SCIENTIFIC ABSTRACTS AND SESSIONS

Transcription

1 AAPM Meeting Program 1982 SCIENTIFIC ABSTRACTS AND SESSIONS SUNDAY, JULY 30 Educational Symposium Valencia A Education Council: AAPM History/Public Education SU-AA-ValA-01 Reflections of the Founding of the AAPM R Gould*, University California San Francisco, San Francisco, CA (No abstract provided) SU-AA-ValA-02 Getting Medical Physicists On Local TV News: The Discoveries & Breakthroughs Program B Stein*, AIP, College Park, MD Discoveries & Breakthroughs: Inside Science (DBIS) is a nationally syndicated television program, consisting of a dozen 90-second news segments per month, produced by the American Institute of Physics in conjunction with partner societies including AAPM. Marketed and distributed to local television stations across the country, DBIS provides a unique opportunity to present science and scientists to local TV news viewers, who represent an important yet underserved segment of the US public for science news. This presentation will show some DBIS news clips on medical physics and explain the stories behind them. It will present quantitative studies, funded by NSF, which have demonstrated the effectiveness of the program. It will show a new website, especially created for AAPM members, that contains dozens of medical-physicsrelated DBIS videos which may be useful for outreach and other purposes. This talk will also explore how to increase opportunities for maximizing coverage of medical physics by the program. Educational Objectives: 1. Understand the format of the Discoveries & Breakthroughs television program, its objectives, and the importance of its target audience. 2. Learn the complete process of how DBIS segments on medical physics are created from story idea to final production. 3. Understand how to submit story ideas for the program, serve as an outside expert for stories under consideration, and generally help DBIS maximize its coverage of medical physics. Professional Symposium Valencia B Professional Council Symposium: Ethics and Conflict of Interest in Publishing, Research and Patient Care SU-BB-ValB-01 Authorship, Competing Interests, and the Responsible Conduct of Research" F Macrina*, Vice President for Research, Virginia Commonwealth University, Richmond, VA (No abstract provided) SU-BB-ValB-02 Ethical Conflicts in the Clinical Workplace D. Jay Freedman, Chair, AAPM Ethics Committee (No abstract provided) Joint Imaging/Therapy Symposium Young Investigators Symposium Valencia A SU-CC-ValA-01 Automatic Comparison Between Reference and On Board Digital Tomosynthesis for Target Localization L Ren*, D Godfrey, J Wu, H Yan, F Yin, Duke University, Durham, NC, Duke University Medical Center, Durham, NC Purpose: Digital tomosynthesis (DTS) is a method for reconstructing 3D images from cone-beam projection data acquired with limited angulation (e.g., 40 o ) of an x-ray source, and is much faster and lower dose than full cone-beam CT (CBCT). We previously developed a method for generating reference DTS images from a planning CT for registration with actual onboard DTS images. This study examines the accuracy of 3D-3D registration of reference and on-board DTS images to assess the potential of DTS for image-guided radiation therapy (IGRT). Method and Materials: We simulated the online positioning of an anthropomorphic chest phantom with 6 noncoplanar reference BBs attached. Planning CT data of the phantom were acquired with a G.E. Lightspeed RT scanner. On-board CBCT projection data were acquired with a Varian 21EX Clinac, equipped with a kv on-board imager. On-board DTS images were reconstructed from a subset of the CBCT projection data (81 projections, 44 o ). True alignment of planning and on-board image data was achieved according to a 3D point-based registration of the 6 reference BBs in the CT and CBCT images. Single-axis rotations up to +/- 10 o and 3-axis translations up to +/- 10 mm were simulated in the planning CT, prior to the generation of reference DTS images. A 67.5mm 162.5mm 20.8mm region of interest surrounding the spinal cord was extracted for registration. Mutual information-based 3D-3D registration of reference and on-board DTS images was performed, and residual registration error was recorded. Results: Registration errors are within 0.7mm and 0.1 degree in all cases. The average registration error was 30% less for translations along the dimension of tomographic motion than for the other two dimensions. Conclusions: 3D-3D rigid-body registration of reference and on-board DTS images is highly accurate, suggesting that DTS may be an effective IGRT technique. Partially supported by a Varian research grant. SU-CC-ValA-02 More Accurate Determinations of Air-Kerma Strength for Brachytherapy Sources W Culberson*, L DeWerd, University of Wisconsin, Madison, WI Purpose: To determine more accurately the air-kerma strength of lowenergy photon-emitting brachytherapy sources. Method and Materials: Air-kerma strength is the agreed upon metric for most brachytherapy sources and is defined as the product of the kerma rate in vacuo at distance d on the transverse axis of a seed (polar angle of 90 degrees), multiplied by the square of this distance d 2. This work introduces new methods to evaluate how anisotropy affects air-kerma strength. First, NaI detector measurements yield in-air anisotropy data that were previously reported only with Monte Carlo transport methods. In addition, multiple aperture sizes are used to evaluate large-angle free-air ionization chamber measurements. Lastly, a new method of Monte Carlo transport, which incorporates the distance-dependent geometry effects of certain brachytherapy seeds, is used to determine more accurately the air-kerma rate on the transverse axis of the source. These methods may be used to evaluate air-kerma strength to a higher degree of accuracy. Results: Wellcollimated NaI detector measurements yield precise in-air anisotropy measurements that may be compared directly with Monte-Carlo transport simulations. Measurements of two seed types show deviations near the transverse axis of the source of at least 5%. Monte-Carlo determined pointdetector simulations yield more accurate estimations of air-kerma strength and show that air-kerma strength is not constant for all distances in vacuo, as would be predicted by its definition. Conclusion: This work shows that with the combination of these new measurement techniques, air-kerma strength may be evaluated to a higher degree of accuracy for low-energy photon-emitting brachytherapy sources. New experimental data on two seeds in-air anisotropy is also presented.

