bluuebunny/crossref_metadata_2025 huggingface dataset:
SU‐FF‐T‐406: Measured Dose Gradients for Extracranial Stereotactic Radiosurgery
Authors: Chase D , Mahan S , Ramsey C
Published: June 2005

Purpose: The main limiting factor in determining the amount of radiation dose that can be prescribed to a vertebral metastasis is the dose to the spinal cord. The goal of this work was to measure the dose gradient for a helical tomotherapy delivery system. Method and Materials: The spinal cord organ at risk (OAR) was a cylinder 10‐mm in diameter that extended 5‐cm superior‐inferior. The PTV was a half annulus 25‐mm thick that also extended 5‐cm superior‐inferior. Five benchmark test cases were created with the separation between the PTV and the spinal cord set to 2‐mm, 4‐mm, 6‐mm, 8‐mm, and 10‐mm. Although the gap between the PTV and the OAR was variable, the thickness of the PTV was held constant. Relative dose measurements were made using calibrated film placed in the phantom. The dose gradient was measured from the slope of anterior‐posterior profiles in the gap region between the PTV and the OAR. Results: For a 10‐mm gap, the calculated maximum OAR dose was 7.5 Gy (25%) for a prescribed PTV dose of 30 Gy. The calculated maximum OAR dose increased to 21.3 Gy (71%) with a separation of 2‐mm. A linear regression yielded a dose gradient of 10.6% / mm (R2 = 0.988). Typically, dose gradients in excess of 5% / mm decrease the PTV uniformity index below the clinically acceptable 20%. However, the benchmark tests in this study yielded a dose gradient in excess of 10% / mm while maintaining PTV uniformity indices of 4, 10, 11, 11, and 11% respectively for the 10, 8, 6, 4, and 2‐mm PTV‐to‐OAR separations. Conclusion: Helical tomotherapy benchmark test cases delivered to a cylindrical phantom indicated that the delivered dose gradient could be as high as 10% per millimeter while providing PTV uniformity of 15% or less.
SU‐FF‐T‐407: Prediction and Optimization of Stereo Tactic Frame Placement for Collision Avoidance in Gamma Knife Radio Surgery
Authors: Lay M , Movahed A , Ahmad S , Mardirossian G
Published: June 2005

Purpose: To develop a positioning assistive device (PAD) and system that optimizes the use of gamma knife stereo tactic frame placement before treatment in avoiding frame‐collimator collisions during treatment. Method and Materials: Prior to stereo tactic frame application, pretreatment MRI or CT images are used for tumor localization. The PAD system has been designed and constructed to predict the optimal frame placement coordinates. The PAD device is used to assist in applying the stereo tactic frame, utilizing optimized frame placement coordinates, with respect to a fiducial marker placed on a phantom prior to scanning. The PAD device has been constructed using high grade plastic curved to accommodate a cervical collar on its inner surface and a steel block and rod system to its outer surface. The block and rod system consists of a horizontal (y‐axis) and a vertical (z‐axis) rod with a 90 degree bend at the top (x‐axis). The two rods are attached via a block with two oblique holes each taped with a screw to allow for adjustment along the respective axis. The x‐axis portion has a block with a dovetail grove to allow attachment of the stereo tactic frame. The block and rod system allows for translation and rotation about each axis enabling six degrees of freedom in a compact design. This ensures that the placement of the stereo tactic frame is level and accurate. Results: The PAD system has been found to optimize frame placement and predict frame/collimator collisions. PAD allowed for accurate and reproducible frame applications. Conclusion: The PAD system will allow the neurosurgeon the ability to apply the stereo tactic frame to the patient in a more accurate and reproducible way. PAD does lead to easier treatment planning, faster treatment delivery, and reduced patient stress. Demonstration of the device will be presented.
SU‐FF‐T‐411: The Diode Volume Averaging Effect and Corrections in Very Small Photon Fields
Authors: Sham E , Devic S , Seuntjens J , Podgorsak E
Published: June 2005

Purpose: To evaluate the volume effect of diode dosimeters. Two p‐type silicon diodes, one with a sensitive volume diameter of 0.6 mm (small diode) and the other with a diameter of 2.5 mm (large diode), were used to measure percentage depth doses and profiles of very small 10 MV radiation beams with diameters of 1.5 and 3 mm. Because of the volume averaging effect, responses of the two diodes differ significantly. In this presentation we describe techniques suitable to correct for the volume effect exhibited by the large diode. Method and Materials: Volume effect corrections to PDD measurements include: (1) simulating diode measurements by Monte Carlo (MC) technique and determining the central axial geometry factors associated with the diode active areas and (2) changing the measurement setup from SSD to SAD followed by converting the large diode TMR data to PDD with the inverse square law. Volume effect corrections to profile measurements include: (1) applying an inverse gradient algorithm to de‐convolve the broadened large diode profile and (2) simulating the diode profiles by film measurements and determining the lateral geometry factors for de‐convolution with image analysis of the film. Results: For the 1.5 mm field, significant decreases in relative errors between the large diode PDD and the (reference) small diode data are observed after applying either MC correction or TMR conversion. In the 3 mm field at the depth of dose maximum, the inverse and film de‐convolution techniques reduce the FWHM of the large diode profile from 3.8 mm to 3.2 mm and 3.0 mm, respectively. Conclusion: The correction techniques we apply to large diode data result in reasonable agreement between the large and small diode data, allowing the use of large diodes for PDDs and beam profiles in very small radiation beams.
MO‐D‐I‐609‐05: Uncertainties in Measuring Activity Concetrations of a Moving Phantom Aquired with Standard Clinical PET/CT Protocol
Authors: Pevsner A , Mageras G , Nehmeh S , Humm J , Erdi Y
Published: June 2005

