J Jin

Image-Guided Localization Accuracy of Stereoscopic Planar and Volumetric Imaging Methods for Stereotactic Radiation Surgery and Stereotactic Body Radiation Therapy: A Phantom Study

PURPOSE To evaluate the positioning accuracies of two image-guided localization systems, ExacTrac and On-Board Imager (OBI), in a stereotactic treatment unit. METHODS AND MATERIALS An anthropomorphic pelvis phantom with eight internal metal markers (BBs) was used. The center of one BB was set as plan isocenter. The phantom was set up on a treatment table with various initial setup errors. Then, the errors were corrected using each of the investigated systems. The residual errors were measured with respect to the radiation isocenter using orthogonal portal images with field size 3 × 3 cm(2). The angular localization discrepancies of the two systems and the correction accuracy of the robotic couch were also studied. A pair of pre- and post-cone beam computed tomography (CBCT) images was acquired for each angular correction. Then, the correction errors were estimated by using the internal BBs through fiducial marker-based registrations. RESULTS The isocenter localization errors (μ ±σ) in the left/right, posterior/anterior, and superior/inferior directions were, respectively, -0.2 ± 0.2 mm, -0.8 ± 0.2 mm, and -0.8 ± 0.4 mm for ExacTrac, and 0.5 ± 0.7 mm, 0.6 ± 0.5 mm, and 0.0 ± 0.5 mm for OBI CBCT. The registration angular discrepancy was 0.1 ± 0.2° between the two systems, and the maximum angle correction error of the robotic couch was 0.2° about all axes. CONCLUSION Both the ExacTrac and the OBI CBCT systems showed approximately 1 mm isocenter localization accuracies. The angular discrepancy of two systems was minimal, and the robotic couch angle correction was accurate. These positioning uncertainties should be taken as a lower bound because the results were based on a rigid dosimetry phantom.

Analysis of outcomes in radiation oncology: An integrated computational platform

Radiotherapy research and outcome analyses are essential for evaluating new methods of radiation delivery and for assessing the benefits of a given technology on locoregional control and overall survival. In this article, a computational platform is presented to facilitate radiotherapy research and outcome studies in radiation oncology. This computational platform consists of (1) an infrastructural database that stores patient diagnosis, IMRT treatment details, and follow-up information, (2) an interface tool that is used to import and export IMRT plans in DICOM RT and AAPM/RTOG formats from a wide range of planning systems to facilitate reproducible research, (3) a graphical data analysis and programming tool that visualizes all aspects of an IMRT plan including dose, contour, and image data to aid the analysis of treatment plans, and (4) a software package that calculates radiobiological models to evaluate IMRT treatment plans. Given the limited number of general-purpose computational environments for radiotherapy research and outcome studies, this computational platform represents a powerful and convenient tool that is well suited for analyzing dose distributions biologically and correlating them with the delivered radiation dose distributions and other patient-related clinical factors. In addition the database is web-based and accessible by multiple users, facilitating its convenient application and use.

Clinical Use of Dual Image-Guided Localization System for Spine Radiosurgery

The recently released Novalis TX linac platform provides various image guided localization methods including a stereoscopic X-ray imaging technique (ExacTrac) and a volumetric cone beam computed tomography (CBCT) imaging technique. The ExacTrac combined with the robotic six dimensional (6D) couch provides fast and accurate patient setup based on bony structures and offers “snap shot” imaging at any point during the treatment to detect patient motion. The CBCT offers a three dimensional (3D), volumetric image of the patients setup with visualization of anatomic structures. However, each imaging system has a separate isocenter, which may not coincide with each other or with the linac isocenter. The aim of this paper was to compare the localization accuracy between Exactrac and CBCT for single fraction spine radiosurgery treatments. The study was performed for both phantom and patients (96 clinical treatments of 57 patients). The discrepancies between the isocenter between the ExacTrac and CBCT in four dimensions (three translations and one rotation) were recorded and statistically analyzed using two-tailed t-test.

