J Bourland

Tradeoffs of integrating real-time tracking into IGRT for prostate cancer treatment

This study investigated the integration of the Calypso real-time tracking system, based on implanted ferromagnetic transponders and a detector array, into the current process for image-guided radiation treatment (IGRT) of prostate cancer at our institution. The current IGRT process includes magnetic resonance imaging (MRI) for prostate delineation, CT simulation for treatment planning, daily on-board kV and CBCT imaging for target alignment, and MRI/MRS for post-treatment assessment. This study assesses (1) magnetic-field-induced displacement and radio-frequency (RF)-induced heating of transponders during MRI at 1.5 T and 3 T, and (2) image artifacts caused by transponders and the detector array in phantom and patient cases with the different imaging systems. A tissue-equivalent phantom mimicking prostate tissue stiffness was constructed and implanted with three operational transponders prior to phantom solidification. The measurements show that the Calypso system is safe with all the imaging systems. Transponder position displacements due to the MR field are minimal (<1.0 mm) for both 1.5 T and 3 T MRI scanners, and the temperature variation due to MRI RF heating is <0.2 degrees C. The visibility of transponders and bony anatomy was not affected on the OBI kV and CT images. Image quality degradation caused by the detector antenna array is observed in the CBCT image. Image artifacts are most significant with the gradient echo sequence in the MR images, producing null signals surrounding the transponders with radii approximately 1.5 cm and length approximately 4 cm. Thus, Calypso transponders can preclude the use of MRI/MRS in post-treatment assessment. Modifications of the clinical flow are required to accommodate and minimize the substantial MRI artifacts induced by the Calypso transponders.

Evaluation of the spatial dependence of the point spread function in 2D PET image reconstruction using LOR-OSEM.

PURPOSEnThe use of positron emission tomography (PET) imaging has proved beneficial in the staging and diagnosis of several cancer disease sites. Additional applications of PET imaging in treatment planning and the evaluation of treatment response are limited by the relatively low spatial resolution of PET images. Including point spread function (PSF) information in the system matrix (SM) of iterative reconstruction techniques has been shown to produce improved spatial resolution in PET images.nnnMETHODSnIn this study, the authors sampled the spatially variant PSF at over 6000 locations in the field of view for a General Electric Discovery ST PET/CT (General Electric Healthcare, Waukesha, WI) scanner in 2D acquisition mode. The authors developed PSF blurred SMs based on different combinations of the radial, depth, and azimuthal spatial dependencies to test the overall spatial dependence of the PSF on image quality. The PSF blurred SMs were included in a LOR-OSEM reconstruction algorithm and used for image reconstruction of geometric phantoms. The authors also examined the effect of sampling density on PSF characterization to design a more efficient sampling scheme.nnnRESULTSnThe authors found that depth dependent change in the amplitude of the detector response was the most important factor affecting image quality. A SM created from a PSF that introduced r (perpendicular to the LOR), d (parallel to the LOR), or r and d dependent blurring across the radial lines of response led to visually identifiable improvements in spatial resolution and contrast in reconstructed images compared to images reconstructed with a purely geometric SM with no PSF blurring. Images reconstructed using a SM with r and d dependent blurring across the radial lines of response showed improved spatial resolution and contrast-noise ratios compared to images reconstructed with a SM that had only r dependent blurring. Additionally, the authors determined that the PSF could be adequately characterized with roughly 85% fewer samples through the use of a better optimized sampling scheme.nnnCONCLUSIONSnPET image reconstruction using a SM made from an accurately characterized PSF that accounts for r and d dependencies results in improved spatial resolution and contrast-noise relations, which may aid in lesion boundary detection for treatment planning or quantitative assessment of treatment response.

A round-robin gamma stereotactic radiosurgery dosimetry interinstitution comparison of calibration protocols.