Purpose: We have evaluated the magnitude of errors associated with measuring activities in moving objects in PET/CT images acquired using standard helical/clinical CT data. Method and Materials: In this study we used 2001 NEMA IEC Body phantom which has six fillable spheres with diameters of 37, 28, 22, 17, 13 and 10 mm. The target‐to‐background 18FDG activity in a phantom was 8:1. A motorized platform moved the phantom with motion amplitude and period set to 2 cm and 5 s. All data was acquired using standard helical/clinical protocol: helical CT follow by PET emission scan. To determine uncertainties in activity measurements in stationary phantom we have acquired PET/CT scans of a phantom at 5 different positions. To evaluated the magnitude of errors in moving phantom we have reconstructed a PET image using emission data for the moving phantom and data for 30 different helical/clinical CT scans of the moving phantom acquired at random. Results: The uncertainties in measuring activities in the stationary phantom ranged from 9% for the largest sphere to 47% for the sphere with smallest diameter. The errors in measuring activity in the sphere in the moving phantom from 30 PET/CT image ranged from 10% to 33 % depending on the sphere's size. We have estimated the maximum possible observable error between the activity in stationary and moving phantom to be 21% for the largest and 75% for the smaller size sphere. Conclusion: The CT acquisition is on an order seconds and is fast enough that it will not acquire a motion‐average CT scan. PET emission scan on the other had is a motion‐average data. As a result, activity measured from PET/CT data acquired with standard helical/clinical protocol will vary depending on a start‐time of the CT acquisition with respect to the phase of the motion cycle.
MO‐E‐T‐6E‐01: Outcome Prediction Models in Radiotherapy: Methods for Improvement and Future Uses
Authors: Deasy J
Published: June 2005

Historically, the ability to predict a patient's outcome to a course of radiation therapy has been very limited. Knowledge of a patient's likely response to a given therapy was quantified by the maximum slope of the sigmoidally‐shaped patient population dose response curve: steep slopes correspond to precisely known individual dose thresholds, whereas shallow slopes correspond to poorly quantified individual dose thresholds. In the 3‐D treatment planning era, the situation has been fundamentally altered: it is no longer adequate to characterize dose distributions by a single number, as the effect of any distribution, especially on normal tissues which are often non‐uniformly irradiated, may depend in a complex way on the spatial distribution. It has been recognized for more than 60 years that irradiated organ volume can dramatically affect response. Efforts to quantify the ‘volume‐effect’ began in earnest in the late 1980's, but have recently gained much more momentum and effectiveness due to the advent of 3‐D treatment planning. In most investigations to date, the dose‐volume histograms of irradiated organs or tissue structures are analyzed to determine the dose parameters which most correlate with outcome. Remarkably, most organ responses appear to fall (very roughly) into just two categories: in Group I, complication risks rise according to the volume given a dose above a relatively high‐dose threshold (typically 50–60 Gy). These organs include the spinal cord, brain stem esophagus, rectum, and small bowel. In this group, organ function is related to organ structure. In Group II, complication risk is correlated with the mean dose or something similar; functional ability is partially impaired even at relatively low doses. This response group includes parotid glands, brain, liver, and lung. Simplified dose‐response models were developed by Kutcher, Burman, Brahme, Niemierko and others which reflected this range of response behavior. However, those analyses are far from utilizing the full potential of image‐based 3‐D treatment planning, based as they are only on cumulative dose‐volume‐histogram (DVH) data. Moreover, they ignore other potential clinical risk factors, such as diabetes, age, etc. In this talk we will estimate the trajectory of evolving dose‐volume‐outcomes models, including the need to search for potential variations in functional sensitivity vs. anatomical location, the inclusion of patient and disease risk factors, the need for advanced modeling techniques, and the future use of other biomarkers (imaging or gene analysis‐based) in combined models. Methods to deal with the crucial need to accelerate inter‐institutional data collection, modeling, and public archiving will be discussed. Lastly, we will discuss the inevitable move towards treatment planning based on outcomes models (e.g., maximizing the probability of tumor control with acceptable complication risks), and how we can get there from here.
TU‐A‐T‐6C‐01: TG‐70: Clinical Electron Beam Dosimetry: Supplement to TG‐25
Authors: Gerbi B
Published: June 2005

