bluuebunny/crossref_metadata_2025 huggingface dataset:
TU‐D‐M100F‐02: Automated Quality Assurance for Helical Tomotherapy Using Exit Detector Data
Authors: Ramsey C , Seibert R , Outten S , Robison B
Published: July 2007

Purpose: The purpose of this work was to develop techniques for utilizing exit detector data in a helical tomotherapy system to automate the QA process. The clinical significance of this study is that an analysis of the exit detector data acquired during treatment delivery could be used to ensure that the correct delivery sequence is being administered to the patient. Method and Materials: Software applications were developed to automatically analyze uncompressed detector data. To test this software, a MLC test sequence was designed that allows each MLC leaf to be tested using the exit detector data. The goals of this test are 1.) To identify MLC problems (stuck leafs, bad valves, etc…) before failure, and 2.) Perform QA Tests on MLC parameters that affect the delivered dose. Additionally, a clinical test case was created using the treatment delivery sequences for a head & neck patient. The original treatment delivery sequence was modified and 12 known MLC errors were inserted in the MLC controller file. The procedures were delivered and the software was used to analyze the exit detector data. Results: The MLC QA Test was delivered on four occasions with two MLCs. A software application developed by the investigators was then used to analyze the exit detector data from these deliveries. The software correctly performed Latency Tests, Projection Centering Tests, and MLC Transit Time Tests. For each delivery, the tests showed that the MLCs were properly functioning. For the Head & Neck test case, the shape‐detection algorithm was able to identify 11 out of 12 known MLC errors >10 msec. Conclusion: A technique was developed for performing automated QA of the MLC and individual patient deliveries using exit detector data in a helical tomotherapy system. With the tomotherapy detector array, errors in MLC position > 10 msec have been detected.
TU‐D‐M100F‐03: Tracing the Sources of Absolute Dose Errors in IMRT Plan Verifications
Authors: Naqvi S , Sun M , Yi B , Lasio G , Ha J
Published: July 2007

Purpose: Several years since the advent of IMRT, most institutions still perform experimental plan verifications with film and ion‐chamber. In the past year of measurements of Pinnacle3 IMRT plans in our clinic, most isocenter dose measurements were verified to be within 3%, but the 10% that were off by 4–7% were repeated or required MU adjustments. In this work, we trace the main sources of discrepancies by measurement and simulation. Methods: We used an independent dose calculation engine (MCKS) that performs kernel‐based superposition using Monte Carlo sampling of photons, interaction points, and corresponding monoenergetic kernels. The photon source for the simulations is a single phasespace plane scored just above the collimator and generated by an EGS/BEAMnrc code. Even though MCKS models the detailed MLC design (curved ends, tongue‐and‐groove, inter‐leaf gap, and leaf offsets) the MLC material density is fine‐tuned with the measurement of “uniform” intensity map delivered as sequence of narrow strips, which amplifies the leakage dose several fold. Results & Conclusion: Using the code and measurement, we found that a major source of error in IMRT plans from Pinnacle3 is the handling of MLC leakage. For example, if a 1.8% MLC transmission is used, then delivering a 10×10 field with ten 10×1 strips produces a dose overestimation by 12% on the central axis. This reduces to 8% if a 1.5% value is used. In typical IMRT plans, the leakage error is less dramatic, but can frequently exceed the 3% acceptance criterion. Also, no single transmission value is suitable for all possible IMRT fields because MLC scatter varies with field size and shape. Some errors also result from the use of lumped empirical square field output factors to complex IMRT segments without regard to the energy or direction of head scatter photons.
TU‐D‐M100F‐09: Breathing Motion‐Induced Dose Delivery Error Evaluations as Applied to Tomotherapy Dose Delivery
Authors: Chaudhari S , Rangaraj D , Goddu S , Malinowski K , Lu W , Parikh P , Low D
Published: July 2007

Purpose: To develop a method for evaluating breathing motion‐induced dose delivery errors in Tomotherapy dose delivery. Methods and Materials: Dosimetric inaccuracy can result from breathing‐induced tumor motion in Tomotherapy treatment delivery. Patient breathing motion patterns were simulated using quantitative spirometry‐measured patient tidal volumes and converting the tidal volume to tumor motion by varying the ratio of tumor motion to tidal volume for 34 patients. Simulations of Tomotherapy deliveries were conducted modifying the previously published techniques by using measured beam profiles instead of step‐function fluences, and couch speeds typical of Tomotherapy treatments. Radiochromic film and our in‐house 4D phantom were used to verify the algorithm. Results: As expected, the breathing motion blurred the dose distributions, but slow drifts in the average tissue position caused detectable dose errors. The dose errors were expectedly largest with the smallest (1.0cm) field size, and could be >10% for motion amplitudes comparable to the field size. As the field width increased relative to the motion amplitude, and as couch‐speed (pitch) decreased, the error also decreased, and as such these settings may be preferable for patient treatments. These slow drifts occurred over time periods that were coincident with the amount of time required for the field to pass a stationary point. Measurements agreed with the simulation. Conclusions: Previous breathing motion studies did not use real patient breathing patterns and therefore did not consider the impact of slow, relatively small drifts in those patterns. The drifts change direction during the breathing measurement, causing dose errors that are both positive and negative. While the individual dose fraction errors can be >10%, they are unlikely to occur in the same place each day, so the average dose is likely to be consistent with earlier studies. Conflict of Interest: This work supported in part by a grant from Tomotherapy, Inc.
TU‐EE‐A4‐02: Exposure Index Calibration for a Portable DR Detector — A Practical Guide for the Clinical Medical Physicist
Authors: Rauch P
Published: July 2007

