work shows the capability of the VAFAC to experimentally measure the effects of and interstitial brachytherapy seed in‐air anisotropy on a large angular measurement.
WE‐C‐T‐617‐08: Beam Hardening and Implications for Dose Calibration of An A‐Si EPID
Authors: Kirkby C , Sloboda R
Published: June 2005

Purpose: This work aims to quantify and minimize a dose calibration curve discrepancy observed for an a‐Si flat panel megavoltage imager that occurs when the spectral quality of a lower energy (6 MV) beam is changed from that for an open field due to beam hardening. Method and Materials: This investigation considered a Varian aS500 EPID. In the first phase, Monte Carlo simulations modeled spectral changes to a generic 6 MV photon beam as it passed through different thicknesses of steel shot compensator material. A comprehensive model of the detector was also used to simulate the dose absorbed by the integral phosphor layer for the conventional detector configuration, and for configurations including an additional, external copper plate placed in direct contact with the EPID and elevated 154 cm above it. The second phase consisted of measuring EPID dose calibration curves in open and steel‐shot attenuated beams for the various EPID configurations. The modulation transfer function (MTF) and the contrast‐to‐noise ratio (CNR) were also monitored for each configuration using a standard quality assurance phantom. Results: The discrepancy between open and attenuated beam calibration curves was observed to be as much as 6% at 6 MV. To reduce the maximum discrepancy to < 4%, copper thicknesses of 1.0 cm or 0.3 cm were required in the contact and elevated configurations, respectively. Adding the copper reduced the CNR by 28% or 8%, respectively, and the MTF for a given spatial frequency by 38 ± 1% or 13 ± 1%, respectively. Conclusion: Beam hardening can cause significant dosimetric discrepancies for a‐Si EPIDs calibrated in open fields. Addition of an external copper plate can substantially reduce the discrepancy, but at a cost of reduced image quality.
WE‐C‐T‐617‐10: Geometry Calibration of An On‐Board KV Imaging System
Authors: Jeung A , Sloutsky A , Virshup G , Gaudio S , Mostafavi H , Munro P
Published: June 2005

Purpose: We have developed a method of geometrically calibrating the MV and kV radiographic isocenters of a Varian linear accelerator equipped with an on‐board kV imaging system. Method and Materials: The calibration uses a cylindrical phantom containing 16 BB's placed in a spiral pattern and ∼400–600 radiographs of the phantom acquired using both kV and MV imaging systems. Since the phantom has a known geometry, the 7 degrees of freedom (DOF) needed to fit the predicted projections of the BB's to the measured projections can be calculated. During MV imaging a partial transmission plate is placed in the accelerator wedge slot to project a reference shape onto the EPID images of the phantom. The reference shape accounts for motion of the EPID and generates the radiographic isocenter for the MV x‐ray beam alone. Geometry correction parameters can be calculated for the kV imaging system that map the center of radiographic images or CBCT projections to the radiographic isocenter of the MV beam. The method has been tested with linear accelerator and lab bench imaging systems. Results: BB identification accuracy is ∼0.3 pixels and is limited by noise in the images. While the method is insensitive to changes in SID (0.3 pixels ∼= 2.5 cm SID change), the method can measure known in‐plane imager displacements to an accuracy of better than 0.1 mm. Imager displacements versus gantry angle are small − 0.75×0.50 mm. CBCT reconstruction using parameters from this method results in spatial frequencies as high as 18 lp/cm and adjustments in parameters of ±0.1 mm reduce the reconstructed spatial resolution. Conclusion: Our method to match MV and kV radiographic isocenters for on‐board imaging systems shows that accelerator flex is quite modest but that the calibration method can improve the spatial resolution of the CBCT reconstructions.
WE‐C‐T‐6E‐05: NEXGEN Restorative Health Care Thru Synthetic Biology and Nanoscience
Authors: Montemagno C
Published: June 2005

