bluuebunny/crossref_metadata_2025 huggingface dataset:
TU‐D‐J‐6C‐08: Enhancing 4D PET Through Retrospective Stacking
Authors: Thorndyke B , Schreibmann E , Maxim P , Loo B , Boyer A , Koong A , Xing L
Published: June 2005

Purpose: Four‐dimensional (4D) PET presents challenges distinct from 4D CT owing to radiotracer dose limitations. A single‐bed field‐of‐view (FOV) PET scan typically requires several minutes to acquire adequate data for reconstruction, necessarily spanning several respiratory cycles and smearing the radiotracer signal within a given lesion over an increased volume. Although prospective or retrospective gating captures the PET image at a single point in the respiratory cycle, restricting the data to events within the gating interval increases the signal‐to‐noise ratio (SNR). We propose a method, coined “retrospective stacking” (RS), to combine the data from the entire respiratory cycle through deformable registration. In addition, we use the transformation maps to generate a 4D PET with statistics comparable to the single RS image. Method and Materials: A single FOV of a pancreatic cancer patient was acquired via the gated PET mode on a GE Discovery ST PET‐CT scanner. These gated images were registered using a mutual information / B‐splines registration algorithm, and superimposed. A 4D PET series spanning the full respiratory cycle was generated, and fused onto a 4D CT. Results: The SNR of the RS image showed an increase of 15% over a single gated reconstruction. Activity‐volume histograms of radiotracer activity surrounding the pancreatic lesion revealed that the ungated PET showed 33% greater tumor volume (using a 40%‐of‐maximum threshold) than the RS image. The reconstructed 4D PET fused well with the 4D CT, providing a clearer view of radiotracer distribution over the respiratory cycle than was possible using gated reconstructions. Conclusion: Retrospective stacking enabled better integration of temporally varying PET and CT series by reducing radiotracer smearing due to respiratory motion, while at the same time increasing the SNR beyond the poorer statistics inherent in gated PET acquisition. Noise‐reduced 4D PET images could also be generated for fusion with 4D CT.1
TU‐D‐T‐617‐01: Comparison of LDR to PDR Dose Distributions: A Monte Carlo Study
Authors: Price M , Gifford K , Horton J , Eifel P , Lawyer A , Jhingran A , Mourtada F
Published: June 2005

Purpose: 1) To validate a Monte Carlo (MC) model of the Ir‐192 pulsed‐dose rate (PDR) source inside the Fletcher‐Williamson (FW) ovoid using radiochromic film measurements. 2) To compare the FW dose distributions in water to those of Cs‐137 low‐dose rate (LDR) pellet inside the Selectron Fletcher‐Suit‐Delclos (FSD) ovoid. Method and Materials: Detailed mechanical drawings of the FW colpostat were obtained from the vendor. MCNPX 2.5e MC code was used to model the small right FW ovoid and an Ir‐192 source (mHDR v2) centered within the ovoid. MC models of actual and virtual Ir‐192 source in air were also run to derive the conversion from contained to apparent activity. MD‐55 radiochromic film was placed in a polystyrene phantom at a plane parallel to and displaced medially 2.0 cm from the long axis of the colpostat. To compare with the FSD ovoid, MC runs in a 30 cm water sphere were run for common ovoid loadings of 5, 10, 15, and 20 mgRaeq. Results: MCNPX calculated dose relative to film measurements is within ±2% or 2mm distance‐to‐agreement for 92% of the dose grid. For the FW simulations in water, the overall shapes were found to be similar in regions away from the shields to those obtained for the FSD ovoid; however, large but localized deviations were found in high dose‐gradient regions close to the rectal and bladder shields, mainly due to differences in source and shields geometry. Conclusion: The dose distributions predicted by MCNPX are in good agreement with the film results. Dose‐rate atlases calculated in clinically‐relevant 2D planes around the FW and FSD ovoids provide information to aid in the transfer of current LDR intracavitary brachytherapy practice to that using PDR for treatment of gynecological cancers. Conflict of Interest: Partial support by Nucletron Corporation.
TU‐D‐T‐617‐03: Assessment of a Prostate Treatment Plan Using Directional Brachytherapy Sources
Authors: Lin L , Chaswal V , Henderson DL , Thomadsen BR
Published: June 2005

Purpose: To evaluate the effect of using newly developed directional sources in a prostate brachytherapy treatment plan. Method and Materials: Directional sources contain a radiation shield in part of its interior that reduces significantly the intensity of radiation emitted in the shielded direction. They have a similar dose distribution as non‐directional sources on the unshielded side. We use an adjoint region of interest based optimization system to generate a plan for 0.546 U non‐directional sources to deliver a prescribed dose (Dp) of 145 Gy to the prostate and then manually replace some sources with the same strength of directional sources. The orientation and position of the directional sources have been selected to maximize V100 (percent of prostate receiving 145 Gy and higher), minimize R90Gy (percent of rectum receiving 90 Gy and higher) and minimize V125 and eliminate V150 for the central prostate regions including urethra (percent of volume receiving 125% of Dp (181 Gy) and 150% of Dp (217 Gy), respectively). Results: V100 of the prostate target region increases from 97.5% to 98.9%, the central prostate region including urethra, receives the full uniform prescribed dose, V125 drops from 61.7% to 49.2% and the V150 of 2.24% is eliminated. There is a large reduction in R90 Gy from 53.7% to 20.6% for the anterior retal wall, which significantly reduces the probability of rectal morbidity. All treatment plan indicators are improved with use of directional sources. Conclusion: Directional sources can allow the conflicting goals of increasing prostate target V100, decreasing R90Gy, decreasing V125 and eliminating V150 of the central prostate regions including urethra to be independently optimized, yielding a better treatment to the prostate with less rectal and urethral morbidity.
TU‐D‐T‐617‐07: Treatment Plan Validation of the Xoft AXXENT™ X‐Ray Source
Authors: Axelrod S , Rusch T
Published: June 2005

