bluuebunny/crossref_metadata_2025 huggingface dataset:
TH‐C‐224C‐09: Using SPECT‐Guidance to Reduce Intensity Modulated Radiation Therapy (IMRT) Dose to Functioning Lung
Authors: McGuire S , Zhou S , Marks L , Dewhirst M , Yin F , Das S
Published: July 2006

Purpose: Single photon emission computed tomography (SPECT) provides a map of the spatial distribution of lung perfusion. Thus, SPECT guidance can be used to deliberately divert dose away from higher functioning lung, thereby potentially reducing lung toxicity. This work presents an algorithmic methodology for achieving this aim, and tests it in intensity modulated radiotherapy (IMRT) treatment planning of five randomly selected lung cancer patients. Method and Materials: IMRT treatment plans were generated with and without SPECT guidance and compared. Both sets of plans were made to adhere to the same dose‐volume constraints. The SPECT‐guided process works by segmenting healthy lung into four regions on the basis of SPECT intensity, and sequentially allowing dose to the target via regions of increasing SPECT intensity. This process results in reduction of dose to functional lung, reflected in the dose‐function histogram (DFH). DFHs quantify the percentage of total function above dose levels. The plans were compared using DFHs and F20 / F30 values (Fx is the functional lung receiving dose above x Gy; 20/30 Gy are significant thresholds for radiation pneumonitis). Results: In all cases, the SPECT‐guided plan produced a more favorable DFH compared to the non‐SPECT guided plan. Additionally, the F20 and F30 values were reduced for all patients by an average of 13.6% ± 5.2% and 10.5% ± 5.8%, respectively. In all patients, for the SPECT‐guided plans, DFHs of the two highest functioning SPECT regions were reduced while DFHs of the two lower functioning regions were increased, illustrating the dose “give‐take” between SPECT regions that is inherent to the redistribution process. Conclusion: SPECT‐guided IMRT shows potential for reducing the dose delivered to highly functional lung regions. This dose reduction could reduce the number of high grade pneumonitis cases that develop after radiation treatment and improve patient quality of life.
TH‐C‐VaIA‐01: Memorial Lecture for Robert Loevinger
Authors: Seltzer S
Published: July 2006

How perfectly appropriate is the Symposium: “How accurately can we measure dose clinically?” to memorialize Bob Loevinger. The clear threads in Bob's career and seminal work was his quest for improved accuracy in dosimetry measurements, his organization of calibration networks to transfer traceable measurement standards to the clinical physicist, and his development of protocols and formalisms to implement accurate measurements of dose at the clinical level. In his professional career of some 55 years, his contributions were many and important. Quiet and unassuming, Bob was the supreme dosimetrist: he designed and constructed extrapolation chambers for beta dosimetry in his early career, and then the Wide‐Angle‐Free‐Air‐Chamber (WAFAC) for prostate‐seed dosimetry in his late career; he developed the MIRD schema with Mones Berman for dose calculations in nuclear medicine, and led the development of the TG 21 protocol for the determination of absorbed dose from high‐energy photon and electron beams; at the IAEA in the 1960s he was instrumental in the creation of their Dosimetry Laboratory and the IAEA/WHO Secondary Standards Dosimetry Laboratory Network that serve the mostly developing member states, and initiated their postal dosimetry service to clinics in the member states; then at the NBS in the 1970s he worked with the AAPM (he was a charter member) to create the network of Accredited Calibration Dosimetry Laboratories to better disseminate US radiation measurement standards to the clinical community in North America. Much of his work was recognized during his lifetime by numerous awards given him by many organizations, but I think he would be most honored by this Symposium, which gathers like‐minded scientists to discuss issues on which he spent his career.
TH‐C‐VaIA‐02: In's and Out's of Non‐Equilibrium Dosimetry
Authors: Seuntjens J
Published: July 2006

Typical procedures for clinical dosimetry are well established for measurements in charged particle equilibrium situations. These situations only form a subset of the measurement challenges of a clinical physicist. Dose measurements in regions of charged particle disequilibrium require special considerations before the measurement can be assumed accurate. These disequilibrium conditions occur in association with measurements performed for standard clinical treatment techniques as well as for special treatment techniques. In the symposium we identified four major areas that complicate accurate clinical dosimetry: (1) the photon build‐up region, (2) narrow photon beams, (3) measurements in heterogeneous phantoms and (4) modulated fields and dynamic measurements.

In the present lecture we describe the principles of accurate dose measurements in general in equilibrium and non‐equilibrium situations. Principles of detector response, energy dependence and characteristics of practical detectors are described with emphasis on the understanding of detector behaviour in non‐equilibrium conditions. By way of introduction, practical examples will be provided of non‐equilibrium measurements in the build‐up region, narrow beams, heterogeneous phantoms and IMRT beams. The lecture will provide general guidelines about detector suitability for commercially available devices for the discussed areas of application. Finally, uncertainties of the procedures in a clinical context will be discussed.

Educational Objectives:

1. To understand the fundamental complications of non‐equilibrium measurements.

2. To understand the impact of detector properties on measurement accuracy in non‐equilibrium situations.

3. To place measurement uncertainties in non‐equilibrium conditions in a clinical perspective.
TH‐C‐VaIA‐04: Accurate Dosimetry in Photon Build‐Up Region
Authors: Abdel‐Rahman W , Devic S , Seuntjens J , Podgorsak E
Published: July 2006

Accurate measurements in the dose build‐up region for high energy photon beams are not easily obtained. While dosimetry in situations where electronic equilibrium exists is well understood, there is in general no consensus on the most accurate method for measuring doses in the build‐up region. In the past, data acquired with an extrapolation chamber were regarded as the benchmark by which the data from other more commonly used dosimeters are evaluated. As extrapolation chambers are clinically impractical devices there is a need to study the behaviour of commercially available and clinically suitable detector systems for accurate dosimetry in the build‐up region.

