Patient Position Correction Research Articles

Beam‐centric algorithm for pretreatment patient position correction in external beam radiation therapy

In current image guided pretreatment patient position adjustment methods, image registration is used to determine alignment parameters. Since most positioning hardware lacks the full six degrees of freedom (DOF), accuracy is compromised. The authors show that such compromises are often unnecessary when one models the planned treatment beams as part of the adjustment calculation process. The authors present a flexible algorithm for determining optimal realizable adjustments for both step-and-shoot and arc delivery methods. The beam shape model is based on the polygonal intersection of each beam segment with the plane in pretreatment image volume that passes through machine isocenter perpendicular to the central axis of the beam. Under a virtual six-DOF correction, ideal positions of these polygon vertices are computed. The proposed method determines the couch, gantry, and collimator adjustments that minimize the total mismatch of all vertices over all segments with respect to their ideal positions. Using this geometric error metric as a function of the number of available DOF, the user may select the most desirable correction regime. For a simulated treatment plan consisting of three equally weighted coplanar fixed beams, the authors achieve a 7% residual geometric error (with respect to the ideal correction, considered 0% error) by applying gantry rotation as well as translation and isocentric rotation of the couch. For a clinical head-and-neck intensity modulated radiotherapy plan with seven beams and five segments per beam, the corresponding error is 6%. Correction involving only couch translation (typical clinical practice) leads to a much larger 18% mismatch. Clinically significant consequences of more accurate adjustment are apparent in the dose volume histograms of target and critical structures. The algorithm achieves improvements in delivery accuracy using standard delivery hardware without significantly increasing total treatment session duration. It encourages parsimonious utilization of all available DOF. Finally, in certain cases, it obviates the need of a robotic couch having six DOF for the correction of patient displacement and rotations.

The Importance of Correct Patient Positioning in Theatres and Implications of Mal-Positioning

Patient positioning in theatre pertains to how a patient is transferred and positioned for a specific procedure. Patient safety is a central focus of care within the NHS and every healthcare practitioner must ensure that patients are protected from harm where possible. Mal-positioning of the patient has important implications in terms of associated problems of pressure sores, nerve compressions, deep vein thrombosis and compartment syndrome, and should be avoided.

Dosimetric consequences of uncorrected setup errors in helical Tomotherapy treatments of breast-cancer patients

Background and purpose The Tomotherapy Hi-Art II system allows acquisition of pre-treatment MVCT images to correct patient position. This work evaluates the dosimetric impact of uncorrected setup errors in breast-cancer radiation therapy. Materials and methods Breast-cancer patient-positioning errors were simulated by shifting the patient computed-tomography (CT) dataset relative to the planned photon fluence and re-computing the dose distributions. To properly evaluate the superficial region, film measurements were compared against the Tomotherapy treatment planning system (TPS) calculations. A simulation of the integrated dose distribution was performed to evaluate the setup error impact over the course of treatment. Results Significant dose differences were observed for 11-mm shifts in the anterolateral and 3-mm shifts in the posteromedial directions. The results of film measurements in the superficial region showed that the TPS overestimated the dose by 14% at a 1-mm depth, improving to 3% at depths ⩾5 mm. Significant dose reductions in PTV were observed in the dose distributions simulated over the course of treatment. Conclusions Tomotherapy’s rotational delivery provides sufficient photon fluence extending beyond the skin surface to allow an up to 7-mm uncorrected setup error in the anterolateral direction. However, the steep dose falloff that conforms to the lung surface leads to compromised dose distributions with uncorrected posteromedial shifts. Therefore, daily image guidance and consequent patient repositioning is warranted for breast-cancer patients.

