bluuebunny/crossref_metadata_2025 huggingface dataset:
TU‐C‐332‐07: Accuracy and Reproducibility of Tumor Position During Prolonged and Multi‐Modality Animal Imaging Studies
Authors: Zhang M , Huang M , Le C , Zanzonico P , Claus F , Kolbert K , Martin K , Ling C , Koutcher J , Humm J
Published: July 2008

Purpose: Dedicated small‐animal imaging devices are being used more frequently for translational molecular imaging studies. However, few studies have investigated the magnitude of animal motion during extended dynamic imaging studies or the precision of animal repositioning in multi‐modality and/or serial imaging protocols. The objective of this work was to determine the positional accuracy and precision with which tumors in situ can be reliably and reproducibly imaged on dedicated small‐animal imaging equipment. Method and Materials: A custom rodent animal cradle with a stereotactic template served to define a coordinate system and to facilitate rigid‐body image registration. Attached to the template were fiduciary markers containing PET tracer and MRI and CT contrast media for visualization on the respective scanners. To quantify animal tumor motion during imaging protocols, “gold standard” point markers were inserted into tumors grown on the hind limb of nude rats. Three types of imaging examination were performed with the animals continuously anesthetized and immobilized: (i) single‐modality imaging (microPET and MRI) in which the animals remained in the same scanner continuously for 2 hours, (ii) multi‐modality imaging studies in which the animals were transported from a microPET to an MR scanner located in another building, and (iii) serial microPET scans in which the animals were removed from the scanner, then re‐positioned and scanned. Results: The animal tumor moved by less than 0.2–0.3 mm over two‐hour microPET or MR imaging sessions. Transporting the animal between instruments introduced additional error of ∼0.2 mm. In serial animal imaging studies, in which the animal was returned to its cage and subsequently re‐positioned, reproducibility within ∼0.8 mm could be obtained. Conclusion: To our knowledge, this is the first study systematically and rigorously evaluating the accuracy and precision with which tumors can be repeatedly imaged in small‐animal imaging devices.
TU‐C‐352‐01: Adapting Traditional Clinical Medical Physics to Digital Radiography
Authors: Willis C , Jones A , Shepard S , Peck D
Published: July 2008

Development of standards for performance verification of digital radiographic systems has been outpaced by commercialization of the technology. Practitioners, operators, vendors, and medical physicists are uncertain as to how to configure, calibrate, and verify systems for the best diagnostic quality images at the lowest practical radiation exposure to the patient. Several manufacturers have fielded commercial systems for performance verification, but long term data indicating trends and action limits is rare. Sources of definitive information on practical clinical methods are also scarce in the scientific literature. This has led to the current situation where clinical imaging operations are routinely conducted using systems that are not optimized.

This workshop will discuss four important aspects of traditional medical physics practice that are modified when dealing with DR systems. These processes are of primary importance to medical physicists, because they affect both the quality of DR images and the ionizing radiation dose to patients. First, we consider how to perform an inspection on a radiographic unit using CR or DR. Second, we discuss how to set up the AEC for CR or DR exams. Third, we address how to verify the exposure indicator for CR and DR. Last, we discuss available phantoms and their characteristics and limitations when used with DR.

Educational Objectives:

1. Understand how some conventional tests should be modified for a digital radiographic system that is integrated into an electronic image management system.

2. Identify key references and standards that can be useful in testing of DR.

3. Appreciate opportunities for improvement of testing of DR systems.
TU‐D‐AUD B‐01: Volumetric Modulated Arc Therapy for Head & Neck Cancer
Authors: Afghan MKN , Cao D , Mehta V , Shepard D
Published: July 2008

Purpose: Volumetric Modulated Arc Therapy (VMAT) is an arc‐based technique that utilizes dynamically modulated arcs to deliver intensity‐modulated radiation therapy (IMRT) treatments on a conventional linear accelerator. In this work, we evaluate VMAT as a treatment technique for patients with carcinomas of the head‐and‐neck. Method and Materials: Five complex head and neck cancer patients, with multiple prescription levels, were selected for this study. Fully inverse planned VMAT plans were optimized for these patients using our homegrown arc‐sequencing software. The software uses simulated annealing to optimize the aperture shapes and weights while minimizing the differences between the optimized (ideal) and sequenced intensities. The optimized plans were compared with step‐and‐shoot IMRT plans generated using the Pinnacle3 treatment planning system. VMAT plan verifications have also been performed using Elekta's Precise Beam Infinity™ control system which has been installed on an Elekta Precise linear accelerator in our clinic. Results: Using our arc‐sequencing tool, VMAT can be used to create highly conformal head‐and‐neck treatment plans. As compared with traditional fixed‐field plans, VMAT was able to reduce the average parotid dose from 85.3 cGy to 73.6 cGy per fraction. Additionally, the average number of monitor units was reduced from 1058.3 to 502.3 per fraction. Initial delivery tests demonstrate that using VMAT complex head‐and‐neck deliveries can be completed in under 6 minutes. Conclusion: VMAT should serve as an important tool in the delivery of radiation therapy for head‐and‐neck carcinomas. By utilizing the dosimetric advantages of rotational IMRT, VMAT can provide more uniform target doses and reduced critical structure doses as compared with fixed field IMRT. Conflict of Interest: Research partially sponsored by Elekta Corporation.
TU‐D‐AUD B‐04: A Dosimetric Comparison of Simultaneous Integrated Breast Radiotherapy Using 3D Conformal, IMRT and Tomotherapy Techniques
Authors: Kirk M , Shah A , Coon A , Jiang L , Dickler A , Griem K
Published: July 2008

