bluuebunny/crossref_metadata_2025 huggingface dataset:
SU‐FF‐J‐42: Analytical Image Guided Radiotherapy for Head and Neck
Authors: Barbiere J , Beninati G , Figueroa C , Hanley J
Published: July 2007

Purpose: Present a general cost‐effective robust clinical process that can analytically evaluate and correct patient setup error for head and neck radiotherapy by comparing orthogonal megavoltage portal images (PI) with corresponding digitally reconstructed radiographs (DRR). Method and Materials: Adobe Photoshop CS2 is used to interactively segment images. Scripts are available to automate many steps, including image enhancement. MATLAB is a software package with many features for rapid matrix computations and image analysis. The closest point distance (CPD) for each PI point to a DRR point forms a set of homologous points. The translation T that aligns the PI to the DRR is equal to the difference in centers of mass. The original PI points are transformed and the process repeated with an Iterative Closest Point (ICP) algorithm. Analytical results are displayed as a cumulative histogram of the percentage of points exceeding CPD. The total area Σ under the histogram is a general error metric. A color coded image is provided to display the anatomical location of the PI points with respect to the DRR and their CPD. Results: The ICP algorithm always converged. The integral Σ was consistently lower for the analytical IGRT, with a mean reduction of 16%. The process was tested retrospectively by three users. Typical user dependence was below 0.5 mm. The time for segmentation of two images and computations was under ten minutes. The process has been integrated with a Visual Basic Toolbox that guides the user through the required all the steps from importing the daily Portal Image, segmentation, computing the translation, and printing a standard report. Conclusion: Inexpensive commercial software has been shown to be useful for on line analytical IGRT. The process developed is robust, consistently better then IGRT, provides documented error metrics, and can be customized by the user for individual applications.
SU‐FF‐J‐46: Dosimetric Effects of Daily Localization for Prostate Cancer Patients Using MV‐CBCT
Authors: Gayou O , Reitz B , Parda D , Fuhrer R , Miften M
Published: July 2007

Purpose: To evaluate the effect of daily shifts observed with mega‐voltage cone beam CT (MV‐CBCT) localization on the IMRT dose distribution received by prostate cancer patients. Method and Materials: Eight patients who received a dose of 77.4 Gy to the PTV, which included the prostate and the seminal vesicles with a 1 cm margin, were selected for this retrospective study. Prior to each daily treatment fraction, the prostate was localized using MV‐CBCT, and the treatment couch position was corrected accordingly in the lateral (RL), longitudinal (SI) and vertical (AP) directions. The shifts for each of the 308 fractions were recorded, and the 308 corresponding dose distributions that the patients would have received if the shifts were not applied were calculated. Dose volume histograms (DVH) and mean dose for target and organs‐at‐risk were derived from these dose distributions, and compared to the treatment plan. Results: The average shifts for each patient were less than 6, 5 and 5 mm in the RL, SI and AP directions, respectively, with standard deviations ranging from 2 to 7 mm. The relative mean dose difference for the prostate was less that 1%, however effects as large as 15% and 20% were observed for the rectum and bladder, respectively. Rectum dose differences were correlated to AP shifts, while the bladder dose was affected by the SI shifts. Conclusion: For IMRT plans with a 1 cm margin, daily localization of the prostate is necessary to reduce the risk of bladder and rectum complication. The dose to these organs is very sensitive to systematic errors, while the effects of random errors cancel each other due to the essentially spherical shape of the dose distribution. Our results show that accurate patient positioning is an important step in any dose‐escalation and/or margin reduction strategy to further improve the therapeutic ratio.
SU‐FF‐J‐48: Three‐Dimensional MRI and Cone‐Beam CT Matching for Localization of Prostate Cancer Treatment
Authors: Wang Z , Wu Q J , Lee W R , Yin F
Published: July 2007

PURPOSES: There is a growing interest using MRI only for prostate cancer treatment planning. This study is aimed to investigate clinical feasibility of matching 3D planning MRI to on‐board cone‐beam CT (CBCT) images for target localization. Materials and Methods: Five patients treated with prostate cancer were analyzed in this retrospective study. In addition to conventional treatment planning CT, planning MRI images were acquired for each of these patients for better soft tissue delineation. The MRI images were fused to the planning CT during the clinical planning process. Nine sets of on‐board CBCT images were acquired prior to treatment from these patients. On‐board patient position correction was performed using planning CT‐CBCT matching for all the treatments. In this study, both CT‐CBCT matching and MRI‐CBCT matching were performed to localize the treatment isocenter. The couch shifts for isocenter placement from each matching were recorded to evaluate localization accuracy. The relative differences of the couch shifts between the MRI‐CBCT matching and CT‐CBCT matching were analyzed to assess the accuracy of the MRI‐CBCT matching relative to the CT‐CBCT matching. Results: The couch shifts in lateral, longitudinal and vertical directions based on the MRI‐CBCT matching are 0.18 ± 0.12 cm, 0.18 ± 0.16 cm, and 0.38 ± 0.12 cm, respectively. The differences of the couch shifts between the MRI‐CBCT matching and CT‐CBCT matching are 0.23 ± 0.10 cm, 0.29 ± 0.19 cm, and 0.30 ± 0.21 cm along the lateral, longitudinal and vertical directions, respectively. Conclusions: Target localization based on matching between 3D planning MRI and on‐board CBCT is feasible for the radiation treatment of prostate cancer.
SU‐FF‐J‐50: Dose‐Guided Radiation Therapy Strategies with Megavoltage Cone‐Beam CT
Authors: Pouliot J , Aubry J , Aubin M , Morin O , Chen J , Nunna C , Celi J
Published: July 2007

Dose Guided Radiation Therapy (DGRT) is an extension of adaptive radiation therapy where dosimetric considerations constitute the basis of treatment modification and validation. The objective of this work is to elaborate two DGRT scenarios, treatment‐time and progressive, and gather all the required functionalities within the same workstation to use it in a clinical environment.

