bluuebunny/crossref_metadata_2025 huggingface dataset:
TU‐EE‐A2‐01: A Linear‐Quadratic‐Linear Formulation to Model Radiation Dose‐Response
Authors: Astrahan M
Published: July 2008

Purpose: Recent technological advances enable radiotherapy to be delivered in a highly conformal manner almost anywhere in the body. This has renewed interest in hypofractionation wherein the tumor is delivered a few fractions of large dose/fraction. Extrapolating clinical experience with conventional fractionations to fractions of high dose is important when designing hypofractionated regimens. Method and Materials: The concept of biologically effective dose (BED) based on the linear‐quadratic (LQ) formulation is useful for intercomparing conventional fractionations but is suspect at high dose because the LQ curve bends continuously on the log‐linear plot. A linear‐quadratic‐linear (LQ‐L) formulation which better fits the final exponential response of experimental dose‐response studies at high dose is described. This new formulation requires only one new term, the dose DT at which the LQ curve transitions to a linear tail. LQ‐L is applied to published dose‐response curves and the clinical implications of LQ‐L are examined across a wide range of fractionations. Results: For fractions of high dose, the LQ formulation underestimates the dose per fraction required to maintain equivalency with conventional regimens. The LQ‐L model fits a wide variety of experimental survival data over a wide range of dose. When , the line tangent to the LQ curve at DT intersects the e−αD and e−βD2 curves at dose α/β and also closely fits the linear response in the high dose region of many in vitro studies. Conclusion: For fractions of high dose LQ‐L gives better estimates of BED than LQ because LQ‐L better fits experimental dose‐response in the high dose region. This is particularly important when planning hypofractionated regimens for reactions with low α/β such as prostate cancer or late sequelae because for these reactions falls within the contemplated range of hypofractional doses.
TU‐EE‐A3‐02: Multi‐Dimensional Prediction of Respiration Motion
Authors: Pitcher G , Seibert R , Ramsey C
Published: July 2008

Purpose: The purpose of this study was to develop a technique to dynamically predict tumor position and uncertainty in three dimensions in real time. Method and Measurements: A novel multi‐dimensional time delay kernel regression (MD‐TDKR) technique was developed to predict motion up to 1.5 seconds into the future. This technique uses historical data and the correlations between the multiple dimensions to predict target position, and allows for continuous memory updates to dynamically retrain the model. This technique could be used in a variety of motion prediction applications. Results: In one example, the algorithm inaccurately predicted tumor position, but was able to identify the corresponding uncertainty increase. When the model was unable to accurately predict position, the 95 percent uncertainty interval becomes quite large. Likewise, the predictions are accurate where the uncertainty is small. This information can be used in determining when to temporarily trigger the beam on‐and‐off. The average percent error was computed at different latencies. For predictions 1.5 seconds into the future the error was 7.83%, for 1 second it was 6.55%, and for 0.5 seconds it was only 5.27%. The average uncertainty for the predictions at 0.5, 1, and 1.5 seconds into the future was 4.99%, 6.18%, and 7.12%, respectively. Conclusion: This study shows that MD‐TDKR can learn complex relationships in multidimensional respiration data. Because of its ease of building and updating the memory matrix, the model can adjust to new operating conditions, such as changes in a patients breathing pattern. It was also able to determine the uncertainty for a given query vector, allowing for the ability to hold the beam off when the prediction is unsure. These factors indicate that MD‐TDKR has great potential for use in radiation oncology. It is also anticipated that the MD‐TDKR paradigm could be easily implemented into commercial respiratory gating systems.
TU‐EE‐A4‐01: Bismuth Shields Vs. MAs Reduction for Decreased Radiation Dose to Breasts in CT Examinations
Authors: Goodsitt M , Christodoulou E , Larson S , Kazerooni E , Bogot N , Frank L
Published: July 2008

Purpose: Bismuth breast shields have been promoted as a means for selectively reducing the radiation dose to the breast by about 30% in CT studies, while maintaining image quality. A study was performed to compare image noise and CT number accuracy with the shields to an alternative dose reduction method of employing 30% less mAs. Method and Materials: A humanoid thorax phantom with simulated breasts was imaged on a GE VCT scanner using: 1) a standard lung cancer screening protocol, 2) the same protocol but with a commercial bismuth breast shield, and 3) 30% less mAs without the shield. Regions‐of‐interest (ROIs) were placed in the images and the mean CT numbers and standard deviations of the CT numbers were compared. Results: Relative to the mean CT numbers in images for the standard technique, use of the breast shield resulted in increases of about 9HU, 19HU, 6HU, and 57HU in ROIs in the heart, anterior left lung, posterior left lung, and right breast, respectively. Corresponding changes for 30% mAs reduction were 1HU, −3HU, −2HU, and 0HU. Ratios of the standard deviations of the CT numbers in the dose reduced images to those in the images using the standard technique for the above ROIs were 1.4, 1.2, 0.9, and 1.8 for the breast shield and 1.3, 1.0, 1.0, and 1.2 for 30% mAs reduction. Conclusion: mAs‐reduction is preferred over bismuth breast shields because: 1) mAs‐reduction has much less effect on mean CT numbers, which is important for quantitative studies such as lung density and coronary calcification assessment, 2) noise in the mA‐reduced images is less, and 3) the images do not suffer from streak artifacts arising from the shields. Additional comparisons in images of human subjects undergoing IRB‐approved coronary calcification studies with the breast shield vs. 30% reduced mAs will be presented.
TU‐EE‐A4‐05: Influence of Photon Scatter Modeling On Image Reconstruction Accuracy in CBCT
Authors: Mainegra‐Hing E , Kawrakow I
Published: July 2008