Purpose: The dosimetric characteristics of simultaneous integrated boost (SIB) breast radiotherapy techniques utilizing 3D‐conformal (3DSIB), linac‐based intensity modulation (IMRT) and Tomotherapy (TOMO) were compared. The SIB techniques were compared to conventional whole breast irradiation (WBI). Method and Materials: Fifteen patients were included in the study. The SIB treatments (3DSIB, IMRT and TOMO) delivered 45Gy(25×1.8Gy) to the whole breast minus the boost bed, PTV(Breast‐Boost), and concurrently delivered 60Gy(25×2.4Gy) to the tumor bed, PTV(Boost). The WBI treatment delivered 45Gy(25×1.8Gy) the whole breast, PTV(Breast), followed by 20Gy(10×2Gy) to PTV(Boost). The WBI and SIB treatment were planned to deliver equivalent dose to the targets. Statistical significance between dosimetric parameters was determined using Wilcoxon signed‐ranks. The dosimetric data were compiled to form population dose volume histograms which display the mean value and 1σ uncertainty. Dose homogeneity was quantified by the parameter D95%−5%, representing the percent difference in dose covering 95% of the target minus the dose covering 5% of the target. Results: Using the D95%−5% parameter the PTV(Breast‐Boost) volume received a more homogeneous dose using TOMO(14.3%) compared to 3DSIB(25.1%), IMRT(27.1%) and WBI(44.7%). The PTV(Boost) volume received a morehomogeneous dose using non‐intensity modulated techniques, WBI(6.9%) and 3DSIB(7.0%), compared to IMRT(11.6%) and TOMO(12.0%). The mean percent volume of ipsilateral lung that received ⩾20Gy was smaller using IMRT(15.8%) compared to 3DSIB(17.7%), TOMO(17.1%) and WBI(19.0%). For left‐sided breast cancer patients, the mean percent volume of heart that received ⩾35Gy was less for intensity modulated SIB techniques, IMRT(0.1%) and TOMO(0.2%), than 3DSIB(4.4%), and WBI(6.2%). The contralateral breast received a smaller mean dose using IMRT(0.39Gy) compared to WBI(0.81Gy), 3DSIB(0.67Gy) and TOMO(1.28Gy). Conclusion: IB techniques delivered a more homogenous dose to the PTV(Breast‐Boost) volume, while non‐intensity modulated techniques delivered a more homogeneous dose to the PTV(Boost) volume. Intensity modulated SIB techniques (IMRT and TOMO) decreased the heart dose relative to 3DSIB.
TU‐D‐AUD C‐04: Response of An Implantable MOSFET Dosimeter to 192Ir HDR Radiation
Authors: Fagerstrom J , Micka J , DeWerd L
Published: July 2008

Purpose: To characterize the response of implantable MOSFET dosimeters to HDR radiation. Method and Materials: Sicel Technologies, Inc. has developed an implantable MOSFET detector that is read telemetrically by an inductively coupled antenna. All tests were performed in Virtual Water™ phantoms. To test detector response to dose rates near those delivered by HDR sources, detectors were irradiated at four SADs by a source. Detector response to relative to energies was tested by alternating fractions between and . The effect of accumulated dose was tested with detectors at 3, 4, and 5 cm from an source. The rotational and longitudinal angular responses were tested by irradiating the dosimeters at varying orientations around the source. Results: Differences in detector response due to varying dose rates were not statistically measurable. The detectors were more sensitive to than energies, and demonstrated a nonlinear accumulated dose effect. This effect was characterized by three third‐order polynomials fit to data from detectors placed at three different distances from the source. The detectors have little angular dependence except when the detector is irradiated with the coil and electronics assembly aligned between the MOSFETs and incident radiation. This orientation causes a −15.3% difference in response. Conclusion: Dose rates between 22 and 84 cGy/min do not measurably affect detector response. The energy response and accumulated dose effect have been characterized. The angular response of the detectors is minimal except when the MOSFETs are irradiated through the coil and electronics assembly. This effect may be minimized by establishing proper orientation in implantation protocols. A calibration curve obtained with would allow the dosimeter to be used for absolute dose measurements with HDR radiation. Conflict of Interest: Funding for this research was provided in part by Sicel Technologies, Inc.
TU‐D‐AUD C‐08: Experimental Proof and Feasibility of a 3D Real‐Time Detector System for Therapeutic Proton Beams
Authors: Archambault L , Beddar S , Sahoo N , Poenisch F , Gillin M , Mohan R
Published: July 2008