2 AAPM Meeting Program 1983 SU-CC-ValA-03 A Directional Algorithm for An Electronically-Collimated Gamma- Ray Detector for Intraoperative Localization of Radiation Sources A Lackie*, K Matthews II, B Smith, W Hill, W-H Wang, M Cherry, Louisiana State University, Baton Rouge, LA Purpose: An electronically-collimated gamma-radiation detector for intraoperative localization of sentinel lymph nodes and metastases is under development. Analogous to Compton telescopes and Compton cameras, localization is achieved using the coincidence detection of Comptonscattered gamma rays. Electronic collimation allows the device to operate without physical collimation, providing high sensitivity while also allowing directional information to be determined. We report on implementation of algorithms to calculate the direction to the source. Methods and Materials: Two approaches to direction reconstruction were evaluated. The first technique backprojects each event onto the surface of a sphere centered on the device s primary detector. To use Fourier filtering methods for deblurring, the sphere s surface is mapped by stereographic projection onto a plane, filtered in Fourier space, and then projected back onto the sphere. The second technique also backprojects events onto the sphere, then determines the rectangle that circumscribes the backprojected cone; localization is obtained by intersection of all circumscribed rectangles. Results: Performance of the algorithm has been evaluated using randomly generated ideal Compton-scatter events from point sources for our detector geometry. Direction angles are calculated within 5% accuracy for source positions up to 45 off-axis for the filtering approach and ~30 o for the circumscription approach. Error in calculated direction angles depends on the arbitrary diameter of the sphere; optimally, the sphere should intersect the source. The circumscription technique converges to an estimate of direction angles in ~50 events; the filtering approach requires ~1000 events. Conclusion: The two methods complement each other in speed and field-of-view. Monte Carlo simulations and experimental testing of a prototype system are ongoing as a separate part of the overall project; data from these will further supplement evaluation of the algorithms. Acknowledgment: Supported by Homeland Security Advanced Research Projects Agency, and Space and Naval Warfare Systems Center San Diego; Contract No. N C-6024 SU-CC-ValA-04 Using Flow Information to Support 3D Vessel Reconstruction From Rotational Angiography I Waechter* 1, J Bredno 2, J Weese 2, D Hawkes 1, (1) University College London, London, GB, (2) Philips Research Aachen, Aachen, DE Purpose: For the assessment of cerebral vessel diseases, it is very beneficial to obtain three dimensional morphologic and haemodynamic information about the vessel system. Our goal is to determine both concurrently using one rotational angiography sequence. To enable the extraction of flow information, the rotational angiography images should show inflow and outflow of contrast agent. Images with this property however, are not well suited to standard volume reconstruction algorithms. This work shows how flow information can support the vessel reconstruction to overcome this conflict. Method and Materials: In our method flow information is used as follows to determine, for every voxel, the likelihood of being inside a vessel: First, the rotational time intensity curve (R-TIC) is determined from the image intensities at the projection points of the current voxel. Next, the arrival time of the contrast agent bolus at the voxel is estimated from the R-TIC. Finally, a measure of the intensity and duration of the contrast enhancement is determined. The likelihood is used to steer the Fast Marching algorithm, which determines the order in which voxels are analyzed. This enables the centreline of the vessels to be extracted by backtracking. The proposed method was tested on 80 computer simulated rotational angiography sequences with systematically varied blood flow and contrast agent injection parameters. Results: The mean error in the 3D centreline and radius estimation was 0.62 mm and 0.28 mm respectively. Pulsatile blood flow was found to increase the error only slightly (0.05 mm). Conclusion: Under pulsatile and non-pulsatile conditions, flow information can be used to enable a 3D vessel reconstruction from rotational angiography with inflow and outflow of contrast agent. Future work will aim to extract more quantitative flow information. Conflict of Interest: Research sponsored by Philips Research Aachen SU-CC-ValA-05 Advanced Integral Method for the Simulation of Diagnostic X-Ray Spectra C Dodge*, M Flynn, Henry Ford Health System, Detroit, MI Purpose: To create a fast and accurate computer algorithm for simulating the emission x-ray spectra from diagnostic tubes as a function of tube voltages, target material, and take-off angles. Method and Materials: The method uses an integral model to determine the radiative losses from an electron as it slows down in arbitrary media. The effect of selfabsorption and backscatter is accurately described by distribution functions for electron number, electron depth, and angular distribution that are functions of electron slowing down energy. The Monte Carlo program PENELOPE was used to determine these three distribution functions. An exact accounting of electron orientation was found necessary due to large variations in the Bremsstrahlung cross-section as a function of emission angle. These are accounted for in the integral model by pre-computing tables based on the Kissel Bremsstrahlung shape function. Characteristic x- ray emissions as a function of over-voltage are described using Monte Carlo results for both direct and indirect production. The computer algorithm is implemented as a part of a larger program for computationally simulating x-ray production, transmission, scattering, and detection for imaging systems (XSPECT, V4.0). Results: We compared our program to the measured x-ray spectra of Mercier, and spectral computations of PENELOPE. We found good agreement and an improvement over prior semi-empirical estimates (Birch & Marshall, Tucker, Storm). Conclusion: We have developed a program that can simulate x-ray spectra from