Purpose: One method for scatter correction in cone beam computed tomography (CBCT) is to compute the scatter with a Monte Carlo simulation. The accuracy of this approach may be influenced by the accuracy of the underlying photon scattering cross sections. The purpose of this study is to investigate the effect of the level of sophistication of photon interaction models on the computed scatter in CBCT and its influence on the accuracy of image reconstruction. Method and Materials: The investigation is performed using egs_cbct, a new EGSnrc based code for use in CBCT imaging. The EGSnrc treatment of Rayleigh scattering is improved to include measured molecular coherent scattering form factors (MCSFF) in addition to the commonly used independent atom approximation form factors (IAAFF). A more accurate algorithm for sampling coherent scattering angles is also added. Three photon scatter models are investigated: Compton scattering according to the Klein‐Nishina equation and no Rayleigh scattering (simple); Bound Compton scattering modeled in the relativistic impulse approximation (RIA) and IAAFF; RIA and MCSFF. Scatter calculation and image reconstruction accuracy is tested for a 30 cm diameter water sphere with and without inserts of varying density and materials for a scan with 180 projections. Results: The simple model is not sufficiently accurate for estimating photon scatter in CBCT. The influence of MCSFF on the computed scatter distributions is small and only noticeable at the edges of the phantom. No significant difference in the accuracy of the reconstructed images is observed between the MCSFF and IAAFF coherent scattering models. Conclusion: Rayleigh scattering must be included in the Monte Carlo simulation to estimate the scatter in CBCT imaging. The inclusion of molecular interference effects in coherent scattering has no significant effect in the image reconstruction process.
TU‐FF‐A1‐01: Quantification of Clinical Limitations of Proton Beam Range Verification Using Offline PET/CT Due to Biological Washout Effects
Authors: Knopf A , Parodi K , Paganetti H , Shih H , Bortfeld T
Published: July 2008

Purpose: The use of offline positron emission tomography/computed tomography (PET/CT) scans after proton beam delivery as quality assurance tool is currently under investigation at the Massachusetts General Hospital. A specific aim is to validate the planned proton range in the patient. This study investigates restrictions on the accuracy of the offline method due to washout effects in perfused tissue. Method and Materials: Imaging was performed at a commercial PET/CT scanner for 30min, starting within 20min after treatment. Measured PET/CT images were coregistered to the planning CT and compared with corresponding PET distributions, obtained from CT‐based Monte Carlo simulations. For 5 patients with tumors in the head and neck region the range was verified at positions where the proton beam ended in bone as well as at positions where the beam stopped in well‐perfused tissue. Besides a range comparison at specific positions in the fall‐off region of the activity depth profiles (pointwise), a range analysis taking the entire fall‐off region into account (shift) was performed. Results: For treatment beams ending in bone it was found that pointwise range verification at the 20% (50%) position in the last activity fall‐off within 1.2mm (2.5mm) and shift range verification within 2.4mm is feasible. In soft tissue pointwise range verification turned out to be not practicable due to the washout of the activity signal. The shift method showed the potential for range verification within 4.3mm in soft tissue and within 2.4mm if only the last 50% of the final activity fall off‐region lay in well‐perfused tissue. However in soft tissue it is complex to distinguish reasons for observed deviations. Conclusion: Offline PET/CT scans permit mm‐accurate range verification only in well‐coregistered bony structures, thus at limited positions. In soft tissue biological washout effects degrade the measured activity distribution and therefore prevent mm‐accurate offline PET/CT range verification.
TU‐FF‐A4‐02: Evaluation of the Performance of the Fast Scanning Platform of An OCT System
Authors: Heard M , Ibbott G
Published: July 2008

Purpose: To ensure that the performance of a fast optical computed tomography (OCT) scanning system is comparable with previous models. Method and Materials: MGS Research Inc. developed an OCT system based on the translate‐rotate method used by early generation x‐ray CT scanners. The performance of the system has been investigated and the system has been used in several published studies. Recently, a new OCT system was developed by MGS Research Inc. that reduces scan times by a factor of 10 or more. Several 3D dosimeters were irradiated using 6 MV photons and imaged on both version of the scanner. The image noise, reproducibility, and spatial accuracy were determined and used to evaluate the performance of the system. Results: The new version of the scanner reduced the scan time per plane from 7 minutes to 30 seconds. Preliminary results showed that noise levels in the images from both models were comparable. The uncertainty in the determination of the optical density values from images acquired with both models was ∼2%. The new model uses Fresnel lenses that may need adjustment prior to an imaging session which can affect the reproducibility of the system. Conclusion: The new version of the OCT scanner shows promise as a replacement for the previous version. Continued improvement in the software and hardware are needed to make the system as robust as the previous version.