The proposed DGRT strategies use a density‐calibrated Megavoltage Cone‐Beam CT (MVCBCT) image acquired at treatment time, the reference dose plan imported from the planning system and an independent dose calculation engine. A DGRT workstation was created to compare the initial plan with the dose distribution of the day. The beam arrangement is copied to the pre‐registered MVCBCT and dose distribution is calculated. Side by side comparison, dose difference maps and DVH differences are used to assess the clinical situation. Phantom experiments and patient data were used to test the DGRT scenarios. A Pinnacle dose calculation was performed to validate the local dose calculation engine.

The DGRT workstation integrates the following steps; image registration, correction and conversion of MVCBCT images into density, dose re‐calculation, dose comparison with the reference plan and a data extraction module for statistical analysis. Dose re‐calculation on MVCBCT images is performed with a precision better than 3%, enabling the progressive DGRT scenario. When performed on a patient treated for a base of tongue tumor with IMRT, the DGRT scenario showed a mean dose increase from 26 to 42 Gy in the left parotid.

DGRT brings together the knowledge of 3D imaging and the 3D dose distribution to assess the clinical situation. By making dose re‐calculation fast and accurate, the DGRT workstation allows dosimetric studies on a large scale. Results from on‐going clinical studies will determine how frequently significant dose differences occur and lead to site‐specific adaptation strategies. Research sponsored by Siemens OCS.
SU‐FF‐J‐54: A Hybrid Multiresolution Method for Automatic Registration Between Reference and On Board Digital Tomosynthesis
Authors: Ren L , Yan H , Godfrey D , Yin F
Published: July 2007

Purpose: Automatic registration between reference and on‐board digital tomosynthesis (DTS) is a critical step for target localization using DTS. We previously developed an approximate method for automatic registration between reference and on‐board DTS, in which reference DTS was not generated iteratively to match with the anisotropic resolution of on‐board DTS. This study aims at developing an accurate method for automatic DTS registration with improved efficiency, robustness and accuracy. Method and Materials: The accurate DTS registration method was realized mainly by hardware acceleration of DRR generation, which was achieved by implementing a 3D texturemap based volume rendering process in graphic card using OpenGL language. The alpha blending mode was chosen to simulate ray casting algorithm, and three color channels of graphic card were used for data storage. With hardware acceleration, the accurate method changes the CT pose and regenerates DRR and reference DTS iteratively to match reference DTS with on‐board DTS based on mutual information. A hybrid multiresolution method was developed to combine approximate and accurate methods in a multiresolution scheme. It was tested on both anthropomorphic chest phantom and H&N patient data with simulated translations and rotations. The registration efficiency, attraction range and accuracy were recorded and compared with previous results. Results: The hybrid multiresolution method is 4 times faster than approximate method. Its registration attraction range is 2–5 times larger than that of approximate method for single‐axis translations and rotations, and 1.5–2 times larger for combined 6‐D rotations and translations. The registration accuracy is within 1.2 deg/mm for phantom data and 1.5deg/mm for patient data through the whole attraction range. Conclusions: The hybrid multiresolution method is much more efficient, robust, and accurate than approximate method. It is potentially a practical and reliable method to clinically implement DTS for target localization.

Partially supported by a Varian research grant.
SU‐FF‐J‐55: Treatment Verification for Lung Cancer Patients Undergoing Fractionated Stereotactic Body Radiation Therapy (SBRT) Utilizing Cone Beam CT (CBCT)
Authors: Hammoud R , Patel S , Ajlouni M , Guan H , Pradhan D , Li S , Movsas B
Published: July 2007