In vivo verification of proton beam path by using post‐treatment PET/CT imaging

The purpose of this study is to establish the in vivo verification of proton beam path by using proton-activated positron emission distributions. A total of 50 PET/CT imaging studies were performed on ten prostate cancer patients immediately after daily proton therapy treatment through a single lateral portal. The PET/CT and planning CT were registered by matching the pelvic bones, and the beam path of delivered protons was defined in vivo by the positron emission distribution seen only within the pelvic bones, referred to as the PET-defined beam path. Because of the patient position correction at each fraction, the marker-defined beam path, determined by the centroid of implanted markers seen in the posttreatment (post-Tx) CT, is used for the planned beam path. The angular variation and discordance between the PET- and marker-defined paths were derived to investigate the intrafraction prostate motion. For studies with large discordance, the relative location between the centroid and pelvic bones seen in the post-Tx CT was examined. The PET/CT studies are categorized for distinguishing the prostate motion that occurred before or after beam delivery. The post-PET CT was acquired after PET imaging to investigate prostate motion due to physiological changes during the extended PET acquisition. The less than 2 degrees of angular variation indicates that the patient roll was minimal within the immobilization device. Thirty of the 50 studies with small discordance, referred as good cases, show a consistent alignment between the field edges and the positron emission distributions from the entrance to the distal edge. For those good cases, average displacements are 0.6 and 1.3 mm along the anterior-posterior (D(AP)) and superior-inferior (D(SI)) directions, respectively, with 1.6 mm standard deviations in both directions. For the remaining 20 studies demonstrating a large discordance (more than 6 mm in either D(AP) or D(SI)), 13 studies, referred as motion-after-Tx cases, also show large misalignment between the field edge and the positron emission distribution in lipomatous tissues around the prostate. These motion-after-Tx cases correspond to patients with large changes in volume of rectal gas between the post-Tx and the post-PET CTs. The standard deviations for D(AP) and D(SI) are 5.0 and 3.0 mm, respectively, for these motion-after-Tx cases. The final seven studies, referred to as position-error cases, which had a large discordance but no misalignment, were found to have deviations of 4.6 and 3.6 mm in D(AP) and D(SI), respectively. The position-error cases correspond to a large discrepancy on the relative location between the centroid and pelvic bones seen in post-Tx CT and recorded x-ray radiographs. Systematic analyses of proton-activated positron emission distributions provide patient-specific information on prostate motion (sigmaM) and patient position variability (sigmap) during daily proton beam delivery. The less than 2 mm of displacement variations in the good cases indicates that population-based values of sigmap and sigmaM, used in margin algorithms for treatment planning at the authors' institution are valid for the majority of cases. However, a small fraction of PET/CT studies (approximately 14%) with -4 mm displacement variations may require different margins. Such data are useful in establishing patient-specific planning target volume margins.

MO‐A‐BRD‐01: Quality Assurance for IGRT

Image‐guided radiation treatment (IGRT) is now widely available and implemented for routine clinical use in many clinics, allowing unprecedented level of precision and accuracy in treatment delivery. These systems bring a substantial change in clinical practice for all the disciplines involved in radiation medicine. One can now correct patient position immediately prior to administration of radiotherapy by registering IGRT images to a reference CT scan, generally the same that has been used for treatment planning. Modern IGRT systems not only display two‐dimensional bony anatomy and airways, like portal imaging, but also three‐dimensional datasets representing the internal anatomy of the patient, implanted markers, soft tissue, and sometimes the target to be irradiated. Growing experience with IGRT has demonstrated the ability to verify, on a daily basis, the position of internal anatomy structures, such as the tumour or its surrogates, the spinal cord, and other organs at risk with respect to the treatment beam geometry for several anatomical sites. It is therefore logical to use IGRT daily for on‐line correction of patient translations and rotations, and to compare successive IGRT images to monitor changes of internal anatomy through the course of therapy. Nevertheless, introducing IGRT within busy radiation therapy clinics requires thoughtful commissioning and quality assurance (QA) protocols, and judicious modification of existing radiation therapy processes and protocols. While the modes of use of these novel systems will continue to evolve, their performance needs to be of the highest level, as they will be depended upon in the treatment process; indeed, increasing the precision of radiotherapy delivery may lead to reduced margins and therefore allow further dose escalation.There are two key features of IGRT systems that require particular attention: geometric accuracy and image quality. First, the chosen IGRT system often does not share a common central axis with the megavoltage treatment beam; therefore, the geometric relation of the IGRT datasets to the megavoltage treatment beam must be assessed and monitored to ensure adequate localization, scaling, and geometric accuracy. Second, image quality metrics define the ability of the IGRT system to consistently produce an image of sufficient quality to localize the structures of interest, through either manual au automatic registration of IGRT images to the reference planning CT scan. A well‐planned QA program integrates closely the IGRT system procedures with linac procedures described in accepted QA standards such as the report of the AAPM task group 40 report.This lecture will present a brief review of IGRT systems, including kilovoltage and megavoltage cone‐beam CT, ultrasound, CT on rails, Tomotherapy, and projection radiography. The emphasis will be on QA issues germane to these systems and the clinical processes relying on their use. QA checks and phantoms will be suggested, and some QA metrics, with their associated tolerance levels based on clinical experience will be presented. Successful strategies for clinical implementation of IGRT will also be discussed.Learning Objectives:1. Understand the technical and process issues related to IGRT systems2. Present a QA program for IGRT systems focusing on geometric accuracy and image quality3. Users will tailor their program according to the clinical use of the device.