The investigation was supported by PHS grant CA 10953 awarded by the NCI, DHHS.
WE‐E‐AUD C‐02: Impact of Image Quality On CT Guided Prostate Radiotherapy
Authors: Morrow N , Lawton C , Li X
Published: July 2008

Purpose: Different CT modalities with varying image quality are being used to correct for interfractional variations in patient setup and anatomy changes, thereby reducing CTV‐to‐PTV margins, for prostate radiotherapy (RT). We explore how CT image quality affects patient repositioning and CTV‐to‐PTV margin. Method and Materials: Three CT‐based IGRT modalities routinely used in our institute for prostate RT are considered in this study: MV fan beam CT (Tomotherapy), MV cone beam CT (MVision, Siemens) and kV fan beam CT (CTVision, Siemens). Daily shifts are determined by manual registration to achieve the best soft tissue agreement. Effect of image quality on patient repositioning was determined by statistical analysis of daily shifts for 65 prostate cancer patients (34 Tomotherapy, 21 CTVision,10 MVision) treated in our clinics. The impact of soft tissue contrast on organ interface identification was evaluated by analyzing contours drawn by 7 users on the scans from each imaging modality. In addition, variability of soft tissue registration between 10 users was evaluated based on the registration of representative scan for each CT modality with its corresponding planning scan. CTV‐to‐PTV margin was defined as 1.96σ. Results: Inferior image quality with MV CT based IGRT leads to increased variations in daily shifts (3, 4, 5 mm for CTVision, Tomotherapy and MVision) and in prostate delineation (6, 3, 10 mm for CTVision, Tomotherapy and MVision). Superior image quality with the kV CT results in reduced variation between 10 users in soft tissue registration. Uniform margin introduced to account for the uncertainty in the identification of prostate edge are determined to be 2, 6 and 5 mm for CTVision, Tomotherapy and MVision. Conclusion: Image quality adversely affects the reproducibility of the manual registration for IGRT and necessitates a margin of 2 mm for kV CT and 6 mm for MV CT to ensure adequate coverage.
WE‐E‐AUD C‐04: Effect of CT Calibration Phantom Size On the Range in Proton Radiation Therapy
Authors: Li H , Liu R , Wu R , Sahoo N , Mohan R , Gillin M , Zhu X
Published: July 2008

Purpose: The range of a proton beam in the patient is estimated using the relationship between CT numbers (HU) and relative linear stopping powers (RLSP). The purpose of this study is to assess the range uncertainties introduced by the size of the phantom used for determining the calibration curve. Method and Materials: CT scans were performed on tissue equivalent inserts placed at the center of two phantoms of different sizes representing human head and body with a GE Lightspeed 16‐slice CT scanner. The HU to RLSP calibration curve was established for each phantom based on a stoichiometric calibration technique. For each planning CT image data set, two plans were generated using head and body calibration curves, respectively. The range difference between two corresponding fields was evaluated using distance‐to‐agreement (DTA) along the beam direction in a homogenous water phantom in water equivalent thickness (WET). The DTAs were calculated with a threshold of percent dose difference of 3%. Plans for four prostate and four lung patients were investigate. Results: The measured CT number for the same insert placed in two phantoms of different sizes could differ up to 200 HU for the bone equivalent materials. The maximum dose difference between two proton plans generated with head and body calibration curves could be up to 33%. The difference in DTA was up to 2.4 mm WET for prostate patients and 5.4 mm WET for lung patients. Conclusion: The calibration phantom size could contribute up to 5.4 mm WET range uncertainties for the lung patients in our study. The current practice is to use average calibration curve to minimize the uncertainty. Advanced imaging techniques such as dual‐energy CT scans could be utilized to reduce the uncertainty in CT numbers. Other factors that introduce range uncertainty in proton therapy will be investigated in future.
WE‐E‐351‐01: Multi‐Dimensional Visualization Challenges in 4‐Dimensional Radiotherapy
Authors: Chen G
Published: July 2008

4D CT is routinely used in radiation therapy planning of moving targets. Its current role is commonly limited to assessing the extent of target motion, and designing an aperture that encompasses this motion. However, much more information can be extracted from 4D imaging, if computational and visualization tools were available. Examples include: 1) regional ventilation analysis in lung irradiation; 2) analysis of serial 4D CT to quantify tumor trajectory variations 3) identification of corresponding airway bifurcations to provide validation of deformable registration algorithms 4) interactive 4D volume rendering to permit visualization of structures within the radiation field. While the core tools of image processing remain the same (segmentation, image registration, change detection, visualization), extrapolation to 4D requires new and powerful implementations.
WE‐E‐332‐01: A Unified Dose Indicator for Tomographic Acquisition Modalities
Authors: Beldjoudi G , Yilmaz E , Bochud F , Pachoud M , Schmidt S , Verdun FR
Published: July 2008

Introduction : Tomographic techniques in diagnostic and interventional radiology or radiation therapy are in rapid development. The CTDI formalism that is currently used to assess dose has been challenged by several groups. The goal of this contribution is to evaluate the required scan length to reach equilibrium on various MDCT units and estimate the average dose delivered within a slice when using systems having a cone beam geometry.