tubes of arbitrary anode materials (including alloys), and target angles for tube voltages of 1 to 400 kv. After generation of target specific tables, the x-ray spectra can be computed in a few seconds. The results are equivalent to Monte Carlo estimates that require days to compute a single spectra. SU-CC-ValA-06 A Novel Approach to Assessing Breast Density Utilizing Sound Speed Measurements C Glide*, N Duric, P Littrup, Wayne State University, Detroit, MI Purpose: Women with high mammographic breast density are at a 4- to 7- fold increased risk of developing breast cancer compared to women with fatty breasts. The purpose of this work is to investigate the potential of assessing breast density via acoustic velocity measurements obtained with ultrasound computed tomography. Method and Materials: A sample of approximately 50 patients was imaged with our computed ultrasound tomography clinical prototype. Each data set was comprised of 45 tomograms ranging from near the chest wall through the nipple region. Whole breast acoustic velocity was determined by creating image stacks and evaluating the sound speed frequency distribution. The acoustic measures of breast density were evaluated by comparing these results to two mammographic density measures: (1) qualitative, as determined by a certified radiologist using the BIRADS Categorical Assessment based on a 1 (fatty) to 4 (dense) scale, and (2) quantitative, via digitization and computerized analysis of archival mammograms. The former involved a radiologist s visual assessment of each mammogram, while the latter required scanning cranio-caudal films with a Vidar VXR-16 DosimetryPro digitizer and implementing semi-automatic segmentation routines. Results: Approximately 60 m/s difference in acoustic velocity was found between the fatty and dense BIRADS categories. This investigation indicated a positive correlation between BIRADS category and acoustic velocity of the breast. In addition, a strong correlation between the mean acoustic velocity and quantitative measures of percent breast density was demonstrated (Pearson correlation coefficient 0.651, p < 0.001). Conclusion: These results support the hypothesis that utilizing acoustical velocity as an analogue to mammographic breast density is feasible. Our approach to evaluating breast density has the potential to provide a safer, non-ionizing, and more quantitative means of evaluating breast density, thus better elucidating the relationship that exists between breast density and breast cancer risk. SU-CC-ValA-07 Air Kerma Rate Measurements From a Miniature X-Ray Source Using Free-Air Ionization Chambers S Davis* 1, J Micka 1, L DeWerd 1, T Rusch 2, (1) University of Wisconsin, Madison, WI, (2) Xoft, Inc, Fremont, CA

3 AAPM Meeting Program 1984 Purpose: To measure the air kerma rate from a miniature x-ray source using free-air ionization chambers (FACs), and to transfer the source air kerma rate to a well-type ionization chamber. Method and Materials: Air kerma rates from several Xoft AXXENT miniature x-ray sources were measured in air along their transverse axes using three different FACs, each with a source-to-aperture distance of 100 cm. The sources were operated at 50 kv and 100 µa beam current. The University of Wisconsin (UW) Attix FAC was used for the initial measurements, and follow-up measurements were performed using the Attix and Ritz FACs at the National Institute of Standards and Technology (NIST). Two different FAC aperture sizes were used for each of the air kerma rate measurements at NIST. Additional measurements for each source were performed using a well-type ionization chamber with a custom-built aluminum source holder. The ratio of air kerma rate at 100 cm to well chamber current was used to determine a well chamber calibration coefficient for each source. Results: The air kerma rates for the smaller aperture were generally within 2% of the rates measured with the larger aperture, indicating that the sources were aligned properly. The well chamber calibration coefficients demonstrated some source to source variation, with an overall standard deviation of 5.3%. The results suggest that most of this variation can be attributed to azimuthal anisotropy around the long axes of the sources, and not differences in the photon spectrum emitted from each source. Conclusion: Both the Attix and Ritz FACs are appropriate for measuring air kerma rates from the miniature x-ray sources, but further work will be necessary to develop methods suitable for traceability to national measurements standards. Conflict of Interest: Funding for this research was provided by Xoft, Inc. SU-CC-ValA-08 Air-Kerma Strength Determination of a 169Ytterbium High Dose Rate Brachytherapy Source J VanDamme*, L DeWerd, J Micka, W Culberson, S Davis, University of Wisconsin Medical Radiation Research Center Purpose: To provide an accurate determination of the new 169 Yb high dose rate (HDR) bracytherapy source in terms of air kerma strength, based on an adaptation of the current, NIST traceable, in air measurement standard in use for 192 Ir HDR sources. Methods and Materials: Several modifications to the seven distance technique, which is the current standard for HDR source strength measurement, were required to adapt it to the 169 Yb spectrum. An Exradin A4 spherical chamber was employed, which has a relatively flat chamber response to the range of energies in the 169 Yb spectrum, and has been verified to accurately measure the air kerma strength of 192 Ir to within the reported uncertainty of the current standard measurement technique. To convert the electrometer readings to source strength, a chamber coefficient, N k, was determined by using the NIST calibrated chamber coefficients from several NIST H-Beams, whose energy spectrums fall strategically within the 169 Yb spectrum. Several correction factors must be applied to these electrometer readings, including corrections for temperature and pressure, air attenuation, air scatter, ion recombination, and corrections for the finite size of the chamber. Results: The decay corrected average of fourteen measurement iterations was x 10-3 Gy-m 2 /hr. Analysis of uncertainty was performed on these experimental 169 Yb air kerma measurements using the standard NIST method for evaluating