SU‐FF‐T‐394: An Image Guided Target Localization System for Brain Radiosurgery and Fractionated Stereotactic Radiotherapy Using a Non‐Invasive Fixation

Purpose: To develop an image guided target localization technique to improve the patient positioning accuracy for brainradiosurgery and fractionated stereotactic radiotherapy using non‐invasive fixation. Materials and method: BrainLab stereotactic localization box system is used for the initial patient setup. The patient is immobilized using a thermal mask with a mouth bite piece. An image guided localization system (Novalis body system, BrainLab) is used to finely adjust the position after the initial setup. The system automatically fuses two X‐ray images with the corresponding DRR and gives the position offsets in 6 dimensions. The accuracy of the image guided localization system was evaluated in a rando phantom with a 2‐mm‐metal‐ball placed as the isocenter, and in 15 patients with total of 48 fractions for image fusion. Results: The phantom study showed that the positioning accuracy is within 1‐mm for various isocenter positions. The image fusion for each patient was checked carefully and could be evaluated easily due to its rigid structure and rich bony features. At least three distinguish features could be identified in each image, and a 1‐mm translational move generally induced a visible dis‐matching in these features. This suggests that the accuracy of alignment to the isocenter of the X‐ray system be better than 1‐mm. Considering that the isocenter of the X‐ray system could be different from the linacs, the overall positioning accuracy for the image guided system would be in the order of 1.4‐mm. The offsets of the fusion results was used for mutual testing between the image guided system and the stereotactic localization box system. The average offsets for the lateral, AP and longitudinal directions were −0.67±1.09, 0.44±1.09, and −0.84±1.44 mm, respectively. This is consistentwith the results of the non‐invasive immobilization techniques reported by other authors. Conclusion: The image guided system could improve the patient positioning accuracy.

SU‐E‐T‐165: Systematic Evaluation of Uncertainties Associated with GAFCHROMIC EBT2 Film Dosimetry for 6MV Photon Beams

Method and Materials:Eight packets of films were exposed to 13.5cm ×13.5cm, 6MV radiation fields in a solid water phantom. Dose levels of 1.1, 3.2, 5.3, 7.4, and 9.5 Gy were delivered to five films in each packet. Films were scanned both before and after irradiation using an Epson flat‐bed scanner (24hr wait‐time for post‐irradiation coloration). Corresponding 2D dose distributions were measured with a detector‐array (MatriXX). Point dose comparisons were performed with an ion chamber. Digitized film images were registered to the 2D dose distribution to generate a correction map that compensated the scanner non‐uniform response as a function of dose. Optical density (OD) and net optical density (NetOD) values were calculated for all images. Dose response curves were established using mean values of a central 0.5cm × 0.5cm region‐of‐interest (ROI). Images were converted to dose, and error uncertainties (1SD) were measured in the central 8cm × 8cm ROI. Results: The overall dosimetric uncertainties (1SD) of the NetOD approach were 2.2%, 1.9%, and 3.5% for red, green, and blue channels, respectively. The corresponding uncertainties of OD were 2.7%, 3.1%, and 8.3%, respectively. For low dose range (<3 Gy), the green channel revealed higher uncertainty (SDgreen= 3.3%) than the red channel (SDred=2.6%). However, for high doses (3∼9 Gy), the green channel showed less variability (SDgreen=1.6%, SDred=2.9%). Minimum SDred and SDgreen were 1.6% at 5.3Gy and 1.3% at 7.4 Gy, respectively. Scanner non‐uniformity correction mitigated the irregular response of scanner detector elements observed initially. Conclusion: NetOD may be a more useful metric for benchmarking EBT2 than OD. We demonstrated that the lowest dose uncertainties were achieved using the red channel for low dose range, while the green channel was preferred for higher doses. Scanner non‐uniformity correction is necessary for higher precision dosimetry.

SU‐FF‐J‐01: Feasibility of a Pre‐Object‐Grid to Reduce Scatter and Improve Image Quality in Cone‐Beam Computed Tomography (CBCT)