PURPOSEnAbsorbed dose calibration for gamma stereotactic radiosurgery is challenging due to the unique geometric conditions, dosimetry characteristics, and nonstandard field size of these devices. Members of the American Association of Physicists in Medicine (AAPM) Task Group 178 on Gamma Stereotactic Radiosurgery Dosimetry and Quality Assurance have participated in a round-robin exchange of calibrated measurement instrumentation and phantoms exploring two approved and two proposed calibration protocols or formalisms on ten gamma radiosurgery units. The objectives of this study were to benchmark and compare new formalisms to existing calibration methods, while maintaining traceability to U.S. primary dosimetry calibration laboratory standards.nnnMETHODSnNine institutions made measurements using ten gamma stereotactic radiosurgery units in three different 160 mm diameter spherical phantoms [acrylonitrile butadiene styrene (ABS) plastic, Solid Water, and liquid water] and in air using a positioning jig. Two calibrated miniature ionization chambers and one calibrated electrometer were circulated for all measurements. Reference dose-rates at the phantom center were determined using the well-established AAPM TG-21 or TG-51 dose calibration protocols and using two proposed dose calibration protocols/formalisms: an in-air protocol and a formalism proposed by the International Atomic Energy Agency (IAEA) working group for small and nonstandard radiation fields. Each institutions results were normalized to the dose-rate determined at that institution using the TG-21 protocol in the ABS phantom.nnnRESULTSnPercentages of dose-rates within 1.5% of the reference dose-rate (TG-21+ABS phantom) for the eight chamber-protocol-phantom combinations were the following: 88% for TG-21, 70% for TG-51, 93% for the new IAEA nonstandard-field formalism, and 65% for the new in-air protocol. Averages and standard deviations for dose-rates over all measurements relative to the TG-21+ABS dose-rate were 0.999±0.009 (TG-21), 0.991±0.013 (TG-51), 1.000±0.009 (IAEA), and 1.009±0.012 (in-air). There were no statistically significant differences (i.e., p>0.05) between the two ionization chambers for the TG-21 protocol applied to all dosimetry phantoms. The mean results using the TG-51 protocol were notably lower than those for the other dosimetry protocols, with a standard deviation 2-3 times larger. The in-air protocol was not statistically different from TG-21 for the A16 chamber in the liquid water or ABS phantoms (p=0.300 and p=0.135) but was statistically different from TG-21 for the PTW chamber in all phantoms (p=0.006 for Solid Water, 0.014 for liquid water, and 0.020 for ABS). Results of IAEA formalism were statistically different from TG-21 results only for the combination of the A16 chamber with the liquid water phantom (p=0.017). In the latter case, dose-rates measured with the two protocols differed by only 0.4%. For other phantom-ionization-chamber combinations, the new IAEA formalism was not statistically different from TG-21.nnnCONCLUSIONSnAlthough further investigation is needed to validate the new protocols for other ionization chambers, these results can serve as a reference to quantitatively compare different calibration protocols and ionization chambers if a particular method is chosen by a professional society to serve as a standardized calibration protocol.

SU‐E‐T‐468: Gamma Knife Perfexion Dosimetry: A Monte Carlo Model of One Sector

PURPOSEnWe have implemented a Monte Carlo (MC) based dose computation model of one sector of the Gamma Knife Perfexion (GK PFX) using the Penelope MC dosimetry codes. The single sector simulation was rotated about the z-axis to model all eight GK sectors. GK dosimetric aspects examined include: 1) output factors (OF) for each of the three GK collimator sizes (4, 8, 16 mm), 2) OFs for each source row and collimator size, and 3) dose distribution profiles along the x- and z-axes, compared to film measurements and dose calculations from the Leksell GammaPlan (LGP) workstation.nnnMETHODSnWe defined the internal GK PFX geometry in Penelope with the aid of vendor-supplied proprietary information. A single source per row was modeled for five rows for each of the 3 collimators (15 beams modeled). MC simulations were carried out on a Linux cluster. Phase space files (PSFs) were collected for the 15 modeled collimators then rotated about the z-axis to model the sector of 24 sources per collimator. 3D dose distributions from the MC model, film, and LGP DICOM-RT dose exports were analyzed using Matlab. For OF calculations, a 16 cm diameter dosimetry sphere was modeled with a virtual detector volume at its center.nnnRESULTSnGood agreement is found for row- and total-output factors (greatest deviation of any type < 4%) compared to reference values. Off-axis factors closely follow LGP predicted dose distributions along the x-axis and differ on the inferior side of the z-axis.nnnCONCLUSIONSnDetailed geometric representations (radiation source, device components) of the GK PFX are required for high fidelity MC simulations. Calculated GK PFX OF values depend on the simulated detector volume size (4 mm OF most dependent). Our model shows strong agreement for the GK PFX OFs and dose profile curves compared to reference values. Non-disclosure agreement for proprietary information with Elekta AB. No financial contribution.