Purpose: The purpose of this talk is to describe the intent and content of Task Group 70: Clinical electron beam dosimetry: supplement to TG‐25. Task Group 70 of the AAPM was formed originally to address the issues of clinical electron beam dosimetry in light of the changes brought about by the new calibration protocol TG 51. The latter task group gives a clear description of the procedure required to establish the dose rate at one point in a radiation beam. However, clinical dosimetry demands the ability to find the dose rate at any point in a radiation treatments field. TG 70 addresses these issues for electron beam treatments. Originally, Task Group 25 addressed these issues at the introduction of calibration protocol, TG 21 and provided direction for all aspects of clinical electron beam application in radiation therapy. Its initial intent was to give recommendations to practicing medical physicists, techniques for them to follow, and reasons for the particular recommendations. Task Group 70 has the same intent as the original TG 25 but to do so in light of not only TG 51 but also to take into account advances in the field since the time of TG 25. Improvements in the accuracy of correction factors such as the incorporation of realistic stopping powers for electron beams, Pwall, Pgr and Pfl, are describe along with improvements in electron beam treatment planning or improvements in techniques that have taken place since the original TG25 report. Each section states clearly the reason for that particular section, the problems that are addressed, or the new information that has been developed since TG 25. The solutions to those problems, the actions needed to be taken by practicing clinical medical physicist, and other direction is provided in each section. Practical clinical aspects of electron beam dosimetry will be discussed including techniques for determining the output factors, percentage depth dose, and output factors and central axis percentage depth dose measurements for small, irregularly shaped electron beams. The final section of the task group report describes clinical applications of electron beams.
TU‐B‐I‐618‐02: Converting the Radiology Department From Film‐Screen to Digital — Making the Transition
Authors: Shepard SJ
Published: June 2005

In recent years, Picture Archiving and Communications Systems have begun to proliferate in the medical community. This is a result of technical advances that have occurred in two areas — advances in computer technology (such as storage density, processing speeds, network bandwidth, compression techniques, and so on), and the maturation of robust standards for communication and transmission of medical images and information (Standards like HL‐7, DICOM, and IHE). In parallel with these developments, the medical imaging equipment and information systems vendors have been developing robust software implementations and more advanced system architecture to take advantage of this emerging technology and these robust standards. As a result of this revolutionary change in the way we operate, it has become increasingly important that we in the Medical Physics community modify our approach to the type of support we bring to these operations accordingly. This presentation will focus on some of the issues the Medical Physicist is likely to face when supporting a clinical operation that is undergoing (or is about to undergo) a conversion from film‐screen base radiography to digitally acquired images. It will cover the educational needs of the Medical Physicist, radiologists and technical staff, how to use photostimulable phosphor imaging to introduce a radiology department to digital imaging, the importance of Modality Work List Management in the filmless radiology environment, selection of a DDR system for clinical use, weaning the referring clinicians off of film and what issues are important to achieving this, quality control programs for digital imaging systems, and finally those features of the radiology reading room environment that are important for soft‐copy interpretation.
TU‐C‐I‐609‐01: The Differences Method for the Tissue Structured Noise Determination in Digital Mammography
Authors: Grabski V , Brandan M‐E , Trujillo F , Villaseñor Y
Published: June 2005

Purpose: To propose a new method for the estimation of the tissue structured noise, based on the differences distributions among neighboring pixels. Method and Materials: Breast tissue structured noise (σT) is determined as the difference between the total and system noise. The system noise (σSYS), which includes quantum and apparatus noise, is determined using digital mammography images without an object. The total noise is obtained from the differences distributions among neighboring pixels in the region where the compressed breast thickness is constant. The range of raw pixel values is split into intervals to suppress the quantum noise variation contribution and σT is determined for each interval. 280 Senographe 2000D clinical images from 100 unselected patients were collected and analyzed. Also based on the differences distributions for a flat phantom, we propose a method to estimate spatial resolution for pixelized detectors. Results: The average measured value of σT /σSYS is 0.45±0.15 for the 10 cycles/mm frequency and for all the pixel value intervals. The absolute value of σT for the same frequency is estimated to be 1.7 ± 1.0 microns in microcalcification thickness equivalent. The σT is also determined down to 1 cycle/mm frequency. The estimated Senographe 2000D spatial resolution is approximately 60 microns. Conclusion: The proposed method for σT and resolution estimation is very simple and fast enough to use in online image processing, not requiring a complex signal/background analysis. The main limitation of the method for low frequencies (less than 1 cycle/mm) is statistical.
TU‐C‐I‐609‐02: Bandpass Observer Detection Mechanisms in Structured Noise
Authors: Abbey C , Eckstein M
Published: June 2005