uncertainty. This analysis established an overall k = 2 expanded uncertainty of 2.10%. Conclusion: It is shown that, with a few modifications, the current standard for high dose rate brachytherapy source calibration could be employed to accurately calibrate the new 169 Yb HDR brachytherapy source in terms of air kerma strength. The uncertainties as analyzed fall within those currently used for 192 Ir calibration. Conflict of Interest: Research funding provided by Implant Sciences Corporation. SU-CC-ValA-09 Radiation Quality in High Contrast Imaging with Orthogonal Bremsstrahlung Beams A Sarfehnia*, K Jabbari, J Seuntjens, E Podgorsak, McGill University, Montreal, Quebec Purpose: To study the characteristics of orthogonal bremsstrahlung photons produced by megavoltage electron pencil beams and to evaluate the suitability of their use for improved radiation therapy imaging. Method and Materials: A 10 MeV electron beam emerging through the research port of a Varian Clinac-18 linac was made to strike targets of carbon, aluminum and copper. The quality of resulting forward and orthogonal bremsstrahlung beams was evaluated using PDD and attenuation measurements, and the experimental findings were compared with Monte Carlo-calculated results using the EGSnrcMP code. Images of contrast objects were acquired with Agfa 400 diagnostic films and their contrast levels were analyzed. Results: Photon yield and mean energy of the forward bremsstrahlung spectra were determined to be essentially independent of the target s atomic number Z. In comparison with forward bremsstrahlung, the yield and effective energy were lower in the orthogonal direction, and this decrease was more pronounced for targets of lower atomic number. The effective energy of a spectrum produced by carbon dropped by a factor of 10 from 1535 kev in the forward direction to 151 kev in the orthogonal direction, while for aluminum it dropped by 77% to 425 kev, and for copper by 37% to 1107 kev. The image contrast of films exposed with orthogonal beams was qualitatively determined to be superior to that obtained using the forward megavoltage beams. Conclusions: Orthogonal bremsstrahlung beams produced by megavoltage electrons have a significantly lower mean energy compared to forward beams. In the orthogonal direction, higher Z targets create higher intensity, while lower Z targets provide a more desirable low energy spectrum. Using their relatively low effective energy, orthogonal bremsstrahlung beams produced by megavoltage electrons striking low atomic number targets yield images with a higher contrast than do forward bremsstrahlung beams. SU-CC-ValA-10 Cone-Beam CT for Radiation Therapy with Reverse Helical Trajectory S Cho*, C Pelizzari, X Pan, The University of Chicago, Chicago, IL Purpose: To propose and study a novel scanning trajectory, named reverse helical trajectory, for a cone-beam CT (CBCT) imager mounted on a linear accelerator (LINAC) treatment system by applying an exact 3D backprojection filtration (BPF) algorithm. Method and Materials: A numerical study using 3D Shepp-Logan phantom was performed. We applied the PI-line-based BPF algorithm to reconstruct exact 3D image from data acquired numerically for a reverse helical trajectory. It was revealed that there is a middle gap in the reconstructed image which is due to lack of PI-lines passing through the gap. Application of a chord-based BPF algorithm showed a reduction in the middle gap. Two kinds of line plus reverse helical trajectories were proposed and tested to reduce the middle gap further. One of them is a reverse helical trajectory with a short line segment between two helices separated apart. The other one is a reverse helical trajectory with a long line segment connecting the end points of the revere helices. Results: The middle gap in the reconstructed image was reduced by employing a chord-based BPF algorithm. The gap was reduced further when we modified the scanning trajectory by inserting a line segment between two helices. The gap was removed completely when we used a reverse helical trajectory with a long line segment connecting the end points of the revere helices. Conclusion: A novel scanning geometry for a LINAC-mounted CBCT imager was proposed, and a preliminary numerical study was performed. The middle gap in the reconstructed image obtained by PI-line-based BPF algorithm was effectively eliminated by using a chord-based BPF algorithm with a line plus reverse helical trajectory. Exhibit Hall F Therapy Moderated Poster Session Moderated Poster - Area 1 (Therapy): IMRT Planning and Dosimetry SU-DD-A1-01 Advances in Co-60 Based Tomotherapy Including Megavoltage CT L Schreiner, J Darko*, C Joshi, M Rogers, N Chng, C Peters, G Salomons, A Kerr, Cancer Centre of Southeastern Ontario, Kingston, Ontario Purpose: To evaluate the potential for Co-60 based tomotherapy including dose delivery and mega-voltage CT (MVCT). Tomotherapy is a rotational implementation of IMRT that provides highly conformal doses and patient setup verification using MVCT. Current tomotherapy is limited to linear accelerators. This poster presents advances in our investigation of Cobalt-60 based tomotherapy, including MVCT. Method and Materials:

4 AAPM Meeting Program 1985 The fundamental components for the Co-60 tomotherapy dose delivery and MVCT imaging experiments are a benchtop motion stage and a clinical Co- 60 MDS Nordion T-780 unit. Film and polymer-gel dosimetry are used to validate the tomotherapy dose delivery planned using in-house software. Imaging is provided by a Varian Portal Vision LC250 EPID. MVCT imaging is demonstrated using a variety of phantoms, including an anthropomorphic head phantom, and various contrast phantoms. EGS Monte Carlo simulation is used to model different beam delivery approaches such as source design for increased radiation output. Results: The computer simulations, film dose measurements, and three-dimensional polymer gel dosimetry all demonstrate that Co-60 tomotherapy provides conformal dose delivery required of modern IMRT techniques. Film measurements show that dose delivery corresponds excellently with treatment plans, validating our in-house planning system. Treatment planning studies show that Co-60 tomotherapy delivery compares favourably with that from linac