Purpose: Establish a method for calibrating and utilizing the exposure indices (EXP, REX) of a Canon portable DR imaging system. Method and Materials: The Canon model CXDI‐50 was investigated in order to understand the nature of the reported EI's, to establish a calibration procedure, and to verify the EI adequacy as an indicator of the detector exposure for clinical procedures. Canon reports two EI's: “REX”, which changes with image processing settings and “EXP” which is independent of image processing changes. However, there was very little documentation of the EXP parameter and no associated calibration procedure. The effect of an undocumented adjustment parameter (“Constant for Exposure Index”) was investigated and ultimately served to calibrate the EXP value for a standard RQA6 beam (80kVp, 27 mmAl). Using an ionization chamber the exposure at the input to the detector was measured as a function of the reported values of EXP and REX for the RQA6 beam, for 70 kVp, 0.5 mmCu+2 mmAl (TG116), and for several combinations of kVp and acrylic attenuator. Results: Both REX and EXP linearly increase with exposure (different slope). REX is associated with the density on a printed film and thus changes with window and level settings. This makes REX a poor indicator of exposure adequacy. The “constant for exposure index” has no affect on REX. To achieve the same EXP value as obtained with RQA6, the detector dose must be increased by 20% for 80kVp, 8″ acrylic, 31% for 120kVp, 6″ acrylic, and 43% for 60kVp, 6″ acrylic. Conclusion: The information presented will be particularly useful to a medical physicist desiring to establish the calibration of EXP, to determine the appropriate target EXP for a particular body part and view, and to calibrate a phototimer to be utilized with the Canon DR system.
TU‐EE‐A4‐05: Comparison of Backprojection and a Penalized Maximum Likelihood Algorithm for Detection of Microcalcifications in Breast Tomosynthesis.
Authors: Das Mini , Feng B , Gifford H , Glick S
Published: July 2007

Tomosynthesis is proving to be a valuable tool for generating three dimensional images of the breast with only a limited number of projection angles. The image quality of the resulting tomosynthesis slices can potentially be improved by using more optimal acquisition geometries and/or improved reconstruction techniques. In this paper we compare the performance of a standard back‐projection (BP) algorithm with an algorithm that maximizes a penalized likelihood (PML) objective function using an optimization transfer principle leading to a simultaneous update algorithm. To compare reconstruction algorithms, a computer simulation is used to model the breast tomosynthesis geometry and human observer performance in detecting micro‐calcification clusters is evaluated. The simulation modeled the compressed breast using a structured breast phantom, with randomly inserted clusters of spherical “micro‐calcifications”. Clinically realistic x‐ray spectra were generated and x‐ray transport through the breast phantom was modeled using ray‐tracing combined with the focal spot and the detector blur of a 200 micron CsI scintillator. The iterative reconstruction used a simultaneous update algorithm where the non‐quadratic penalized likelihood objective function (which is difficult to maximize) is replaced by a surrogate paraboloidal function. A Huber prior was used as the potential function with the ability to control edge preserving factors and smoothness. A standard back‐projection technique that is computationally faster (in comparison to the iterative method) was used as a comparison. To evaluate microcalcification detection accuracy of the methods, an N‐alternative forced choice (NAFC) based observer study was performed. The results of NAFC studies performed with 3 observers and 63 pairs of images for each method shows that the PML based technique gave a percent correct improvement by 0.32 over the FB technique. The observers read the PML based images about 4 times faster than the FB technique indicating higher visibility of calcifications in the former.
TU‐EE‐A4‐06: Experimental Evaluation of Effective Detective Quantum Efficiency for Digital Radiographic Imaging Systems
Authors: Ranger N , Mackenzie A , Honey I , Dobbins J , Ravin C , Samei E
Published: July 2007

Purpose: To develop and evaluate an experimental methodology for measuring the effective detective quantum efficiency (eDQE) of digital radiographic systems which reflects the actual signal‐to‐noise performance of the system per unit exposure. Method and Materials: A NEXT phantom, simulating the scatter and attenuation properties of an adult human thorax was used to measure the resolution, noise, and scatter performance of a digital radiographic system (GE xQi) under conditions approximating those seen in clinical chest radiography. The resolution was measured in terms of the modulation transfer function (MTF) using an edge device placed at the phantom surface closest to the x‐ray tube. The noise was measured in terms of the noise power spectrum (NPS) of the region corresponding to the phantom center, acquired at three exposure levels. The scatter fraction (SF) was evaluated using a beam‐stop technique. These measurements along with measures of phantom attenuation and estimates of x‐ray flux and exposure were incorporated in the computation of the effective Detective Quantum Efficiency (eDQE). Results: The phantom exhibited a broad‐beam transmission fraction of 18.65%. The measured scatter fraction in the presence of grid and phantom was 33%. The MTF of the system dropped by 25% at 1.0 cycles/mm when the edge was placed at the phantom surface due to scatter and focal spot blurring. The computed eDQE was assessed to be 0.038 and 0.028 at 0.5 and 1.0 cycles/mm, respectively (for E= 5.6 mR). Conclusion: Conventional DQE measurements performed under relatively idealized conditions do not accurately represent the relative performance of digital radiographic imaging systems in routine clinical use. A more appropriate metric, the eDQE, measured under conditions that reasonably approximate those encountered clinically reflects the additional contributions from scatter, grid, and focal spot blurring, and provides a better estimate of the relative clinical performance of digital radiographic imaging systems.
TU‐EE‐A3‐02: Registration of Ultrasound Tissue‐Typing Images with CT Images for Image‐Guided Prostate Cancer Radiation Therapy
Authors: Zhou J , Zhang P , Schiff P , Kutcher G , Liu T
Published: July 2007