SU-FF-T-80: Hybrid Image Guided Radiation Therapy for Hypofractionated Prostate IMRT: Feasibility Study

Purpose: In prostate IGRT employing hypofractionation, two methods are commonly used: patient re-positioning and re-planning. We propose a hybrid IGRT technique in this study. The dosimetric benefits and efficiency improvement of the technique is quantitatively evaluated by comparing with the two conventional techniques. Method and Materials: Five prostate patients were studied, each had 1 planning CT and 11 daily CBCTs. The structure-of-interest (SOIs) including CTV (prostate and seminal vesicles), bladder, and rectum were contoured on CT and CBCTs. Initial IMRT plans were generated on Eclipse. Three IGRT techniques (“re-positioning”, “re-plan”, and “hybrid”) were applied on each CBCT. “Re-positioning” utilizes target soft-tissue matching for patient position correction. “Re-plan” re-aligns the patient then reoptimizes the original plan using daily SOIs. “Hybrid” first finds the best-fit plan from plan-library for the daily CBCT. If no suitable plan found, “re-plan” is performed and new plan is added to library. The daily and cumulative doses were calculated and compared for all three techniques. Results: (1) Cumulative dose: For CTV and bladder, the difference among the three IGRT techniques are <1% and <5% at all dose levels. “Re-positioning” slightly underdoses the CTV. For rectum, the differences between “re-plan” and “hybrid” are <2% at all dose levels. The Dmean/D30 doses to the rectum of the “re-positioning” are 5%/10% higher than those of “re-plan”. (2) Daily dose: The average(min-max) D30 dose for “re-positioning” and “hybrid” are [rectum: 84%(67%–94%) and 70%(51%–78%)] and [bladder: 57%(42%–75%) and 59%(46%–70%)]. (3) “Hybrid” required the re-optimization to be performed 6, 4, 1, 2, and 3 times for patient 1 to 5, respectively. Conclusion: Hybrid-IGRT retains the target coverage and critical structure sparing of the “re-plan”, yet it reduces the workload of the re-optimization by >70% and therefore is a viable option for hypofractionated prostate IGRT. Conflict of Interest: Research sponsored by Varian Medical Systems.

Comparison of online IGRT techniques for prostate IMRT treatment: Adaptive vs repositioning correction

This study compares three online image guidance techniques (IGRT) for prostate IMRT treatment: bony-anatomy matching, soft-tissue matching, and online replanning. Six prostate IMRT patients were studied. Five daily CBCT scans from the first week were acquired for each patient to provide representative "snapshots" of anatomical variations during the course of treatment. Initial IMRT plans were designed for each patient with seven coplanar 15 MV beams on a Eclipse treatment planning system. Two plans were created, one with a PTV margin of 10 mm and another with a 5 mm PTV margin. Based on these plans, the delivered dose distributions to each CBCT anatomy was evaluated to compare bony-anatomy matching, soft-tissue matching, and online replanning. Matching based on bony anatomy was evaluated using the 10 mm PTV margin ("bone10"). Soft-tissue matching was evaluated using both the 10 mm ("soft10") and 5 mm ("soft5") PTV margins. Online reoptimization was evaluated using the 5 mm PTV margin ("adapt"). The replanning process utilized the original dose distribution as the basis and linear goal programming techniques for reoptimization. The reoptimized plans were finished in less than 2 min for all cases. Using each IGRT technique, the delivered dose distribution was evaluated on all 30 CBCT scans (6 patients x 5 CBCT/patient). The mean minimum dose (in percentage of prescription dose) to the CTV over five treatment fractions were in the ranges of 99%-100% (SD = 0.1%-0.8%), 65%-98% (SD = 0.4%-19.5%), 87%-99% (SD = 0.7%-23.3%), and 95%-99% (SD = 0.4%-10.4%) for the adapt, bone10, soft5, and soft10 techniques, respectively. Compared to patient position correction techniques, the online reoptimization technique also showed improvement in OAR sparing when organ motion/deformations were large. For bladder, the adapt technique had the best (minimum) D90, D50, and D30 values for 24, 17, and 15 fractions out of 30 total fractions, while it also had the best D90, D50, and D30 values for the rectum for 25, 16, and 19 fractions, respectively. For cases where the adapt plans did not score the best for OAR sparing, the gains of the OAR sparing in the repositioning-based plans were accompanied by an underdosage in the target volume. To further evaluate the fast online replanning technique, a gold-standard plan ("new" plan) was generated for each CBCT anatomy on the Eclipse treatment planning system. The OAR sparing from the online replanning technique was compared to the new plan. The differences in D90, D50, and D30 of the OARs between the adapt and the new plans were less than 5% in 3 patients and were between 5% and 10% for the remaining three. In summary, all IGRT techniques could be sufficient to correct simple geometrical variations. However, when a high degree of deformation or differential organ position displacement occurs, the online reoptimization technique is feasible with less than 2 min optimization time and provides improvements in both CTV coverage and OAR sparing over the position correction techniques. For these cases, the reoptimization technique can be a highly valuable online IGRT tool to correct daily treatment uncertainties, especially when hypofractionation scheme is applied and daily correction, rather than averaging over many fractions, is required to match the original plan.