based 6MV tomotherapy. Dose volume histograms show identical coverage and avoidance of target critical organs. Imaging results show that Co-60 CT provides sufficient contrast and resolution for image guidance. Results from Monte Carlo studies show that it is possible to increase beam output for a dedicated Co-60 tomotherapy unit by modifying the source geometry. Conclusion: Co-60 is well suited to tomotherapy and imaging applications; the development of clinical implementations of Co-60 tomotherapy is warranted and work continues in our centre along these lines. SU-DD-A1-02 Variations of Energy Spectra and Water-To-Material Stopping-Power Ratios in Three-Dimensional Conformal and IMRT Photon Fields S Jang* 1, H Liu 1, J Siebers 2, R Mohan 1, (1) MD Anderson Cancer Center, Houston, TX, UT M.D. Anderson Cancer Center, Houston, TX (2) Virginia Commonwealth University, Richmond, VA Purpose: Complex dose distributions and dose gradients in IMRT may cause spatial variations in photon- and electron-energy spectra. This study examined the change of photon- and electron-energy spectra, and their effects on dosimeter response and water-to-material stopping power ratios (SPR) for 3D and IMRT beams. The later term is an important factor for dosimetry protocols and obtaining dose-to-water conversion in Monte Carlo dose calculations. Method and Materials: The Monte-Carlo BEAM-EGSnrc system was used to simulate external-beam photon fields with 3D or IMRT features. Electron and photon energy fluences and spectra were calculated on a voxel-by-voxel basis using track-length estimation for 3D and IMRT treatment plans. The water-to-material SPR were averaged over the voxel of interest with the electron spectra using the Spencer-Attix theory. The relative response of ion chambers, films, and TLDs were modeled using the photon and electron spectra. Results: There was a strong spatial dependence of photon-energy spectra in both the 3D and IMRT fields. The low-energy (<100 kev) component of the photon spectra increased inversely with doses because of the contribution of the scattered photons. A similar effect was observed for electrons but to a much smaller extent. As a result, the response of film could increase by more than 10% in the low-dose region, whiles the changes of ion chamber and TLD response were within 3%. On the other hand, the variation of the water-to-material SPR with energy spectra and spatial locations was not clinical significant (< 1%) for soft tissue, cortical bone, and lung, and was less than 2% for dry air. Conclusion: Photon- and electron-spectra are spatial- and dose-dependent in 3D and IMRT photon fields. The spectra variation should be considered for certain dosimeters whose responses are energy dependent. For patient-like materials, the water-to-material SPR was relative stable in spite of the spectral variation. SU-DD-A1-03 IMRT Quality Assurance: Dosimetric Assessment of Three Current Methods J Hauger*, M Beach, D Diez, D Duggan, J Durant, G Ding, C Coffey, Vanderbilt University Medical Center, Nashville, TN Purpose: To compare traditional IMRT quality assurance using film dosimetry and small volume ionization chamber measurements with two new commercial products, the Wellhofer MatriXX ionization chamber array and Varian Portal Dosimetry. Available analysis software, hardware requirements and approximate operator times for data preparation, measurement and analysis will also be examined. Method and Materials: Fluence patterns from several 6X and 18X IMRT treatment plans for pelvis and head and neck radiotherapy patients were measured using radiographic film, the MatriXX array, and the Portal Dosimetry array. In each case the fluence pattern predicted by the treatment planning system was compared to the measured fluence pattern using ordinary γ-analysis. Absolute dose at a point in a low-gradient region of the fluence was also measured in the solid water phantom with an ionization chamber and compared to the dose prediction of the TPS. The absolute dose measured at the same point by the MatriXX array was also compared. Results: The absolute dose measurements made in a region of low-gradient using an ionization chamber were, on the average, within 3% of the TPS predicted dose. The absolute dose measurements made using the MatriXX were, on the average, within 5% of the predicted dose. The ion chamber and MatriXX agreed to within 3%. An average of about 4% of pixels failed an ordinary γ-analysis using 5% dose agreement and 3mm DTA criteria for both film and MatriXX measurements. A smaller percentage of pixels measured using Portal Dosimetry failed. The time spent preparing the data was comparable for all methods. Data measurement and analysis times were significantly reduced using the MatriXX and Portal Dosimeter procedures. Conclusions: This work indicates significant time savings for the new methods. In addition, the MatriXX system measures absolute dose at each chamber position. SU-DD-A1-04 In Vivo Prostate IMRT Dosimetry With MOSFET Detectors Using Brass Build-Up Caps N Varadhan* 1, B Garrity 1, J Miller 1, M.Weber 2 1) Minneapolis Radiation Oncology, Fridley, MN, 2) Methodist Hospital, St. Louis Park, MN Purpose: To develop a general formalism with various correction factors to predict dmax entrance dose with the new hemispherical brass buildup caps to be used with MOSFET detectors in anterior prostate IMRT fields and thereby integrate in vivo IMRT dose measurement as part of routine QA process in IMRT radiotherapy Method and Materials: We have used the new wide energy hemispherical build-up caps for this study. Due to its high density and high atomic number it provides the minimal amount of metal needed to achieve full build-up at Dmax for a range of photon energies. We have developed a general formalism to predict Dmax entrance dose by applying necessary correction factors after studying the response of MOSFET with brass build up caps for energy, dose rate, dose reproducibility, SSD and patient specific IMRT correction factor. Results: In vivo Prostate IMRT dose measurements with MOSFET detectors using brass buildup caps was performed and compared against dose predicted by two different treatment planning systems. We used both 6 MV and 10 MV for this study and compared the in vivo MOSFET