MO‐D‐230A‐04: Dedicated PET‐CT and MR‐Simulators in a State‐Of‐The‐Art Radiation Treatment Facility

Purpose: To provide dedicated, integrated PET‐CT and MR simulation and imaging devices in the radiation treatment clinic for purposes of advanced oncologicimaging.Method and Materials: A planning team was established for design of radiationoncology facilities as part of a new comprehensive cancer center. Physicist input included emphasis on combined biological‐anatomical (termed “bioanatomic”™) imaging for a research program in Bioanatomic Imaging and Treatment (BAIT), provision of state‐of‐the‐art treatment devices for IMRT, radiosurgery, and HDR, and analyses of digital medical informatics. PET‐CT and MR simulator specifications were delineated at a time of rapid technology development for both modalities, and included capabilities for gated PETCT acquisition and high‐resolution MRspectroscopy.Results: Facility design includes dedicated rooms for Conventional, PET‐CT, and MR simulation. BAIT simulator devices selected are 8‐slice PET‐CT and 3.0T MR, each with “marking” lasers and virtual simulation tools. PET‐CT s“imulation includes respiratory gating. 3.0T MR simulation includes spectroscopic, diffusion, and perfusion imaging.Radiation safety aspects include shielding for ionizing radiation (PET‐CT) and radiofrequency and magnetic fields (3.0T MR). PET‐CT and MR simulators are centrally located to facilitate patient flow and physician access. PET‐CT and MR simulations are being performed under the auspices of multidisciplinary clinical and research oversight committees. Operators are paired as one imaging technologist (PET‐CT or MR) and one radiation therapist per simulator. Conclusion: Vision for the Bioanatomic Imaging and Treatment Program has been coupled with the opportunity for a new comprehensive cancer center facility to provide multi‐slice PET‐CT and 3.0T MR simulation in the radiation treatment clinic. Using a collaborative multidisciplinary approach, image‐based research protocols have been developed for specific disease sites, and experience is being gained with use of dedicated, integrated PET‐CT and MR simulation. Conflict of Interest: BAIT Program research partners include Varian Medical Systems and GE Healthcare.

SU‐D‐BRB‐02: Monte Carlo Modeling of the Gamma Knife Perfexion

Purpose: For dosimetric and research irradiation studies, we have implemented a Monte Carlo (MC)dose computation model based on the physical and radiological characteristics of the Gamma Knife Perfexion (GK PFX) using the Penelope MCdosimetry codes. GK dosimetric aspects examined include: 1) output factors (OF) for each of the three GK collimator sizes (4, 8, 16 mm), 2) OFs for each source row and collimator size, and 3) dose distribution profiles. Methods: Vendor proprietary information facilitated our modeling of the GK PFX irradiation geometry, which was mathematically defined within Penelope. MC simulations were carried out on a Linux cluster. 3D dose distributions were analyzed using Matlab. A 16 cm diameter dosimetry sphere was modeled with a virtual detector volume at its center. Detector volume varied from 33 to 590 mm3 to study detector volume effects. A single source per row was modeled for five rows for each collimator (15 beams modeled). Single‐source dose distributions were rotated about the z‐axis of the axially symmetric geometry and summed to simulate all 192 sources. Results: Good agreement is found for row‐ and total‐output factors (greatest deviation <2% for the 4 mm collimator) compared to reference values. Simulated and measured full‐width at half‐ max values of 3D dose distribution profiles show sub‐millimeter differences (0.4 mm, 4 and 8 mm collimators; 0.9 mm, 16 mm collimator). There is excellent agreement for integrated profile shapes. Conclusions: Detailed geometric representations (radiation source, device components) of the GK PFX are required for high fidelity MC simulations. Calculated GK PFX OF values are dependent on the simulated detector volume size (4 mm OF most dependent). Our model shows strong agreement for the GK PFX OFs and dose profile shapes compared to reference values. Acknowledgement: Non‐disclosure agreement for proprietary information with Elekta AB. No financial contribution.