Purpose: Recent increase in hypofractionated radiation therapy for early stage lung cancer necessitates accurate patient setup. The acquisition of CBCT prior to treatment provides a three dimensional setup and verification tool. We assessed the utility of CBCT for SBRT through examination of dosimetric parameters. Methods and Materials: Four patients with early stage non small cell lung cancer were used for the analysis. Each patient underwent a 4DCT simulation for internal target volume (ITV) determination. A five to seven field IMRT plan was generated for PTV = ITV + 3mm. Four fractions were delivered (12 Gy/fraction) and CBCT was acquired prior to each fraction. Online image registration between CBCT and simulation CT (simCT) was performed and the resulting shifts were applied to move the couch prior to treatment. In the offline evaluation, the GTV was contoured on each CBCT and compared to the ITV. To assess the ITV coverage, we overlaid the ITV onto CBCT (CBCT‐ITV) based on the shift data and performed dose calculations using the respective simCT plan. Dosimetric parameters including volumes receiving 95% (V95), 90% (V90), and minimum (min) dose were used for analysis. Results: The average set up shifts in the AP, SI, and LR directions were −0.16±0.33, −0.29±0.44, and 0.33±0.34 cm, respectively. CBCT volumes varied when compared to the respective ITV with a range of 0–50% decrease in volume. Due to this variability ITVs were used for the dosimetric analysis. The average PTV/CBCT‐ITV coverages for V95, V90, and min were: 94%/94.9%, 99.3%/96.7%, 85.4%/88.3%, respectively. Conclusion: The CBCT volumes were not representative of the ITV as seen by the volume discrepancy. Our dosimetric analysis showed good correlation between PTV and CBCT‐ITV coverage, supporting our current PTV margin (3mm). Our data affirms that CBCT provides further assurance in regards to target localization for hypofractionated SBRT of lung cancer.
SU‐FF‐J‐57: A Phantom Study On the 3‐D Target Localization Accuracy Using CBCT of An On‐Board Imager
Authors: Zhang L , Yan H , Yin F
Published: July 2007

Purpose: To evaluate the three‐dimensional target localization accuracy using CBCT of an On‐Board Imager (OBI) system by determining the detection error and residual error associated with image registration and patient repositioning. Method and Materials: An anthropomorphic pelvis phantom was used to simulate a range of shifts in the three translational directions and rotations around each of the three axes. First a simulated shift or rotation was applied. Then a CBCT scan was followed by 3D3D matching between the CBCT 3D volumetric image with the planning CT image. The detected shift vector was compared to the actual shift vector to calculate the detection error. Next the phantom was automatically realigned to the planned position by the OBI system. Subsequently, a second CBCT scan and 3D3D matching was performed to verify the re‐positioning accuracy (residual error). In each of the three translational direction, simulated shifts of 1, 2, 5 and 10 mm were evaluated. In the longitudinal direction large shifts of 20 and 50 mm were evaluated to see whether accuracy was dependant on shift magnitude. Simulated rotations of 1, 3 and 5 degrees around the three axes were also evaluated. Results: On the average, the detection error for translational shifts is 0.4+/−0.3 mm. The average of the corresponding residual error is 0.6+/−0.4 mm. For simulated rotations the average detection error is 0.2+/−0.1 degrees. The average of the corresponding residual error is 0.4+/−0.5 mm. Conclusion: The results indicate that CBCT is capable of detecting target shifts and patient rotation to within 1 mm and 0.5 degree respectively. The shifted target could be realigned to the planned isocenter to within 1mm accuracy when correcting for translational shifts and within 1.8 mm when correcting for rotations.

This study is partially supported by a Varian research grant.
SU‐FF‐J‐62: Analysis of the Respiratory Motional Effect On the Cone‐Beam CT Image
Authors: Song JY , Nah BS , Chung WK , Ahn SJ , Nam TK , Yoon MS
Published: July 2007

Purpose: To evaluate the respiratory motional effect on the cone‐beam CT (CBCT) image quality in the region of inhomogeneous structures. Method and Materials: A phantom system was constructed in order to simulate respiratory motion. One part of the system is composed of a moving plate and a motor driving component which can control the motional cycle and motional range. The other part is solid water phantom containing a small cubic phantom (2 cm × 2 cm × 2 cm) surrounded by air which simulate a small tumor volume in the lung air cavity. CBCT images of the phantom were acquired in 20 different cases and compared with the image in the static status. The 20 different cases are constituted with 4 different motional ranges (0.7 cm, 1.6 cm, 2.4 cm, 3.1 cm) and 5 different motional cycles (2, 3, 4, 5, 6 sec). The difference of CT number in the coronal image was evaluated as a deformation degree of image quality. Results: The relative pixel intensity values as a compared CT number of static CBCT images were 71.07% at 0.7 cm motional range, 48.88% at 1.6 cm motional range, 30.60% at 2.4 cm motional range, 17.38% at 3.1 cm motional range. The tumor phantom sizes which were defined as the length with different CT number compared with air were increased as the increase of motional range (2.1 cm: no motion, 2.66 cm: 0.7 cm motion, 3.06 cm: 1.6 cm motion, 3.62 cm: 2.4 cm motion, 4.04 cm: 3.1 cm motion). Conclusion: This study shows that respiratory motion in the region of inhomogeneous structures can degrade the image quality of CBCT and it must be considered in the process of setup error correction using CBCT images.
SU‐FF‐J‐63: Comparison of Daily Setup Corrections Using On‐Board Cone‐Beam Computed Tomography, and Two Planar Kilovoltage Imaging Systems
Authors: Saleh H , Dogan N , Williamson J
Published: July 2007