Purpose: To develop an image registration scheme, which combines both rigid and deformable registration techniques, to map the ultrasonic tissue typing (UTT) images to the computed tomography (CT) images for tumor targeted prostate IMRT treatment. Method and Materials: UTT spectrum analysis can identify the cancerous regions inside the prostate to guide prostate radiation therapy. Current IMRT treatment plans for the prostate are designed on CT images. We developed a rigid registration followed by a deformable registration scheme for tumor region mapping between CT and UTT images. The rigid registration was achieved by adjusting the relative position of the two image sets until the mutual information between them was maximized. The deformable registration was based on a biomechanical model and finite element method (FEM). The prostate from both image sets was outlined by the clinician and the surface deformation between the two contour sets were fed into the FEM software algorithm to derive the volumetric displacement inside the prostate. When the voxel points correspondence between two image sets were known, the tumor area detected with UTT was mapped onto the CT images and used for 3D planning for dose escalation. Results: The algorithm was validated using a tissue mimicking deformable prostate phantom and ten prostate specimens. The urethra served as a marker for verifying the registration process. For phantom and ex vivo study, the displacement of the urethra matched well between the CT images and the deformed ultrasound images. The distribution of the 2D matching errors for the urethra central point had a mean and standard deviation of 2.0mm ± 1.1mm. We started to apply our registration scheme using in vivo prostate scans.

Conclusion: We validated our registration scheme with our phantom and ex vivo studies. We demonstrated the feasibility of clinically employing this UTT method for intra‐prostatic tumor dose escalation.
TU‐EE‐A3‐05: Assessment of Treatment Site‐Specific Setup Accuracy and Reproducibility in Patient Positioning Using Daily MVCT Imaging
Authors: Schubert L , Paliwal B , Westerly D
Published: July 2007

Purpose: To assess treatment site‐specific corrections in patient positioning from TomoTherapy Hi‐ART® megavoltage images. Method and Materials: Initial positioning corrections, determined by fusion of daily pre‐treatment MVCT to planning CT images, were analyzed for different anatomical treatment sites.Over 600,1400, and 1200 fractions of head and neck (H&N) and brain, lung, and prostate tomotherapy treatments, respectively, were assessed. Translational and rotational, per‐fraction setup corrections were retrospectively compiled from individual patient archive files. Setup corrections were compared amongst anatomical treatment sites. Setup reproducibility was assessed by analyzing standard deviations and histograms of setup corrections. Setup accuracy was also assessed by analyzing 3D vector lengths and magnitudes of corrections. Results: Large variations in setup corrections were seen in all three disease sites. H&N treatments had a significantly smaller (p<0.001) vector length standard deviation of 2.54 mm compared with 6.99 mm and 6.46 mm for lung and prostate treatments, respectively, but had no significantly different standard deviation in roll rotations. The frequency of translations of vector lengths ⩾ 10 mm were 48.9% and 48.1% of all lung and prostate treatments, respectively, whereas corrections of that magnitude occurred in only 1.56% of H&N treatments. Frequencies of roll rotations were more comparable among the three disease sites. Conclusions: Analysis of patient positioning corrections indicates large variations between patients and treatments, suggesting a role for imaging every patient per‐fraction to ensure the most accurate reproducibility. H&N treatments had a higher setup reproducibility and accuracy in the lateral, longitudinal, and vertical directions. Even so, they were not significantly different rotationally than prostate and lung treatments, suggesting a role for daily imaging in all three treatment sites. In quantifying site‐specific positioning reproducibility and accuracy, this work may be useful for assessing new treatment planning margins for image‐guided procedures or for developing adaptive radiotherapy techniques.
TU‐EE‐A1‐03: Dosimetric and Clinical Benefits of Image Guided Patient Setup for Hypofractionated Radiotherapy (HFRT)in Lung
Authors: Guan Y , Della Biancia C , Chang J , Yorke E , Rosenzweig K , O'Meara W , Amols H
Published: July 2007