detector reading with dose predicted by Philips Pinnacle ( 6 MV) and CMS XiO ( 10 MV ) treatment planning systems respectively. We achieved a overall accuracy of better than ± 5% on measured patient doses. Conclusion: Routine IMRT QA in most institutions today only involves verifying the optimized fluence map delivered to the patient in a test phantom at a certain preset depth. Based on our work here, we believe adding in vivo IMRT dosimetry with MOSFET detectors using the new brass build up caps along with routine fluence map verification in phantoms and MLC quality assurance offers greater accuracy and confidence in actual dose delivered to the patient. SU-DD-A1-05 A Ray Tracing Method to Generate Initial Conditions for IMAT Optimization M Oliver*, A Gladwish, J Craig, J Chen, E Wong, London Regional Cancer Program, London, ON, CA Purpose: To investigate the utility of using ray tracing to extract intrinsic information from CT, contour and primary dose data in order to determine initial conditions (number of arcs, arc weights, arc ranges and leaf positions) that can be input into an Intensity Modulated Arc Therapy (IMAT) optimization routine. Methods and Materials: Patient CT and contour data was ray-traced to determine PTV and PTV-OAR arcs. An additional arc was determined by the calculation of a ray importance factor (RIF) through ray tracing of the primary dose ray-tracing of the PTV. All three sets of arcs were then input into a previously described leaf position optimization algorithm. This method was tested on two geometries by ray tracing 27 equi-spaced beams. The optimized arc deliveries (number of arcs, arc weights, arc ranges and leaf positions) were then input into a fast

5 AAPM Meeting Program 1986 dose calculation algorithm, NXEGS (NumeriX LLC) for dose calculation and comparison with primary dose as calculated by ray tracing. Results: RIF arc addition reduced the objective function by 20% for geometry 1 and 8% for geometry 2. Leaf position optimization further reduced the objective function by 27% for geometry 1 and 29% for geometry 2. Calculation of dose using NXEGS provides accurate dose distributions for IMAT. Conclusions: Ray tracing can quickly provide information about number of arcs, arc ranges, arc weights and leaf positions with very little user input. Leaf position optimization can improve leaf positions once the initial number of arcs and arc ranges are determined. Together these two steps can produce intensity modulated arcs for further optimization with a more accurate dose calculation algorithm. SU-DD-A1-06 Prostate IMRT Dose Escalation with Urethra Sparing: Dose Painting with IGRT M Zhang*, V Moiseenko, M Liu, BC Cancer Agency, Surrey, BC, CA Purpose: With IGRT, the geometric uncertainty in treatment can be reduced, which makes it feasible to implement IMRT dose painting with a reasonable resolution. In this cancer center, an on-line realignment protocol is utilized for prostate cancer patients. This IGRT protocol is based on use of implanted gold fiducial markers and EPI. In this study, dose escalations with urethra sparing have been tested by using IMRT dose painting. Method and Materials: CT scans of three patients were chosen from the IGRT group. The original 3D-CRT plan (74Gy/37fr, 10mm PTV margin) was used as a reference. In test IMRT plans two PTVs were generated. PTV1 was defined as 5mm extension of prostate. PTV2 was generated from PTV1 with 5mm margin subtracted for bladder, rectum, and urethra. Two raw plans were generated. Plan 1 was 74Gy/37fr to PTV1, and Plan 2 was 74Gy/37fr to PTV2. Then, the urethra sparing IMRT boost plan was generated as a weighted sum of the two raw plans, e.g. Real Plan=w 1 Plan1+ w 2 Plan2. Different combinations of weighting factors were tested: w 1 [0.6, 1], w 2 [0.1, 0.5]. The dose to each organ was calculated with organ motion simulated based on actually recorded EPI image mismatches. The tumor control probability (TCP) and effective dose were used to evaluate the plans. Results: To achieve the same urethra D50 (minimum dose to 50% volume) as the reference plan, the highest weighting combination was w 1 =0.7, w 2 =0.5 (Prescription Dose= ( ) 74Gy=88.8Gy). This yields significant dose reduction in bladder and rectum. For the considered patient the TCP increases from ~74% to ~95%. Conclusion: With IGRT, the urethra sparing IMRT dose painting is superior to the 3D-CRT plan. The total prescription dose can be as high as 88Gy, with TCP of ~95% and lower GI complication. Since urethra has been spared, the GU complication will be less. Exhibit Hall F Joint Imaging/Therapy Moderated Poster Session Moderated Poster - Area 2 (Joint): Modeling of Intra-Fraction Organ Motion SU-DD-A2-01 On the Accuracy of a Moving Average Algorithm for Tracking Respiratory Motion During Radiation Therapy Treatment Delivery R George* 1, J Williamson 1, M Murphy 1, E Weiss 2, P Keall 1, (1)Virginia commonwealth University, Richmond, VA, Virginia Commonwealth University, Richmond, VA, (2) Georg-August-Universität, Göttingen, DE Introduction: Real-time motion tracking (RTT) treatment delivery has several advantages toward the improvement of accuracy for radiotherapy. However, currently there are certain limitations to this technique. The purpose of this study was to investigate an alternative treatment scenario using a moving average algorithm (MA) for treatment which could potentially be approaching the accuracy of RTT. Method: A comparison was performed between three different treatment scenarios (1)RTT: Xest () t = Xact ( t RT) ; (2)MA: Xest ( t) = mean[ Xact ( t RT) : Xact ( t RT n)] ; (3)Static beam delivery (SB) Xest ( t) = mean[ Xact (0) : Xact ( n)] Where Xest ( t) and Xact ( t) are the estimated and actual position at time t, n in seconds is the averaging period (5-25 seconds range). The data used for this analysis was 331 respiration-motion traces from 24 lung-cancer patients acquired using three different breathing types (free breathing(fb), audio coaching(a) and audio-visual biofeedback(av)). The metrics used for comparison were the group systematic error( M ), the standard deviation(sd) of the systematic error( Σ ), and the root mean square of the random error( σ ). The averaging period was varied to study the effect on the various metrics. Margins were