Purpose: The purpose of this study is to compare bony landmark‐based setup corrections derived from a linac gantry‐mounted kilovoltage (kV) imaging system that can either be used in orthogonal radiography (OBI) or kV circular orbit computed tomography (CBCT) imaging mode. A third commercial image‐guided radiotherapy system, consisting of orthogonal room‐mounted planar x‐ray imaging system, Exac Trac (ET), was also tested. In addition to daily setup corrections, imaging doses were compared. Methods and Materials: Both patient and phantom data were used to perform this study. Five patients were selected and couch shifts ware determined using both the OBI and ET systems. Using a pelvic phantom, intentionally introduced shifts of 0, 5, and 20 mm in the anterior/posterior (AP/PA), superior/inferior (S/I), and lateral directions, respectively, were applied. The estimated couch shifts were compared using all three Image Guided Radiotheraphy (IGRT) systems. Two cranial cases, one liver case, one lung case, and one prostate case were selected and the estimated couch shifts using the OBI and the ET systems were determined while the patient is in place. Couch shifts are not applied until all imaging modalities are performed. Results: Comparison of simulated shifts using all three imaging modalities showed a standard deviation of setup uncertainties is within 1 mm in all directions. Patients' data showed that the biggest difference is in the order 4 mm in the superior/inferior direction for liver patient and 2 mm in the superior/inferior direction for cranial patient. Conclusions: This study showed that while using 3D/3D image fusion is an excellent choice for online daily patient setup, it is possible to determine couch shift to high degree of accuracy using either 2D/2D or 2D/3D image fusion. For lung and liver patient, tracking of organ motion must be used to any imaging modality.
SU‐FF‐J‐64: The Role of Seminal Vesicle Motion in Target Margin Assessment for Online Image Guided Prostate Cancer Radiotherapy
Authors: Liang J , Wu Q , Yan D
Published: July 2007

Purpose:. For intermediate and high risk prostate cancer, both prostate gland and seminal vesicle (SV) should be included in CTV. However, motion patterns of prostate and SV could be different resulting in different margin requirement. Our purposes are to investigate motion of SV and its role at the proper margins requirement on two online image guided correction methods. Method and Materials: Twenty‐five patients with at least 16 daily CTs per‐patient were included in this study. Contour to contour based image auto‐registrations were performed to obtain organ daily motions.

Two online image guided correction methods were simulated by using conventional 4‐field treatment plan. Margins were designed to ensure that the minimal CTV cumulative dose after correction was within 2% dose reduction for at least 95% of patients. The first guidance used prostate daily translational displacement for patient online correction. In this case, two margin designs were analyzed. Margin‐A was designed as uniform margin for whole CTV. Margin‐B was designed for SV only meanwhile a fixing 3 mm uniform margin for prostate. The second guidance used the whole CTV translational displacement to determine the online correction. In this case, a uniform margin‐C was designed for whole CTV. The cumulative dose was constructed based on deformable organ registration. Results: The motion of SV was (0.1±1.7, 0.0±6.6, 0.1±3.7) mm, which was significantly larger than the prostate motion of (−0.2±1.0, −1.2±4.4, −0.1±3.1) mm. The margin‐A, margin‐B and margin‐C produced similar PTVs which were PTV=CTV+4mm, (Prostate+3mm)+(SV+4mm) and CTV+3mm. The corresponding PTV volumes were 129±30, 121±28 and 116±28cc accordingly. Conclusion: SV can move significantly larger than prostate, additional margins are needed but less essential in online correction with conformal dose delivery of prostate cancer treatment mainly due to the shallower dose gradient in the SV region.

Partially supported by NIH grants CA118037 and CA091020.
SU‐FF‐J‐66: Inter and Intra‐Fractional PTV Margins for Non‐Sedated Intracranial Pediatric Patients
Authors: Beltran C
Published: July 2007

Purpose: To investigate the proper PTV margin for non‐sedated intracranial pediatric patients based on inter and intra‐fractional motion using a reflector array radiocamera system. Method and Materials: The patients were immobilized and simulated with a custom thermoplastic facemask and bite block. Attached to the bite block was a frame containing an array of fiducial reflectors. The treatment planning CT was registered with the fiducial frame software. During the first day of treatment, films were taken to ensure proper setup and to record the baseline position of the array. The position of the array was continuously tracked by infrared cameras and their deviations from baseline recorded at a frequency of 10 Hz. Based on the method described by van Herk et al, an inter‐fractional, intra‐fractional, and combined margin were calculated. Results: Twenty one patients were treated in 2006 with an average of 25.7 +/− 6.9 treatment fractions for which motion data was acquired. The average age was 15.7 +/−4.3 years. There was an average of 4.2 +/− 1.4 couch positions. With half of the inter‐fractional data analyzed, the preliminary inter‐fractional margins are 2.16 mm PA, 2.29 mm LR, and 2.28 SI. Based on one quarter of the intra‐fractional data, the preliminary intra‐fractional margins are 0.58 mm PA, 0.37 mm LR, and 0.62 SI. The preliminary combined margins are 2.24 mm PA, 2.32 mm LR, and 2.37 mm SI. Conclusions: Based on the preliminary data, a PTV margin of 2.5 mm is feasible for intracranial pediatric patients treated with radiocamera monitoring. If the patient is re‐positioned after each couch movement, a 0.75 mm PTV margin for intra‐fractional motion is potentially achievable. The ability to re‐position to the sub millimeter level requires further investigation.
SU‐FF‐J‐69: A Unified Framework for Planning Uncertainty and Delivery Uncertainty in Inverse Planning
Authors: Chen Q , Lu W , Chen M , Ruchala K , Olivera G
Published: July 2007