Purpose: Accuracy of delivered dose compared to BrachyVision™ generated treatment plans was tested using both a calibrated miniature air ionization chamber to measure dose at the prescription point, and radiochromic film to map isodose contour lines. Quantitative and qualitative comparisons of the total dose and contour shapes were made. Method and Materials: Xoft applicator balloons were held in a water phantom with a Solid Water™ fixture. A PTW model 34013 ionization chamber connected to a PTW UniDos electrometer, and read into a computer, were used to measure the dose at the prescription point. Films were held in the fixture in the plane of the source, offset by 5 mm. The Xoft Axxent controller performed sample treatments including stepping of the source for four combinations of balloon size and operating voltage. Film samples were digitized and converted to dose values through a specially developed calibration curve. Comparison of isodose contours with the film‐measured dose was accomplished with specially written image analysis algorithms, which compared the measured values with predicted along the isodose contours. Results: Dosimeter based dose measurements agreed with prescription to within 11% worst case, with an RMS error of 8%. Film analyses quantified deviation from isodose contours as a function of polar angle, with relative RMS deviations well under 10%. However limitations in the accuracy of film calibration limited absolute accuracy to just over 20%, with systematic behavior attributable to limitations of the fitting function. Conclusion: Validation of treatment plans were performed using direct dosimetry and film based measurements. Novel image analysis tools were developed to allow numerical evaluation of film dose to isodose contours. Conflict of Interest: Research was supported by Xoft, Inc.
TU‐EE‐A3‐03: Performance of a Bench‐Top, Megavoltage CT (MVCT) Scanner Using Cadmium Tungstate‐Photodiodes
Authors: Monajemi T , Tu D , Fallone B , Rathee S
Published: June 2005

Purpose: To evaluate the imaging performance of a prototype fan‐beam megavoltage CT (MVCT) scanner in and 6 MV beams. Method and Materials: The 80‐element detector is fabricated by tiling 8‐element (element size 0.275 × 0.8 × 1 cm3) and photodiode arrays and arranging them on an arc (radius = 110 cm). A precision rotary stage and its control are added to create a third generation CT scanner. The attenuation of and 6 MV beams was measured as a function of solid water thickness, and fit to a second order polynomial to correct for spectral hardening artifacts. A calibration procedure was established to remove ring artifacts caused by the distinctly asymmetric line spread functions at the ends of 8‐element blocks. The low contrast resolution (LCR) as a function of dose and object size, the signal to noise ratio (SNR) as a function of dose, and the linearity of CT numbers with density were quantified. Results: Throwing away one‐ninth of collected projection angles to reduce the dose per image adversely affects the resolution in 6 MV images; however, 15 mm targets at 1.5% level are still visible at 7 cGy. The low contrast target of 1.5% at 6 mm diameter is visible in images at 2cGy. The LCR in the objects stays approximately constant with the dose reduced from 17 to 2 cGy. In general, the contrast decreases as the target diameter decreases. The SNR2 obtained from a uniform phantom increases linearly with dose (R2=0.9977). CT numbers as a function of the density show a linear trend (R2= 0.9923). Conclusion: The prototype detector performance is satisfactory to achieve the ultimate goal of this project of creating a focused 2‐D MV detector with high detective quantum efficiency such that reasonable LCR at low dose can be obtained in MVCT.
TU‐EE‐A4‐02: Impact of Ultrasound‐Guided Patient Setup On OAR Dose in Conformal Radiation Therapy for Prostate Carcinoma
Authors: Wang Z , Rajagopalan B , Malhotra HK , Kuettel MR , Podgorsak MB
Published: June 2005

Purpose: To evaluate the impact of daily ultrasound‐guided patient repositioning on bladder and rectal dose in the treatment of prostate carcinoma with conformal radiation therapy. Method and Materials: Standard‐of‐care for the treatment of low and intermediate risk prostate carcinoma currently includes 3D conformal radiation therapy (3D‐CRT). During external beam therapy, ultrasound‐guided isocenter realignment can be performed daily to correct patient setup for prostate motion (up to 1 cm) relative to bony anatomical landmarks. In this study, five patients with substantial ultrasound‐documented inter‐fraction prostate motion during their treatment course were selected. Starting with the original treatment plan, two additional plans were retrospectively generated for each patient. In one set, using the ultrasound documented displacements, organ contours were moved for each fraction, thus simulating positioning with misalignment caused by organ motion if ultrasound guidance were not used. In a second set of plans, the isocenter was shifted, as were the organ contours, simulating realignment based on the ultrasound image. In all cases, the number of planned monitor units was set to those of the original plan. For a given patient, isodose distributions, DVHs and EUDs were calculated for the prostate, bladder, and rectum for each fraction and then combined for each shift condition. Results: In all reconstructed plans, there were no substantial changes in dose coverage of the prostate (<0.21 % change in EUD) compared to the original plan. However, in some cases with no realignment, higher dose maxima were predicted in the bladder and rectum, with the consequent EUD for each organ significantly greater compared to the original plan. Conclusion: Although fractional organ motions may have little effect on the overall dose delivery to the prostate, the effect on the dose to OARs (rectum and bladder) can be significant if no daily realignment is performed.
TU‐EE‐A1‐04: Development of a Customizable Neural Network Brachytherapy Nomogram Equivalent
Authors: Silvern D , Wojcicka J , Kudynski R , Bagan E
Published: June 2005