This lecture will provide an overview on the characteristics of the dose build‐up region measurements for photon beams, the suitability of several clinical dosimeters for build‐up dose measurement and limitations in measurement accuracy.

Educational Objectives:

1. Understanding the reasons why doe measurements in the build‐up region are challenging.

2. Grasp issues of the suitability of several dosimeters for measurements in the dose build‐up region.

3. Discussion of ongoing research on dosimetry in the dose build‐up region.

4. Discussion of the associated uncertainties and their clinical relevance.
TH‐C‐VaIA‐07: Moderated Discussion & Concluding Remarks
Authors: Seuntjens J
Published: July 2006

Radiation therapy is rapidly moving into the direction of image guidance, functional target volumes, hypofractionation, dose escalation and 4D deliveries. In this era of joint imaging‐therapy developments, accurate dosimetry techniques albeit often overlooked, increasingly require dealings with complex charged particle disequilibrium measurements and their interpretation. In this symposium, clinical dose measurements have been reviewed with emphasis on accuracy in these charged particle disequilibrium conditions. After a brief introduction and review of fundamentals of dosimetry, clinical accuracy requirements, four distinct areas of dosimetry in charged particle disequilibrium and their clinical relevance have been discussed. Guided by the main conclusions of the contributions in the four areas, moderated discussions will be conducted around clinical relevance of dosimetric accuracy in on non‐equilibrium conditions, detector suitability, technical aspects and other areas of application.
TH‐C‐230C‐01: Mammography QA Workshop I and II
Authors: Shaw C , Geiser W
Published: July 2006

Purpose: to explore the impact of DR mammography on the practice of medical physics. Historical and new methods for providing scheduled and unscheduled mammography services for facilities using DR mammography will be presented. Method and Materials: clinical medical physics services provided for DR mammography facilities from various manufacturers will be presented. Quality control activities approved by the FDA for each individual manufacturer will be compared. The status of a proposed “Alternative Standard” to allow for a more uniform approach to medical physics mammography services will be reviewed. Case studies will be presented demonstrating methods and cost considerations for providing medical physics services under the “minimum standard” (prescribed by MQSA regulations) and “best practices” model. Results: Medical physics services provided in the “minimum standard” and “best practices” model have implications for the quality and cost of mammography services. Conclusion: Medical physics services may be provided in a professional and valuable manner, using combinations of the minimum standard of practice and the “best practices” model. The professional medical should consider multiple parameters when determining the appropriate model under which to deliver services.
TH‐D‐330A‐01: Annual Gamma Camera QC: Techniques And Phantoms To Simplify The Implementation Of The NEMA NU‐1 Procedures For Tests Recommended By The ACR
Authors: Hamacher K , Goldsmith S
Published: July 2006

The objective of this work is to present techniques to perform the NEMA tests that are suggested by the ACR for annual gamma camera QC. Since the recommendation contains nine tests, the goal is to devise simple and efficient procedures to reduce the required time without compromising the quality. Also included is a comparison of test equipment conforming to NEMA specifications to equipment made in‐house.

The time to complete the annual QC is markedly reduced by the availability of specifically designed phantoms. They provide a reliable means to compare the cameras and to establish a performance baseline. For two tests, Multi Window Spatial Registration and SPECT Resolution with scatter, the NEMA prescribed equipment is compared with readily available devices. The comparison of the commercial MWSP device and the one produced in‐house showed that the average maximum difference of the center of mass for the three Ga‐67 peaks measured at various points across the crystal face is 0.35 mm vs. 0.33 mm for the commercial and the in‐house device, respectively. For the SPECT resolution using three capillary tubes filled with Tc‐99m, a NEMA phantom is compared to a modified Jaszczak phantom. The FWHM is 4.78 pxl vs. 4.79 pxl (radial) and 4.144 pxl vs. 4.167 pxl (azimuthal) for NEMA and Jaszczak phantom, respectively.

Further topics include sensitivity, count rate performance, comparison of fit methods, and monitor/printer QC.
TH‐D‐330A‐03: Development of a High Resolution Imaging System for Nuclear Medicine
Authors: Roa D , Smither R , Shieh Y , Nie K , Zhang X , Al‐Ghazi M , Milne N , Caligiuri P
Published: July 2006