Purpose: Fast and accurate quality assurance of complex proton treatments is challenging, but could be achieved with a real‐time three‐dimensional detector system. The purpose of this work is to study the feasibility and to provide experimental proof of principle that a liquid scintillator (LS) dosimetry system can measure three‐dimensional dose distributions of proton beams in real‐time. Method and Materials: A plastic tank of 7×7×16 cm3 filled with BC‐531 liquid scintillator was irradiated with a proton pristine Bragg peak. Scintillation light produced during the irradiation was measured with a cooled CCD camera. Acquisition rates of 20 or 10 frames per seconds (fps) were used to image consecutive frame sequences. These measurements were then compared to ion chamber measurements and Monte Carlo simulations. Results: We have shown that real‐time image acquisition was possible with our current dosimetry system design. A field of view of 12×9 cm2 was measured with the CCD camera. Dose measured from the images acquired at rates of 20 fps and 10 fps have a standard deviation of 1.1% and 0.7% respectively in the plateau region of the Bragg peak. Differences were seen between the raw LS signal and the ion chamber due to quenching effects of the LS. We have shown that this effect can be accounted for and corrected by Monte Carlo simulations. Once corrected, dose distributions measured with the LS were in good agreement with ion chamber measurements. Conclusion: We have shown the feasibility of using real‐time image acquisition to measure proton dose distributions in LS. Like many other detectors, LS response depends on the energy of the proton beam, but we have shown that Monte Carlo can be used to correct LS measurements. Such system would be extremely useful for fast and accurate quality assurance of proton treatments.

Supported by the NCI (1R01CA120198‐01A2).
TU‐D‐AUD C‐09: In‐Vivo Dosimetry Using Aluminum Oxide Detectors
Authors: Danzer J , Harris C , Usynin A , Ramsey C
Published: July 2008

Purpose: The purpose of this study was to determine if aluminum oxide detectors could be used for in‐vivo dosimetery. The first specific aim of this project was to characterize the performance of a commercially available aluminum oxide detector system for in‐vivo dosimetry. The second specific aim of this study was to compare the accuracy of the new aluminum oxide detector to a commercially available MOSFET system in side‐by‐side patient measurements. Method and Materials: The dosimeters were evaluated for: 1) Signal decay; 2) Field size dependence; 3) Energy dependence; 4) Angular dependence; and 5) their reusability using the Landauer, InLight MicroStar system. In‐Vivo dosimetry measurements were taken for 53 patients treated on a Varian 21EX using dosimeters and 67 patients using OneDose Mosfets. Results: The variation between dosimeters was evaluated and found to be ±1.6%. The dosimeters appeared to over‐respond in the first 10 minutes, however, after 10 minutes the chips were within 1 percent of the steady‐state reading. Unlike other detectors, the dosimeters showed no field size, energy, or angular dependence. In testing the reusability, it was found that the half life of the OSL material was 5.14 ± 0.01 hours. The agreement between the dosimeters and the calculated doses for the in‐vivo dosimetry patients was −1.9±5.9%. The OneDose agreement between mosfet and calculated dose was found to be −1.3±8.5%. Conclusion: dosimeters can be a convenient, inexpensive alternative to TLDs, MOSFETS, and Diodes. The agreement between calculated and measured doses for in‐vivo dosimetry QA is comparable to TLDs, MOSFETS, and Diodes. The dosimeters can be quickly read and analyzed after 10 minutes (to allow time for signal decay). The dosimeters do not appear to have an energy, field size, angular, or sensitivity dependence. In addition, under specific conditions, the detectors can be erased and re‐used.
TU‐D‐351‐01: Image‐Guided Therapies: From Fundamentals to New Frontiers
Authors: Siewerdsen J , West J , Sherar M , Yan D
Published: July 2008

Concurrent advances in medical imaging, therapeutics, and understanding the biological basis of disease progression and treatment response point convincingly to a transformative approach to medical interventions in the decades ahead. Such interventions are marked by a dramatic increase in the utilization of image information from multiple modalities and demand accurate coregistration of structural and functional information across a broad range of spatial and temporal scales. The scope of advanced therapeutic approaches enabled by such advances is broad, ranging from high‐precision image‐guided radiation therapy and surgery to minimally invasive target ablation, cell‐based therapies, and other forms of novel therapeutics. Moreover, the information acquired during the course of information will drive patterns of therapy delivery that are increasingly adaptive and patient‐specific. This symposium focuses on the scientific principles of imaging, guidance, treatment delivery, and feedback / adaptation in the context of advanced medical interventions and looks ahead to new technologies being developed for novel image‐guided therapies. Four expert speakers have been invited to present on topics central to such advancement: Dr. Siewerdsen will discuss principles of imaging performance, imaging task, and new imaging technologies; Dr. West will address principles of geometry, registration, and uncertainty (e.g., target registration error) in relation to real‐time tracking and navigation; Dr. Sherar will discuss a broad spectrum of technologies for therapeutic delivery, including physical, molecular, chemical, and cellular means by which a therapeutic insult can be delivered to disease under image guidance; finally, Dr. Yan will describe principles of feedback and control in the development of adapative therapies. Together, these topics present a “closed‐loop” in issues of imaging, navigation, therapy delivery, and adaptation that are central to the advancement of image‐guided interventions.
TU‐D‐332‐06: Quantitative in Vivo Measurements of Treatment Response in Murine Tumors Using Diffusion‐Weighted Magnetic Resonance Imaging
Authors: Marsh R , Terry N , Bankson J , Hazle J
Published: July 2008