Purpose: The objective is to develop a software brachytherapy nomogram equivalent. Although modern techniques obviated much nomogram usage, situations arise where nomograms prove useful. If site dimensions are unknown, they can be measured followed by nomogram treatment planning. Regulatory agencies mandate independent checks. Deploying this software could easily meet this requirement. A graphical nomogram is fixed and cannot be modified. In contrast, the nomogram equivalent is configurable. This field‐customizable tool should further empower clinicians. Method and Materials: The nomogram equivalent is a three‐layer feed‐forward neural network (NN). The inputs consist of three site dimensions and volumes. The hidden layer has ten processing elements (PE's). The output layer is a single PE representing air kerma strength. The PE inputs are weighted, summed and applied to the sigmoid function, defining the PE output. A training algorithm based on differential evolution was developed. A group of fifty competing NN's are continuously evolved by simulating biological evolutionary processes. The weights are initially assigned random values. During training they are evolved to decrease the error between the NN output and desired value. Training data consist of three dimensions, volumes and air kerma strengths from past implants. The software cycles through the training data, altering the weights. The sum‐of‐squares difference between the outputs and air kerma strengths drives the training process. Once trained the NN can predict air kerma strengths for new implants. Results: The differential evolution training algorithm successfully determined NN weights for predicting required air kerma strengths. In most cases the network predicted air kerma strength within 11 percent. Consistent training data representing the full range of possible implant site dimensions and volumes are important for obtaining accurate predictions. Conclusion: The software is ready for clinical use. The authors believe that the NN paradigm is a sound method for implementing a field‐customizable nomogram equivalent.
TU‐EE‐A2‐03: Evaluation of Dose Calculation in KV‐Cone Beam CT Data Sets
Authors: Tücking T , Nill S , Oelfke U
Published: June 2005

Purpose: The accuracy of dose calculation in CT data sets derived from a linac attached kV‐cone beam CT (kV‐CBCT) equipment was analyzed by comparing dose profiles, depth dose curves, and dose volume histograms of scanned objects with the dose parameters calculated in the respective fan beam CT cubes. Method and Materials: To perform kV‐CBCT acquisition for adaptive radiotherapy an x‐ray tube and a flat panel imaging detector were attached at a linac on opposing sides of the isocenter. Several phantoms, namely a water filled cylindrical phantom, a special contrast phantom with several inlays of different materials, and an Alderson head phantom were scanned. Furthermore a kV‐CBCT was performed for a real head and neck patient with the intention to correct for patient setup‐errors. The data were reconstructed with an in‐house software tool not executing any scatter corrections. Dose distributions were calculated using a clinically approved pencil beam algorithm for modulated or actual treatment plans. Attenuation coefficient to electron density conversion for kV‐CBCT dose calculation was done with help of the contrast phantom using a simple calibration procedure. Dose contours and DVHs were compared to reference calculations on fan beam CT data. Additionally a γ‐criteria analysis was performed. Results: For all analyzed cases absolute dose values and DVHs calculated on the CBCT data sets were in good agreement with the reference data and no differences greater than 3% were found in the considered profiles. Therefore the comparisons also passed the 3% and 3mm γ‐criteria in nearly all voxels. Conclusion: Although CBCTs may hardly be comparable in image quality to standard diagnostic ones they might nevertheless be good enough for a sufficiently accurate dose calculation and therefore be well suited for dose guided radiotherapy. Even better results may be possible by additionally including scatter correction. Conflict of Interest: Supported by Siemens‐OCS.
TU‐FF‐A3‐02: Two Male Tomographic Models Segmented From MR and CT Images
Authors: Lee C , Lee J
Published: June 2005