Purpose: To construct a diagnostic imaging system for nuclear medicine that can generate images with spatial resolutions of 1–2 mm and hence improve the image quality in diagnostic nuclear medicine. Method and Materials: Tumors of 1–2 mm can be detected using photon diffraction to focus the gamma rays emitted by the radiopharmaceutical in the tumor region onto a detector, much like using a magnifying glass to focus sunlight on a small area. The gamma rays from the tumor volume are focused by an array of lenses, each comprised of small copper crystals arranged in concentric rings of increasing diameter. Specific crystalline plane orientations can be used for each ring so that the diffracted gamma rays converge on a small area on a detector's sensitive region. No collimation is needed since each lens in the system will focus one gamma‐ray energy at one time. Results: Preliminary data collected with a prototype lens using Co‐57 and Tc‐99m not only show that photon diffraction is a viable approach to focusing gamma rays, but yielded a spatial resolution of 3 mm. Sensitivity estimates based on these data indicate that a 9‐lens array could detect a 1 μCi spherical source of 0.01cc in volume with a signal‐to‐noise ratio of 6‐to‐1. Conclusion: The use of photon diffraction in diagnostic imaging in nuclear medicine is a strong and innovative technique that only this imaging system possesses. Photon diffraction eliminates the need for external collimation, which is the cause of limited sensitivity in gamma and SPECT cameras, because the crystal elements in a lens prevent photons with energies other than the specified energy from reaching a detector. Finally, detection of a tumor site at its inception can allow for earlier initiation of treatment and wider treatment options which can potentially improve the chances of a cure.
TH‐D‐330D‐01: BIROW — Biomedical Imaging Research Opportunities Workshop: Intersociety Project to Accelerate Biomedical Imaging Discovery and Application
Authors: Fullerton G , Bourland J , Pelizzari C , Ling C , Jaffray D , Eckelman W , Patt B , Hendee W , Mulvaney J , Mackie T , Fenster A , Nagy P , McNitt‐Gray M , Chan H
Published: July 2006

The Biomedical Imaging Research Opportunities Workshops (BIROW) held over the past 4 years examined the leading edge of research opportunities in biomedical imaging, to recommend strategies to advance imaging research and thereby improve imaging applications in preventive medicine, medical diagnosis and treatment. Funded through grants from the National Institute of Biomedical Imaging and Bioengineering (NIBIB) and the Whitaker Foundation, and held each spring in Bethesda MD, the BIROW workshops were sponsored by 5 organizations: the American Association of Physicists in Medicine, Radiological Society of North America, Biomedical Engineering Society, Academy for Radiology Research, and the American Institute of Medical and Biological Engineers.

This session introduces the purpose and accomplishments of BIROW and plans for its future, followed by discussions by several leading researchers on topics of substantial current interest in biomedical imaging research: Image‐Guided Therapy (Drs. Bourland, Pelizzari, Ling and Jaffray); Small Animal Imaging Systems (Drs. Fullerton, Eckelman and Patt); Medical Imaging Technology: From Concept to Clinic (Drs. Hendee, Mulvaney, Mackie and Fenster); and Informatics and Computerized Image Analysis (Drs. Nagy, McNitt‐Gray and Chan). Participants in this session will gain insight into the types of research challenges and opportunities present in BIROW workshops through the presentation of these examples.

1:30 – 1:50

Introduction Session: Purpose of BIROW

1:50 – 3:20

Image‐Guided Therapy

Small Animal Imaging Systems

4:00 – 5:30

Medical Imaging Technology: From Concept to Clinic Informatics and Computerized Image Analysis.
TH‐D‐VaIB‐02: Skin and Body Dose Measurements for Varian Cone‐Beam CT (CBCT) During IMRT for Prostate Cancer
Authors: Wen N , Guan H , Hammoud R , Pradhan D , Nurushev T , Chen Q , Li S , Movsas B
Published: July 2006

Purpose: With the increased use of CBCT for daily patient setup, kV dose delivered to patient should be investigated. This study is to measure skin and body dose from Varian daily CBCT for prostate patients. Methods and Materials: CBCT scans were acquired in half‐fan and pulsed‐fluoro mode with a half bow‐tie mounted. A technical setting of 125kV, 80mA and 25ms was used. Skin and body doses were first measured for a Rando pelvic and an IMRT QA phantom, set centrally, with TLD and a cylindrical chamber. Then skin dose for 7 prostate patients undergoing daily CBCT was measured. To avoid the ring artifacts centered at prostate, the treatment couch was dropped 3cm from patient's tattoo. TLD capsules were placed on patient's skin at 3 sites: AP, Lt Lat and Rt Lat. Phantom measurement was also made for this setup. The absorbed dose was determined by the air‐kerma‐based calibration method recommended by TG61. Results: For phantoms set centrally, measured skin dose was ∼6 cGy, ∼5.6 cGy, ∼3.7cGy at AP, Lt Lat, and Rt Lat, respectively. Body dose at the center was ∼3–4 cGy. With table dropping (TD), only AP skin dose was increased ∼12%. Patient AP skin dose varied with separation, ranging 4–6 cGy for thicker patients (AP 23 – 33 cm) and 6 – 8 cGy for thinner patients. Minimum changes were observed on lateral dose for patients with different size. Lt Lat skin (4cGy) and bone (9cGy) doses were higher than Rt Lat skin (3cGy) and bone dose (6cGy) Conclusions: Daily CBCT provides better patient setup but it increases skin and body dose. The dose can range from 120 – 330 cGy for skin and 120 – 380 cGy for body during the 42 daily fractions delivered for IMRT prostate patients.
TH‐D‐ValB‐07: Development of a Novel High Quantum Efficiency Flat Panel Detector for Megavoltage Cone Beam CT/DT: Construction and Evaluation of a Prototype Single‐Row Detector
Authors: Pang G , Mei X , Rowlands J A
Published: July 2006