Purpose: Post‐irradiation changes in tissue structure can lend insight into how a tumor is responding to therapy. Here, diffusion‐weighted magnetic resonance imaging (DW‐MRI), which observes Brownian motion within a tissue sample, was used to quantify changes in the diffusion properties of irradiated tumors in vivo. Method and Materials: MCa‐29 mammary carcinoma was grown in the hind legs of mice. Tumors were grown to a diameter of 8mm, treated with 20Gy of gamma‐rays (mean energy = 662keV), and imaged with DW‐MRI. After this initial scan, half the mice were sacrificed and the tumors excised for histological analysis. The remaining animals were re‐scanned three days later before being sacrificed for histological analysis. Diffusion data were collected on a 4.7T Bruker Biospin scanner (Billerica, MA) and analyzed using a two‐compartment model, representing MR signal as originating from a slow‐diffusing component, often associated with intracellular space, and a fast‐diffusing component, often associated with extracellular space. The analysis yielded rates of diffusion for the slow‐ and fast‐diffusing components and the volume fraction occupied by slow‐diffusing component, or cellular space (CVF). Results: Between imaging sessions the mean rates of diffusion in the slow‐ and fast‐diffusing components increased 96% (p=0.03) and 62% (p=0.30), respectively. The DW‐MRI‐based CVF remained approximately unchanged (CVF=0.40±0.04 versus CVF=0.43±0.08, p=0.88). This did not correlate with the histology‐based CVF measurements which, during the same time period, decreased significantly from 0.76±0.03 to 0.56±0.01 (p<0.001). Conclusion: Although there was disagreement between the CVF measurements made by DW‐MRI and histology, a qualitative analysis indicated regional increases in tumor homogeneity after treatment. Also, the rates of diffusion of both tissue components increased after radiation treatment, suggesting decreased structural organization in the tumor, a finding supported by histology. Thus, DW‐MRI has potential as a non‐invasive in vivo method of observing structural changes in irradiated tissues.
TU‐EE‐A2‐03: Target Motion Tracking with a Scanned Particle Beam: Calculation and Experimental Validation of Biologically Effective Doses in the Presence of Motion
Authors: Schmidt A , Bert C , Saito N , Chaudhri N , Iancu G , von Neubeck C , Rietzel E
Published: July 2008

Purpose: A beam tracking system for scanned ion beams was developed to compensate intrafractional motion. A novel method to calculate biologically effective doses for charged particles (especially carbon ions) in the presence of motion was implemented and experimentally validated. Method and Materials: The tracking system adapts pencil beam positions according to target motion in quasi real time. Lateral position adaptation is performed with the beam scanning system, longitudinal range compensation with a dedicated energy modulation system that includes absorber material of adjustable thickness. To validate the calculation of biologically effective dose distributions in the presence of motion, a medium‐filled container with Chinese hamster ovary cells was irradiated on a sliding table (3D biological dosimetry). Cell survival was measured for three irradiation schemes: 1. stationary target, 2.moving target without tracking, 3. moving target with tracking. Measured cell survival was converted to biologically effective doses and compared to calculations. Furthermore, measurements of the stationary setup and tracking irradiation under motion were compared directly to validate the functionality of the beam tracking system. Results: The average relative differences between measured and calculated biologically effective doses D (Dcalc/Dmeas‐1) in the target area were −5%±9%, 5%±9% and −6%±8% for the stationary, moving target and tracking irradiation, respectively. Differences between calculations and measurements were comparable to relative variations in biologically effective doses of the experimental data of 11% (individual measurements). The difference between stationary and tracking irradiation (1‐Dcomp/Dstat) was 6%±9%. Conclusion: The performance of the beam tracking system was succesfully validated with 3D biological dosimetry by cell survival measurements. Results of the novel method to calculate biologcially effective dose distributions are in good agreement with measured data. Conflict of Interest: Research sponsored in part by Siemens Healthcare.
TU‐EE‐A3‐04: Registration of On‐Board Digital Tomosynthesis and Planning CT for Partial Breast Irraadiation Patient Setup Verification with Surgical Clips
Authors: Winey B , Cormack R , Zygmanski P , Lyatskaya Y
Published: July 2008

Purpose: This study examines the accuracy of On‐Board Digital Tomosynthesis (OB‐DTS) imaging by the localization of individual surgical clips and registration to planning CT instead of a reference‐DTS (RDTS). Using planning CT instead of a RDTS for registration might decrease the uncertainty of the registration resulting from the added edge blurring of DTS and simplify the setup verification process. Method and Materials: A solid water breast phantom containing four surgical clips was imaged with CT and OB‐DTS modalities. The OB‐DTS was acquired for a 40° lateral angle (kV source: 250°–290°). An open‐source GPU based FDK filtered backprojection algorithm was used for reconstruction of DTS image sets. The centroids of all the clips were localized in the DTS image sets and locations were compared with the clip locations in CT image sets. Using a filtered cross‐correlation algorithm, the OB‐DTS was registered with the CT and RDTS image sets and the registration accuracy of OB‐DTS was calculated. Results: Clip centroids were found to be within 1 mm in DTS and CT image sets. The OB‐DTS was registered with the CT within 1 mm ± 2 mm along the x‐axis (direction of projection) and less than 1 mm ± 1 mm along the two orthogonal (y and z) axes with a maximum translation of ±1.4 cm in the orthogonal axes and ±4.7 cm along the x‐axis. The OB‐DTS was registered with the RDTS within 4 mm ± 11.5 mm along the x‐axis, 1.4 mm ± 2.8 mm along the y‐axis and 0.5 mm ± 0.7 mm along the z‐axis, with a maximum translation of ±1.4 cm in the orthogonal axes and ±2.8 cm along the axis of propagation. Conclusion: OB‐DTS can be registered to CT with equal or better accuracy than when registered with RDTS and with greater translations along the propagation axis.
TU‐EE‐A3‐06: Impact of Respiratory Velocity On Target Volume Using 4DCT
Authors: Nakamura M , Narita Y , Matsugi K , Matsuo Y , Nakata M , Mizowaki T , Hiraoka M
Published: July 2008