Method and Materials: Measurements were performed on three MDCT systems (GEMS 8, 16 and 64‐row), a flat panel fluoroscopy (Allura — Philips), an IGRT (Synergy, Elekta ltd) and on a tomotherapy (TomoTherapy Inc.) using two standard CTDI phantoms and a home made phantoms (PMMA cylinder of 30 cm filled with water) with a conventional small volume ion chamber (0.6 cc Farmer type) and a standard pencil ion chamber. Dose profiles were also recorded at various positions within the slice to study the impact of scatter as a function of the distance between the centre of the phantom and its periphery. Results: For CT units a theoretical length of 400 mm is required to reach the equilibrium at the center of the phantom. The measurements show that a dose plateau was reached after 420 mm. The average dose within a slice measured in our home made phantom for standard abdominal CT (120 kV, 210 effective mAs) was 15.3 mGy; 26.9 mGy for the fluoro‐CT (122kV, 274mAs, 20.7s); 39.0mGy for IGRT system (120kV, 219mAs, 27×27cm2 field size, one rotation) and 13.1 mGy for the Tomotherapy system (3.5 MV, pitch=0.8). Conclusion: The CTDI concept should be replaced by a more generic methodology, such as the one presented here, that could be used in all cone beam geometries. Measurements should be done using water equivalent phantoms.
TH‐SAMS‐AUD B‐06: TG113: Improving Treatment Consistency and Data Quality for Clinical Trials
Authors: Moran J
Published: July 2008

The goal of AAPM Task Group 113 is to provide guidance to physicists, QA centers, and others involved in clinical trials on methods to improve the consistency and quality of data generated for trials involving external beam radiotherapy.

To date, there are no universally agreed upon standards for the physics practices related to clinical trials. In addition, as treatment techniques become more sophisticated, it is even more challenging to comprehensively maintain consistency across multiple institutions. Since hundreds of institutions may be involved in clinical trials, it is critical to review the entire treatment planning and delivery process and to identify areas where improvements can be made to ensure that high quality and consistent data are acquired from all institutions treating patients on clinical trials.

This presentation will focus on factors that impact data quality for the treatment planning and delivery process. In addition, methods to help individual physicists improve the consistency of clinical trials will be discussed. The scope of the task group includes image acquisition for volume definition, treatment planning systems, patient localization, treatment guidance and delivery, and credentialing for clinical trials.

Educational Objectives:

1. To describe the goals of TG♯113 Physics Practice Standards for Clinical Trials.

2. To highlight factors that directly impact clinical trials that involve IMRT and IGRT.
TH‐A‐351‐01: Digital Mammography: A Physician's Perspective
Authors: Parikh J
Published: July 2008

Full field digital mammography (FFDM) represents an exciting new frontier in the evaluation of breast cancer. FFDM separates out the components of image acquisition, image processing and image displays compared to traditional film screen mammography, allowing a variety of potential technical advantages. In the ACRIN study FFDM had higher sensitivity for detecting breast cancer compared to SFM in premenopausal women, perimenopausal women, and women with dense breasts. Other reported advantages of digital mammography include higher contrast resolution, reduced noise, reduced need for repeat exposures and thus lower radiation dose to the patient, rapid soft‐copy display image interpretation, linkage to PACS systems and advanced applications such as CAD, telemammography and new‐modality imaging such as contrast‐enhanced digital mammography, dual‐energy subtraction mammography, stereo‐mammography, and tomosynthesis.

Digital mammography has not yet received widespread clinical implementation because of several barriers. Significant costs associated with hardware purchase, additional equipment, professional retraining and development and technical support are incurred by facilities that choose digital mammography. Ongoing operational issues include comparing digital versus film screen images in the radiologist and technologist work stations. Lastly, while storage may be rapid, reliable and convenient with digital mammography, significant sizable amounts of computer memory will ultimately be required to store the amount of data required to maintain high‐quality images.

Educational Objectives:

1. To review the clinically relevant functional components of digital mammography.

2. To introduce some of the advanced applications of digital mammography.

3. To discuss some barriers to widespread clinical use of digital mammography.

4. To review data from clinical trials rerarding digital mammography.

Outline:

(1) Why digital?

(2) Functional components

(3) Advanced applications

(4) Barriers

(5) Clinical trials
TH‐SAMS‐332‐04: Dose Descriptors and Dose Assessment for Cone Beam CT Scanners
Authors: Geleijns J
Published: July 2008

Computed tomography (CT) dosimetry should be adapted to the fast developments in CT in order to keep pace with new CT technology. Recently, a cone beam CT scanner that challenges the existing Computed Tomography Dose Index (CTDI) dosimetry paradigm was introduced. The appropriateness of existing CT dose metrics for a cone beam CT scanner will be assessed and to new approaches for CT dosimetry will be suggested.

Dose measurements with a small Farmer type ionization chamber, and 100mm and 300mm long pencil ionization chambers can be performed free‐in‐ air to characterize the cone beam. According to the most common dose metric in CT, measurements can also performed in 150mm and 350mm long CT head and CT body dose phantoms with 100mm and 300mm long ionization chambers. Effects that cannot be measured with ionization chambers can be explored with Carlo calculations of the dose distribution in 150mm, 350mm and 700mm long CT head and CT body phantoms. To overcome inconsistencies in the definition of CTDI, doses are also expressed as CT_Dose100 (CT_Dose100=160/100 × CTDI100). CTDI300 is chosen as the standard of reference against which results were assessed.