Purpose: To estimate the clinical advantages of reduced field margins made possible by image guided patient setup and monitoring. Method and Materials: The limitations in patient setup using skin marks and the uncertainties of intrafractional motion are well known. In this study, kV cone‐beam images were taken before and after treatment on 10 lung patients (1–3 fractions/patient). Pre and post treatment images were registered according to bone and to tumor. In 4 of 10 patients we observed GTV shifts, intrafractional motion, and/or differences between bony and tumor registrations exceeding 5mm and as large as 16mm. These 4 patients were retrospectively replanned using GTV‐PTV margins of 5, 10, 15, and 20 mm, on the assumption that Image Guidance (IG) enables such reductions compared to conventional patient setup. The same optimization constraints and beams were used as in the original plans. Lung V20, dmean (mean physical dose), fdam (functional damage model), and NTCP were calculated for HypoFractionated (HF) prescriptions of 12Gy×4f, 20Gy×3f, and 30Gy×1f. Results: For a small tumor in the lateral apex, little clinical gain was found by reducing field margins from 20 to 5 mm even for the highest prescription doses with dmean <5Gy, V20<8%, fdam<0.09, and NTCP<0.25 for all treatment plans. For a larger tumor near midline even a modest HF dose (12Gy×4f) resulted in dmean of 24Gy and fdam=0.43 for 20mm margins. These parameters could be reduced to 10 Gy and 0.23 respectively with IG and 5mm margins. Conclusions: The use of IGRT for HFRT in lung is beneficial to some but not all patients.For small peripheral tumors reduced field margins made possible by IGRT result in real but clinically minimal benefits. For larger, more mobile tumors, the benefits of IG setup and reduced field margins enable significant dose escalation, making more patients eligible for such treatments.
TU‐EE‐A1‐05: Using a Commercial MOSFET System for TBI in Vivo Dosimetry: Characterization, Calibration and Mid‐Plane Dose Calculations
Authors: Cranmer‐Sargison G , Lapointe C
Published: July 2007

Purpose: The goal of this work was to: (1) analyze the response characteristics of a commercial MOSFET system, (2) develop an accurate MOSFET calibration convention and (3) develop a TBI mid‐plane dose calculation method that relies only on entrance dose measurements. Method and Materials: Using a nominal 6 MV photon beam, MOSFET response reproducibility, linearity and stability were analyzed. A calibration coefficient was calculated for each detector using a method that relates MOSFET and ion chamber response in an acrylic jig to dose to water at an equivalent depth — calibration coefficients were verified for all MOSFETs by measuring the dose in water at various depths. We derive a simple method of calculating TBI mid‐plane dose that relies only on MOSFET entrance dose measurements. The method was tested by calculating the mid‐plane dose to a 30×30×30 cm3 water tank and an anthropomorphic phantom. Results: The detectors showed a reproducibility of ±2.0% for a dose to water at dmax greater than 65.0 cGy. Response linearity was excellent and a correlation between measurement reproducibility and detector response established. However, detector response did decrease as a function of accumulated dose. The dose to water at a depth of 10 cm was verified by ionization chamber measurement to be 67.3 cGy — the mean response of the 15 MOSFETs was 68.0 ± 2.0 cGy. Dose to the mid‐plane of the water tank was confirmed with an ion chamber to be 50.0 cGy — using the formalism set out in this work the dose was calculated to be 51.6 cGy. For a prescribed 100.0 cGy the mid‐plane dose to the anthropomorphic phantom was calculated to be 99.0 cGy. Conclusion: The unique calibration convention and TBI mid‐plane dose calculation methods were found to be accurate and easily implemented as part of any clinical TBI program.
WE‐C‐L100J‐06: Linear Systems Analysis for a New Solid State X‐Ray Image Intensifier (SSXII) Based On Electron‐Multiplying Charge‐Coupled Devices (EMCCDs)
Authors: Kuhls A , Yadava G , Patel V , Bednarek D , Rudin S
Published: July 2007

Purpose: To objectively evaluate and optimize performance of a new SSXII using linear‐systems analysis with varying constituent elements. Method and Materials: An imaging module of the SSXII consists of a fluorescent phosphor, a minifying fiber‐optic taper (FOT), and a fiber‐optic plate (FOP) coupled directly to the sensor of an EMCCD camera. An array of such modules is used to achieve the desired field‐of‐view (FOV). Linear‐systems analysis was applied to a module to determine system performance for various combinations of components whose properties were estimated using known physical constants and manufacturer specifications. A 350μm thick CsI(Tl) structured phosphor deposited on an additional FOP was considered in this analysis; however the phosphor may be grown directly on the FOT for improved optical transfer efficiency. New back‐thinned sensors, offering high optical quantum efficiencies (>90%), are also considered. Various FOT minifications were studied to evaluate the tradeoffs between a larger field‐of‐view (FOV) per module and potential DQE degradation. Results: Initial calculations indicate elimination of a FOP in the imaging chain could improve the integral DQE by 30–80%, depending on the FOT minification. Use of a back‐thinned sensor could offer additional improvements of 10–25% over a front‐illuminated EMCCD. Increasing the FOT minification factor from 3:1 to 6:1 would tend to decrease the integral DQE by ∼50%, which could be more than compensated for by making the above two design improvements. Calculated MTFs and DQEs will be presented for various minifications, exposures, and gains. A prototype SSXII (16μm pixels) demonstrated 20 lp/mm bar‐pattern resolution radiographically and fluoroscopic image sequences with exposures of <3 μR/frame. Conclusion: The linear‐systems analysis indicates current designs for the SSXII will provide high‐resolution, low‐noise, real‐time imaging. With component optimization, an acceptable DQE can be maintained with taper minifications as large as 6:1.