Purpose: To assess the impact of respiratory velocity on target volume using four‐dimensional computed tomography (4DCT). Method and Materials: A 20 mm diameter object in a QUASAR™ phantom sinusoidally moved with 10 mm amplitude along the longitudinal axis of the CT couch. The motion period was set in the range of 2–12 sec at 2 sec intervals. 4DCT data were acquired on a General Electric 4‐slice Lightspeed RT CT scanner in an axial cine mode. Respiratory motion was recorded by a Varian Real‐time Positioning Management system. A CT slice thickness and image acquisition time were 1.25 mm and 0.5 sec, respectively. The cine duration was set to the motion period plus 2 sec. The number of 10 images per each couch position was reconstructed. Measurement repeated 3 times for each pattern. The object was automatically segmented using threshold on CT images. Volumetric analysis was performed to evaluate variations in the object size by different periods. Results: The maximum volume of the object was 6.35 ml at a maximum instantaneous velocity (Vmax) of 30.11 mm/sec, which was larger by 51.2% than true volume. While the probability that a difference between imaged volume and true volume was more than 5%was 37.3% at the velocity of ⩽ 10.68 mm/sec corresponding to the Vmax with the period of 5.87 sec, it increased to 96.3% at the velocity of >10.68 mm/sec. A significant difference was seen between the mean volume with the period of ⩽ 10.68 mm/sec and >10.68 mm/sec (P<0.01). Conclusion: Severe motion artifacts are more pronounced at higher respiratory velocity. Even if the respiratory period is slow, motion artifacts remain as long as the object moves during CT data acquisition.
TU‐EE‐A4‐03: Calculation of the Dose of the Day Using Megavoltage Cone‐Beam CT
Authors: Aubry J , Cheung J , Yom S , Gottschalk A , Nunna C , Morin O , Aubin M , Descovich M , Beaulieu L , Pouliot J
Published: July 2008

Purpose: The purpose of this work is to show that megavoltage cone‐beam CT (MVCBCT) images can provide accurate dose recalculation and be used to verify the daily dose distribution received by patients treated for head‐and‐neck (H&N) and prostate cancers. Method and Materials: Corrections for the cupping and missing data artifacts seen on MVCBCT images were developed for both H&N and pelvic imaging. MVCBCT images of six H&N and two prostate patients were acquired weekly during the course of their treatment. Several regions of interest were contoured including: the prostate and rectum and for H&N cases the spinal cord and parotids. Dose calculation was performed with the corrected MVCBCT images using the planned treatment beams and variations from treatment plan dosimetric endpoints were analyzed. Results: MVCBCT image correction and calibration for the H&N (pelvic) region shows standard deviations in dose calculations between kVCT and MVCBCT images of 1.9% (0.6%). The mean dose to the right parotid of H&N patients had an average increase of 18% during treatment. Increases of up to 52% were observed. The maximum dose to 1% of the spinal cord went up by 2% on average, although increases of up to 10% were noted. For prostate patients on one fraction an undetected setup error caused the dose received by 95% of the prostate to diminish by 3%. One patient had an average increase of 3.6% of the maximum dose received by 1% of the rectum. Conclusion: MVCBCT was used succesfully to verify the daily dose distribution for H&N and prostate patients. A substantial increase in the mean dose to the parotid glands was observed during treatment. For prostate patients the impact of setup errors on the prostate dose coverage was observed, along with the dosimetric consequences of volume changes in normal tissues. Conflict of interest: Supported by Siemens.
TU‐E‐350‐02: Medical Imaging Physics: Career Opportunities and Considerations
Authors: Barnes G
Published: July 2008

Medical imaging is big business — a multibillion dollar business, both for the manufacturers of medical imaging equipment and for health care providors. It plays an important role in diagnosis and in therapy. For these and other reasons medical imaging physics offers a number of highly satisfying career paths — clinical support in small to moderate sized medical centers, consulting clinical support, academic radiology positions, and positions in industry. For the past 30+ years, the career path an individual takes depends primarily on individual interest and capabilities and to a lesser extent on job availability. This session focuses on the responsibilities of the medical imaging physicist in these different career paths. Presented are examples of different career paths. Discussed are the responsibilities that medical physicists typically have, minimum educational and experience requirements, and desirable skills and credentials.

This Meet the Expert Session will be moderated by Gary T. Barnes, Ph.D. Dr. Barnes has worked for more than thirty‐five years in medical imaging. His experience encompasses routine clinical support, radiology resident teaching, mentoring of young medical physicists, research, and prototype medical imaging equipment development. He is a Fellow of the AAPM, ACR and AIMBE, and is currently Professor Emeritus, Department of Radiology, UAB Medical Center, President of RAD Physics, Inc., a medical physics consulting company he founded in 1978, and President of X‐Ray Imaging Innovations, a technology development company he founded in 1998. For the fifteen years prior to his retirement from UAB and becoming Professor Emeritus, he was the Director of the Physics and Engineering Division of the Department of Radiology. The Division included medical physics faculty, medical imaging equipment service engineers, QC technologists and technicians, computer programmers, and other informatics specialists. At the time of his retirement the Division had twenty‐five members. Dr. Barnes is the author or coauthor of 100+ peer reviewed papers and has several issued U.S. patents.
TU‐FF‐A1‐05: Treatment Planning to Reduce the Impact of Delivery Errors in Helical Tomotherapy
Authors: Westerly D , Soisson E , Chen Q , Woch K , Schubert L , Olivera G , Mackie T
Published: July 2008