Measurements free‐in‐air reveal good‐to‐excellent correspondence between CTDI300air and CT_Dose100air; CTDI100air substantially underestimates CTDI300air. Measurements in phantoms and Monte Carlo calculations reveal good correspondence between CTDI300w, CT_Dose100w and CTDI600w; CTDI100w substantially underestimates CTDI300w. Measurements at different positions within CT dose phantoms with an ionization chamber that is smaller than the cone beam are fundamentally different compared to readings of pencil chambers that are longer than the cone beam.

CT_Dose100air and CT_Dose100w are pragmatic metrics for characterizing the dose of the cone beam CT scanner; these quantities can be measured with the widely available 100mm pencil ionization chambers and 150mm long CT dose phantoms. CTDI300air and CTDI300w in a 350mm long CT dose phantom can serve as appropriate standards of reference for characterizing the dose of the cone beam CT scanner. Simple geometrical considerations, supported by Monte Carlo calculations, explain the fundamental differences of different CT dose metrics when measurements are performed at different position within CT dose phantoms.
TH‐A‐352‐01: Management of Interventional Equipment and Suite: Cradle to Grave
Authors: Strauss K
Published: July 2008

The life of any fluoroscopic imaging device begins with someone's perceived need for it and ends when it is removed from the clinical environment. In between these “cradle and grave” milestones, the machine must meet clinical objectives, provide good image quality, and reduce radiation dose to both patients and personnel. An organized program that achieves cost effective quality improvement is required to achieve these objectives. The first step identifies the clinical requirements of the imager. This identification assists obtaining adequate project funding. One acquires the imager by identifying the vendor of choice, negotiating the purchase, and issuing a purchase contract. One concurrently must plan an effective and efficient supporting facility followed by monitoring of the progress of renovation/construction. After the unit is installed, the owner acceptance tests the imager followed by staff training. A comprehensive program of routine testing, maintenance, repair, and record keeping to assure equipment performance begins after commissioning of the unit and continues throughout its life.

Educational Objectives:

1. Understand how to help physicians identify equipment attributes that address clinical needs.

2. Understand important steps in selecting, negotiating, and contracting the purchase of imaging equipment.

3. Understand basic elements of facility design necessary to support selected imaging equipment.

4. Understand the objectives of acceptance testing of state‐of‐the‐art imaging equipment.

5. Understand the basic elements of a program to assure equipment performance after its commissioning.
TH‐B‐350‐01: Scintillation Dosimetry: Review, New Innovations and Applications
Authors: Beddar S , Beaulieu L
Published: July 2008

Interest in the development of plastic scintillation detector systems for dosimetry has been evolving for more than a decade. Plastic scintillation materials have many properties that make them ideal for dosimetry including water equivalence and energy independence for MV photons, linearity with dose, dose rate independence, and high spatial resolution. Therefore, these detectors do not require the usual conversion and/or correction factors used for other commonly used detectors to convert the dosimeter reading to absorbed dose. The only disadvantage of plastic scintillation detectors is the spurious effect arising from Cerenkov radiation produced in the optical fiber that guides the scintillation light, which has been solved. This evolution started with point detectors and is leading to matrix arrays to respond to the ever‐increasing complexity of radiotherapy treatment fields such as IMRT. Small fields, high dose gradients and other challenging conditions could soon require the development of commercial scintillation detectors. Moreover by using a photodetector with a large sensitive area (e.g. a CCD camera), arrays of several hundreds of scintillation detectors could be made to simplify long and arduous quality assurance tests.

This lecture will provide an overview of the dosimetric characteristics and properties of plastic scintillation detectors when exposed to high‐energy photon, electron as well as proton beams. We will discuss both forms of plastic‐based scintillation detectors: plastic scintillators and plastic scintillating fibers. Finally, we will present few applications for plastic scintillation detectors in clinical radiotherapy: stereotactic radiosurgery, quality assurance and in vivo dosimetry applications.

Educational Objectives:

1. Review the underlying physics of scintillation materials.

2. Review the properties of plastic scintillation materials used in radiation dosimetry.

3. Understand the principles of this method and recent innovations in scintillation dosimetry.

4. Identify applications that could benefit from scintillation detectors.
TH‐C‐350‐03: A Dosimetric Validation of RapidArc Treatment Plans for 5 Treatment Sites
Authors: Verbakel W , Senan S , Lagerwaard F , Hoffmans D , Cuijpers J , Slotman B
Published: July 2008