TU-H-BRB-01: Physics and Dosimetry for Radiation Countermeasure Research

The US government has substantial research and development activities underway for medical countermeasures that will insure the long-term safety and survival of the countrys population after unfortunate large-scale biological, chemical and radiological and nuclear events. Preparedness includes research and development of medical countermeasures to address radiation-induced cutaneous and internal injury from radiation and nuclear events as well as for minimizing radiation risks incurred during and after travel in space. Other important research and development efforts include the repurposing of countermeasures and development of radioprotectors and mitigators to improve the outcome of radiation treatment. Participating agencies include NIAID, BARDA, NCI, and NASA, with examples of research and development funding that includes the Centers for Medical Countermeasures against Radiation (CMCR) consortia (NIAID) and primary and sub-contracts with commercial entities (BARDA). Each of these programs requires substantial medical and health physics effort in collaboration with biology colleagues to provide a range of radiation sources, dosimetry instrumentation and assessment methods, and animal models for specific radiation-induced effects and injuries. Radiation countermeasure activities for government agencies will be reviewed, the importance of model development will be stressed, example radiation countermeasure research projects will be reviewed, and the roles for medical physicists will be discussed.nnnLEARNING OBJECTIVESn1. Review US national radiation countermeasure activities. 2. Review the roles for medical physicists in radiation countermeasures research and development. 3. Understand specific physics challenges in radiation research that need solutions. Research for JD Bourland supported in part by: NIH/NIAID, U19 AI67798 (subcontract 131714). NIH/NCI, R01CA155293. HHSO10020130019C, BARDA/Argentum Medical LLC.; J. Bourland, Research supported in part by: NIH/NIAID, U19 AI67798 (subcontract 131714); NIH/NCI, R01CA155293; HHSO10020130019C, BARDA/Argentum Medical LLC.; P. Prasanna, Supported by the Radiation Research Program, National Cancer Institute.

MO-A-209-02: A Tale of Two Journals: Open-Access and Subscription-Based Publications

Scientific publishing is a major endeavor of the AAPM. Although publishing requires substantial resources, it has the potential to generate both reputation and revenue to further the missions of our society. The AAPM owns and publishes two journals: Medical Physics and the Journal of Applied Clinical Medical Physics (JACMP). Medical Physics, with the tagline The international journal of medical physics research and practice, publishes research concerned with the application of physics and mathematics to the solution of problems in medicine and human biology, with an emphasis on theoretical and experimental approaches. The JACMP is an applied journal that publishes papers designed to help clinical medical physicists perform their responsibilities more effectively and efficiently for the greater benefit of patients. The two journals reflect two different missions, reach two different (yet often overlapping) audiences, engage two different publication approaches, and operate under two different financial models. A journal must serve both producers (authors who submit manuscripts to - and ultimately have their work published in - the journal) and consumers (readers - either individuals who access journal content or subscribing institutions that make the journals content available to its members). Any viable publication model and cost-recovery structure must satisfy the interests of both groups without sacrificing the needs of the scientific community. The purpose of this symposium is to explore the roles of the AAPMs two journals in advancing the scientific and clinical missions of the society and to understand the complex interplay of economic factors that impact financial decisions governing the journals and the returns they bring to the AAPM.nnnLEARNING OBJECTIVESn1. Appreciate the complementary roles of Medical Physics and JACMP in advancing the scientific and clinical missions of the AAPM. 2. Understand AAPM resources committed to the two journals and the return on these investments. 3. Understand the role of the AAPMs Journals Business Management Committee in the financial stability of the two journals. 4. Explore the potential benefits and pitfalls of various economic and publication models for the two journals. J. Bourland, Member, Journals Business Management Committee, AAPM.