Purpose: For radiation therapy, two kinds of uncertainties exist. The first is planning uncertainty, the uncertainty of identifying tumor from the diagnostic images. This uncertainty may come from tumor's microscopic spread, the image distortion, etc. The second is delivery uncertainty. This includes setup errors, patient intrafraction and interfraction motions. The ICRU adopted a simple solution to the problem, adding hard margins to GTV to form CTV, ITV, and PTV. Margin improves tumor coverage but increases dose to OAR. Attempts have been made to discard the hard margin around CTV and incorporate the delivery uncertainty into the inverse planning and let the optimizer to decide what the best is for patient. However, the planning uncertainty was still handled with hard margins. In this work, we developed a framework that integrates both effect in the same model. Methods: We establish our study in the dose delivery coordinate system, each voxel has certain probability being tumor, and certain probability being OAR. This probability can be a result of setup error, organ motion/deformation, microscopic spread of tumor, or a combination of all the effects. For each voxel receive certain amount of dose, we calculate its contribution to objective function as tumor weighted by the probability of it being tumor and its contribution as OAR weighted by the probability of it being OAR. This objective function is used for inverse planning. Results: We proved that our framework is equivalent to previous approaches in handling motion uncertainty. Furthermore, our framework is more computationally efficient than previous approaches. Conclusion: We developed a unified framework to handle planning uncertainty and delivery uncertainty inside the inverse planning process. This framework allows us to determine the optimized plan for patient with the consideration of all uncertainties. It is a better approach than simply adding a predetermined hard margin.
SU‐FF‐P‐07: Monitor and Reducing Patient Radiation Exposure From Fluoroscopically‐Guided Procedures in a Teaching Hospital
Authors: Kuan H , Rivera H , Ferretti J , Lawson W
Published: July 2007

Purpose: Radiation dosimetry information from fluoro‐guided procedures was monitored timely for patient safety and for training of residents. Methods: Fluoro‐guided procedures were regularly conducted in our hospital‐based radiology core area (R/F rooms for GI, BE and other procedures), operating rooms, radiology special procedure suites, cardiac catheterization labs, and out‐patient clinics (ambulatory surgery, orthopedics, pain management, urodynomics, etc). Quality control and patient dosimety programs were managed by medical physicists. Results: Records on fluoro times/techniques for all procedures were reviewed on daily basis, and results discussed with respective attendings/residents who performed the procedure. Analysis of accrued data showed that all residents, after a learning period, were able to complete a general procedure in less than 5 minutes fluoro time. Interventional procedures were found to have some patient exposures at high radiation doses. Conclusions: Vigilant monitoring and closely interacting with practicing physicians and technologists were essential in helping to improve a physician's technique toward reducing fluoro times necessary. Additional attention was warranted to patient exposure in interventional procedures since some high doses were found.
SU‐FF‐T‐01: Effective Source Distance and Virtual Source Location for MLC Based Electron Radiotherapy
Authors: Tynan P , Stathakis S , Esquivel C , Shi C , Papanikolaou N
Published: July 2007

Purpose: To determine the effective source distance (ESD) for electron fields used in modulated electron radiotherapy (MERT) using an existing photon multileaf collimator and to examine their behavior with changes in field size and beam energy. Methods and Materials: In order to accurately calculate the MU needed for MERT treatment the electron effective source distance should be determined. ITwo methods were used to determine the virtual source positions of electron beams at energies of 6, 9, 12, 16, and 20 MeV. For field sizes varying from 2×2 cm2 to 10×10 cm2, parallel plate ion chamber measurements in a plastic water phantom were taken, and the inverse slope method was used to determine the ESD for each field at each energy. In addition, beam profiles measured with film were used to determine the virtual source location (VSL). Results: The ESDs calculated using the inverse slope method increased with increasing field size and energy. It was observed that the ESD increase with field size was less steep at high energies than at lower energies. Comparing the ESD and VSL measurements for a 10×10 cm2 field, it was observed that the VSL was slightly larger than the ESD at both 9 and 16 MeV electron beams. Conclusions: The ESDs measured were strongly dependent on both field size and electron beam energy. Differences between ESD values may be due to differences in the electron beam scatter that is energy dependent. Discrepancies between ESD and VSL values may be due to the method used to measure ESD which more closely matches clinical conditions than the film VSL method. Determination of the effective source distance can be used for correct MU calculations for modulated electron radiotherapy.
SU‐FF‐T‐03: 3D Characterization of Small Megavoltage Radiation Fields Using An Optical CT Scanner with Ferrous Xylenol Orange Gels
Authors: McNiven A , Babic S , Jordan K , Battista J
Published: July 2007