Purpose: To develop two new tomographic models of Korean typical adult males from magnetic resonance (MR) and computed tomography (CT) images of living subjects, and to calculate photon dose conversion coefficients for external irradiation geometries. Method and Material: Two healthy volunteers, subject A (172 cm in height, 65 kg in weight) and subject B (172 cm in height, 68 kg in weight), whose body size was within the range of Korean average values (170.9±3.7 cm in height, 67.9±4.8 kg in weight), were recruited. Whole body MR images were obtained from one subject and CT images from the other. The source images were semi‐automatically segmented to construct three dimensional matrices. The resulting matrices were ported into Monte Carlo transport code, MCNPX2.4, and photon dose conversion coefficients for external monoenergetic photon beams with the energies from 0.015 MeV to 10 MeV were calculated for idealized irradiation geometries. Results: The resulting Korean tomographic models were named KTMAN‐1 and KTMAN‐2. Up to 80% of organs and tissues in both models were within 30% of average values of Korean adult male. 21 and 23 organs and tissues were segmented in KTMAN‐1 and KTMAN‐2, respectively. Dose conversion coefficients were computed for all organs and tissues in external irradiation geometries: antero‐posterior (AP), postero‐anterior (PA), right lateral (RLAT), and left lateral (LLAT). Through investigation of differences in organ doses between tomographic models and stylized model (ICRP74 data), unrealistic position of thyroid, testes, arms, and stomach in stylized models were revealed. Conclusion: Two new tomographic models constructed in this study provided more realistic dose conversion coefficient than stylized models. Although difference of organ mass and position between the Korean models and western reference model caused insignificant discrepancies of organ doses for external dosimetry, the Korean models will be more meaningful for internal dosimetry.
TU‐FF‐A2‐01: A Heterogeneity Inclusive Algorithm for Calculation of Central‐Axis Absorbed Dose in Finite‐Size Pencil Beams
Authors: Pino R , Nizin P
Published: June 2005

Purpose: To develop algorithms for calculating central‐axis absorbed dose from high‐energy photon beams in heterogeneous media of arbitrarily varying density. Method and Materials: We employ and extend a phenomenological model for the central‐axis absorbed dose in therapeutic photon beams (Nizin, Med. Phys. 26, 1893–1900 (1999)) in order to describe absorbed dose in heterogeneous media. The parameters defined by the model are extracted from beam data for a homogeneous water phantom. For heterogeneous media, the model's parameters are determined by rescaling these for water using the local values of the relative density of the medium. Results: The phenomenological model was generalized for arbitrarily varying density media by introducing a differential equation that modulates the field size dependent normalization factor. This factor is multiplied by the photon fluence, which decays exponentially with the radiological depth. The generalized model was tested using slab geometries where layers of water and lung‐like material alternate. We found very close agreement between absorbed dose calculated using the generalized model and Monte Carlo simulations (Kawrakow et al, NRCC PIRS‐701 (2003)) for finite‐size pencil beams and for a wide range of field sizes for broad beams. Conclusion: Accurate algorithms for calculation of central‐axis absorbed dose in heterogeneous media are proposed. For both narrow and broad photon beams, the model generates results that are in close agreement with Monte Carlo data for slab geometries. Conflicts of Interest: This research was supported by NOMOS Radiation Oncology Division of North American Scientific.
TU‐FF‐A2‐06: The Use of MVCT Images for Dose Calculation in the Presence of Metallic Objects
Authors: Meeks S , Langen K , Wagner T , Willoughby T , Kupelian P
Published: June 2005

Purpose: Metallic objects such as dental fillings and prostheses cause artifacts in kilovoltage CT (kVCT) studies. The problem with these artifacts is twofold. Firstly, they obscure soft tissue structures. Secondly, the artifacts create artificial CT numbers that compromise the accurate calculation of absorbed dose. Megavoltage CT (MVCT) imaging reduces these artifacts, and this study investigates the impact of such artifacts. Method and Materials: The MVCT to electron density curve for a Hi‐ART II helical tomotherapy unit was extended to include high electron density materials using aluminum, titanium, and copper targets. MVCT images of a prostate patient with a hip prosthesis that is treated on a helical tomotherapy unit are then used to evaluate the impact of kVCT artifacts on the calculation of absorbed dose. At the time of treatment planning the kVCT image was used for planning. For actual patient treatment the artificial hip was generously contoured and no beam entrance though this region was allowed for treatment planning. Daily MVCT image of the patient were acquired for patient alignment. Retrospectively, a treatment plan was generated that allowed beam entrance through the hip prosthesis. This second plan was recalculated on the MVCT image to determine the dosimetric effect of the artifacts. Results: The recalculated dose in the MVCT image shows that the absorbed dose in the hip prostheses and immediately next to the prosthesis is lower in the MVCT image than in the kVCT image. This dose differential can be as large as 15%. Soft tissue areas that are affected by the kVCT image artifacts have higher absorbed dose in the MVCT based recalculation. Conclusion: In the presence of metallic artifacts, significant differences in the absorbed dose computation exist between dose calculations based KVCT images and that calculated in megavoltage images.
WE‐C‐I‐609‐02: Investigation of Various Methods for Determination of Similarity Measures for Pairs of Clustered Microcalcifications On Mammograms
Authors: Muramatsu C , Li Q , Schmidt RA , Suzuki K , Shiraishi J , Newstead GM , Doi K
Published: June 2005