Verification of target position in the post‐prostatectomy cancer patient using cone beam CT

We present the results of a pilot study designed to investigate methods that may be applied to develop a patient position correction protocol for the post-prostatectomy patient receiving radiotherapy. Imaging was carried out with cone beam CT (CBCT) to investigate its suitability for detecting changes in rectal and bladder volumes and movements of these organs relative to the treatment planning CT. Eligible patients were imaged daily during the first week of treatment and weekly thereafter. Surrogate explanatory variables, including distance from the isocentre to the anterior rectum and bladder length, were tested for their potential to substitute for contouring entire organs and predict for changes in coverage of the planning treatment volume (PTV) by the 95% isodose (PTV95) and the maximum dose delivered to 50% of the rectal volume (RECTD50). The PTV defined on the CBCT images was larger than that defined on the planning CT and resulted in a decrease in the PTV95. Bladder length correlated with bladder volume and changes in bladder volume were associated with a decrease in PTV95. Rectal volumes changed randomly during treatment. There was a trend for the rectum to move anteriorly as treatment progressed. CBCT may be used to define the PTV, rectum and bladder though the reason for an apparent increase in PTV on CBCT requires further investigation. The bladder length and distance to the anterior rectal wall are potential surrogate explanatory variables. Further studies will be designed to test values of these surrogates that predict the need for a change in isocentre position.

TH‐A‐AUD A‐01: Quality Assurance for Image‐Guided Radiation Therapy

Image‐guided radiation treatment (IGRT) is now widely available and implemented for routine clinical use in many clinics, allowing unprecedented level of precision and accuracy in treatment delivery. These systems bring a substantial change in clinical practice for all the disciplines involved in radiation medicine. One can now correct patient position immediately prior to administration of radiotherapy by registering IGRT images to the reference planning CT scan. IGRT information displays not only two‐dimensional bony anatomy and airways, like portal imaging, but also three‐dimensional datasets representing the internal anatomy of the patient, implanted markers, soft tissue, and sometimes the target to be irradiated. Growing experience with IGRT has demonstrated the ability to verify, on a daily basis, the position of internal anatomy structures, such as the tumour or its surrogates, the spinal cord, and other organs at risk with respect to the treatment beam geometry for several anatomical sites. It is therefore logical to use IGRT daily for on‐line correction of patient translations and rotations, and to compare successive IGRT images to monitor changes of internal anatomy through the course of therapy. Nevertheless, introducing IGRT within busy radiation therapy clinics requires thoughtful commissioning and quality assurance (QA) protocols, and judicious modification of existing radiation therapy processes and protocols. While the modes of use of these novel systems will continue to evolve into the distant future, their performance needs to be of the highest level, as they will be depended upon in the treatment process; indeed, increasing the precision of radiotherapy delivery may lead to reduced margins and therefore allow further dose escalation.There are two key features of IGRT systems that require particular attention: geometric accuracy and image quality. First, the chosen IGRT system often does not share a common central axis with the megavoltage treatment beam; therefore, the geometric relation of the IGRT datasets to the megavoltage treatment beam must be assessed and monitored to ensure adequate localization, scaling, and geometric accuracy. Second, image quality metrics define the ability of the IGRT system to consistently produce an image of sufficient quality to localize the structures of interest, through either manual au automatic registrat5ion of IGRT images to the reference planning CT scan. A well‐planned QA program integrates closely the IGRT system procedures with linac procedures described in accepted QA standards such as the report of the AAPM task group 40.This lecture will present a brief review of IGRT systems, including kilovoltage and megavoltage cone‐beam CT, ultrasound, CT on rails, Tomotherapy, and projection radiography. The emphasis will be on QA issues germane to these systems and the clinical processes relying on their use. QA procedures and phantoms will be suggested, and some QA metrics, with their associated tolerance levels based on clinical experience will be presented. Successful strategies for clinical implementation of IGRT will also be discussed.Educational Objectives:1. Understand the technical issues related to IGRT systems.2. Understand the impact of IGRT on clinical processes.3. Present a QA program for IGRT systems focusing on geometric accuracy and image quality.