Purpose: Most electronic portal imaging devices (EPIDs) developed so far have low x‐ray absorption, i.e., low quantum efficiency (QE) of 2–4% for megavoltage x rays. A significant increase of QE is desirable for applications such as megavoltage cone‐beam CT (MV‐CBCT) and digital tomosynthesis (MV‐CBDT). Our overall goal is to develop a new generation of area detectors for MV‐CBCT/DT, with a QE an order of magnitude higher than that of current EPIDs and yet an equivalent spatial resolution. To this end, a novel direct‐conversion design of such a high QE detector was introduced recently [Pang and Rowlands, Med. Phys. 31, 3004 (2004)]. The purpose of this work is to construct and evaluate a prototype single‐row detector. Method and Materials: A prototype single‐row detector was constructed based on the novel design. It consists of a single custom‐made printed circuit board (with microsize cavities, charge collection electrodes and microstrip spacers) sandwiched between two identical tungsten plates. The detector array has 128 pixels each with dimensions of 0.45mm (width)×0.6mm (length)×22mm (height). The detector array was placed inside a sealed vessel filled with Xe gas (ionization medium) and then connected to a data acquisition board (XDAS, Electron Tubes Ltd.) for readout. Some fundamental imaging properties including QE, noise power spectrum (NPS) and detective quantum efficiency (DQE) were measured with a 6MV beam. Results: Phantom images were obtained using a dose as low as one Linac pulse. The QE of the prototype is ∼ 66% at 6MV. The DQE at zero frequency is more than an order of magnitude higher than that of current EPIDs. Conclusion: This work demonstrated the feasibility of our novel design for a high QE MV detector. Construction and evaluation of a prototype flat‐panel area detector is in progress.

Conflict of Interest: Supported by the DOD Prostate Cancer Research Program (DAMD17‐04‐1‐0276).
TH‐D‐224C‐01: A Quality Assurance Procedure to Monitor Mechanical Stability and Image Quality of An On‐Board KV Cone‐Beam CT Imager
Authors: Ali I , Lovelock D , LoSasso T , Amols H
Published: July 2006

Purpose: To develop Quality Assurance (QA) procedures that monitor mechanical stability and image quality performance of a new kV on‐board‐imager (OBI) and cone beam CT (CBCT) system. Material and Methods: QA of mechanical stability includes measurements of the OBI kV and Linac MV isocenters, shifts resulting from gantry rotation, translational of imager, flexing of support arms, and reproducibility of couch shifts. Image quality QA includes measurement of noise, CT number uniformity, linearity, spatial and contrast resolutions. Results: Our system has a systematic shift between kV and MV isocenters of ∼1.4 mm. Translational motion of the OBI is accurate to ∼0.9 mm and rotational motion to ∼0.2 mm. Couch positioning accuracy has an error of ∼0.9 mm longitudinally. CT numbers are less uniform than for conventional CT with full fan (without filter) CBCT scans producing better results than other modes (CT number uniformity ∼2.5%). σ for CBCT is about 1% worse than for conventional CT and thus low‐contrast level objects such as supra‐and sub‐slice targets with < 0.5% nominal contrast in the contrast resolution module are not resolved in CBCT. CBCT numbers agree with simulator CT within 3% in the range −1000 to +1000 for scans in air. High‐contrast resolution of the OBI cone‐beam CT is comparable to the conventional CT simulator. Conclusions: Systematic shifts of the OBI isocenter from radiation isocenter must be considered for patient setup and IGRT procedures using CBCT. Systematic isocenter shifts caused by rotational and translational motions of couch and gantry must also be corrected for to achieve sub‐millimeter target localization accuracy. Image quality of kV cone‐beam CT is inferior to conventional simulator CT in terms of uniformity, and low‐contrast resolution, but has comparable CT number linearity and high‐contrast resolution. Conflict of Interest: This work is supported by NCI Grant P01‐CA59017.
TH‐D‐224C‐02: The State of Radiotherapy Physics Through The Eyes of a Quality Auditor
Authors: Followill D , Molineu A , Alvarez P , Ibbott G
Published: July 2006

Purpose: To assess the radiotherapy programs of institutions participating in NCI sponsored clinical trials and assist them to implement remedial actions. Materials and Methods: The Radiological Physics Center (RPC) has developed an extensive Quality Assurance (QA) program over the past 38 years. This program includes on‐site dosimetry reviews where measurements on therapy machines are made, records are reviewed and personnel are interviewed. The program's remote audit tools include mailed dosimeters (TLD) to verify output calibration, comparison of dosimetry data with RPC “standard” data, evaluation of benchmarkd and patient calculations to verify the treatment planning algorithms, review of institution's QA procedures and records, and use of anthropomorphic phantoms to verify tumor dose delivery. The RPC assists institutions in finding the origins of discrepancies, and in resolving them. Results: The percent of institutions receiving dosimetry recommendations has been level at 70% for the past 5 years. The most frequent recommendations were for maintenance of an appropriate QA program, beam calibration, depth dose and wedge factors. Since TG‐51 was published, the number of reference calibrations meeting the RPC's ±3% criterion has decreased. The TLD program shows that only ∼3% of the beams are outside our ±5%/±5mm criteria, but these discrepancies are distributed over nearly 15% of the institutions. There continues to be a 33% percent first time failure rate for the IMRT H&N anthropomorphic phantom. The other anthropomorphic phantoms, i.e. pelvic IMRT, lung stereotactic and liver stereotactic, have higher pass rates. Conclusion: Numerous dosimetry errors continue to exist and the RPC's QA program plays an important role in identifying and helping institutions resolve these errors to improve not only the quality of clinical trial patients' treatments, but also all patients treated at the participating institution.