Purpose: Presentation of images similar to an unknown lesion on mammograms may be useful to radiologists in their image interpretation. The purpose of this study is to investigate different methods for determination of similarity measures that would agree with radiologists' subjective similarity ratings. Method and Materials: We used 881 ROIs that included clustered microcalcifications. We selected 19 “unknown” images each of which was compared with six “known” images to produce 114 pairs. Nine breast radiologists provided similarity ratings based on the overall impression for diagnosis for the 114 pairs. Subjective ratings were marked on a continuous rating scale between 0 and 1, where 0 and 1 correspond to two lesions not similar at all and almost identical, respectively. A number of image features were extracted from the lesions for determination of similarity measures. Two similarity measures were determined by use of the Euclidean distance without and with unique strong features of an unknown image. A psychophysical similarity measure was determined by use of an ANN that can learn the relationship between radiologists' similarity ratings and feature values. The correlation values between the subjective ratings and the similarity measures were determined to evaluate the usefulness of similarity measures. Results: The Euclidean‐distance‐ based measure with unique features provided the improved correlation values for four cases, but degraded correlation values for three cases, compared with that without unique features. When ANN was employed, the correlation coefficients were further improved by use of a leave‐one‐out test method. Conclusion: Simple similarity measures such as those based on the Euclidean distance would not be a reliable measure. The selection of features for a specific unknown case and the use of ANN have the potential to improve the determination of reliable similarity measures. Conflict of Interest: RAS and KD: shareholders, R2 Technology Inc.; KD: shareholder, Deus Technology Inc.
WE‐C‐I‐609‐07: On Constructing Priors and Likelihoods for Deformable Shape Models
Authors: Joshi S , Merck D , Tracton G , Stough J , Broadhurst R , Pizer S , Chaney E
Published: June 2005

Purpose: Explicit deformable shape models (DSMs) can be used in a Bayesian statistical framework to provide a priori information for posterior optimization to match the DSM against a target image for automatic segmentation. In this approach a DSM is initialized in the target image and undergoes a series of deformations to closely match the target object. Deformation is driven by optimizing an objective function with terms for geometric typicality (prior) and model‐to‐image match (likelihood). The purpose of this work was to develop strategy, methodology, and tools for constructing the geometric prior and intensity likelihood for a particular form of DSM called m‐reps. Method and Materials: Geometric truth is defined for an object of interest by a statistically significant collection of expert human segmentations of training images. M‐reps are fit to the human drawn contours by minimizing the distance between the surfaces of the m‐rep and the contours under added conditions that lead to positional correspondence across training cases. The geometry of the resulting set of training m‐reps is analyzed in non‐Euclidean space using an approach called principal geodesic analysis (PGA) to yield a set of eigenmodes that define the geometric prior. The intensity likelihood is constructed by registering each training m‐rep with the corresponding gray scale image and collecting regional intensity information that is statistically characterized over all training cases. The intensity information can be in several forms including linear profiles and regional histograms. Results: PGA produces modes that include natural deformations such as local twisting, bending, bulging, and constricting. Unlike analysis in Euclidean space, improper shapes are avoided. The form of the intensity prior can be customized to each object of interest for optimal performance. Conclusion: These methods are powerful, robust and generalizable to other DSMs. Conflict of Interest: The presenting author has a financial interest in Morphormics, Inc.
WE‐C‐J‐6C‐09: Cone Beam Digital Tomosynthesis (CBDT): An Alternative to Cone Beam Computed Tomography (CBCT) for Image‐Guided Radiation Therapy
Authors: Pang G , Au P , O'Brien P , Bani‐Hashemi A , Svatos M , Rowlands JA
Published: June 2005

Purpose: To investigate the feasibility of using a new imaging technique, i.e., Cone Beam Digital Tomosynthesis (CBDT) as an alternative to the Cone Beam Computed Tomography (CBCT) approach for creating 3D cross sectional images of a patient in the radiotherapy treatment room. Method and Materials: Similar to the CBCT approach, the CBDT uses an x‐ray source (either a kV source or a MV source on a Linac or another radiation‐emitting device) and an x‐ray detector to acquire projection data by rotating them around a patient simultaneously. Unlike CBCT, CBDT utilizes partial scans, typically in the range of 20–60 degrees of gantry arc. The main advantage of the CBDT approach is that it takes less time to perform image acquisition and reconstruction. An experimental CBDT system has been built on a Linac with a recently developed flat‐panel detector. A novel filtered backprojection algorithm was developed for CBDT reconstructions. CBDT phantom images have been generated for different degrees of gantry arc and beamline configurations and compared to those from CBCT. Results: Cross‐sectional images of a Rando phantom were generated with comparable image quality to CBCT using as small as ∼ 22 degree gantry arc. The planes of CBDT reconstruction are orthogonal to the x‐ray beam at the midpoint of the arc. These reconstructed planes are of particular relevance to image‐guided radiation therapy, because they depict anatomy along planes that are most relevant to the treatment beam, i.e., orthogonal to the beam axis. Conclusion: We have demonstrated the feasibility of using CBDT to acquire cross‐sectional images of an object in the treatment room to guide radiotherapy treatment. Compared to CBCT, the CBDT approach is much faster with acceptable image quality in the planes most relevant to the treatment. Conflict of Interest: This work was supported by Siemens Medical Solutions USA, Inc.
WE‐C‐T‐617‐07: Using X‐Ray Spectrometry to Quantify the Effects of Emission Anisotropy On Air‐Kerma‐Strength Measurements of Prostate Brachytherapy Seeds
Authors: Mitch M , Seltzer S
Published: June 2005