Recent advances in our ability to manipulate matter at the scale of individual molecules have created an incredible level of excitement in both the scientific community and the general population. The excitement over this new capability, commonly labeled nanotechnology, is vested in the expectation of the development of new materials and systems that offer unparalleled functionality. Materials that autonomously adapt their shape and physical properties in response to the their surroundings, computers that instead of operating by switching the flow of electrons, manipulate information through the management of the ethereal world of quantum states and, molecular sized machines that actively repair damage to our bodies and function as molecular scale prosthetics are all expectations of nanotechnology. While the question of whether or not this vision is truly achievable is still open, the truth is that much of the expectations for nanotechnology are already realized in living systems. Living systems however, are more than a product of matter manipulation at the molecular scale; the richness of functionality associated with living systems is a direct product of the information generated from both the interactions between molecules and the overall supra‐molecular structure of the system. In essence living systems are “living” because of the fusion of nanotechnology and informatics. Living systems result from the precision assembly of matter with prescribed modalities for the transport and transduction of information among supra‐molecular clusters. Recent advances in surgical and diagnostic tools emanating from nanotechnology will be presented in the context of Integrative Technology. Integrative Technology is the intersection of the precision assembly of matter, nanotechnology, coupled with the functional building blocks of nature, biotechnology, fused by the network flow of spatiotemporal information, informatics. The power of Integrative Technology is illuminated through the creation of Synthetic Biological Systems. Hybrid biotic/abiotic systems that enable the restoration of loss biological function thru designed communication among supra‐molecular assemblies. Illustrative examples will be presented that include the engineering of muscle‐powered Biobots and the creation of nano‐sized excitable vesicles with the ability to intrinsically process information. These devices have the potential of replacing lost biological function in physiologic structures such as the Sinoatrial node or damaged neurons, and highlight a new modality for disease therapy. Technology that will expand surgical applications beyond the arena of structural repair to the realm of physiological manipulation.
WE‐D‐I‐6B‐02: High DQE Megavoltage Imaging Using Active Matrix Flat‐Panel Imagers Incorporating Polycrystalline Mercuric Iodide
Authors: Zhao Q , Antonuk LE , El‐Mohri Y , Du H , Li Y , Sawant A , Su Z , Wang Y , Yamamoto J
Published: June 2005

Purpose: In order to explore the potential for significantly improving the DQE of megavoltage active matrix flat panel imagers (AMFPIs), two forms of thick, polycrystalline mercuric iodide photoconductors have been investigated. Method and Materials: Prototype direct‐detection AMFPIs with an effective pitch of 508 μm and a pixel format of 192×192 were coated with two types of : a “PVD” form created through physical vapor deposition and a “PIB” (particle‐in‐binder) form involving screen‐printing. Results were compared to those from a conventional indirect‐detection megavoltage AMFPI employing a phosphor screen. Data were obtained fluoroscopically, using a 6 MV beam at very low doses, equivalent to ∼0.004 to 0.04 cGy. Results: Compared to PVD, the PIB array exhibits much lower dark current, lower dark drift, slightly higher lag and similar non‐uniformity, linearity and DQE. For both PVD and PIB, MTF and DQE results are in a good agreement with theoretical expectations, and the MTF is higher than that from the conventional megavoltage AMFPI. Moreover, the DQE results show input‐quantum‐limited behavior, even at very low doses. Finally, zero‐frequency DQE values are ∼1.4% and 1.2% for PVD and PIB, respectively, matching the theoretical upper limits for the thickness of the converters used. Given the modest photoconductor thickness of these early prototypes, the DQE values compare favorably to values of ∼1% obtained from conventional megavoltage AMFPIs. Conclusion: These initial studies indicate that AMFPIs incorporating polycrystalline have the potential for significant dose reduction in megavoltage imaging. The use of offers the promise of increased quantum efficiencies through the development of substantially thicker converters than have thus far been achieved, without significant degradation of the spatial resolution. In addition, compared to PVD, PIB appears to provide better prospects for such thick coatings, leading to higher DQE. This work is supported by NIH grant R01‐CA51397.
WE‐D‐I‐6B‐08; Deformable Registration of the Planning Image (KVCT) and Daily Treatment Images (MVCT) for Adaptive Radiation Therapy
Authors: Lu W , Chen Q , Kupelian P , Langen K , Meeks S , Ruchala K , Olivera G
Published: June 2005

Purpose: The incorporation of daily images into the radiotherapy process leads to Adaptive Radiation Therapy (ART), in which the treatment is evaluated periodically and the plan is adaptively modified during the whole course of radiotherapy. Deformable registration between the planning image (usually KVCT) and the daily image is the key component of ART. The MVCT is one of the most informative yet convenient daily image modalities. In this presentation, we developed a fast technique for deformable registration between the KVCT image and the MVCT images. Method and Materials: The method is extended from the state‐of‐art deformable registration technique, which is fast and accurate for same modality registration. Considering the higher noise and lower contrast nature of MVCTs, special techniques such as “edge‐preserving smoothing” and “reference based histogram calibration” are applied before the deformable registration process. The whole process is around 2–3 minutes for typical MVCT size (256×256×64) when runs on a single processor PC. We retrospectively studied daily MVCTs from commercial TomoTherapy machines in different clinical centers. These data include 3 lung cases, 5 head‐neck cases, 3 prostate cases and 2 pelvis cases. Each case has one KVCT image and 30–40 MVCT images. We registered the MVCT images with their corresponding KVCT image. Results: The similarity measures and visual inspection of contour matches by physicians validate this technique. The applications of deformable registration in ART, including “accumulative dose calculation”, “automatic ROI re‐contouring” and “tumor growth/shrinkage monitoring” through the whole course of radiotherapy are studied. Conclusion: Deformable registration between the KVCT and MVCT images is an essential step towards ART. Through the combination of conventional image processing techniques and the fast intensity based deformable registration, such task becomes feasible. Extensive tests based on the daily MVCT data from the TomoTherapy machines validate such technique. Several key components of ART are developed.