Purpose: To investigate the source of delivery quality assurance (DQA) errors observed for a subset of patients planned for treatment on Tomotherapy. Method and Materials: Six patients planned on Tomotherapy were selected for analysis. Three patients had passing DQA plans and three had DQA plans with ion‐chamber measurements that deviated from the expected dose by more than 3%. All patients were planned using similar parameters, including a 2.5 cm field width and pitch values ranging from 0.143–0.215. Machine output was determined not to be the problem so normalized leaf timing sinograms were analyzed to determine the mean leaf open time (LOT) for each plan. This analysis suggests the observed discrepancies are associated with plans having predominantly low LOTs. To test this, patients with failing DQA measurements were replanned using an increased pitch of 0.287. After replanning, new DQA plans were generated and ion‐chamber measurements performed. Exit fluence data was also collected during the DQA delivery using the onboard MVCT detectors for dose reconstruction purposes. Results: Sinogram analysis showed increases in mean LOTs of 30–85% for the higher pitch plans. In addition, ion‐chamber measurements showed a reduction in point dose errors of 1.9–4.4%, bringing all patient plans within the ±3% acceptance criteria. Dose reconstruction results were in excellent agreement with ion‐chamber measurements and clearly illustrate the impact of leaf timing errors on plans having predominantly small LOTs. Conclusion: The impact of leaf timing errors on plans with low mean LOTs can be significant. This becomes important for plans using low pitches, or potentially for hyper‐fractionated treatment schedules. The ability to reduce the impact of these errors by increasing the plan pitch is demonstrated. In addition, the efficacy of dose reconstruction in diagnosing delivery errors is established. Conflict of Interest: Some authors have a financial interest in TomoTherapy, Inc.
TU‐FF‐A2‐05: 3D Dose Distribution of the Elekta Gamma Knife
Authors: Meltsner S , DeWerd L
Published: July 2008

Purpose: To design and implement a novel three‐dimensional (3D) radiochromic film phantom for use with the Gamma Knife (GK). Use of radiochromic film and a flat bed white light scanner will allow greater resolution than gel‐based 3D dosimeters. In addition, this method does not require sophisticated imaging tools such as CT or MRI machines. Method and Materials: A spherical Virtual Water phantom was constructed to hold up to a 4.7cm thick stack of radiochromic film made of film slices 5cm in diameter. This stack size is large enough to capture the entire dose distribution resulting from the GK fields. The resolution capable with this phantom depends on the scanner in use and the thickness of the radiochromic film, which in this case allows a resolution of up to 0.004×0.003×0.2mm3. Monte Carlo methods show that the presence of a thick stack of film does not significantly disturb the measured dose distribution. Results: Using this film phantom it is possible to obtain a three dimensional representation of the dose distribution of a GK unit. Due to the size of the films and the thickness of the stack, all the dose distribution information can be obtained in one exposure. This technique allows a higher scanning resolution to be achieved than is commonly obtained with gel dosimetry, which is on the order of 1×1×2mm3. Conclusion: Fully characterizing any radiation field requires knowledge of the dose distribution as well as the absolute value of the dose. This film phantom provides a method for mapping the dose distribution of a three dimensional radiation field with excellent resolution.
WE‐E‐AUD B‐03: Analysis of Dose Perturbation Factors for An NACP‐02 Parallel‐Plate Ionization Chamber in Clinical Electron Beams
Authors: Chin E , Seuntjens J , Bailey M , Shipley D , Palmans H , Dusautoy A , Verhaegen F
Published: July 2008

Purpose: To investigate the dose perturbation factors pwall, pcav, pQ in water of an NACP‐02 parallel‐plate ionization chamber in clinical electron beams (4–18 MeV). These perturbations factors are assumed unity in current dosimetry protocols, but recent publications have cast doubts on this. Method and Materials: An EGSnrc Monte Carlo model was developed for the NACP‐02 parallel‐plate ionization chamber. Perturbation factors for the presence of the non‐water wall pwall, the presence of an air cavity pcav and the total perturbation factor pQ were calculated in water. The influence of the front and back walls were investigated. The influence of electron beam energy and depth in water were investigated. Results: pwall is significantly different from unity for low energy electron beams, and decreases with energy. pcav is close to unity for all beam energies. Both pwall and pcav increase significantly with depth in water, leading to an increasing pQ, which reaches a value of 1.23 at a depth of R50 for 4 MeV electrons. This was, however, found to depend strongly on whether scaling of the non‐water equivalent front wall of the chamber was performed; pQ decreased from 1.23 to 1.02 for 4 MeV electrons at R50 when scaling is done. At the reference depth zref, little effect of the window scaling was noted. The back and front wall of the ion chamber were found to have opposite influences on pwall. The effect of in‐scattering and track obliquity on the depth dependence of pcav was investigated. Conclusion: The findings of an increased pQ with depth in water could have important consequences for clinical depth dose measurements. However, the issue of front window scaling needs further study, and recommendations need to be issued. The behaviour of pcav and pwall with depth and energy were explained. Studies like these may allow improved chamber design.
WE‐E‐AUD B‐06: Monte Carlo Study of Proton Beam Energy Spectra Downstream of Density Heterogeneities and the Influence of Multiple Coulomb and Nuclear Scatterings On the Bragg Peak Distal Falloff
Authors: Sawakuchi G , Titt U
Published: July 2008