Purpose: To investigate the feasibility of using a 3D dosimetry system consisting of a radiochromic ferrous xylenol orange (FX) gel and a prototype commercial cone beam optical‐CT scanning system (Vista™, Modus Medical Devices, Inc., London, ON) to dosimetrially characterize small megavoltage photon fields. Method and Materials: An FXG gel was irradiated using 6MV x‐ray fields (30, 10 and 5mm diameter) produced via custom stereotactic collimators mounted on a Varian Clinac 2100EX accelerator. Measurements were also completed using radiochromic (Gafchromic EBT) and radiographic (Kodak EDR2) film, and a Exradin #a1SL ion chamber for comparison purposes. The gel was optically CT scanned 10, 20, 30 and 40 minutes post‐irradiation to assess the effects of xylenol orange/ferric ion complex diffusion in gel. Lateral dose profiles and percentage depth dose (PDD) curves were extracted from the 3D image data sets for analysis. Results: FX gels together with fast optical CT scanning allows for the measurement of all dosimetric parameters required as pencil beam input for treatment planning systems in a single irradiation. The experimental set‐up is simple, and the gel allows for dose measurements in a tissue‐equivalent and energy‐independent medium. Lateral beam profiles as measured in the FXG gel matched those measured using both types of film within 2 mm. PDD curves measured with gel and ion chamber were also in good agreement (within 3 mm) up to 65mm depth. Conclusion: The radiochromic FX gel and fast optical CT scanner is a promising dosimetry system for the characterization of small x‐ray fields for the purposes of commissioning treatment planning systems. The advantage is that a full 3D data set can be acquired quickly and efficiently. Conflict of Interest: Two of the authors (KJ and JB) have a licensing agreement with Modus Medical Devices, Inc. for the commercialization of the Vista™ scanner.
SU‐FF‐T‐05: 4D Dosimetry in Radiotherapy Using Optical Flow and Monte Carlo Techniques
Authors: Huang T , Park K , Forster K , Stevens C , Zhang G
Published: July 2007

Purpose: Thoracic tumors present difficulties in dosimetry due to the respiratory motion and inhomogeneity. This study demonstrates that the combination of Monte Carlo dose calculation and 4D dose handling using deformable image registration provides a way to accurately calculate 4D dosimetry for such cases. Method and Materials: Breath‐hold and 4D CT images of lung cancer cases were used in this proof‐of‐concept study. A 4D CT image set of a phantom is also included in this study to compare the calculated 4D dose with measurements. A 3‐D conformal treatment plan was designed on the maximum expiration image volume. The treatment plan isocenter was placed at the tumor centre of the maximum expiration image set. BEAMnrc, a Monte Carlo simulation user code based on EGSnrc, a package for the Monte Carlo simulation of coupled electron‐photon transport, was used in the dose calculations for multiple respiration phases. The 3D optical flow program, which uses image‐intensity‐gradient based deformable registration algorithm, was applied to the calculation of the displacement matrices between the CT images of different respiration phases. The matrices were then used to map the dose distributions of all the respiration phases to the total dose. Results: The 4D phantom study showed that the calculated doses and the measurements agreed with in 2% in the center of the tumor insert. In a left lung tumor case, the tumor motion range was about 0.5 cm. Although the dose at the center of the tumor did not change much, the 4D integrated isodose lines moved in the direction of the tumor motion comparing to the static plans, which is consistent to the expectation. Conclusion: 4D dosimetry using Monte Carlo simulation and deformable image registration provides a promising way to handle thoracic cancer radiation dose calculation.
SU‐FF‐T‐09: A Convolution Method to Evaluate Setup Uncertainties in External Radiotherapy
Authors: Perez‐Rozos A , Sainz I Jerez , Rodríguez J Carrasco
Published: July 2007

Purpose: We are going to evaluate the setup uncertainty in external radiotherapy using a convolution method, comparing the processed dose matrix with the original one. Confidence intervals in dose volume histograms and dose to points can be made and visualized. Method and Materials: The dose matrix calculated with Philips Pinnacle (v. 7.4f) treatment planning system is exported in binary format. Using OREX CR portal image system the setup uncertainty of different localizations has been determined and approximated by Gaussian probability distributions N(0,σ). These distributions are used as a convolution kernel. . The convolution of this kernel and the dose matrix estimates the setup uncertainty. We also calculate the standard deviation, , and applying the central limit theorem, we can establish 95% confidence intervals on dose to points and dose volume histograms. The new calculated dose matrix, D+2σ, D−2σ, and |2σ|/DC are introduced in the treatment planning system for comparison purposes. The process of convolution and auxiliary calculations is performed with a home made Fortran program that can be executed directly by the treatment planning system using a script. Results: We applied this method to several localizations. The approximation of setup uncertainties to Gaussian distribution must be assessed previously, in other case the user must use a different convolution kernel. The convolution method fails near the surface because the differences between calculated doses and setup doses are not valid, but bearing in mind this, the method shows an useful estimation of the uncertainty. Conclusion: With this method the estimation of the setup uncertainty, with real site data, can be evaluated directly in the treatment planning system. The margins to organ at risks and PTVs could be evaluated, and confidence intervals in dose volume histogram could be established. The comparison of the uncertainty between treatment plans could support a clinical choice.
SU‐FF‐T‐10: A Distributed‐Computing Framework Using Nested Partitions for Beam‐Angle Optimization in IMRT
Authors: Nazareth D , Jones M , Liu J , Kuettel M , Furlani T , Podgorsak M
Published: July 2007