Purpose: The use of medical images to detect and classify disease involves interpretation of images with spatially correlated variability. These variations can arise from physical processes in detection such as electronic and quantum noise, from patient to patient variability (often referred to as “patient structured” or “anatomical” noise), and from image processing algorithms such as image reconstruction and contrast enhancement. A number of linear observer models have been suggested as predictive of human observer performance for the purposes of optimizing imaging systems for diagnostic performance in noise limited tasks. We investigate some of these models in three simple detection tasks in Gaussian textures with lowpass, highpass, and uncorrelated (white) power spectra. Method and Materials: We use a recently developed methodology to directly estimate a linear observer template in two‐alternative forced‐choice (2AFC) detection tasks with white noise, lowpass noise, and highpass Gaussian noise. In all cases the signal to be detected was a small Gaussian “lesion”. After training, three human observers participated in the studies which involved 2,000 2AFC trials per condition. Results: The observed linear templates exhibit a bandpass structure in all cases. The peak of the spatial frequency band used by observer's shifts under different correlation structures. Various models, including Hotelling, Channelized Hotelling, Nonprewhitening, and Eye‐filtered Nonprewhiteing models were compared to the human observer data. Conclusion: Human observers adopt different visual strategies in response to the correlation structure of images that is reasonably well modeled as a bandpass filter. Many suggested models of detection performance do not fully capture this filter.
TU‐C‐I‐611‐02: New Development of Imaging Theory and Algorithms in Computed Tomography
Authors: Pan X
Published: June 2005

Computed tomography (CT), one of the most widely used imaging modality in medicine and other areas, is currently in a period of renaissance largely due to the advent of the helical cone‐beam data acquisition technology. This new development has opened up tremendous opportunities for the design and applications of CT imaging protocols in medicine and other areas that would otherwise be impossible. On the other hand, the cone‐beam CT also poses numerous theoretical and numerical challenges on the algorithm development. One of such challenges is to the need of new theory for obtaining accurate images from helical cone‐beam data to which the conventional CT theory is no longer applicable. In this talk, following the introduction of the basic principle of CT imaging, the recent advances of CT technology and imaging theory/algorithms will be discussed. Emphasis will be placed on the description of the new concepts, theory, and algorithms that we have recently developed for image reconstruction from cone‐beam data acquired with a wide variety of scanning configurations, including the helical scanning configuration. We also believe that one of the important trends in CT imaging is the so‐called targeted imaging of a region of interest (ROI) within the subject from truncated data. Such a targeted imaging strategy would substantially reduce the radiation dose delivered to the subject, scanning effort, and/or data contamination from motion and other artifacts. Therefore, I will describe our approaches to and algorithms for exactly reconstructing ROI images from truncated or reduced data in cone‐beam CT.
TU‐C‐I‐611‐03: Computed Tomography — What systems deliver vs. What clinicians Need
Authors: Vannier M
Published: June 2005

Computed tomography is the most important modality for many patients, since CT is the first and only examination they have before therapy or for followup. As the technology evolves, we observe changes the range of CT applications and its clinical utilization. We are in the midst of rapid expansion of clinical CT imaging use, due to increased speed and coverage, necessitating an evaluation of its current and potential future contributions. The needs of clinical medicine not yet met by CT, but potentially achievable in screening, diagnosis, therapy and followup are valid goals for technology developers. Despite intrinsic limitations due to use of ionizing radiation and substantial fixed costs, CT has room for growth and its technological evolution is not near the end.
TU‐C‐T‐6C‐04: Quantifying the Tradeoff Between Complexity and Conformality
Authors: Bortfeld T , Craft D , Halabi T , Trofimov A , Monz M , Küfer K
Published: June 2005

Purpose: Complex intensity maps in an IMRT plan may have several undesirable effects: increased leakage and integral dose, higher sensitivity to delivery errors, longer treatment time. On the other hand, the potentially negative impact of a reduction of the complexity on the dose conformality is not a priori clear. We propose to study the interplay between complexity and dose conformality in an interactive multi‐objective planning approach. Method and Materials: Two surrogate measures of complexity have been studied in previous approaches: The number of multileaf collimator (MLC) segments (in step and shoot IMRT delivery), and the number of monitor units (MUs). The first is difficult to incorporate into optimization strategies because it is an “NP hard” problem and the calculation would be time prohibitive. Fortunately, one MLC vendor has proven that the MLC hardware and control software can be designed such that the pure number of MLC segments is not a limiting factor anymore. We therefore use the number of MUs as the measure of complexity. Mathematically, the number of MUs is given by the sum of positive gradients of the intensity map along the direction of leaf motion. We include the sum of positive gradients as a linear objective function in our plan optimization framework. In our approach we find solutions that are Pareto optimal, i.e., their complexity cannot be further reduced without compromising conformality. The results are incorporated into an interactive plan navigator by means of a “complexity slider”, which allows the user to interactively explore the impact of a change of the plan complexity on the dose distributions. Results: Examples of optimized IMRT plans in the head and neck region and in the prostate will be presented. The potential of reducing complexity is case dependent but in comparison with current IMRT planning approaches a substantial reduction of the number of MUs (in the order of 20%) is often achievable without compromising the dose distribution. The complexity slider allows one to explore the whole spectrum of plans including the extrema, i.e. simple but not conformal or highly conformal but complex plans. Conclusion: The complexity of an IMRT plan can be limited or reduced with various methods. In general there is a price to be paid for this. Multi‐objective optimization can help to find the most suitable tradeoff. Supported by the NCI (1 R01 CA103904)
TU‐C‐T‐617‐03: A Geometrical Method for Lesion Placement in Functional Radiosurgery
Authors: Cernica G , Podgorsak M , Plunkett R , Kuettel M , de Boer S
Published: June 2005