Introduction: Scattered radiation is a major contributor to the degradation of image quality in Cone‐beam Computed Tomography(CBCT). The purpose of this study is to demonstrate a novel technique that samples scattered radiation during imaging acquisition, and subsequently account for it during image processing, thereby significantly improving CBCTimage quality. Methods and material: A grid composed of multiple, interspaced, lead strips is placed between the x‐ray source and the imaging object. The grid alternately blocks half of the entrance radiation and divides the imaging field into multiple strips of fan‐beam fields. In each projection, scatter is sampled in the shadowed strips, then interpolated and subtracted from the original imaging data in the fan‐beam fields. Half of the 3D‐CBCT images in an alternate pattern can be obtained from these projection data with one rotation. A complete set of CBCTimages can be obtained by two rotations in axial mode, or one rotation in helical mode. We have tested the feasibility of the method by comparing an incomplete set of CBCTimages of an enlarged Catphan phantom using the Varian Trilogy CBCT system using 3 settings: (A) without the grid; (B) with the grid, but without scatter correction; (C) with the grid including scatter correction. Results:CBCTimage quality improved dramatically using setting C (grid with scatter correction). Streaking artifacts around the air and high‐density inserts, apparent in A and B, were almost completely removed in C. CT number linearity (R2) was improved from 0.880 (A) and 0.967 (B) to 0.998 (C). Contrast scales were 0.42, 0.71 and 1.47 for A, B and C respectively. The corresponding contrast‐to‐noise ratios were 4.29, 6.60 and 6.42, respectively. Conclusion: Preliminary results suggest that the proposed pre‐object‐grid method can be used to substantially reduce scatter in projection data and therefore improve image quality in CBCT.

SU‐EE‐A2‐06: An Integrated Software Platform for Treatment Documentation and Outcome Analysis for Stereotactic Radiosurgery and Hypo‐Fractionated Stereotactic Body Radiotherapy

Purpose: Outcome analysis is an important and challenging task in radiationoncology. We have developed an integrated software platform that facilitates evaluation of outcomes for patients treated with stereotactic radiosurgery(SRS) and hypo‐fractionated stereotactic body radiotherapy(SBRT).Method and Materials: An outcomes‐study database was designed, under IRB‐approval, to store medical information of patients who undergo SRS or SBRT. The outcomes‐study database is integrated with a record‐and‐verify system (ARIA, Varian Medical Systems) and with an in‐house developed hospital database. A software package with GUI was developed using Visual Studio.Net (Microsoft, USA) to evaluate treatment plans. Biological dosemodels including TCP, EUD (Niemierkos model), NTCP (Lyman, Fdam, relative seriality models, etc.) as well as biologically effective doses are determined from dose distributions and DVHs, automatically imported from the treatment planning systems. Patient clinical data (from physician on‐treatment and followup visits) and image data (including planning CT scans and followup CT scans) are imported for outcomes analysis. Results: Thus far approximately 700 SRS/SBRT patients medical records have been populated into the outcomes database. The database contains all medical information, including demographic, social, diagnostic, treatment and follow‐up information. As it is linked to the hospital database,treatmentinformation encompasses multi‐disciplinary‐based treatments including surgery and chemotherapy. The web‐based interface enables access and information management remotely. The calculation of biological dose indices provides useful means to correlate dose distributions with clinical outcome with respect to tumor control and healthy tissue complications assessed from followup physical examinations as well as image data used to inspect possible recurrent disease or radiation‐induced damage. Conclusion: The goal of the database is to study the various factors of significance related to the outcomes of patients treated with SRS and SBRT, using non‐standard, hypo‐fractionated RT doses. Parameters specific to biological dosemodels will be updated using maximum likelihood analysis. Acknowledgement: NIH‐R01CA106770.

SU‐FF‐J‐79: Implementation of Four Different Image‐Guided Radiotherapy (IGRT) Systems in a Radiotherapy Department

Purposes: to implement and compare four newly developed image‐guidedradiotherapy systems (Varians Cone‐Beam CT, BrainLAB ExacTrac, Restitu Ultrasound (U/S)‐Sim and Guide, and in‐house stereovision) in one department. Methods and Materials: The cone‐beam CT(CBCT) and the ultrasound(US) systems provide volumetric images of the target at daily setup. The ExacTrac system acquires the biplanar radiographs at patient setup. Both the US and ExacTrac systems are integrated with infrared‐tracking systems for patient‐couch positioning. The in‐house stereovision system captures 3D surface images of the patient at the instants of daily patient setup and during individual beam irradiation. All of four IGRT systems have used treatment planning volumetric imaging information for target position verification and adjustment. Electronic portal images are routinely used for patient position verification. External markers and possible internal markers such as seeds or small cysts or calcifications can be localized and used for additional verification. Results: Emerging data from several institutional IRB‐approved clinical trials demonstrate that the target reposition error and dose delivery uncertainties can be significantly reduced by using such image‐guided systems, each of which may be most useful in specific clinical situations. Conclusions: Our customized stereovision system, which, like US, involves no radiation exposure, is extremely efficient (<2 minutes) and accurate (<2 millimeters) for superficial sites, such as breast cancer. The ExacTrac system appears ideal for lesions associated with bony structures, such as spine and skull. The US and CBCT may be most useful for deformable internal structures, such as prostate cancer. Special methods for dealing with imaging artifacts, such as ring patterns in CBCT, shadow casts and multiple reflections in stereovision and US, and patient motion in ExacTrac and stereovision will be presented.