This work was supported by PHS grants CA10953 and CA081647 awarded by NCI.
TH‐D‐224C‐04: Mechanical, Radiation and Imaging Isocenter Quantification On Four Varian On‐Board Imager Clinacs
Authors: Liengsawangwong P , Kudchadker R , Wu R , Session R , Diel T , Aleman G , Dong L , Gillin M , Zhu X
Published: July 2006

Purpose: Varian has recently introduced the On‐Board Imager (OBI) based image guided radiation therapy (IGRT). The OBI system comprises of a kV x‐ray source and an amorphous silicon detector mounted on robotic arms perpendicular to the gantry. Our aim was to quantify and verify the coincidence of the mechanical, radiation and imaging isocenters for four OBI Clinacs (including one Trilogy linac) we recently purchased at our institution. Method and Materials: The mechanical isocenter (gantry, collimator and couch) was determined using the Radac 2100 device (Medtec). Once the exact mechanical isocenter of the linac was confirmed, the lasers were aligned precisely through the isocenter. The radiation isocenter was then measured using the classical star‐shot technique as well as using the Winston Lutz technique. The Varian supplied isocenter cube device (with a ball in the center) was then positioned on the exact couch at the mechanical and radiation isocenter coincidence point. kV images were then taken at the four cardinal gantry angles 0, 90, 180 and 270 degrees to determine the imaging isocenter. Results: The coincidence between the mechanical, radiation and imaging isocenter are within 1 mm for all four Clinac's. The gantry and collimator mechanical isocenter was typically much less than 1 mm radius for the linacs (> 0.5 mm for the Trilogy). The radiation isocenter was also less than 1 mm radius for all the linacs. The imaging isocenter was less than 0.8 mm for all linacs. Conclusion: The added weight of the robotic arms, x‐ray source and detector does not seem to have a negative effect on the mechanical and radiation isocenter of the Clinac's. The coincidence between the mechanical, radiation and imaging isocenter give confidence to shifts made using OBI based IGRT.
TH‐D‐224C‐06: An Optimal MLC Quality Assurance Procedure Using EPID
Authors: Mamalui‐Hunter M , Li H , Low D
Published: July 2006

Purpose: IMRT QA is extremely time consuming and labor intensive, while IMRT delivery is complex. The purpose of this work is to develop an automated and quantitative IMRT QA process utilizing an EPID. One of the aspects of this is testing of MLC leaf position accuracy. The Memorial‐Sloan‐Kettering strip test was modified to provide an automated EPID‐based process.

Method and Materials: To provide a baseline fluence map, Kodak XV film was irradiated to obtain an open field 2D beam profile, which was fitted with a Gaussian and used in the image correction routine. We used a strip test consisting of seven adjacent segments with an intentional gap of nominally 0.5 mm. The gap was intentionally varied for the positions located at regularly placed hexagonal regions in order to create a set of calibration curves. A fitting technique using a Lorentzian type function was developed to determine the dose‐peak‐height, dose‐peak‐position and FWHM. Results: The standard deviation of dose‐peak‐height at each abutment for traditional Memorial‐Sloan‐Kettering pattern was of the same order of magnitude as that of the average over the entire image. In contrast, in our pattern a difference of an order of magnitude was observed, which indicated the reducing of cross‐talk effect from adjacent leaves. The leaf positioning error can be identified by using standard calibration curves of dose‐peak‐height vs. gap. The coordinate of dose‐peak‐position is found to be an extremely sensitive indicator of leaf error with sub‐pixel standard deviation. Conclusions: This procedure is able to identify MLC positional errors less than 0.5 mm by using a fitting technique and by reducing the cross‐talk effect. Conflict of Interest: Research sponsored by Varian Corporation.
TH‐D‐224C‐07: Focal Source Size Measurement for Monte Carlo Simulations of Percentage Depth Doses in Very Small Photon Fields
Authors: Sham E , Devic S , Seuntjens J , Podgorsak E
Published: July 2006

Purpose: To measure the focal spot of a 10‐MV Clinac‐18 linac for use in Monte Carlo (MC) calculation of percentage depth doses (PDDs) in a very small (1.5 mm diameter) field. Methods and Materials: A technique using a translational slit‐assembly system was developed to measure the linac focal spot. The assembly consists of two lead blocks fastened together to produce a 300 μm wide slit. A small field diode was centered below the slit, reading radiation signals transmitted through the slit while the system (mounted at a distance of 65 cm from the target) was translated linearly across an open beam. The linear translation was achieved through a screw driving mechanism and the position of the slit was measured with a digital caliper. The source dimension was estimated from the FWHM of the Gaussian fit to scanned profiles corrected for background transmission signal. Results: Scans in various angular orientations perpendicular to the beam central axis show that the linac focal spot is elliptical with principal axes of (1.1 ± 0.1) mm and (2.1 ± 0.1) mm. Isodose distribution measured on the solid water phantom surface in a plane perpendicular to the beam central axis for the 1.5‐mm diameter, 10 MV photon beam is also elliptical and oriented in accordance with the measured source shape. A circular Gaussian source model with a FWHM of 1.5 mm, approximating the measured focal spot, was used in the MC calculation of PDDs for the 1.5 mm beam. The MC‐calculated PDDs agree within 2% with measured data. Conclusions: In contrast to standard radiotherapy fields, MC‐calculated PDDs for very small fields show a strong dependence on source dimension. The good agreement between the measured and MC‐calculated PDDs for the 1.5 mm diameter 10 MV beam validates the MC simulation technique using appropriate focal source size.
TH‐D‐224C‐08: Dose Variation Within Lung Tumor and in Its Fingerlike Extension as a Function of Incident Photon Angle
Authors: Sadagopan R , Titt U , Tailor R
Published: July 2006