Purpose: To compare calculated and delivered dose distributions from RapidArc™ (a form of volumetric modulated arc therapy, Varian Medical Systems). RapidArc is a novel approach for delivering single arc therapy in less than 2 minutes, while achieving dose distributions comparable to current IMRT. We report the first detailed dosimetric validation for 5 different tumour sites with RapidArc plans. Method and Materials: Clinical RapidArc plans were generated with a pre‐clinical version for single cases with glioblastoma multiforme, multiple brain metastases, nasopharynx‐, oropharynx‐ and pancreas carcinoma. All five plans were delivered with a Varian Linac and measured in a solid water phantom for 5 coronal planes, 2 cm separated, using double Gafchromic®EBT films. Plans were also measured using ionisation chamber arrays (MatriXX). Measured and calculated dose distributions were compared using 2D gamma evaluation with limits of 2mm and 3.5% (of typical PTV dose in phantom). Results: All 25 film measurements showed high agreement with calculations, with a mean gamma of 0.29 and on average 1% (maximum 3%) of the film surface exceeding a gamma of 1.0. Relatively strong spatial dose modulations could be measured, within 95–107% dose range in the PTV, which were not completely predicted by calculations. This could lead to local dose changes >3% when changing a plane by 2mm. MatriXX measurements corresponded better with dose calculations than film measurements, which may be due to the limited resolution of 7.6mm of MatriXX. Conclusion: RapidArc accurately delivers the planned dose distributions. Film measurements may be preferred for dosimetric verification as more dose modulation is detectable than with ionisation array measurements. A “2.5D” gamma evaluation taking into account multiple adjacent dose planes would show better agreement for film dosimetry. Our results indicate that RapidArc can be introduced into clinical practice. Conflict of Interest: Research was a collaboration with Varian Medical Systems.
TH‐C‐AUD A‐02: The Air‐Kerma Strength Standard for 192Ir HDR Sources at the University of Wisconsin ADCL
Authors: Rasmussen B , Davis S , Micka J , DeWerd L
Published: July 2008

Purpose: To present a compilation of a seven‐year study of measurements using several HDR afterloaders and to provide a comprehensive analysis of the various published methodologies for interpolating between NIST standards to determine the air‐kerma calibration coefficient for . Ultimately an update of the current interim standard will be considered. Method and Materials: An acrylic apparatus for performing the seven‐distance measurement technique equipped with laser alignment was used to acquire all datasets. Measurements were performed with an Exradin A3 spherical ion chamber that had been calibrated at NIST for beam qualities of M250 as well as . A total of four different afterloaders were measured during multiple trials and a comparison of the results was made to assess any trends in measurements due to source geometry. Recently published interpolation methods were compared to the method used in the original establishment of the interim standard in 1991 with proper accounting of the revisions in the NIST air‐kerma standards in 2003. Three different methods for solving the non‐linear system of equations were compared to assess stability and minimize uncertainty. Results: Depending on the interpolation method, deviations of −1.12% to −0.37% from the long‐standing air‐kerma calibration factor were observed. In comparing the measurements from the last seven years, (2000–2007), to the well chamber transfer standards, (1991), certain trends between various source models were identified, but the overall effect was found to be in the range of — 0.95% to 0.18%. Conclusion: Based on the data recorded it is reasonable to assume, given the uncertainty in the method, that a single calibration factor would indeed be appropriate for all source models. The possibility of a formal update to the standard will be considered.
TH‐C‐AUD A‐10: LDR Brachytherapy Dosimetry: Monte Carlo Code and TG‐43 Comparisons
Authors: Furstoss C , Poon E , Reniers B , Carrier J , Pignol J , Williamson J , Beaulieu L , Verhaegen F
Published: July 2008

Purpose To study the Monte Carlo codes PTRAN_CT and MCNPX2.5 and compare the dosimetry results with the TG‐43 formalism for a permanent seed implant of a breast brachytherapy. Method and Materials: The geometry validation of a model 6711 iodine seed was studied calculating the radial dose function in water and the energy spectrum of the seed in air. The results for the calculated spectrum were compared with an experiment carried out with an Amptek XR‐100T spectrometer. The calculation times for MCNPX and PTRAN_CT were analyzed by calculating the figure of merit for water phantoms with different voxel numbers. The absolute dose was validated comparing the absolute dose in water with TG‐43 and the absolute isodoses obtained from EBT Gafchromic film. The results of a treatment plan for a breast brachytherapy were compared with TG‐43 calculations. Results: The discrepancy between the calculated and published radial dose function values is less than 3% for the two MC codes. The comparison of the energy spectrum with the experiment reveals a contribution of the detector diode for the energy inferior to 5 keV. The calculation time comparison between MCNPX and PTRAN_CT shows that PTRAN_CT is 10%–30% faster than MCNPX for a voxel number between 200,000 and 500,000. A good agreement is obtained for the absolute dose calculated by the two MC codes compared to the TG‐43 calculation in water and the absolute isodoses measured in the Gafchromic film. The breast cancer patient plan shows that the MC results differ 12.7% in comparison with the TG‐43 results. Conclusion: MCNPX and PTRAN_CT simulations agree with the absolute dose in water obtained with TG‐43 and experiments. Moreover, the patient dosimetry study reveals the interest to use a MC code where the tissue composition and the interseed attenuation are taken into account.
TH‐C‐AUD C‐07: Prediction of Fiducial Motion in Respiratory Tumors for Image‐Guided Radiotherapy
Authors: Riaz N , Wiersma R , Mao W , Xing L
Published: July 2008