Purpose: Commercial IMRT treatment planning software currently solves the dose optimization (DO) problem, but does not consider additional variables, such as gantry, collimator, or couch angles. These parameters are generally specified by clinicians based on experience, and are often arbitrary and suboptimal. Including the additional variables in the treatment planning process increases the size of the combinatorial problem, and would require access to vast computational resources and novel optimization techniques. We have formed a collaboration with a leading academic supercomputer center and implemented the Nested Partitions (NP) algorithm, a versatile and efficient method, to produce IMRT plans which employ optimal beam angles. Method and Materials: NP is a metaheuristic algorithm which is flexible enough to guide the search of a heuristic or deterministic dose optimization algorithm, and is well suited for implementation on distributed‐computing platforms. We implemented the NP framework in conjunction with the dose‐optimization algorithm employed in a commercial treatment planning system. We performed a study on a 5‐field prostate case by executing the programs on four machines simultaneously, and evaluated the resulting treatment plans based on DVH constraints for the target and critical structures. Results: After only three iterations of the NP algorithm, the resulting plan realized an improvement of 14% in a measure of beam‐angle‐sample quality over a clinical plan employing equi‐spaced beams. Since each iteration was implemented simultaneously on multiple machines, a decrease in execution time by a factor of about 4 was observed. Conclusion: Including additional variables, such as beam angles, in the IMRT optimization problem results in a vastly increased search space. However, our results indicate that the use of a distributed‐computing platform combined with a powerful metaheuristic optimization algorithm can result in treatment plans which provide better target coverage and normal tissue sparing, and can be generated more efficiently than those currently produced.
SU‐FF‐T‐15: A Kerma‐Dose Hybrid Scheme for Variance Reduction in Monte Carlo Dose Calculations of Photon Beams
Authors: Sheu R , Chui C , Mageras G , Lovelock D , Yang J
Published: July 2007

Purpose: To improve the performance of Monte Carlo dose calculation for photon beams, a kerma‐dose hybrid dose calculation scheme is introduced in this work. Material and methods: Due to the high scoring efficiency of kerma, the statistical uncertainty of kerma can be reduced to a lower level than for dose with the same number of histories. However, kerma only equals dose in regions where charged particle equilibrium (CPE) exists. Our approach is to first determine the regions where CPE exists and then replace the dose by kerma to improve statistical accuracy in these voxels. To perform this hybrid kerma‐dose calculation, the kerma and the energy imbalance function are simultaneously recorded for each voxel at the time of dose calculation. The energy imbalance function (Q) is defined as , where Ei is the initial energy of the i‐th charged particle starting its track in the voxel and edepj is the energy deposited by the j‐th charged particle passing through this voxel. In a voxel where CPE exists, Q should be close to zero. In the current work, dose is replaced by kerma in voxels where Q is less than 10% of kerma. Results: Calculations were done in a multi‐layer slab geometry and a CT voxel phantom. In both geometries, the results show that the energy imbalance function is sufficiently sensitive to distinguish regions where CPE exists. The replacement of dose by kerma significantly reduces the statistical uncertainty (by a factor of 3) and the isodose curves become much smoother in those regions. Conclusions: The proposed hybrid scheme can significantly reduce the statistical uncertainty in the dose calculation while accurately accounting for the inhomogeneity effect caused by loss of CPE.
SU‐FF‐T‐16: A Linear Programming Model and Solution for Radiation Shielding Design of Radiotherapy Treatment Vaults
Authors: Newman F , Asadi‐Zeydabadi M
Published: July 2007

Purpose: To optimize shielding material combinations and thicknesses for megavoltage radiotherapy accelerator vaults for any photon energy using linear programming by minimizing the costs of shielding materials while satisfying the constraints pertaining to regulatory doses and available space. Methods: Radiotherapy accelerator vault construction usually requires an experienced physicist to design the appropriate thicknesses of concrete, steel or lead to satisfy the regulatory limiting doses, available thicknesses for shielding and the budget allocated for the project. We have developed a model and solution using linear programming that minimizes the costs of the shielding materials while satisfying regulatory dose limits and space available for the project. The model considers primary, leakage and scatter x‐ray radiation and neutron contamination using calculation techniques as in NCRP 151. The optimization routine will insert the required tenth value layers (TVLs) of steel or lead depending on the demands and the cost of these materials. The optimization considers neutron contamination as well. We discuss a parameterized version of the model that is never infeasible and which allows the user to weight the space versus cost according user preference. Results: The results of the linear program can be easily checked for correctness using standard calculation techniques. If there is abundant space for the wall thickness, concrete only is the result of the optimization since it has the lowest cost. If the allowed wall thickness is too small for concrete only, the optimization will return combinations of concrete, steel or lead such that all constraints are satisfied and costs are minimized. Existing thicknesses of concrete, for example, are easily incorporated into the model as are special requirements for IMRT. Conclusion: The linear program returns reliable, viable and optimized shielding thicknesses of concrete, steel and lead for primary barriers as well as for leakage and scatter radiation barriers.
SU‐FF‐T‐17: A Manual MU Check Calculation Procedure for Step‐And‐Shoot IMRT
Authors: Loyd M
Published: July 2007