Purpose: To utilize the results of prostate brachytherapy seed x‐ray spectrometry to quantify the effects of emission anisotropy on air‐kerma‐strength measurements. Method and Materials: Air‐kerma strength was calculated from x‐ray spectra measured with a high‐purity germanium detector with the seed rotated at 90‐degree intervals about an axis perpendicular to the mid‐point of the long axis of the seed. The “air‐anisotropy ratio”, αS, was formed by taking the quotient of the air‐kerma strength of the seed with the long axis perpendicular to the detector face and the air‐kerma strength of the seed with the long axis parallel to the detector face. The resulting quantity αS is a quantitative measure of in‐air anisotropy, which has a significant effect on the relative responses of secondary‐standard well chambers and the National Institute of Standards and Technology (NIST) Wide‐Angle Free‐Air Chamber (WAFAC) primary standard. Results: Values of αS for various seed models are influenced by the presence of end welds, seed internal geometry, and the incorporated radionuclide. In comparing αS among seeds, a lower value indicated that the output of the seed was more directional (along the transverse axis), such that less emission was “missed” by the WAFAC (8 degree half‐angle) that was detected by the well chamber (approximate 4π geometry). Conclusion: The quantity αS has proven to be a useful parameter for explaining differences in well‐chamber response observed for different seed models having the same emergent spectrum on their transverse axis. It also has been used to identify aberrantly produced seeds that are not representative of those calibrated previously by NIST. Characterization of seed anisotropy is necessary in order to maintain accuracy in the transfer of air‐kerma‐strength standards from NIST to the AAPM Accredited Dosimetry Calibration Laboratories and to seed manufacturers.
WE‐C‐T‐617‐09: The Radiological Physics Center's Anthropomorphic Quality Assurance Phantom Family: New Developments
Authors: Followill D , Lowenstein J , Jhingran A , Roll J , Hernandez N , Ibbott G
Published: June 2005

Purpose: To develop quality assurance (QA) phantoms that simulate specific treatment sites for the purpose of credentialing institutions for participation in NCI‐sponsored advanced technology clinical trials. Method and Materials: The Radiological Physics Center (RPC) has developed an extensive credentialing program for institutions wishing to participate in clinical trials that use advanced technologies such as IMRT. This program includes questionnaires and irradiation of an anthropomorphic QA phantom specifically designed for the trial. These QA phantoms typically are water‐filled plastic shells with imageable targets, avoidance structures, and heterogeneities that contain TLD and radiochromic film dosimeters. Three new trials are presently under development; 1) extracranial stereotactic radiotherapy for liver metastases, 2) IMRT for endometrial or cervical cancer and 3) advanced RT for mesothelioma. New phantoms or target/dosimetry inserts have been designed to meet the credentialing needs for the three trials listed above. Results: A new liver phantom has been designed and constructed. The liver phantom contains two targets within the liver and three organs at risk (OAR); the stomach, kidney and spine. This phantom will be placed on a 2D reciprocating table to include target motion in the AP and SI directions. A new target/dosimetry insert has been designed for the RPC's existing thorax phantom that will include an OAR for the liver and dosimeters in the lung and in the chest wall. Finally, another target/dosimetry insert has been designed for the pelvic phantom that will include a vagina, cervix, bladder and rectum. Conclusion: The RPC has been and will continue to be proactive and responsive to the needs of the study groups as new treatment modalities are used for new clinical trials. This work was supported by PHS grant CA10953 and CA081647 awarded by NCI, DHHS.
WE‐C‐T‐6E‐04: Nanotechnology for Molecular Imaging
Authors: Lanza G
Published: June 2005

Developments in molecular science are pushing the temporal detection horizon of medical diagnosis and therapy back from the anatomical sequelae of disease to its earliest physiological and biochemical manifestations. The emerging field of “Molecular Imaging” may be envisioned as the in vivo diagnosis of complex pathological processes by detection of unique biochemical signatures. The concept is analogous to microscopic detection of specific epitopes with immuno‐histochemistry techniques translated into a complex and hostile in vivo environment and detected with noninvasive medical imaging systems. We have developed a novel multi‐modal site‐targeted contrast agent for sensitive and specific imaging of molecular epitopes and local therapy. This “platform” approach comprises a nanoparticle that is applicable to at least three common noninvasive imaging modalities: ultrasound (native particle), magnetic resonance (gadolinium conjugated), and nuclear imaging (radionuclide conjugated). Homing ligands linked to the surface of the nanoparticle an injectable agent with long circulating half‐life and high signal amplification upon binding to a molecular target. This novel platform has been used to detect angiogenesis, fibrin, tissue factor and collagen III and to locally deliver therapeutic agents through a unique contact facilitated mechanism. Over the next decade, molecular imaging, in conjunction with rational targeted therapies, will likely change many clinical paradigms in medicine.
WE‐D‐I‐6B‐01: Soft Tissue Visualization Using a Highly Efficient Megavoltage Cone Beam CT Imaging System
Authors: Ghelmansarai F , Bani‐Hashemi A , Pouliot J , Calderon E , Hernandez P , Mitschke M , Aubin M , Bucci K
Published: June 2005