calculated using the formula by Stroom et al. (IJROBP 1999;43(4)) Results: M and Σ are negligible for both MA[ M (-0.01,0), Σ (0,0.01)] and RTT[ M (0), Σ (0)] compared to SB[ M (-0.15,-0.02), Σ ( )]. MA( ) has a slightly reduced σ than SB( ). Negligible improvements were found by varying the average periods for M and Σ. σ was found to be insensitive to the different averaging periods( for A). From the margin calculations FB is most affected by the different treatment scenarios. (All values in cm). Conclusions: MA has accuracy advantages over SB and practical advantages over RTT. MA significantly reduces M and Σ compared with SB. MA and SB require less margins for AV than that for FB and A. The margins required for RTT are independent of breathing training type. There is a group systematic error caused by intrafraction motion during FB. SU-DD-A2-02 Variability of Waveforms and Probability Distributions in External Respiratory-Surrogate Marker Data A Trofimov*, G Sharp, T Bortfeld, Massachusetts General Hospital, Boston, MA Purpose: To investigate intra- and inter-fractional, inter-subject variability in the motion patterns of external respiratory-surrogate markers. A strong correlation between the motion of external markers and internal targets has been previously reported. Method and Materials: Varian real-time position management (RPM) system is used clinically to monitor external marker motion. We analyzed over 450 RPM datasets (traces) from 186 4D- CT, and 6 gated radiotherapy subjects (mean length: 235 seconds). Aperiodic (long-term) motion components were subtracted by applying high-pass filtering to Fourier transform of the data. Probability distribution functions (PDF) of the marker position were constructed, and variability bounds were calculated for the realized distributions. Trace-average waveforms (TAW) were constructed from cumulative PDF, calculated separately for leading and trailing edges of motion cycles within the trace. Results: Inter- and intra-fractional variability of PDF were reduced where the aperiodic motion components were subtracted from the data. The distribution of aperiodic shifts was approximately Gaussian over multiple fractions. Comparison between the data from various subjects showed that the PDF (when normalized to the mean amplitude of individual traces) was remarkably stable, indicating rather limited inter-fractional and intersubject variability. While intra-fractional variability of PDF appeared to be typically larger than either inter-fraction or inter-subject, as a wide variety of waveforms were realized within each trace. Conclusion: The marker position PDF and its variability bounds, constructed based on a single trace (e.g., pre-treatment 4D-CT), may serve as a conservative estimate of the expected variability in the PDF realized during a fractionated treatment. This information can be used in robust optimization of treatment planning for moving targets. The TAW may potentially be useful in subject classification by respiratory personality, and prediction of the realized PDF for a given expected uncertainty in the trace extrema positions (full exhale and inhale). Supported in part by the NCI grant 5P01-CA SU-DD-A2-03 Assessment of Four-Dimensional CT Image Acquisition Quality G Starkschall*, N Desai, P Balter, K Prado, D Luo, D Cody, T Pan, The University of Texas M. D. Anderson Cancer Center, Houston, TX The purpose of the present work is to develop and validate a series of tests to assess the quality of four-dimensional (4D) computed tomography (CT) imaging as applied to radiation treatment planning. Using a commercial motion phantom and two programmable moving platforms with a CT phantom, we acquired 4D CT datasets on two commercial multislice helical CT scanners using different approaches to 4D CT image reconstruction.

6 AAPM Meeting Program D CT image data sets were obtained as the platform moves in different patterns designed to simulate various breathing patterns. Known inserts were contoured and statistics were generated to evaluate properties important to radiation therapy, namely phase-binning accuracy, geometric accuracy, volume accuracy, and CT number accuracy. Phase-binning accuracy varied by as much as 5% for a 4D procedure in which images were reconstructed, then binned, but exhibited no variation for a 4D procedure in which projections were binned prior to reconstruction. Geometric distortion was found to be small as was volume error. Partial volume effects in the direction perpendicular to the axial planes of reconstruction affected volume accuracy, however. CT numbers were reproduced accurately, but 4D images exhibited significantly more noise than static CT images. Characterization of such properties can be used to better understand and optimize the various parameters that affect 4D CT image acquisition. SU-DD-A2-04 A Simple Method to Reconstruct a Representative Mid-Ventilation CT Scan From 4D Respiration Correlated CT Scans for Radiotherapy Treatment Planning of Lung Cancer Patients J Wolthaus*, C Schneider, J-J Sonke, M van Herk, J Belderbos, M Rossi, J Lebesque, E Damen, The Netherlands Cancer Institute / Antoni van Leeuwenhoek Hospital, Amsterdam Purpose: Four-dimensional (4D) imaging techniques can be used to obtain (respiration) artifact-free CT images of the thorax. However, its use in radiotherapy is limited since clinical treatment planning systems are currently not able to use the full 4D data. The purpose of this study was to reconstruct a representative single 3D CT scan from the 4D data set (with tumor closest to the mean position) for use in radiotherapy planning of lung tumors to enable reduction of treatment error margins. Method and Materials: After acquisition of the 4D CT scan (10 frames), the tumor is manually segmented (roughly) in the first frame and automatically (grayvalue) registered to the tumor in the subsequent frames. This gives the motion of the tumor during the respiratory cycle in 3D. Subsequently, from the cranio-caudal (CC) tumor motion curve, the mean tumor position and its corresponding mid-ventilation (MV) time-percentage are calculated. The CT scan for planning is reconstructed at this time-percentage. As indication of the merit of this concept, its effect on margins from CTV to PTV and on the PTV volume