Purpose: Report the dose variation within a lung tumor and its finger‐like extension as a function of incident photon beam angle. Method and Materials: A phantom with 3‐cm diameter acrylic sphere (tumor) inside a cork medium was designed so that the sphere could be rotated. TLDs were placed at various locations within the sphere and just outside the sphere. The outer TLDS simulated the tumor's finger like extension. Two 2.8cm thick acrylic plates (chest wall) sandwiched the phantom. The TLDS were exposed to a 6 MV beam of 10×10 cm2 with the sphere centered at isocenter (100 SAD). The field size was such that the dose at the periphery of the acrylic sphere was not influenced by the beam penumbra. The exposures were repeated at every 30° of the fingerlike extension with respect to the incident beam angle. The doses at the TLDs were also calculated using MCNPX Monte Carlo codes and CC convolution algorithm. Results: The measured and calculated doses were normalized to the dose at the center of the sphere. Then ratios of MC and convolution values to measured values were calculated and plotted as a histogram. The standard deviations for MC and convolution to TLD ratios were 3.5% and 1.1% respectively. The standard deviations of random error in TLD, MC and convolution methods were 2, 3 and 1.5% respectively. Conclusions: MC and CC convolution values agreed well with measured data. The data would be presented as function of incident beam angle. In addition, analysis of a PB algorithm and an 18 MV beam data would be presented.
TH‐D‐224C‐10: Commissioning and QA of Cone Beam CT for Image Guided Radiation Therapy
Authors: Lehmann J , Perks J , Semon S , Purdy J
Published: July 2006

Purpose: To report the results of commissioning tests and the development of a quality assurance program for the commercial version (i.e., non‐research model) Elekta Synergy Cone Beam CT (CBCT) system. Methods and Materials: Our institution was one of the first sites in the US to install a commercial version of the Elekta Synergy Cone Beam CT system. A commissioning procedure was developed consisting of six parts: (1) System safety, (2) Geometric accuracy (i.e., agreement of MV and kV beam isocenters), (3) Image quality, (4) Algorithm accuracy, (5) Dose to patient, and (6) Quality assurance procedures. Results: The system passed all safety tests and agreement of MV/kV isocenters was found to be within 1 mm. Using a precisely moved skull phantom, reconstruction and alignment algorithm was found to be accurate within 1mm and 1 degree in each dimension. For 12 measurement points spanning a 9×9×15cm3 volume in a RANDO phantom, average agreement in x, y, and z coordinates was 0.010cm, −0.012cm, 0.022cm (SD: 0.021cm, 0.055cm, 0.021cm; Largest deviations: 0.06cm, 0.10cm, 0.05cm, respectively). Larger deviation for the y‐component can be partially attributed to CT slice thickness of 1 mm along that direction. Dose to patient depends on machine settings and was found not to exceed 6.3cGy in the highest setting used in our clinic (120kVp, 40mA, 40ms per frame, 360 degree scan, S20 field of view). A more typical dose for a full 360 degree scan is 4cGy. Conclusions: The procedures discussed in this presentation were applied to the Elekta Synergy CBCT system. They have been useful in identifying difficulties with the CBCT operation operation, including data transfer issues. System safety, geometric accuracy, and image quality checks are recommended to be performed on a monthly basis; algorithm accuracy and dose to patient checks are recommended to be performed annually.
TH‐D‐ValA‐01: Preliminary Investigation of Multi‐Pass Respiratory Gated Helical Tomotherapy (MRG‐HT)
Authors: Kim B , Kron T , Chen J , Battista J
Published: July 2006

Purpose: To investigate the feasibility of multi‐pass respiratory gated helical tomotherapy (MRG‐HT) technique by measuring the dosimetric degradation caused by target motion, and the resulting dosimetric improvements made by the MRG‐HT technique Method and Materials: MRG‐HT was simulated on the HT unit (Tomotherapy Inc.) by delivering a sequence of HT plans modified by blocking beam projections remaining outside the gating window (50 % duty cycle, end‐expiration gating). After each plan delivery, the couch was reset and the succeeding plan was commenced at a different respiratory phase to allow the previously “blocked” beam projections to fall within the gating window. EDR2 film was placed inside a moving phantom (Modus Medical Devices Inc.), which was driven sinusoidally in the longitudinal direction. For each of the two scenarios with different experimental parameters, the resulting dose profiles along the axis of gantry rotation were obtained for a stationary target treated with the original plan, a moving target treated with the original plan, and a moving target treated with the MRG‐HT technique. Results: In the first scenario, target motion led to underdosing 7.5 +/− 0.5 mm of the target and overdosing additional 2.0 +/− 0.5 mm of normal tissues. With the gating technique, these values were reduced to 4.5 +/− 0.5 mm and 1.0 +/− 0.5 mm, respectively. Further improvement could be achieved using a smaller gating window. In the second scenario, selected parameters caused significant dose modulations of 8.5 +/− 1.0 % inside the target, which was effectively eliminated by the MRG‐HT technique. Conclusion: MRG‐HT seems to be a very promising technique for gated HT. Its dosimetric improvement was demonstrated in the form of reducing target underdosing and dose modulations. In future, more sophisticated experiments will be made to simulate lung cancer treatment, including the effect of non‐sinusoidal motion and lung inhomogeneity.
TH‐D‐ValA‐03: An Improved Lung Model, Incorporating Realistic Random Anatomical Features, for Monte Carlo‐Based Dosimetry
Authors: Liang L , Larsen E , Chetty I J
Published: July 2006