WE-EF-207-08: Improve Cone Beam CT Using a Synchronized Moving Grid, An Inter-Projection Sensor Fusion and a Probability Total Variation Reconstruction

Purpose: To present a cone beam computed tomography (CBCT) system, which uses a synchronized moving grid (SMOG) to reduce and correct scatter, an inter-projection sensor fusion (IPSF) algorithm to estimate the missing information blocked by the grid, and a probability total variation (pTV) algorithm to reconstruct the CBCT image. Methods: A prototype SMOG-equipped CBCT system was developed, and was used to acquire gridded projections with complimentary grid patterns in two neighboring projections. Scatter was reduced by the grid, and the remaining scatter was corrected by measuring it under the grid. An IPSF algorithm was used to estimate the missing information in a projection from data in its 2 neighboring projections. Feldkamp-Davis-Kress (FDK) algorithm was used to reconstruct the initial CBCT image using projections after IPSF processing for pTV. A probability map was generated depending on the confidence of estimation in IPSF for the regions of missing data and penumbra. pTV was finally used to reconstruct the CBCT image for a Catphan, and was compared to conventional CBCT image without using SMOG, images without using IPSF (SMOG + FDK and SMOG + mask-TV), and image without using pTV (SMOG + IPSF + FDK). Results: The conventional CBCT without using SMOG shows apparent scatter-induced cup artifacts. The approaches with SMOG but without IPSF show severe (SMOG + FDK) or additional (SMOG + TV) artifacts, possibly due to using projections of missing data. The 2 approaches with SMOG + IPSF removes the cup artifacts, and the pTV approach is superior than the FDK by substantially reducing the noise. Using the SMOG also reduces half of the imaging dose. Conclusion: The proposed technique is promising in improving CBCT image quality while reducing imaging dose.

WE-EF-207-04: An Inter-Projection Sensor Fusion (IPSF) Approach to Estimate Missing Projection Signal in Synchronized Moving Grid (SMOG) System

Purpose: A synchronized moving grid (SMOG) has been proposed to reduce scatter and lag artifacts in cone beam computed tomography (CBCT). However, information is missing in each projection because certain areas are blocked by the grid. A previous solution to this issue is acquiring 2 complimentary projections at each position, which increases scanning time. This study reports our first Result using an inter-projection sensor fusion (IPSF) method to estimate missing projection in our prototype SMOG-based CBCT system. Methods: An in-house SMOG assembling with a 1:1 grid of 3 mm gap has been installed in a CBCT benchtop. The grid moves back and forth in a 3-mm amplitude and up-to 20-Hz frequency. A control program in LabView synchronizes the grid motion with the platform rotation and x-ray firing so that the grid patterns for any two neighboring projections are complimentary. A Catphan was scanned with 360 projections. After scatter correction, the IPSF algorithm was applied to estimate missing signal for each projection using the information from the 2 neighboring projections. Feldkamp-Davis-Kress (FDK) algorithm was applied to reconstruct CBCT images. The CBCTs were compared to those reconstructed using normal projections without applying the SMOG system. Results: The SMOG-IPSF method may reduce image dose by half due to the blocked radiation by the grid. The method almost completely removed scatter related artifacts, such as the cupping artifacts. The evaluation of line pair patterns in the CatPhan suggested that the spatial resolution degradation was minimal. Conclusion: The SMOG-IPSF is promising in reducing scatter artifacts and improving image quality while reducing radiation dose.