Purpose: To investigate the proton beam energy and angular spectra downstream of density heterogeneities and the influence of multiple Coulomb scattering (MCS) and nuclear scattering (NS) on the Bragg peak distal falloff. Method and Materials: The investigation was performed using Monte Carlo (MC) simulations with MCNPX. Six geometries including a homogeneous slab of bone and five different heterogeneities were studied. The contribution of MCS and NS processes on the energy and angular spectra, and distal falloff width was investigated using different physics options in the simulations including: i) MCS and NS processes turned on; ii) MCS turned off and NS turned on; iii) MCS turned on and NS turned off; and iv) MCS and NS turned off. Results: The distal falloff width shows an increase for all the heterogeneities when compared to the homogeneous geometry. Using different physics options it was confirmed that MCS is the main process that leads to the increase in the distal falloff width. On one hand, the contribution of MCS in the distal falloff width depends on the heterogeneity. On the other hand, the contribution of NS has only a small dependence on the heterogeneity. The shape of the energy and angular spectra shows a strong dependence on the geometry. Conclusion: It is confirmed that the degradation of the Bragg peak and consequent increase of the distal falloff width is mainly due to MCS process occurring in the heterogeneities. Further analysis showed that NS process contributes to about 5 % of the distal falloff width and has only small dependence on heterogeneity. A correlation between the distal falloff widths and the shape of the energy spectra is found. It is proposed a methodology based on the shape of the energy spectra to predict the distal falloff width for simple heterogeneities.
WE‐E‐351‐04: Image Registration Solves Everything?
Authors: Kessler M
Published: July 2008

Ongoing developments in multimodality and 4D imaging technologies are providing opportunities to improve tumor localization and patient modeling for treatment planning and to reduce setup variations and margins for treatment delivery. Proper use of these technologies can provide greater sparing of dose to normal tissues and permit safer escalation of tumor doses. Imaging techniques are also being developed to help predict response to treatment during therapy when interventions and adaptations are still possible. The explosive growth in imaging technology has been followed by growth of technologies to manage the plethora of data now being generated.

One key technology is image registration, which can be thought of as producing the “glue” to allow quantitative integration of anatomic, functional and dosimetric information from different imaging studies. Indeed, most modern imaging workstations, treatment planning systems and even treatment delivery systems now provide tools for image registration and data integration. These tools range in sophistication from simple manual registration involving just a few degrees of freedom (DOF) to fully automated registration techniques which produce deformation fields at the resolution of the image data.

Unfortunately, widespread use of these techniques for all but simple situations has been impeded by lack of more realistic models which can accommodate processes other than just deformations such as tissue loss, gain, and sliding. The situation is also complicated by the realization that the optimal number of DOF and other registration parameters vary from tissue to tissue; using too few or too many DOF or inappropriate parameters can cause inaccurate or unphysical results. Fortunately, developments aimed at overcoming these obstacles continue to emerge and will pave the way to broader clinical use of image registration. These developments as well some of the remaining hurdles will be discussed.
TH‐SAMS‐AUD B‐05: Image Guided Radiation Therapy
Authors: Galvin J
Published: July 2008

Paralleling the introduction of Intensity Modulated Radiation Therapy (IMRT) in the early to mid 1990s, Image Guided Radiation Therapy (IGRT) is an important but problematic new tool for the Radiation Oncology community. The challenges are similar to IMRT in that IGRT is a general classification that covers many different approaches with numerous variations. Where IMRT ranges from tomotherapy to conventional MLC‐based dose delivery with many different treatment planning approaches, IGRT covers a broad spectrum of techniques ranging from stereoscopic x‐ray imaging with rigid‐body registration to cone‐beam volume imaging with deformable fusion. Thus, like IMRT, guaranteeing the safe and effective use of this new modality in a clinical trial setting will require procedures that are as comprehensive as the ones currently in place for IMRT. As quality assurance techniques are developed for routine clinical use of advanced technologies like IGRT, they must also be appropriately standardized for use in clinical trials.

Image Guidance is defined for RTOG protocols as the steps of obtaining daily in‐room images of the patient in the treatment position, matching or fusing these images to the ones obtained during the treatment planning process, and the procedure of manually or automatically changing the patient's position based on the fusion information. Notice that this description does not consider the patient as a deformable object. Using this simple definition, a number of Radiation Therapy Oncology Group (RTOG) protocols have employed or are including IGRT for targeting the lesion. These protocols require institutions to verify their IGRT methodology through a credentialing process. This process is aimed at guaranteeing that each institution understands the protocol requirements relative to the use of their fusion software or manual shift capabilities.