(Partial support: the UB Foundation and NIH grants R01‐EB002873, R01‐NS43924.)
WE‐C‐L100J‐07: Investigation of Fourier‐Based, Hypothesis‐Testing Detection and Discrimination Tasks in Dual‐Energy Imaging
Authors: Richard S , Siewerdsen J , Tward D
Published: July 2007

Purpose: To relate theoretical descriptions of detectability index to imaging performance as assessed by real observers for a variety of detection and discrimination tasks. Dual‐energy (DE) imaging provided a foundation for the research, incorporating a variety of decomposition techniques that significantly affect the noise‐equivalent quanta (NEQ). Method and Materials: The NEQ was derived using cascaded systems analysis, extended to DE imaging with a flat‐panel detector. Three idealized task functions were considered: sphere detection, shape discrimination (sphere vs. disk), and texture discrimination (uniform vs. crenellated disk). Detectability index was derived by integrating the task function over the NEQ. These idealized tasks were physically emulated with a chest phantom. DE images were acquired over a range of conditions and noise‐reduction techniques, and the performance of real observers was assessed in rank‐order and alternative‐forced‐choice (AFC) tests. The degree to which theoretical detectability index correlated to human observer performance was investigated. Results: Decomposition technique profoundly affected the NEQ. Theoretical detectability index agreed qualitatively with human observers. In DE soft‐tissue images, noise reduction (e.g. anti‐correlated noise reduction) had a minor effect on sphere detection, but a significant effect on shape and texture discrimination tasks, consistent with a factor of ∼2 increase in detectability. In DE bone‐only images, noise reduction improved the detectability index for all tasks, particularly for detection of small spheres and texture discrimination. The results were consistent with phantom images where, for example, a 1.6 mm sphere became conspicuous (Az∼1.0) upon application of simple‐smoothing on the high‐kVp image. Conclusions: Analysis of DE imaging performance provided a useful probe of the complicated interplay between frequency‐dependent NEQ and various imaging tasks, evident in both theoretical and real observer studies. The results provide initial validation that theoretically derived detectability index represents a meaningful surrogate for observer performance and is a useful objective function for system optimization.
WE‐C‐L100J‐10: Three Dimensional Vessel Centerline Reconstruction and Imaging System Calibration From a Single Vessel in Angiograms
Authors: Walczak AM , Hoffmann KR , Singh V , Dashkoff N , Kassab M , Iyer VS
Published: July 2007

Purpose: Three dimensional (3D) vessel reconstructions can be useful in assisting clinicians with diagnosing and treating vascular disease, providing information about vessel diameter, length, and tortuosity. Reconstruction of 3D vessels from two views requires knowledge or calibration of the geometry relating the two imaging systems. We introduce a geometry calibration technique that performs this calibration using only a single indicated vessel segment in the angiograms. Methods: Vessel segments of interest were indicated in two angiographic views. The initial imaging geometry is estimated using each view's gantry information, and corresponding points along the vessels in both views are determined using epipolar constraints. The 3D position of the vessel is determined by triangulation of the found corresponding points. The geometry is corrected by varying nine of the imaging system's parameters using the Nelder‐Mead Downhill Simplex Method, with an objective function that minimizes the distance between the reprojection of the reconstructed 3D vessel centerlines and the 2D indicated vessel centerlines in both views. Results were compared with the enhanced‐Metz‐Fencil (EMF) geometry correction technique, which requires identification of additional corresponding points in both views. Results: Variations in the shapes obtained from our single vessel technique (SVT) and the EMF were comparable, median RMS of 0.47 and 0.34 mm, respectively, with magnification variations of 2.2% and 0.7%, respectively. Median errors in 2D reprojections of the 3D data for our technique and the EMF were both 0.12 mm, indicating very good agreement with the 2D indicated centerlines. Conclusion: We have developed an imaging geometry correction technique for two views based on alignment of reprojected 3D data with its respective 2D image information using only a single indicated vessel segment. This technique is reliable and comparable to other geometry correction techniques requiring additional user input.

Supported by NIH grant ♯HL52567 and Toshiba Medical Systems Corporation.
WE‐C‐M100J‐02: Image‐Guided Photodynamic Therapy of Prostate Cancer
Authors: Zhu T
Published: July 2007