Purpose: To investigate the effects of incorporating a more physically‐realistic lung model, preserving random anatomical features of the lung, on MC‐based dose distributions. Methods: A random lung model was built based on morphological data. The model homogenizes the lung parenchyma with structures of “chunk” sizes less than 0.05 cm, and models all larger chunks (branches of the bronchial and vessel trees, up to ∼1.5 cm) as randomly‐positioned 2‐D cylinders. The MC code PENELOPE was employed to calculate dose distributions in a water phantom containing a lung region, modeled by either a homogenized lung (as used in conventional planning) or the random lung model. Dose calculations used 6 and 18 MV photon beams with four different field sizes. Results: Depth dose curves in the random lung model illustrate significant perturbations when the structure size is comparable to the field size. For the 1×1 cm field size, large differences (up to 34% of Dmax) exist in the largest structures due to the loss of CPE with small field size. For large field sizes (10×10 cm or higher), little difference is observed between the random and the homogeneous models. The additional attenuation of the large structures also results in a region of dose reduction behind the lung. Conclusion: A new random lung model reveals significant dose perturbations from the homogeneous model, and shows that the homogeneous model breaks down when the field size is comparable to the structure size. This work is of importance in IMRT planning, where beamlets are used, or in the treatment of small tumors, where small field sizes are used in the planning. This work suggests that in such cases, a more precise description of the lung geometry, e.g. a high resolution CT‐based pixel‐by‐pixel density map, may be necessary for accurate dosimetry.
TH‐D‐ValA‐06: A Novel, Heterogeneity Inclusive, Pencil‐Beam Based Algorithm to Improve Lung IMRT Using the Corvus Planning System
Authors: Romesberg M , Pino R , Rubin R , Denisi J , Nizin P
Published: July 2006

Purpose: We investigate a new finite‐size pencil‐beam algorithm for calculating absorbed photon dose in heterogeneous media of arbitrarily varying density for inverse planning in CORVUS treatment planning system and evaluate its performance modeling heterogeneous systems and in optimization of an IMRT lung plan. Method and Materials: A new FSPB is developed by extending a phenomenological model (Med. Phys 26:1893–1990, 1999) for the central‐axis absorbed dose in therapeutic photon beams for heterogeneous media. The model's parameters are rescaled based on the density of the medium. A differential equation is introduced to model the interface build‐up processes of CAX primary and scatter dose. Primary dose profiles are calculated using density‐dependent kernel integration, interpolated in the FSPB axis direction and evaluated depending on the density at the point of interest. Scatter dose profiles are computed using scatter integration and evaluated locally. Results: The new heterogeneity inclusive FSPB was implemented in a development version of CORVUS. Original and new FSPB dose calculations were compared with Monte Carlo calculations performed using PEREGRINE. For a heterogeneous semi‐slab phantom and for an IMRT lung plan, the dose distribution generated by the new FSPB agrees well with MC results, while the original one shows substantial discrepancies. IMRT plan optimizations were carried out using both original and new FSPB, and then a final dose calculation was performed using PEREGRINE. The plan calculated using the new FSPB shows better target conformality than the one computed using the original FSPB. Conclusion: The new FSPB possesses greatly improved accuracy as demonstrated in a variety of phantom and patient cases, both for dose calculation and IMRT optimization. FSPB best features were preserved with little extra computational overhead promising accurate and fast inverse planning and real‐time dose sculpting and dose volume histogram manipulation.

Research sponsored by North American Scientific, NOMOS Radiation Oncology Division.
TH‐D‐ValA‐07: Clinical Use of Monte Carlo in Proton Therapy: Pencil‐Beam Algorithm Vs Monte Carlo for Proton Therapy of Skull‐Base and Para‐Spinal Tumors
Authors: Paganetti H , Parodi K , Adams J , Kooy H
Published: July 2006

Introduction: Pencil beam algorithms rely on kernels to model proton range in density‐scaled water equivalent material. Monte Carlo dose calculation methods are more accurate by design. This study addresses the issue of clinical significance of differences between a commercial pencil‐beam algorithm and Monte Carlo dose calculation.

Skull‐base or para‐spinal tumors are challenging for dose calculations due to interfaces between high and low density areas in the irradiated volumes. In addition, for para‐spinal cases, there are often metallic implants that not only distort the CT image but also affect the accuracy of dose calculations. Methods: Proton treatment planning is done at our institution by using FOCUS/XiO (CMS Inc.). To do full Monte Carlo based dose calculation the treatment head was modeled including a simulation of the modulator wheel rotation as well as aperture and compensator. The patient CT data information was converted into materials with explicit element composition and density. All secondary particles were tracked.

Treatment plans designed with the treatment planning program were recalculated with the Monte Carlo code and compared by analyzing dose distributions and dose volume histograms. Results: In general, we found good agreement. Small differences are in part due to the difference between dose to medium and dose to water. Significant differences were found in and near air cavities as well as in areas affected by high density implants. Conclusion: For some tumors in the head and neck region and near the spine, Monte Carlo based dose calculation shows significant differences compared to pencil‐beam algorithms. Consequently, at our institution, a sub‐set of the patient population receives Monte Carlo dose calculation to assist in treatment planning.
TH‐D‐ValA‐08: Experimental Evaluation of the Accuracy of Contralateral Lung Dose Calculations for IMRT Plans
Authors: Schofield D , Court L , Allen A , Hacker F , Czerminska M
Published: July 2006