Purpose: Recent developments in two‐dimensional x‐ray detector technology have made volumetric Cone Beam CT (CBCT) a feasible approach for integration with conventional medical linear accelerators. The requirements of a robust image guidance system for radiation therapy include the challenging combination of soft tissue sensitivity with clinically reasonable doses. Previously, low contrast objects have generally not been perceptible with MV energies due to relatively poor signal to noise ratio (SNR) performance. We have developed an improved imaging system that is optimized for MV CBCT and acquire CBCT images containing soft tissue contrast using a 6MV x‐ray beam. Method and Materials: Many factors, such as image noise, x‐ray scatter, improper calibration and acquisitions have a profound effect on the imaging performance of CBCT. In this study attempts were made to optimize these factors in order to maximize the SNR. A QC‐3V and contrast/resolution phantoms were used to determine the contrast to noise ratio (CNR) and f50 of a single 2‐D projection and the contrast and spatial resolution of the reconstructed images. Results: The computed f50 was 0.43 lp/mm and the CNR for a radiation dose of 0.02cGy was 43. Relative electron density as low as 1% can be resolved with clinically reasonable radiation doses. Clinical Megavoltage CBCT images acquired with this system demonstrate that anatomical structures such as the prostate and optic nerves are visible using radiation doses in range of 4 to 8cGy. Conclusion: We have developed an imaging system that is optimized for MV and acquire Megavoltage CBCT images containing soft tissue contrast using a 6MV x‐ray beam and irradiation doses in range of 4 to 8cGy. This system can also be used for routine portal imaging applications without risk of saturation for high dose/high energy treatment/verification imaging, or dosimetry applications. Conflict of Interest : Sponsored by Siemens.
WE‐D‐I‐6B‐05: A Technique for Respiratory‐Gated Radiotherapy Treatment Verification with An EPID in Cine Mode
Authors: Berbeco R , Neicu T , Rietzel E , Chen G , Jiang S
Published: June 2005

Purpose: Many clinics are performing respiratory‐gated treatment based on external surrogates. Verification of these treatments is very important for ensuring the delivery of the planned dose distribution. Method and Materials: We have developed a new technique for treatment verification using an Electronic Portal Imaging Device (EPID) in cine mode for gated 3D conformal therapy. Implanted radiopaque fiducial markers inside or near the target are required for this technique. The markers are contoured on the planning CT set, enabling us to create Digitally Reconstructed Radiographs (DRR's) for each treatment beam. During the treatment, a sequence of EPID images are acquired without disrupting the treatment. Implanted markers are visualized in the images and their positions in the beam's‐eye‐view are calculated off‐line and compared to the reference position by matching the field apertures in corresponding EPID and DRR images. The precision of the patient setup, the placement of the beam‐gating window, as well as the residual tumor motion can be assessed. This technique has been demonstrated in a case study, with three implanted markers. Results: For this patient, the intra‐fractional variation of all marker positions in the gating window has a 95% range of 4.8 mm in the SI direction (the primary axis of motion). This is about the same (5 mm) as the residual motion considered in the planning process. The inter‐fractional variation of the daily mean positions of the markers, which indicates the uncertainty in the setup procedure, is within +8.3/−4.5 mm (95% range) in the SI direction. Conclusion: The proposed technique will be used for gated treatment verification. The results of many cases will help us determine appropriate treatment margins for gating. Conflict of Interest: This work was supported, in part, by a grant from Varian Medical Systems, Inc.
WE‐D‐I‐6B‐07: Human and System Error Analysis of a 3D Optical Laser Scanning System for Image‐Guided Patient Positioning
Authors: Sohn J , Kim S , Monroe J , Chvetsov A , Wessels B
Published: June 2005

Purpose: The final goal of this research is to develop a patient repositioning system using a 3D optical laser scanner. We investigated the magnitude of human and system errors, which determines the accuracy of the patient repositioning system. We also scanned four breast patients to test the efficacy of this system. Method and Materials: A handheld 3D surface scanning system acquires patient surface images, which are registered to a reference image using a rigid body transformation. The resultant image registration generates the patient position adjustment to match the images. We evaluated human and system error associated with this positioning technique. To measure human error, ten operators performed ten experiments to acquire the same reference point. To evaluate the system error, we attached the receiver (serving as the origin of the surface image data) to the X‐Y‐Z positioning system and measured the discrepancy between the measured physical position and the scanner reported distance in three axes. We also scanned four breast patients and analyzed the repositioning errors. Results: The mean human error and standard deviation were evaluated to be 1.86mm and 0.66mm, respectively. The system mean error and standard deviation were found to be 0.4mm and 0.32mm (n=45), respectively. Image translation and rotational shifts from the patient study ranged from 5mm to 15mm and −6° to 11°, respectively. Conclusion: We have experimentally evaluated human and system error associated with a new 3D hand‐held laser scanner positioning system. As shown in the result, human error was much more significant than system error. This handheld 3D optical scanning system has proved its capability as a tool for patient repositioning between fractions. Future investigations will examine the relation between surface images and internal target localization.
WE‐D‐I‐609‐02: Characterization of a Novel Dual‐Panel PET Scanner for Use in Small Animal Imaging
Authors: Souris JS , Cho S , Penney BC , Chen C‐T , Kao C‐M
Published: June 2005