was calculated covering respiratory motion, respiratory baseline variation and setup errors (systematic and random). Results: Based on 13 patients, the worst tumor position accuracy (with respect to the mean tumor position) in the mid-ventilation CT scan occurred in the anterior-posterior direction: -0.7±0.8 mm (due to hysteresis). For these patients, the errors in conventional free-breathing CT were estimated to be 0±3.4 mm (CC) and 0±1.4 mm (AP). The mid-ventilation concept resulted in margin reduction up to 45% and a PTV volume reduction up to 35%. Conclusion: The mid-ventilation concept, based on tumor motion, is a simple method to obtain an artifact-free CT scan with smaller systematic errors compared to conventional CT scans. Significant reduction of the PTV volume can be achieved. SU-DD-A2-05 Impact of Fiducial Marker Placement for the Purpose of Phase Definition of the Respiratory Cycle for 4D-CT Image Reconstruction J Killoran*, A Allen, R Berbeco, Y Lyatskaya, Dana-Farber / Brigham and Women's Cancer Center, Department of Radiation Oncology, Harvard Medical School, Boston, MA, USA Purpose: Most 4D-CT acquisition methods rely on an externally measurable quantity proportional to the breathing cycle (e.g. chest wall excursion), for 4D-CT image reconstruction. Typically, the position of a single reflective marker placed on the patient's chest is monitored. The marker location is often chosen primarily to maximize the measurable motion irrespective of proximity to tumor location. We examine the behavior of motion of multiple markers, at different locations, placed on the patient surface during 4D-CT acquisition and evaluate the impact of marker location on respiratory cycle phase definition for 4D-CT reconstruction and its subsequent application to radiotherapy planning.method and Materials: An infrared guided positioning system (igps), capable of tracking multiple reflective fiducials in 3 dimensions, has been adapted to provide respiratory phase information for 4D-CT reconstruction. Data for 3-5 marker positions, placed at different locations on the patients chest, from 10 patients receiving 4D-CT was examined. Results: For most patients (9/10) motion of 3-5 markers is reasonably well synchronized suggesting no significant effect of the fiducial location. For one patient, we observed a marker on the abdomen switch from being completely in-phase to being completely out of phase relative to a marker on the center of the chest. This dramatically illustrates that the phase of a specific external marker may not correspond to the motion, external or internal, near the volume of interest. Conclusions: The position of a fiducial marker may affect not only the amplitude of motion but also the observed phase for some patients. The importance of this phase shift depends on how the resulting 4D-CT is ultimately applied to radiotherapy planning. In particular, if specific phases (e.g. extremes) are selected for radiotherapy target definition, special attention should be paid to the location of the fiducial marker and its role in image reconstruction. SU-DD-A2-06 The Effect of Respiratory Rate and Radiation Timing On Dose Coverage in Dynamic Breast IMRT C Ding*, X Li, M Huq, C Saw, D Heron, N Yue, University of Pittsburgh Cancer Institute, Pittsburgh, PA Purpose: IMRT has been shown to be capable of delivering plans with desirable homogeneous dose distribution for breast cancer treatment. However, the dose distribution may be influenced by interplay between dynamic MLC and respiratory motion. The purpose of this study was to investigate the impact of respiratory rate and radiation timing on the dose distribution of breast dynamic IMRT. Method and Materials: Using similar setup configuration, a helical CT and 4DCT image sets for six breast cancer patients were collected and contoured. Dynamic IMRT plans were designed using the helical CT images. The planned MLC sequence was segmented according to the respiratory phases with a series of respiratory rates (7.5-30/min) and radiation timing (evenly distributed in respiratory cycles). The segmented dynamic MLC sequences were applied to the radiation fields on the corresponding 4DCT phases. A program was developed to calculate the cumulative dose distribution from all the phases. Results: For normal breathing rates (15-20/min), the dose coverage didn t change significantly regardless of radiation starting time. The change of target V 90 was less than 2%. However, for extremely slow respiratory rates (7.5-10/min), the dose distribution and V 90 changed significantly depending on the radiation timing. The change of target V 90 was more than 10%. There was no significant dose coverage change for the underlying heart regardless respiratory rate or radiation timing. Conclusions: For breast patients treated with dynamic IMRT, if the respiratory rate of the patient is within the normal range then the impact of such respiration on dose coverage of the target was found to be statistically insignificant. However, the dose distribution may change significantly when patient has a slow breathing rate. Respiratory gating may be required to obtain satisfactory dose coverage for such cases. There was no significant dose distribution change for heart regardless respiratory rate or radiation timing. Exhibit Hall F Joint Imaging/Therapy Moderated Poster Session Moderated Poster - Area 3 (Joint): Tomographic Imaging for Therapy Localization SU-DD-A3-01 Kernel Classification for Assessing Inter-Fraction Motion in IGRT B Robison* 1, C Ramsey 1, R Seibert 1, D Garvey 2, (1) Thompson Cancer Survival Center, Knoxville, TN, (2) University of Tennessee, Knoxville, TN Purpose: To develop a method that identifies an IGRT imaging session as either normal or problematic based solely on the amount of right-left, anterior-posterior, and superior-inferior repositioning of the patient over the treatment session. Methods and Materials: A retrospective data set containing over 1100 anterior-posterior, right-left lateral, and superiorinferior patient shift values for 29 prostate patients was examined using a non-parametric kernel regression classification method to determine if a patient was normal or problematic. The treatment sessions were grouped as either being "normal", or affected because they were