Purpose: Several groups have investigated monitoring respiratory tumor motion during a radiotherapy treatment session by fluoroscopically tracking implanted markers. However tracking requires prediction because there is a mechanical latency involved in either shutting the beam off or moving the position of the beam to adjust for the tracking results. Investigators have examined predicting respiratory tumor motion using various linear, non‐linear, and adaptive techniques. Here, we run a pilot study and try a new non‐linear regression method for prediction and compare it with linear prediction on three patients with respiratory tumors. Method and Materials: We examine two methods to predict the future location of the tumor, moving linear regression and moving support vector regression. By trial and error, we find that using 8 prior locations of the tumor is optimal for the linear model. The support vector regressor is non‐linear because we use a radial basis kernel function to expand the input space. Like the linear model, it also uses 8 prior locations to predict the future location. The loss function is the ε‐insenstive. We test our models on data from 3 patients with respiratory tumors. The motion data was collected with Accuray's Synchrony system at 30Hz. Results: We predict the location of the tumor 1 second ahead. The root mean square error of no prediction, linear regression, and support vector regression respectively is 7.41 mm, 1.93 mm, and 1.47 mm. Conclusion: On this small set of patients, we appear to predict tumor motion further into the future than previously reported. Although this might be because of the small sample size, what remains significant either way is the fact that support vector regression out‐performed the linear method for predicting tumor location for each of the three patients.
TH‐C‐351‐03: A Novel Small Radiotherapy Facility to Irradiate Partial Body of Zebrafish Embryos
Authors: Rodriguez M , Mathias J , Huttenlocher A , Jeraj R
Published: July 2008

Purpose: To develop a new facility to irradiate partial body of zebrafish embryos and test the system with an actual radiobiology experiment. Method and Materials: This micro‐irradiator uses a 50 kV photon beam, miniature x‐ray, Xoft Inc. The source is inserted in a cylindrical brass collimator of 3 cm diameter and 3 cm long. A pinhole of 1 mm diameter along the central axis produces a well‐focused pinpoint beam with a sharp penumbra. A photodiode monitors the beam and provides readings for dose calculation. Specimens are irradiated at 6 mm from the collimator and they are accurately positioned on the beam using a video camera and a computer‐controlled movable table. The system was used to irradiate total and partial body of zebrafish embryos at 3 days post‐fertilization to investigate radiation induced apoptosis and microphages recruitment at 40 Gy for both irradiation modalities. Results: This irradiation facility is portable and can fit in any radiobiology lab. The image‐guidance and high precision of the movable table enable accurate specimen position. The beam monitoring system provides exact, fast, and easy dose determination. Total body zebrafish irradiation at 40 Gy shows a severe post‐treatment cell ablation effect and substantial apoptotic increase after 3 days post‐irradiation. For partial body irradiation, there is an increase in apoptotic cells and remarkable macrophages recruitment after 3 days post‐irradiation. Conclusion: This robust, simple, and effective image‐guided micro‐irradiator is an appropriate tool to accurately irradiate partial body of zebrafish embryos, cell cultures or any other small specimen used in radiobiology studies. The tests comparing total and partial zebrafish embryo irradiation revealed significant difference in cell response. In general, this novel micro‐irradiator has expanded the radiation modalities for very small animals used in radiobiology studies and opened the possibility to adventure deeper in radiotherapy research.
TH‐C‐351‐05: Quantitative Methods for Contouring On PET‐CT Images
Authors: Day E , Betler J , Mohammadi S , Reitz B , Parda D , Miften M
Published: July 2008

Purpose: FDG‐PET imaging is routinely used to diagnose and stage cancer patients. It is also gaining wide acceptance as a tool to assist in tumor delineation in radiotherapy (RT) treatment planning. However, target volume definition is subject to inter‐observer variability. The objective of this study was to evaluate several existing auto‐contouring methods and develop a technique that would reduce inter‐observer variability. Method and Materials: Eighteen rectal and anal cancer patients who had undergone PET‐CT imaging and received RT were retrospectively reviewed. For each patient, a FDG‐PET avid (AVID) region was contoured by an experienced clinician without the use of the CT scan. The AVID volume was compared to volumes derived by the automated methods. Three automated methods were used: a fixed SUV cutoff of 2.5, previously suggested in the literature, a percentage of the maximum SUV (%SUVmax), and an in‐house derived mathematical technique. A 43% threshold was found for %SUVmax using a phantom study with cylinders of known volumes filled with varying concentrations of FDG. The mathematical approach generated 3D volumes using a Confidence Connected Region Growing (CCRG) technique that calculated the mean and standard deviation from pixel intensities contained in a 3D volume grown from a seed pixel. Results: The class solution of using a single value of SUV or a %SUVmax proved limited. These two methods depend on the correct threshold being applied and need to be different for each patient. The resulting volume differences ranged from 1%–129%. The CCRG based volumes were within 8% of the AVID volumes with a range of 1%–23%. Conclusion: Assuming that the same seed pixel is chosen, the CCRG method reduces inter‐observer contouring variability on FDG‐PET images and provides a viable clinical solution by always growing the same volume.