Purpose: To develop a rigorous mathematical method enabling precise navigation through stereotactic space to functional radiosurgery targets not visualized by any imaging modality. Method and Materials: In radiosurgery for functional disorders, Schaltenbrand‐Wahren scaling formulas are often used to identify the appropriate target relative to imaged brain anatomies. Stereotactic frame alignment relative to internal brain anatomy is therefore critical, and, consequently, the success of the treatment can be limited by frame misalignments. A mathematical method through which one can define directions in the brain was developed. Such a method enables accurate navigation to appropriate targets based on visible anatomic landmarks regardless of frame positions. The procedure was tested for GammaKnife thalamotomy, where the target is the ventrolateral nucleus of the thalamus. Small simulated targets were placed between three consecutive slices of a head phantom to provide CT‐visible anatomy consistent with the Schaltenbrand‐Wahren brain atlas. Scaling formulas were then used to navigate from the anterior and posterior commissures to the ventrolateral nucleus, where an additional marker was placed. Other markers were placed along the brain's midplane. A stereotactic headframe was then attached to the phantom four separate times: once with optimal alignment based on external anatomical markings, and 3 times with varying misalignments. The phantom was CT‐scanned (1 mm slice thickness) with each frame attachment. Results: Over all image sets, the average distance between the predicted ventrolateral nucleus and its imaged location was 0.64 mm (maximum of 0.95 mm with greatest frame misalignment), consistent with imaging uncertainty. Conclusion: It is difficult to align a stereotactic frame to internal anatomy based on external landmarks. A mathematical method independent of frame placement has the potential to improve the accuracy of functional radiosurgery and to lessen the importance of ideal frame placement. Furthermore, this method can be used to confirm lesion placement in functional radiosurgery.
TU‐C‐T‐617‐06: Importance of Pre‐Fraction Helical CT Isocenter Verification in Extracranial Stereotactic Radiosurgery
Authors: Hope A , Alaly J , Liu J , Deasy J , Bradley J , Drzymala R
Published: June 2005

Purpose: To quantify the impact of pre‐fraction helical CT isocenter verification vs. setup based on planning CT in fractionated extracranial stereotactic radiosurgery. Method and Materials: Treatment plans (Elekta PrecisePlan) and pre‐fraction isocenter verification helical CT scans for 12 patients (40 fractions) were recovered from treatment plan archives. All structures were contoured by a single physician at the time of treatment. Each plan was imported into a customizable treatment plan analysis suite (CERR). Using CERR, pre‐fraction isocenter verification CT scans were fused with the original treatment plan using the external body frame as a reference. The original planned dose distribution was then translated from original treatment plan isocenter to pre‐fraction verification isocenters in each fraction. Dose and volume parameters for pertinent structures were automatically extracted using both registration methods (planned or pre‐fraction scans) for the original treatment plan and for all subsequent fractions. All patients were treated using the pre‐fraction verified isocenter rather than pre‐calculated body frame fiducials as per our institutional policies. Results: GTV volumes on pre‐fraction CTs varied from original planned GTV volume (64%–203%, mean=101.8+/−26.5%) largely due to helical sampling of a mobile target. Using the external body frame as the only setup reference would have resulted in geographic misses (<80% coverage of 95% of GTV) in 7/40 (17.5%) fractions. Pre‐fraction isocenter verification resulted in improved D95 GTV coverage (88–102%, mean=99.3% +/−2.4%) with no geographic misses. Conclusion: The current RTOG protocol (0236) evaluating extracranial stereotactic radiosurgery does not require pre‐fraction CT tumor position verification. Our institutional policy is to verify isocenter/tumor position prior to each fraction via CT. Although helical scanning artifacts are present, pre‐fraction CT‐based isocenter verification may provide more consistent tumor coverage than setup to planned body frame fiducials. Conflict of Interest: Support for this research was provided in part by Elekta, Inc.
TU‐C‐T‐6E‐01: Accurate IMRT Verification with Fluoroscopic EPIDs: A Generally Applicable Method Based On Direct Imaging of Cross‐Talk Kernels
Authors: Franken E , de Boer H , Heijmen B
Published: June 2005