Purpose: To experimentally determine the accuracy of contralateral lung dose calculated by Eclipse for mesothelioma IMRT plans. Materials and Methods: Two planning techniques were used to create 9 mesothelioma IMRT plans in Eclipse, which were then applied to a Rando torso phantom. Group A consisted of 5 plans with 160° of the contralateral side free from entrance beams while the 4 Group B plans had 80°–90° free. MOSFETs were placed at 10 coplanar points within the contralateral lung of the phantom, and doses measured for each field for each plan. These were compared with doses calculated using the pencil beam algorithm (modified Batho) in Eclipse. Two controls (ipsilateral AP/PA and 40×40cm AP/PA) were evaluated. Results: The 40×40cm AP/PA control plan showed good agreement between Eclipse and experimental results with the difference being 1.04% +/− 3.62% (1 stdev), while the ipsilateral only AP/PA plan showed a disagreement of −33.3% +/− 6.9% (Eclipse was cold compared with measurements) suggesting poor scatter modeling. The overall average difference between measured and calculated daily doses of Group A and B plans was −21.1% +/− 15.7% (−6.98cGy +/− 5.2cGy) and −9.8% +/− 29% (−6.58cGy +/− 19.5cGy), respectively. The differences were dependent on whether the points were in or out of individual fields. The average difference for points in field for Group A and B plans were −5.5% +/−23.4% (−1.169 +/− 4.99cGy) and +3.04% +/− 37.9% (1.57 +/− 19.6cGy), respectively. This increased to −49.2% +/− 10.9% (−5.8 +/− 1.28cGy) and −41% +/− 14.9% (−8.66 +/− 3.12 cGy), respectively, for points outside the field. Conclusions: Eclipse adequately models lung dose in field, but underestimates scatter dose out of field by up to 50%. This should be considered when evaluating contralateral lung DVH's, especially when attempts are made to use IMRT to lower mean lung dose.
TH‐E‐330A‐01: Computational Evaluation of Breast Geometry From Breast CT
Authors: Huang S , Yang K , Kwan A , Boone J
Published: July 2006

Purpose: Estimation of breast skin thickness and breast shape using a radial‐geometry segmentation algorithm on breast CT images. Method and Materials: Forty‐two breast image data sets were obtained from a prototype breast CT scanner, and were used to evaluate breast skin thickness and effective diameter. Patients were imaged at 80‐kVp using x‐ray tube currents (0.7 – 7.6 mA) depending on the patient's breast size. For each coronal breast image, the breast silhouette was segmented using a threshold value computed by a histogram‐based iterative algorithm. Breast area was also computed from the thresholded coronal images. A 360‐degree radial scan, originating at the center of mass of each breast CT image and continuing to the image edge, produced a radial profile of breast tissue intensity as a function of angle. A derivative filter was used to identify the inner and outer breast skin layers. In order to accurately estimate breast skin thickness, a tangent‐finding algorithm was developed to correct the thickness measurement in non‐circular breast geometries. A standard‐deviation‐based iterative algorithm was also implemented to reduce noise in the skin thickness estimation. Results: Among 42 patients, breast skin thickness was determined to be between 1.50 – 1.55 mm. Plots of effective breast diameter as a function of posterior‐anterior position serve as a concise method for characterizing idealized 3D breast shape, and these parameterized curves are reported for breasts of different size classes based on the cup size metric. Conclusion: Breast CT acquisition techniques, combined with algorithms designed for determining specific breast metrics, were useful for classifying breast shape and skin thickness. Most breast dosimetry coefficients (DgN) are based on the assumption of a 4 mm skin thickness, and the thinner skin dimensions found in this study will likely have a small but significant influence (increase) on breast dosimetry in mammography.
TH‐E‐330A‐06: Scatter Characterization in Cone‐Beam CT Systems with Offset Flat Panel Imagers
Authors: Virshup G , Suri R , Star‐Lack J
Published: July 2006

Purpose: X‐ray scatter significantly degrades the quality of cone‐beam CT (CBCT) reconstructions by introducing cupping and streaking artifacts. Simple correction techniques, based on subtracting a constant value across a projection, fail when the flat‐panel detector is transaxially offset as is required to increase the reconstruction field‐of‐views for body scans. The purpose of this study was to measure x‐ray scatter profiles for transaxially offset detectors and to characterize the resulting artifacts. Method and Materials: Data were collected on a table‐top CBCT system. A pelvic phantom was imaged with a Varian 4030CB imager offset by 16 cm. Scatter was estimated by subtracting a nearly “scatter‐free” projection data set, obtained by narrowing the axial collimator blades, from the full CBCT data set obtained with the blades in their fully open position. The resulting scatter estimate, valid in the narrow region of overlap, was extrapolated to generate a scatter estimate across the entire axial extent of the detector. This scatter estimate was then subtracted from the original CBCT data to generate scatter‐corrected images. Results: The scatter profile in full‐fan projections is relatively flat and symmetric. In contrast, in the half‐fan configuration the measured scatter profile was asymmetric decreasing monotonically from the phantom‐air boundary through the phantom center to the imager edge. The slope of this profile varied smoothly from the AP to lateral views resulting in reconstructions with abnormally bright and dark regions. Scatter correction using the measured profile proved effective. Cupping and doming amplitudes were reduced by 2/3. The average reconstruction error in the prostate region was reduced from over 120 HU to less than 40 HU. Conclusions: The asymmetries introduced by an offset detector result in non‐uniform scatter profiles that generate unusual cupping artifacts. Our technique provides a means of characterizing these profiles. Conflict of Interest: Funding provided by Varian Medical Systems.