Research partially supported by Siemens Medical Solutions.
TH‐C‐351‐08: Clustering Analysis of Dynamic 11C‐Methionine PET in GBM for RT Target Definition
Authors: Wang P , Piert M , Lee I , Gomez‐Hassan D , Junck L , Rogers L , Hayman J , Haken R Ten , Lawrence T , Tsien C , Cao Y
Published: July 2008

Purpose: To assess tumor differentiation in patients with glioblastoma multiforme (GBM) using dynamic 11C‐methionin (D‐MET) PET and fuzzy c‐means (FCM) clustering analysis; and to evaluate the added value of D‐MET PET in radiation therapy (RT) target definition. Method and Materials: D‐MET PET images were obtained prior to RT in 25 patients with GBM. Each scan was composed of 15 phases acquired at 0–50 minutes following injection. Conventional MRI was also acquired before RT for target volume definition and after RT for evaluation of treatment outcome. D‐MET PET data were normalized to the mean uptake of each individual's cerebellum. Volume of interest (VOI) for the analysis was defined based on pre‐RT FLAIR‐MRI and extended to incorporate regions of high uptake of MET. Time‐activity curves of MET uptake in the VOI were classified using a FCM clustering algorithm with spatial constraints. The optimal number of clusters was determined for each dataset by calculating several clustering validity indices. The results of classification were reviewed by experts; and were also correlated to the patterns of local failure after RT. Results: Using the FCM clustering algorithm, time‐activity curves of MET uptake in the VOI were successfully partitioned into tumor, normal brain tissue, inflammation response, surgical cavity and edema. Heterogeneous MET uptake in the tumor was also differentiated. In 15 of the 25 patients who had tumor progression, the pre‐RT PET in the clusters correspondent to the locations of recurrence had a median uptake value of 1.47 (last dynamic phase), which involves clusters beyond the hottest ones. Conclusion: This study demonstrated that dynamic MET‐PET is capable of differentiating active tumors in patients with GBM. It is also promising in providing extra information for RT target definition.

Supported by NIHP01CA59827.
TH‐C‐332‐03: Noise Power Properties of a Cone‐Beam CT System for Breast Cancer Detection
Authors: Yang K , Kwan A , Huang S , Boone J
Published: July 2008

Purpose: To investigate the noise power properties of a cone‐beam breast CT (bCT) system. Method and Materials: Polyethylene cylinders were scanned under different acquisition conditions. Normalized noise power spectra (NNPS) were calculated from difference images by subtracting two identical scans. Multi‐dimensional NNPS were used to evaluate the noise properties of the bCT under different acquisition and reconstruction parameters including kVp, mA, number of projections, cone angle, object size, interpolation method, reconstruction filter, field of view (FOV), matrix size, and slice thickness. Results: Findings from the analysis: The bCT NNPS is rotational symmetric within the coronal plane and its shape is determined by the interpolation method and filter. For a cone angle range from 0° to 14°, the shape of the NPS curve changes slightly. The image variance increases slightly with increasing cone angle. Noise aliasing can be avoided if the sampling frequency of CT image is beyond a specific threshold. The coronal plane noise power decreases with increasing slice thickness. If the radiation dose (mA) is below a specific threshold, the electronic noise from the detector plays a dominate role and the system is no longer quantum limited. The threshold value decreases with increasing object size. When the radiation dose is beyond this threshold, the system is quantum limited. The image variance has a power law relationship of approximately −1.1 with the total dose. If the dose is increased by linearly increasing the number of projections but not the tube current, the noise power has an inverse linear relationship with the dose. When the dose level is kept constant, the image noise also depends on the x‐ray spectrum and the object size. Conclusion: A thorough investigation of the noise power was performed. Quantitative results provide guidance for the bCT system operation, optimization and data reconstruction.
TH‐C‐332‐04: Comparison of Residual Motion Artifacts in Phase‐ and Amplitude‐Binned Onboard 4D‐CBCT Imaging
Authors: Lu J , Zheng D , Keall P , Weiss E , Bartee C , Williamson J
Published: July 2008

Purpose: Linac‐mounted 4D‐Cone Beam CT (CBCT) imaging is an important tool for extending 3D IGRT techniques to respiration‐motion synchronized treatments. Our purpose is to evaluate the effects of two different projection binning strategies, phase and amplitude binning, on the image quality of our recently implemented 4D‐CBCT method using the Varian On Board Imager and RPM breathing motion surrogate. Method and Materials: We have implemented a 4D‐CBCT imaging procedure that adjusts the acquisition parameters to the breathing period. The Varian Real‐time Position Management system (RPM) was used to establish temporal coincidence between the respiratory motion and the CBCT projections. Projections were sorted into 10 bins. For the projections sorted into each bin, the variances about the RPM amplitude and phase means was calculated and compared to the image quality achieved in CBCT images. 4D‐CBCTs were studied with a phantom with irregular motion cycle. The phantom consists of two acrylic spheres and one cylinder on a motion platform to mimic the superior‐inferior tumor motion mostly seen in patients. Results: With breathing cycle period variation but not amplitude variation, we found that amplitude binning produced fewer blurring artifacts which was consistent with RPM analysis which showed significantly smaller amplitude variances for amplitude binning. For the patient subjects, the RPM amplitude but not phase variances were smaller for amplitude binning. However, image quality was significantly better when phase binning was used. We hypothesize that the under‐sampling artifacts play a major role over the motion artifacts which is the case for amplitude binning, while the phase binning has a good control of aliasing artifacts. Residual motion artifacts persist in both phase‐ and amplitude‐ binning 4D‐CBCT images. Conclusion: Optimizing the binning strategies requires understanding complex interplay between residual motion in each bin and other factors such as uniformity and adequacy of angular sampling.

Supported by PPG‐NIH‐P01116602.