Purpose: CCD camera‐based electronic portal imaging devices (CEPIDs) are well suited for absolute dosimetric measurements and are applied routinely for IMRT verification in our institute. CEPID dosimetry is mainly hampered by cross‐talk, which originates predominantly from optical photons reflected within the EPID structure. Previously, this cross‐talk was corrected for based on specific assumptions. However, the sensitive cameras in our present CEPIDs allow for direct imaging of the low‐amplitude, spatially extended cross‐talk kernels. Thus complete, position dependent characterization of the cross‐talk has become feasible. Here, we investigate the accuracy with which portal dose images (PDIs) can be derived from CEPID images (EPIs), using this cross‐talk characterization. Method and Materials: Cross‐talk kernels were obtained from EPIs of 2×2 cm2 ‘pencil’ beams, centered on a 30‐position grid covering the EPID sensitive area. In the experimental set‐up and post‐processing, care was taken to exclude contributions of low‐magnitude long‐range incident MeV‐radiation to the kernels. By interpolation of the measured kernels, a high resolution kernel‐database was generated. Using this database, PDIs can be derived from EPIs by iterative deconvolution of the cross‐talk contribution. Absolute PDI calibration was based on EPIs and corresponding PDIs, obtained by scanning a linear diode array (LDA), for two reference fields. To test the accuracy of this method, we compared EPI‐derived PDIs with LDA‐based PDIs for a series of symmetric and asymmetric fields, including Alderson‐phantom thorax and IMRT fields. Results: The measured cross‐talk kernels were asymmetrical and position‐dependent. PDIs derived from EPIs with these cross‐talk kernels matched with LDA‐based PDIs to within 1% (1 SD) for all investigated fields (including IMRT), both inside and outside the field penumbras. Conclusion: We have developed a generally applicable method to determine position‐dependent cross‐talk kernels via direct imaging. With these kernels, we can derive accurate PDIs from EPIs that are well‐suited for dosimetric IMRT verification.
TU‐C‐T‐6E‐02: A Computational Algorithm for Independent Planar Dose Verification of IMRT
Authors: Lu B , Li J , Palta J , Liu C
Published: June 2005

Purpose: The purpose of this work is to develop a computational algorithm for independent planar dose verification of IMRT plans delivered with an MLC as part of the IMRT quality assurance procedure. Method and Materials: A pencil beam convolution method was adopted for the algorithm. The general scheme of the calculation consists of three parts. The first part calculates the delivered fluence in air based on the positions of the leaves and jaws specified in the RTP file. We employed a modified three‐source model to obtain the planar fluence at the detector plane based on the projected source integration on the source plane shaped by the combined positions of the jaws and MLC. In the second part, dose kernel in phantom was determined by fitting the convolution results with a series of square‐field profiles. Thirdly, the mid‐leaf and interleaf transmissions, rounded leaf end, and tongue‐and‐groove effects were considered to improve the accuracy of the algorithm. All of these effects were taken into account by semi‐empirical methodology and the parameters were obtained by fits to measured data. To verify the accuracy of the algorithm, we compared the calculation results with the MapCHECK measurements. Results: The accuracy of the algorithm was verified by comparing the calculations with the MapCHECK measurements for several head‐and‐neck cases. The passing rate for most of the cases is above 95% with the 2% in absolute dose or 2‐mm distance‐to‐agreement criteria. The agreement was better than the agreement between the results from a commercial planning system (Pinnacle3, version 7.0g) and MapCHECK measurements for all the cases tested. Conclusion: An algorithm has been developed for independent planar dose verification of IMRT plans as part of the IMRT quality assurance procedure. Our methods could be implemented on other kinds of machines with minor modifications.
TU‐C‐T‐6E‐03: Adapting the Victoreen 7200(TM) Device as a Tool for Daily QA of the Helical Tomotherapy Hi‐Art(TM) Unit
Authors: Detorie NA , Teslow T , Schultz J , Bell R , Ridenour V
Published: June 2005

Purpose: To provide a simple procedure for RTT staff performing daily QA of a Helical Tomotherapy Hi‐Art Unit™ using a Victoreen 7200 ™ ion chamber device. Method and Materials: Intended for use on conventional linacs, the Victoreen 7200™ consists of a flat paddle containing 10 ion chambers embedded 5 mm deep and spanning a 20cm × 20 cm field size. There are 2 perpendicular linear arrays containing 5 chambers each with a common center chamber. The 10th chamber is located diagonally away from the center and may be covered with a stainless steel attenuator. Firmware permits entry of baseline values, temperature and pressure for measurement corrections, and computationally provides beam output, symmetry, flatness, and energy constancy information with only one exposure. By diagonal alignment of this device with the green lasers of the Hi‐Art ™ Tomotherapy Unit, a single, fixed gantry angle (0) beam exposure of 30 seconds using a 5 cm jaw width provides radiation constancy information within established tolerances for daily QA. The device is covered with a 1 cm acrylic build up sheet and a 15 mm stainless attenuator is used for energy constancy. Measurements are made with the treatment couch lowered so the SSD to the top of acrylic sheet is 103.8 cm. The diagonal alignment permits monitoring of the 5 cm jaw width. With a built in RS232 port, this information is easily downloaded into software applications permitting QA analysis and tracking with pass/fail criteria. (Our information is downloaded into ARGUS™.) Results: Analysis of more than 20 data sets indicates reliable and reproducible device performance. The procedure is simple and information rich. Conclusion: The Victoreen 7200™ device is easily adapted to Helical Tomotherapy Daily QA. Conflict of Interest: A collaborative research relationship is currently being developed with TOMOTHERAPY, Inc.
TU‐C‐T‐6E‐06: First Clinical Experience with Pre‐Treatment and in Vivo IMRT Verification Using EPID Dosimetry
Authors: McDermott LN , Wendling M , Sonke J , Stroom J , van Asselen B , van Herk M , Mijnheer B
Published: June 2005