Purpose: Characterization of a novel dual‐panel PET scanner for use in small animal imaging. Method and Materials: A prototype PET system comprised of two non‐rotating HRRT panel detectors — each possessing a single‐layer 72 × 104 array of 2.1mm × 2.1mm × 7.5mm LSO crystals — was evaluated for its suitability in small animal imaging. Resolution measurements were made by translating a 190mm long, 3mm diameter 21.5MBq 〈sup〉68〈/sup〉Ge line‐source across various planes parallel to the detector's face, while sensitivity measurements were made in a similar fashion but using a 0.5mm diameter 65KBq 〈sup〉68〈/sup〉Ge point‐source. To emulate small animal imaging, a mouse phantom was constructed from an 11.25mm length of 25.4mm diameter polycarbonate rod that contained a 3.5mm diameter hole — radially offset by 6.7mm and parallel to the rod's central axis — to accommodate either the 〈sup〉68〈/sup〉Ge line‐source or 〈sup〉18〈/sup〉FDG samples. Results: Activity‐independent scattered fractions were estimated by inserting the 〈sup〉68〈/sup〉Ge line‐source into the mouse phantom, averaging counts in each crystal column parallel to the line‐source, and second‐order polynomial fitting the resulting projection data after excluding its central peak. Noise Equivalent Count Rates (NECRs) were then derived for various energy windows and 〈sup〉18〈/sup〉FDG activities in the mouse phantom, to quantify scanner performance. Preliminary image reconstructions, performed using a 50‐iteration Expectation Maximization algorithm, demonstrated reconstruction resolutions of ∼1.6mm. Sensitivity measurements exhibited significant dependence upon energy window selection; with the 250–750keV energy window resulting in both the highest absolute sensitivity (6.8%) and the highest NECR (142kcps at 1.25MBq/cc). Conclusion: Although the measured sensitivity's energy dependence suggests additional energy calibrations are needed, preliminary data show both device sensitivity and resolution to be comparable to or better than that of current commercial systems. Moreover the dual‐panel scanner's simple fixed geometry and large field‐of‐view are especially well suited for whole‐body imaging and concurrent imaging of multiple animals.
WE‐D‐I‐609‐08: Multimodality Small Animal Imaging: Registration of Functional EPR Images with MRI Anatomy
Authors: Haney C , Parasca A , Pelizzari C
Published: June 2005

Purpose: Electron paramagnetic resonance imaging (EPRI) is a spectroscopic imaging modality being developed for functional and molecular imaging in small animals, and has potential for ultimate translation to clinical imaging. By utilizing contrast agents (“spin probes”) whose EPR spectra are modified by the local environment, it is possible to produce images which directly measure important physiologic quantities. In studies of tumor response to radiation‐mediated gene therapy we have utilized EPR oxygen images, which provide 3D oxygen maps with resolution of ∼1mm spatially and ∼3 Torr in oxygen partial pressure. Like functional nuclear medicine images, EPR oxygen images do not depict anatomy directly. Analysis is facilitated by the ability to use an anatomic image as a “roadmap” for interpretation of the functional images. We have developed methods for registration of EPRI with MRI to allow anatomically‐based analysis of these functional images. Method and Materials: EPRI is performed using locally developed spectroscopic imaging systems. MRI is performed on a 4.7T dedicated small animal system. Several registration techniques have been newly developed or adapted from clinical multimodality imaging. Fiducial markers visible in both EPRI and MRI can be attached to the animal or to the immobilization device. Simple point‐to‐point registration is possible using this method. Surface‐based and intensity‐based registration methods have also been applied. Customized immobilization devices are fabricated using a polymer dental impression material, analogous to foam cradles used in radiotherapy. Results: Both anatomically directed analysis of functional EPR images and analysis of serial changes in defined anatomical regions during a course of therapy are enabled by the use of customized immobilization and anatomic/functional image registration. Conclusion: Image registration is critical for accurate interpretation of multimodality anatomic/functional small animal imaging. Techniques analogous to those in clinical use can be used with success in this setting.
WE‐D‐T‐617‐04: Development of a New Fiber Optic Scintillator Dosimeter System
Authors: Sliski A , Mitch M , Soares C
Published: June 2005

Purpose: To develop a new fiber optic scintillator dosimeter system with high sensitivity and spatial resolution for absorbed dose measurement of low‐energy x‐ray emitting brachytherapy sources in water. Method and Materials: A novel fiber optic scintillator dosimeter system has been developed to obtain the dose distribution in three dimensions in real time around low‐energy x‐ray‐emitting prostate brachytherapy seeds. A unique combination of small sensitive volume (0.5 mm diameter × 0.5 mm thick), novel scintillator geometry and low noise detector electronics allows unprecedented resolution in dose‐mapping of brachytherapy sources. High sensitivity and wide dynamic range is achieved by proprietary detector technology and signal processing methods, allowing measurements of the very low dose rates at distances of up to several centimeters from a seed mounted in a water phantom. A simple USB interface connects the dosimeter to a host PC for data acquisition and analysis. Results: The detector has been mounted in a water phantom and the dose distributions around I‐125 and Pd‐103 prostate brachytherapy seeds have been measured. A comparison of the results to published TG‐43 data for several seed models shows excellent agreement in most cases. The lower limit of practical dose‐rate measurements for low‐energy x‐ray emitting sources is estimated to be 1mGy / h. Conclusion: The development of this new, high sensitivity, high‐resolution fiber optic scintillator dosimeter, has allowed real‐time characterization of the dose distribution around prostate brachytherapy seeds in water. Compared to the state‐of‐the‐art TLD systems currently in use, this fiber‐optic scintillator‐based dosimeter system allows more rapid dose distribution measurements directly in liquid water instead of tissue‐equivalent plastic. Conflict of Interest: This work supported by a NIST SBIR grant SB1341‐03‐W‐0815