Purpose: This study analyzes the results of image‐guided prostate photodynamic therapy (PDT) that integrates PDT dosimetry, light source optimization, computerized light power adjustment, and volumetric real‐time light fluence calculation to deliver uniform photodynamic dose to the target volume (prostate) and spares the critical structures (rectum and bladder). Methods and Materials: All procedures are under the image guidance of transrectal ultrasound. The PDT dosimetry includes multi‐channel real‐time in‐vivo light dosimetry, absorption and fluorescence spectroscopy for 3D optical properties, drug concentrations, and tissue oxygenation. Drug concentration is also determined using fluorescence from a single optical fiber. These measurements are made before and after motexafin lutetium (MLu)‐mediated photodynamic therapy (PDT) using a computerized step motor. The light fluence rate distributions are also measured along the catheters during PDT and compared to the 3D volumetric light fluence calculations. Real‐time light fluence calculation was performed on the 3D target volumes using ultrasound image guidance. An optimization algorithm determines the light source strength, lengths, location, and retraction for cylindrical diffusing fibers (CDF) based on the 3D heterogeneous optical properties. The resulting light source power is feedback into a 12‐channel beamsplitter that is connected to a motorized attenuator system to control the light source intensity interactively during PDT. Results: Preliminary data have shown widespread heterogeneities of optical properties and photosensitizing drug distribution. As a result of these heterogeneities, methods to quantifying the three‐dimensional (3D) distributions of these quantities in individual prostate are essential for the successful application of PDT. Comparison of light fluence rate between real‐time measurements and calculation is performed in heterogeneous medium and the standard deviations are within 30% with a simplified model and better than 11% for an improved model. Conclusions: We have shown the rational and potential for an integrated system that is capable of obtaining critical parameters (light, drug, and oxygenation) and using the PDT dosimetry result as feedback to optimize treatment delivery. We concluded that a real‐time dosimetry and feedback system for monitoring PDT dose during treatment is both achievable and required for clinical interstitial PDT applications.

Educational Objectives:

1. To explain the basic principle of PDT dosimetry.

2. To review explicit PDT dosimetry techniques to characterize tissue optical properties, drug concentration, tissue oxygen concentration, and PDT efficacy.

3. To discuss the rational and requirement for a feedback system incorporating PDT dosimetry, PDT dose optimization, and computerized light delivery.

4. Summarize the clinical results of Prostate PDT.
WE‐C‐AUD‐01: Differences in Predicted Beam Range, Lateral Scatter and Absolute Dose Between Proton Monte Carlo and Pencil‐Beam Dose Calculation Considering Single Fields and Full Plans for Head and Neck Tumors
Authors: Paganetti H
Published: July 2007

Introduction: Monte Carlo based dose calculation is considered to be more accurate than analytical methods. We were interested in the potential impact of the clinical use of proton Monte Carlo dose calculation in particular in areas of frequent tissue interfaces and density variations, i.e. in the head and neck region. Methods: The Monte Carlo code Geant4 was used to simulate the treatment head geometry, patient geometry based on CT as well as treatment room setup and treatment plan parameters. Ten patient plans (para‐spinal tumors and tumoprs in the nasopharynx and para‐nasal sinus area) were analyzed in terms of dose distributions and dose‐volume histograms. Analysis was done considering individual fields and total plans. Results: In general, the Monte Carlo and the pencil‐beam algorithm agree very well in soft tissue in terms of the beam range and the lateral fall‐off. The 80%–20% fall‐off and the position of the respective isodose lines are typically within 1 mm. However, the agreement in range is not as good in the presence of large density variations in the beam path. Differences are pronounced if the end of range is within bony structure and downstream of thick bony inhomogeneities. Further, clinically significant discrepancies in range prediction are visible if density variations are parallel to the beam direction. By interpreting the results one has to take into account that the pencil‐beam algorithm reports dose‐to‐water while the Monte Carlo calculates dose‐to‐tissue. Further, our planning system only specifies relative doses while the Monte Carlo dose distribution is in absolute values. Conclusion: The pencil‐beam algorithm is less sensitive to geometrical complexities and frequent density variations, i.e. bone‐soft tissue, bone‐air or air‐soft tissue interfaces. Significant discrepancies can be seen in the case of inhomogeneities that are tangential to the beam.
WE‐C‐AUD‐03: Investigation of Fast Monte Carlo Dose Calculation for CyberKnife SRS/SRT Treatment Planning
Authors: Ma C , Li J , Deng J , Fan J
Published: July 2007

Purpose: Advanced stereotactic radiosurgery (SRS) and stereotactic radiotherapy (SRT) treatments require accurate dose calculation for treatment planning especially for treatment sites involving heterogeneous patient anatomy. In this work, we have implemented a fast Monte Carlo dose calculation algorithm for SRS/SRT treatment planning with the CyberKnife® system. Methods and Materials: Our system employs a superposition Monte Carlo algorithm. Photon mean free paths and interaction types for different materials and energies as well as the tracks of secondary electrons are pre‐simulated using the EGS4 code system. Photon interaction forcing and splitting are applied to the source photons in a patient calculation and the pre‐simulated tracks are repeated with proper corrections based on the tissue density and electron stopping powers. Electron energy is deposited along the tracks and accumulated in every voxel of the simulation geometry. Scattered and bremsstrahlung photons are transported, after applying the Russian Roulette technique, in the same way as the primary photons. Dose calculations are compared with full Monte Carlo simulations and the CyberKnife treatment planning system (TPS) for lung and head & neck treatments. Results: Comparisons with full Monte Carlo simulations show excellent agreement (within 0.5%). Significant differences in the target dose are found between Monte Carlo simulations and the CyberKnife TPS for SRS lung treatment. The calculation time using our superposition Monte Carlo algorithm is reduced up to 62 times (46 times on average for 10 typical clinical cases) compared to full Monte Carlo simulations. Conclusions: SRS/SRT dose distributions calculated by simple dose algorithms may be significantly overestimated for small lung target volumes, which can be improved by accurate Monte Carlo dose calculations. Properly implemented fast Monte Carlo algorithms can improve dosimetric accuracy with little or no compromise to computational efficiency.
WE‐C‐AUD‐07: Investigation of the Impact of Leaf Design On the Radiation Leakage Through a Multileaf Collimator for Use in Proton Radiotherapy
Authors: Ainsley C , Avery S , Maughan R , McDonough J , Bloch P , Goulart D , Ingram M
Published: July 2007