Purpose: The aim of this study was to verify the first IMRT prostate plans made in our clinic with a newly commissioned treatment planning system (TPS, Pinnacle 7.4f) using an amorphous silicon electronic portal imaging device (a‐Si EPID, Elekta iViewGT), both pre‐treatment and in vivo. Method and Materials: For pre‐treatment verification, the plans of 8 patients were re‐calculated on a polystyrene slab phantom. An in‐house developed back‐projection algorithm was used to estimate the dose distribution at the phantom/patient isocentric mid‐plane (perpendicular to the beam‐axis) with the EPID. Each plan was also validated at the isocentre with ionization chamber measurements. Separate fields were measured with film and EPID, with gantry angle =0°. The in vivo mid‐plane dose was estimated with the EPID for the first 3 fractions and weekly thereafter. γ‐evaluation was used to assess 2D dose distributions with criteria of 3% dose difference (of maximum dose) and 3mm distance‐to‐agreement. The evaluated area included all points within the 20% isodose line of each EPID field. Anatomy changes for in vivo measurements were assessed using cone‐beam CT acquired prior to each verified fraction. Results: For pre‐treatment verification, the dose distributions of EPID vs. plan and EPID vs. film agreed within 3% or 3mm for 99.2% and 100% of points, respectively. The average ratio of the measured and planned isocentre dose was 0.987 ±0.003(SD) for ionization chamber and 0.997 ±0.009(SD) for EPID. For the in vivoyfields, 96.7% of dose points were in agreement. Examples of discrepancies were due to variation in gas pockets during treatment and problems calculating the dose distribution in a small area of overlapping segments (1cm2). Conclusion: These results show that an a‐Si EPID can be used to accurately verify IMRT prostate treatments in the mid‐plane of the phantom or patient, both pre‐treatment and in vivo.
TU‐D‐I‐611‐01: A General Method for Micro CT System Calibration with Phantom Scan
Authors: Yang K , Miller D , Boone J
Published: June 2005

Purpose: System calibration of a micro CT scanner for anatomic imaging of small animals. Method and Materials: A phantom was constructed with lead ball bearings (200 μm in diameter) and scanned with a micro CT scanner designed and built in our laboratory. A general mathematical algorithm was developed to locate the X‐ray focal spot and measure the source to detector distance (SID) and the azimuthal rotation of the detector with information acquired from the projection image of the phantom. A wire phantom (0.070mm Ni‐Cd) was scanned to calibrate the angle measurement of the stepping motor which drives the rotating stage of the scanner, and also to measure the source to iso‐center distance (SIC). An array of different objects was scanned and reconstructed with these calibrated parameters to evaluate the calibration result. Results: With the calibration procedure described, critical system parameters required for cone beam reconstruction including SIC, SID, x‐ray focal spot location, detector rotation angle, and stage rotation angle were measured with high precision. Subsequent phantom testing demonstrated the accuracy of this calibration method by qualitative artifact assessment, and spatial resolution metrics (MTF) also demonstrated excellent performance. Conclusion: A simple but precise method for system calibration of a cone beam micro CT system using flat panel detector was developed. This method is likely useful for CT systems across different scales (from micro CT to human CT).
TU‐D‐I‐611‐02: Volumetric Cone Beam Reconstruction Engines for 4D Image Guidance
Authors: Maltz J , Gangadharan B , Hristov D , Svatos M
Published: June 2005

Purpose: Limited angle tomography (LAT) may be used to rapidly reconstruct longitudinal slice images from cone beam datasets. These images are useful for image guidance in radiotherapy applications. By sampling a limited angular range (AR) imaging time is reduced, particularly in prospectively gated studies. We compare the LAT performance, as a function of AR, of the standard FDK algorithm and the simultaneous algebraic reconstruction technique (SART). We propose an imaging protocol to minimize dose and imaging time, while providing image guidance. Method and Materials: We employ LAT reconstruction (LATview) to produce high contrast images of a target tumor in the presence of large amounts of overlying and underlying anatomy. The performance of the SART and FDK s compared for ARs of 15–60 degrees. Results: We demonstrate using a 4D clinical thorax dataset that a lung tumor may be well delineated from within the ribcage along longitudinal planes utilizing as few as 8 projections over 15 degrees of arc using FDK. Assuming an average patient respiration rate of 0.3Hz, gated imaging is completed within 24 seconds. This constitutes an 8‐fold reduction in imaging time when compared to a full AR tomographic acquisition involving 60 projections over 200 degrees. Conclusion: LATview offers significant speed advantages over full angular sampling for 4D studies. It is also more versatile than techniques of digital tomosynthesis as it is able to simultaneously generate multiple image planes of the target volume. Both FDK and SART appear useful for image guidance. Although the SART images subjectively provide more detail, this detail may reduce the tumor‐background contrast (TBC). As a result, TBC is higher for FDK for the lowest AR (15 degree) acquisitions, while SART performs better in terms of TBC for ranges of more than 30 degrees. Conflict of Interest: Supported by Siemens Medical Solutions USA, Inc.
TU‐D‐I‐611‐07: A Scanning Sampled Measurement (SSM) Technique for Scatter Measurement and Correction in Cone Beam Breast CT
Authors: Liu X , Shaw C , Altunbas M , Wang T
Published: June 2005