This presentation will discuss the process used by the RTOG to credential institutions for the use of IGRT in their protocols, and it will present the problems and considerations that will have to be addressed as the above definition of IGRT expands to include moving targets and deformable fusion.
TH‐C‐350‐04: Characterization of a Delivery System for Volumetric Modulated Arc Therapy
Authors: Matuszak M , Sui H , Yan D
Published: July 2008

Purpose: To evaluate the capabilities and characteristics of a delivery system for volumetric modulated arc therapy (VMAT). Method and Materials: A linear accelerator control system capable of dynamic MLC motion, variable gantry speed, and dose rate modulation during arc delivery was evaluated. The speed and stability of MLC and gantry motion, and the stability of the dose rate was analyzed through a series of test arcs. The transition efficiency between variable dose rates and gantry speeds, reproducibility, and delivery performance of a clinical prostate VMAT treatment were also analyzed. Delivery characteristics and mechanical reproducibility were evaluated by analyzing dynamic delivery log file output and dosimetric agreement was evaluated using a cylindrical phantom with two orthogonal diode arrays. Results: The VMAT control system was capable of maintaining a constant dose rate within 3–5% after a 40° stabilization distance during constant MU/degree arcs from 0.21 to 3.33 MU/deg. Dose rate fluctuations were accounted for by gantry speed regulation during delivery. Leaf motion was stable with maximum positional errors of < 1 mm as the leaves traveled to their maximum extent and changed direction. Leaf errors were independent of gantry angle. Transitions between different MU/deg during delivery were smooth, with stabilization at the new dose rate in < 5 degrees. Dosimetric analysis of a prostate VMAT delivery had a gamma index (3%/3mm) of < 1 for 98.4% of diodes receiving > 4% of the maximum dose. Conclusion: The linear accelerator control system was capable of delivering VMAT plans with constant and variable MU/deg. Small dose rate fluctuations are inherent in the system and are compensated for by dynamic changes of the gantry speed. Dynamic MLC motion is stable and not dependent on the gantry angle. Evaluation of the system on additional clinical plans is warranted.
TH‐C‐AUD A‐06: Evaluation of 3 Tesla MR Image Distortion and Artifacts in a Titanium Applicator Presence: Toward 3T MRI Guided HDR Brachytherapy for Cervical Cancer
Authors: Kim Y , Muruganandham M , Bayouth J , Modrick J , Bhatia S , Jacobson G
Published: July 2008

Purpose: Characterize 3 Tesla (T) magnetic resonance image (MRI) distortion and artifacts induced from a titanium applicator presence. Method and Materials: Based on the ASTM International method, a titanium tandem and ovoids (Varian) was placed in a reference phantom, and embedded in a solution (30L distilled water, 1.5g/L CuSO4). A reference phantom was designed to be free from distortion, to suspend an applicator, and to provide a reference for distortion. MR images were scanned for transverse, sagittal, and coronal views; and also generated both with and without the applicator in place. Image artifact and artifact width were quantified for all three datasets to determine maximum width. For the purpose of this study we used three tandems to simulate an applicator. Two different gels (both water‐soluble) were tested around tandems: lubricating jelly for ultrasound image and white petrolatum gel. Results: Image artifacts were evaluated for pixels changing their intensity by ⩾ 30% and found at mainly three regions; the tip (its artifact width ⩽ 4mm) of tandem and the shoulder region (⩽ 5mm) of tandem and the triangular area (its image artifacts area ⩾ 0.8cm2) surrounded by the three tandems. A shoulder region is located inferior‐outside of uterus and a triangular region also represents the gauze‐packed space in the vagina. Hence, their impact on tumor delineation is minimal. At the tip of tandem, the artifacts width (4mm) potentially leads to limiting microscopic tumor delineation but is within the tolerance (5mm) of MR images registration (AAPM TG53). The distortion was determined to be no more than 1.2mm. The gels described above were found to be helpful in determining the boundary but not in improving artifacts. Conclusion: Artifacts and distortion from a titanium applicator presence were found within the tolerance. 3T MR image is feasible to be implemented into brachytherapy planning process.
TH‐C‐AUD A‐07: Evaluation of the Correction Factor Due to the Lack of Full Scatter Conditions in Cs‐137 and Ir‐192 Brachytherapy Dosimetric Studies
Authors: Ballester F , Granero D , Perez‐Calatayud J , Rivard M , Melhus C , Pujades M
Published: July 2008

Purpose: Use of a finite phantom to derive dose rate distributions around brachytherapy sources implies a lack of backscattering material near the phantom periphery. Conventional planning algorithms and newly‐developed 3D correction algorithms are based on physics data under full scatter conditions. Presently, most published Monte Carlo dosimetric studies have been obtained using either a spherical phantom (15cm in radius) or a cylinder phantom (40×40cm2). The study objective was to derive a simple relationship to correlate the radial dose function, g(r), obtained for each one of these phantoms to that obtained for an unbounded phantom. Method and Materials: Assuming bare point sources of and , kerma was calculated using Monte Carlo GEANT4 code for 1) a spherical phantom of 40 cm in radius, R, which is assumed an unbounded phantom for r ⩽ 20cm, and 2) spherical phantoms of R=15cm and R=21cm. The later size mimics the scatter conditions of a 40×40cm2 cylindrical phantom for both radionuclides. From the ratio of the dose rate distributions for unbounded/bounded phantoms we derived the relationship between g(r) for both phantoms. Results: Phantom size correction results to g(r) were obtained and fit to 3rd order polynomials (R2 > 0.999) valid for r ⩽ 10cm, which is the clinical range of interest. To validate the method, published dose‐rate distributions for two and sources in bounded/unbounded phantoms were compared with the fits of this study. Agreement was typically within 0.2% over all distances studied. Conclusion: In order to compare the dose rate distributions published for different phantom sizes, a simple expression based on fits of the dose distribution ratios for bounded/unbounded phantoms was developed for and . Using these relations, it was possible to correlate g(r) between bounded and unbounded phantoms for improved accuracy and consistency of clinical dosimetry.