TU‐G‐BRD‐04: A Round Robin Dosimetry Intercomparison of Gamma Stereotactic Radiosurgery Calibration Protocols

Purpose: The purpose of this multi-institutional study was to compare two new gamma stereotactic radiosurgery (GSRS) dosimetry protocols to existing calibration methods. The ultimate goal was to guide AAPM Task Group 178 in recommending a standard GSRS dosimetry protocol. Methods: Nine centers (ten GSRS units) participated in the study. Each institution made eight sets of dose rate measurements: six with two different ionization chambers in three different 160mm-diameter spherical phantoms (ABS plastic, Solid Water and liquid water), and two using the same ionization chambers with a custom in-air positioning jig. Absolute dose rates were calculated using a newly proposed formalism by the IAEA working group for small and non-standard radiation fields and with a new air-kerma based protocol. The new IAEA protocol requires an in-water ionization chamber calibration and uses previously reported Monte-Carlo generated factors to account for the material composition of the phantom, the type of ionization chamber, and the unique GSRS beam configuration. Results obtained with the new dose calibration protocols were compared to dose rates determined by the AAPM TG-21 and TG-51 protocols, with TG-21 considered as the standard. Results: Averaged over all institutions, ionization chambers and phantoms, the mean dose rate determined with the new IAEA protocol relative to that determined with TG-21 in the ABS phantom was 1.000 with a standard deviation of 0.008. For TG-51, the average ratio was 0.991 with a standard deviation of 0.013, and for the new in-air formalism it was 1.008 with a standard deviation of 0.012. Conclusion: Average results with both of the new protocols agreed with TG-21 to within one standard deviation. TG-51, which does not take into account the unique GSRS beam configuration or phantom material, was not expected to perform as well as the new protocols. The new IAEA protocol showed remarkably good agreement with TG-21. Conflict of Interests: Paula Petti, Josef Novotny, Gennady Neyman and Steve Goetsch are consultants for Elekta Instrument A/B; Elekta Instrument AB, PTW Freiburg GmbH, Standard Imaging, Inc., and The Phantom Laboratory, Inc. loaned equipment for use in these experiments; The University of Wisconsin Accredited Dosimetry Calibration Laboratory provided calibration services.

WE-D-BRE-01: A Sr-90 Irradiation Device for the Study of Cutaneous Radiation Injury

PURPOSEnTo determine dosimetric character for a custom-built Sr-90 beta irradiator designed for the study of Cutaneous Radiation Injury (CRI) in a porcine animal model. In the event of a radiological accident or terrorist event, Sr-90, a fission by-product, will likely be produced. CRI is a main concern due to the low energy and superficial penetration in tissue of beta particles from Sr-90. Seven 100 mCi plaque Sr-90 radiation sources within a custom-built irradiation device create a 40 mm diameter region of radiation-induced skin injury as part of a larger project to study the efficacy of a topical keratin-based product in CRI healing.nnnMETHODSnA custom-built mobile irradiation device was designed and implemented for in vivo irradiations. Gafchromic™ EBT3 radiochromic film and a PTW Markus chamber type 23343 were utilized for dosimetric characterization of the beta fluence at the surface produced by this device. Films were used to assess 2-dimensional dose distribution and percent depth dose characteristics of the radiation field. Ion chamber measurements provided dose rate data within the field.nnnRESULTSnThe radiation field produced by the irradiation device is homogeneous with high uniformity (∼5%) and symmetry (∼3%) with a steep dose fall-off with depth from the surface. Dose rates were determined to be 3.8 Gy/min and 3.3 Gy/min for film and ion chamber measurements, respectively. A dose rate of 3.4 Gy/min was used to calculate irradiation times for in vivo irradiations.nnnCONCLUSIONnThe custom-built irradiation device enables the use of seven Sr-90 beta sources in an array to deliver a 40 mm diameter area of homogeneous skin dose with a dose rate that is useful for research purposes and clinically relevant for the induction of CRI. Doses of 36 and 42 Gy successfully produce Grade III CRI and are used in the study of the efficacy of KeraStat™. This project has been funded in whole or in part with Federal funds from the Biomedical Advanced Research and Development Authority, Office of the Assistant Secretary for Preparedness and Response, Office of the Secretary, Department of Health and Human Services, under Contract No. HHSO100201200007C.