Purpose: To investigate the impact of leaf design on the radiation leakage through a multileaf collimator (MLC) for use in proton radiotherapy. Method and Materials: Using the Geant4 toolkit, a simulation program was developed to study the dose inadvertently administered to patients undergoing proton radiotherapy as a result of radiation leakage through the closed leaves of a MLC. In the model, the MLC comprised two opposing banks of tungsten‐alloy leaves, arranged perpendicularly to the beam axis. Each leaf was composed from a cuboid base, together with a step along the beam direction—in order to cover the gaps that would otherwise exist between neighbors—in both of the sides that abut leaves in the same bank, and in the end that faces the opposing bank. A simplified beam nozzle was modeled to provide a uniform fluence of protons over the upstream face of the MLC. Energy deposits were recorded in a voxelized water phantom adjoining the downstream face of the MLC. The radiation leakage was assessed as a function of gap and step size, leaf material and beam properties. Results: For a 5 × 5 cm2 field of 230 MeV protons incident on the MLC in the absence of interleaf gaps, the mean dose deposited in a 40 × 40 × 40 cm3 water phantom is 0.66 % of that when the field impinges directly on the phantom (“intraleaf leakage”). With realistic gaps of width 0.125 mm, this increases by a further 0.38 % if no steps are added, reducible to 0.04 % with steps of size 0.2 mm (“interleaf leakage”). The interleaf leakage dose distribution correlates highly with leaf design. Conclusion: Interleaf leakage through a proton MLC can be effectively removed by incorporating steps into the leaf design. Intraleaf leakage can be minimized by appropriate choice of leaf material.
WE‐C‐M100F‐06: Dosimetric Comparison of High‐Z Inhomogeneity in IMRT: A Collaborative Study
Authors: Das I , Chang S , Cheng C , Chopra K , Desrosiers C , Hasson B , Jiang Z , Mitra R , Murphy S
Published: July 2007

With increased longevity, more prostate cancer patients with hip prosthesis are anticipated. Prosthetic devices have high atomic numbers (Z) and produce dose perturbation that is dependent on Z, beam energy, and depth. TG‐63 provided recommendations for high‐Z prosthetic devices in 3D conformal therapy. In IMRT, the dosimetric implication of the high‐Z prosthesis remains uncertain which is investigated in this collaborative study with 10 different treatment planning systems (TPS). Planning target volume (PTV), and the organs at risk (OAR) namely, the bladder, the rectum and a bilateral titanium hip‐prosthesis were contoured on a CT data set of a patient and sent to each collaborator with proper guidelines for beam arrangements, energy and dose volume constraints for planning. Due to significant streaking artifacts in the CT data, users were encouraged to use their own method to correct for redistribution of CT numbers, and assign the appropriate electron densities. Since dose perturbation is significant for low energy and less sensitive with multiple fields, equally distributed 7‐fields were planned. Beam energy was also studied for comparison. One common constraint, 95% PTV must receive at least 95% of dose was strictly followed by each planner. A variety of dose algorithms were used in different TPS, such as pencil beam, superposition, and convolution. Although the results of some planning systems are closer to each other, in general, there is a wide variation in dose distribution in PTV and the OARs, as well as the minimum, the maximum and the median doses which are commonly used in plan evaluation. The variation in MU and the number of segments also vary significantly. High energy beam provided slightly better but not significant dose distribution. Ranking of TPS cannot be established based on a single clinical case. A well‐controlled phantom study is planned to validate the merit of each TPS.
WE‐D‐L100J‐05: CuATSM and FLT PET Images Stabilization Assessment
Authors: Simoncic U , Jeraj R
Published: July 2007

Purpose: The level of correlation of a molecular imaging marker to the underlying biological information is critical, and depends on several factors, one of them being pharmacokinetics of the radioisotope uptake. The main aim of this work to assess the level of Cu‐diacetyl‐bis(N4‐methylthiosemicarbazone) ([Cu‐64]‐CuATSM) and 3′‐Deoxy‐3′‐fluorothymidine ([F‐18]‐FLT) uptake stability as a function of imaging time post‐injection to find an acceptable time window for image acquisition. Method and materials: Five canine patients (three nasal carcinomas, two sarcomas) with spontaneous tumors were scanned with CuATSM (hypoxia marker) and FLT (cell proliferation marker). The FLT scans were dynamic up to 70min. The CuATSM scans were dynamic for 3h and followed with a repeat scan at 24h. Correlation coefficients were calculated between individual frames of a dynamic scan and a reference PET image. For the CuATSM scan, the reference PET image was the CuATSM scan at 24h. For the FLT scan, the reference PET image was either the last frame (70min), or the image obtained with a three‐compartment kinetic modeling. The correlation was calculated based on the volume of interest, defined as a CT‐visible tumor with a small margin. Results: Our results indicate relative stability of the CuATSM uptake after approximately 1h, when the correlation coefficient increases over 0.9. Relatively low level of interpatient variability was observed. The FLT uptake analysis shows relatively fast stabilization, typically achieved within first 30min. The stability behavior was tumor‐type dependant. Relatively high correlation between the SUV (60–70 min) and kFLT was observed, warranting further investigation of true benefit of kinetic analysis. Conclusions: This work investigates spatial and temporal stability of CuATSM and FLT uptake in several tumor types. Due to significant variation in observed time required for image stabilization, we have to be aware that spatial distribution in different tumors can last longer than the initial transient indicates.
WE‐D‐L100J‐09: Assessment of the Radiation Shielding for a Busy PET/CT Facility
Authors: Kappadath S C , Wendt R E
Published: July 2007