Purpose: Present a method to calculate IMRT plan weight or dose to a point from IMRT plan MU settings, capable of being performed manually or with a simple spreadsheet. Applicable to step and shoot IMRT plan MU check calculations. Show how this method corelates to QA measurement results. Method and Materials: Employ beam data tables commonly available for calculating MU settings for 2D and 3D plans. Use simplifying assumptions on the beam equivalent size and the number of MU to apply to producing dose directly and from scatter or leakage to the point of interest based on geometric considerations. The procedure and assumptions will be described. The MU check results will be compared to the beam IMRT plan QA measurement pass/fail results. A 2D diode array was employed for IMRT plan QA measurements. Results: 42 IMRT step and shoot plans have been evaluated. Plan point dose calculated with this procedure fell within 2.5% of the plan value for 35 of the 42 cases. None of these plans failed the measured QA criteria of 3%dose / 3.0 mm DTA gamma evaluation or the 5% point dose measurement criteria. 7 plans had MU point dose results differing by > 2.5%. 2 of these 7 plans failed either the dose measurement QA test or the gamma test. Conclusion: This calculation method shows promise as a plan evaluation tool.
SU‐FF‐T‐19: A Method for Extracting the Relevant MRI Information From Normoxic Polymer Gels Exposed to Low Doses
Authors: Martin R , BenComo J , Martin M , Bankson J , Heard M , Kaluarachchi K , Webb D , Wells N , Ibbott G
Published: July 2007

Purpose: To extract the contribution due to the formation of polymer at low dose from the MRI multi‐echo signal using a numerical method for Laplace transform inversion and to analyze the transverse relaxation rate spectrum Materials and Methods: Several samples of MAGIC gel (9% by weight of methacrylic acid) were prepared and irradiated with and 6 MV photon beams to doses in the range of 0.1 to 0.5 Gy. The samples were scanned in a 7 Tesla MRI Bruker BioSpec using a Carr‐Purcell‐Meiboom‐Gill sequence. The signals were analyzed with the inversion algorithm. Samples of pure gelatin and a combination of gelatin and methacrylic acid were scanned in an MR spectrometer from Bruker. The samples were also scanned in an optical CT. Conclusions: Data analysis showed that polymer formation can be used as a better parameter than the average transverse relaxation rate for polymer gel dose calibration at low dose. The chemical shift spectrum showed that gelatin was not affected by radiation. The optical CT scan correlated well with the other results.
SU‐FF‐T‐21: A Method to Determine the Gantry Isocentricity with a Portal Imager
Authors: Easterling S , Ting J , Scarbrough T
Published: July 2007

Introduction: Historical method of verifying the linear accelerator gantry isocenter depended on the use of “star shots” with film. This method is subjective and its accuracy is in doubt when the expected results are in the range of 1.0 mm or less. In addition, collimator jaw symmetry and movements affect the measurement accuracy of the gantry isocentricity.

Modern linear accelerators are capable of isocentricity less than 1.0 mm radius and modern radiation therapy departments are film‐less.

The purpose of this investigation is to establish a different method to verify the isocenter accuracy during the initial acceptance testing and routine physics QA. Because modern linear accelerators are equipped with electronic portal imagers, this investigation uses the portal imager to determine the gantry isocenter accuracy. Methods and Materials:Out‐line of the procedure:

(1) Check the cross‐hair mechanical rotational isocentricity.

(2) Setup a metallic object at the isocenter using the light‐field and the collimator cross‐hair.

(3) Take a static portal image of this metallic object with gantry pointing vertically down.

(4) Without moving the portal imager, take a portal image of this object while the gantry makes a 360‐degree arc rotation with the same monitor unit as the static exposure.

(5) Analyze the full‐width‐half maximum (FWHM) of each or subtracting these two images. Results and Discussions: If the gantry isocentricity is perfect, the two images would be identical and the FWHM will be identical. But, if the gantry isocentricity is a 2.0 mm diameter, the FWHM will reflect that as well. Advantages of this method are:

(1) Less subjective and less dependent on observers.

(2) Able to detect isocentricity less than 1.0 mm diameter.

(3) Results are immediately available for analysis.

(4) No films.

(5) Results are electronic and can be analyzed by using readily available tools and methods.
SU‐FF‐T‐28: A New Concept of Multileaf Collimator (dual‐Layer MLC)
Authors: Oh S , Jung W , Suh T
Published: July 2007

Multileaf collimator (MLC) is widely used dose delivery device for intensity‐modulated radiation therapy (IMRT). The MLC is convenience to use and to adjust the field shape and beam fluence. However, there are some limitations to improve. One of them is longer beam‐on time. As beam‐on time is longer, patient's integrated dose is unnecessarily increased because of inter‐leaf leakage. In this study, a new concept of MLC (dual‐layer MLC) was proposed to solve this limitation in the aspects of total delivery intensity or monitor unit. The dual MLC consists of independently movable two MLC layer; upper and lower. Their thicknesses are 1 half value layer (HVL) for upper MLC and 5 HVL for lower. The initial intensity (I) of photon beam is attenuated half intensity (0.5I) if the beam is passing through the upper MLC only. Therefore the intensities are able to be delivered three steps; 0, 0.5, and 1. The sequencing algorithm was modified to dual‐layer MLC in step and shot technique. To evaluate the usefulness of a new concept, the total delivery intensity and number of segments were compared with conventional MLC with by three field sizes (5×5, 10×10, 15×15, and 20×20) and four intensity levels (5, 8, 10, and 16). 10000 intensity maps were randomly created for each field size and intensity level. As a result, the total delivery intensity was reduced by 9% to 22% in all field size and number of segment was reduced by 25% to 45%. The unnecessary patient' dose is reduced effetely with dual‐layer MLC because the required total delivery intensity was successfully reduced. The treatment time is also reduced.