Purpose: To implement and investigate a scanning sampled measurement (SSM) technique used to obtain sampled scatter measurement for scatter correction in cone beam breast CT. Method and Materials: Breast phantom or specimen is mounted on the rotating stage in a stationary gantry experimental cone beam CT system. A slightly tilted 2‐D array of 1.2‐mm diameter lead beads, with the beads 1 cm apart from each other, was placed between the object and x‐ray source. A series of projection images were acquired as the phantom is rotated (1 degree per projection view) and the lead beads array shifted by 1.2‐mm from one projection view to the next. Image signals in the lead bead shadow were used to obtain sampled scatter measurements which are then interpolated to form an estimated scatter distribution. The image data in the lead bead shadows are restored by interpolating image data from the two adjacent projection views to form complete (lead bead free) projection images. The estimated scatter distribution is then subtracted from the corresponding restored projection image to obtain the scatter corrected projection images. Results: Sampled scatter measurements were successfully made in each projection image and the accuracy of scatter measurements was verified with a larger beam blocker placed between the lead beads and x‐ray source. The SPRs in the projection images of a breast phantom are found to range from 0.1 to 0.5. Using scatter distribution interpolated from scanning sampled measurements and restored projection image data, scatter correction was successfully performed on all projection images. The resulting scatter corrected projection image data resulted in more accurate reconstruction of the linear attenuation coefficients and reduced the cupping effects. This work is supported in part by a research grant EB000117 from the NIBIB & Bioengineering and a research grant CA104759 from the NCI.
TU‐D‐J‐6C‐01: A Comparison Between Amplitude Sorting and Phase Sorting Using External Respiratory Measurements for 4D CT
Authors: Lu W , Parikh P , Bradley J , Low D
Published: June 2005

Purpose: To compare amplitude sorting and phase sorting techniques using external respiratory measurements for 4D‐CT for patients undergoing quiet breathing. Method and Materials: We have developed a 4D CT technique for mapping respiratory motion in radiotherapy treatment planning. A 16‐slice CT scanner was operated in ciné mode to acquire 25 scans consecutively at each couch position. The scans were sorted into 12 respiratory‐windows based on the amplitude and direction (inhalation or exhalation), and on the phase (0–360°) of a synchronized external respiratory measurement. An air content measure (the amount of air in a 16‐slice CT segment, used as a surrogate for internal motion) was correlated to the respiratory amplitude and phase throughout the lung. Images reconstructed based on the two sorting techniques were displayed for a qualitative comparison. Also, the variations in the amplitude of the respiratory measurement during the entire scan session were compared using 8, 12, 24, and 48 respiratory windows. Results: The air content showed a higher correlation with the respiratory amplitude than with the respiratory phase for most cases. Images reconstructed based on the amplitude sorting technique displayed fewer artifacts, especially at the lung‐diaphragm boundaries, than images reconstructed based on the phase sorting technique. The variations in the respiratory amplitude were much smaller with amplitude sorting than those with phase sorting. These variations decreased significantly with finer amplitude respiratory windows while showed insignificant changes with finer phase respiratory windows. Conclusion: The amplitude sorting was generally better than phase sorting, especially for patients whose breathing was less reproducible. The use of finer respiratory windows did not improve the consistency for phase sorting. Keywords: 4‐D CT, respiratory sorting, motion, radiotherapy.
TU‐D‐J‐6C‐03: Image Segmentation in 4D CT Based On a Deformable Image Registration Model
Authors: Xing L , Schreibmann E , Yang Y , Boyer A , Li T
Published: June 2005

Purpose: While 4D CT provides a powerful tool to study the anatomy change caused by respiration, a hurdle in fully realizing its potentials is how to segment the involved organs efficiently for all breathing phases. Here we investigate a strategy of using deformable image registration for mapping the contours delineated at a known phase to all other phases of the 4D CT images. Method and Materials: 4D CT image sets for four patients (one esophageal and three lung patients) were acquired by using a GE LightSpeed‐QX/I scanner. Ten phase bins were used in the data acquisition. The involved organs were delineated by a physician on the 3D image sets corresponding to the inhale phase point. The images at different phases were registered using a BSpline deformable model implemented in the VTK/ITK platform. The contour points on the inhale phase were automatically mapped to the corresponding points on the images of other phases following the mapping relation established by the deformable model. The performance of the auto‐segmentation tool was evaluated by comparing directly with the contours outlined by the physician on selected phases. Results: The deformable model was capable of accommodating significant variability of structures over time and across different individuals. In all patients, the mean measured error was 1mm for clearly differentiated organs such as the lungs, and 2 mm for medium‐sized organs like heart. A maximum error of 4 mm was observed locally for noisy or binning‐artifact degraded voxels. Additionally, the method was used to deduce tumor path at any phase, permitting close tailoring of the margins of the tumor targets in the presence of respiration. Conclusion: Contour evolution in 4D images can be easily defined and tracked using a BSpline deformable model, with no user interaction required. The method provides millimeter‐accuracy while eliminating the labor‐intensive segmentation procedure.