Purpose: To verify the adequacy of shielding installed in a PET/CT facility and to validate the predictions of the computer program used to design it. Method and Materials: Optically stimulated luminescence dosimeters (Landauer, Glenwood, IL) were placed at various locations and integrated over three month intervals within a PET/CT facility containing two PET/CT scanners (GE Healthcare, Waukesha, WI), two empty bays for expansion, and 16 patient uptake rooms. Software written by us was used to design the radiation shielding based upon high projected workloads in all four scanner bays. The software produces a two‐dimensional plot of weekly radiation dose superimposed on the facility floor plan. Typical lead thicknesses ranged from 10–25 cm (2–5 HVL at 511 keV). The facility was re‐analyzed using the current patient workload (32 patients/day, 18‐FDG radiopharmaceutical, 60 minute uptake, 30 minute scan) and the as‐built shielding thicknesses. A cluster of four dosimeters was deployed in one location to assess the inter‐dosimeter variation. Results: Measured (predicted) weekly mrem doses were 67(86) inside an uptake room, 25(14) and 43(13) inside the scanner rooms, 1(7) outside of the scanner rooms, 2.2(1.5) at a nursing station, 1.3(2) in a central hallway, and <0.1(<1) in the public areas adjacent to the clinic. The variation among the cluster of four essentially identically exposed dosimeters was 12%. Conclusion: Two‐dimensional dose maps help in visualizing the context of and explain the point measurements. In addition to the conservatism built into the program, x‐rays and variability in source locations (primarily dosed patients) may explain the discrepancies between the calculations and the measurements. At maximum workload the radiation dose levels within the uptake rooms would likely exceed the regulatory limit for occupational exposure. Movement of dosed patients within the facility constitutes a significant and complicated source of radiation that should be modeled for better accuracy.
WE‐D‐M100F‐03: MRI/MRS for Chemo/radiation Therapy Response Assessment
Authors: Jackson E
Published: July 2007

Several functional MR techniques, including dynamic contrast agent enhanced MRI (DCE‐MRI), dynamic susceptibility change MRI (DSC‐MRI), diffusion MRI, blood oxygen level dependent (BOLD) MRI, and in vivo spectroscopy, have been proposed and utilized to monitor the effects of conventional and targeted therapies in a longitudinal, non‐invasive manner. This symposium session will briefly review the physical basis of each of these techniques and then will review representative pre‐clinical and clinical trial findings obtained using these methodologies to assess response to treatment by radiation and/or chemotherapy.

Objectives:

1. Review the physical basis of DCE‐MRI, DSC‐MRI, diffusion MRI, BOLD MRI, and MR spectroscopy techniques.

2. Review representative applications of the techniques described above to pre‐clinical and clinical trial applications for early assessment of radiation therapy, both as a single regimen as well as in combination with chemotherapeutic and/or targeted therapies.
WE‐D‐BRB‐01: Support for Staffing and Assuring Quality in Radiation Oncology
Authors: Mills M , Hogstrom K , Dunscombe P
Published: July 2007

Standards for commissioning and quality control of radiation therapy equipment have traditionally been established through the consensus of “experts”. The recently implemented Canadian approach to standards (medphys.ca) has broadened the expert panel to include the entire Canadian medical physics community through web‐based consultation. This process and the standards so developed will be discussed. A limitation is that while such standards developed to date in Canada and in many other jurisdictions undoubtedly have made major contributions to the safety and quality of radiation therapy, they could not be described as being evidence based. An approach to “evidence based” commissioning of a treatment planning system, in which the Equivalent Uniform Doses of targets and organs at risk are used as outcome surrogates, will be discussed. The resulting Quality Control Standards, whether using the novel Canadian approach or the more traditional AAPM Task Group approach, require a determinable amount of time that must be performed by medical physicists. Staffing at appropriate levels to accomplish these tasks is the focus of all activities in medical physics. Proper staffing is therefore crucial to patient care, and necessary to meet regulatory requirements. The results from the Abt study of medical physicist work values for radiation oncology physics services: round III may be used to justify personnel and defend staffing to administration for a number of venues of practice. A work model is created to allow the medical physicist to defend QMP work based on both routine and special procedures service mix. The model may be used to develop a cost‐justification report for setting charges for radiation oncology physics services.

Objectives:

1. Understand alternative options to develop Quality Control Standards for Radiation Oncology Physics.

2. Understand what new information will be provided in the Abt III study.

3. Understand how to use the Abt studies to justify medical physicist work and staffing.