Evaluating Digital Verification Images by Radiation Therapists and Radiation Oncologists. Do We Still Agree?

In radiation therapy, verification images are taken to ensure correct field and patient positioning. A previous study examining the concordance between radiation therapists (RTs) and radiation oncologists (ROs) when evaluating hard copy verification films found excellent agreement. There was very little in the published literature about how this information could be transferred to digital imaging field verification. This study was undertaken to address this gap and also to confirm that the process of digital imaging would maintain a high level of agreement for quality assurance. In practice, at the Toronto Sunnybrook Regional Cancer Centre, images are stored in Multi-Access®, as well as the evaluation and any subjective comments regarding these images. A database of 14,721 images was acquired from a wide variety of disease sites and analyzed. A total of 5,174 true image pairs were evaluated for concordance between the ROs and RTs and overall, there was a 90 percent agreement. Rates of agreement are high; however, a true gold standard for image verification has yet to be established. With the evolution to image guided radiotherapy, transferring the responsibility of image approval to RTs would certainly be beneficial, however it will be up to individual centres to establish their own policies surrounding treatment field verification.

TH-B-M100F-01: Quality Assurance for IMRT and IGRT

Image-guided radiation treatment (IGRT) units are now available in the community and are capable of routine clinical use, offering unprecedented level of precision and accuracy in treatment delivery. These systems bring a substantial change in clinical practice for all the disciplines involved in radiation medicine. One can now correct patient position using image information displaying not only bony anatomy and airways, like portal imaging, but also implanted markers, soft tissue structures within the patient, and sometimes the target to be irradiated. Growing experience with kilovoltage IGRT systems has demonstrated the ability to verify, on a daily basis, the position of internal anatomy structures with respect to the treatment beam geometry for several anatomical sites. Kilovoltage IGRT systems further allow, using daily imaging sessions, on-line correction of patient translations and rotations, and the comparison for successive daily volumetric images permits to track changes of anatomy through the course of therapy. Introducing kV systems within busy radiation therapy clinics requires thoughtful testing and quality assurance protocols (QA) of the device, and judicious modification of existing radiation therapy processes and protocols. While the modes of use of these novel systems will continue to evolve into the distant future, their performance needs to be of the highest level, as they will be depended upon in the treatment process. There are two key features of the kilovoltage IGRT systems require particular attention: geometric accuracy and image quality. First, the kilovoltage IGRT system may not share a common central axis with the megavoltage treatment beam; therefore, the geometric relation of the kilovoltage imaging datasets to the megavoltage treatment beam must be assessed and monitored to ensure adequate localization, scaling, and geometric accuracy. Second, image quality metrics define the ability of the kilovoltage IGRT system to consistently produce an image of sufficient quality to localize the structures of interest. A well-planned QA program integrates closely the IGRT system procedures with linac procedures described in accepted QA standards such as the AAPM task group 40 report. This lecture will present a brief review of kilovoltage IGRT systems, and will focus on a suitable QA program, with special emphasis on issues germane to the system and the clinical processes relying on its use. Daily and monthly procedures, with phantoms constructed for these QA activities, will be suggested, and some quality assurance metrics, with their associated tolerance levels based on three years of experience on such devices will be presented. These levels are based on data accumulated over four years of experience on ten linear accelerators equipped with kilovoltage IGRT devices in our multi-vendor environment. Successful strategies for clinical implementation of IGRT will also be discussed. Educational Objectives: 1. Understand the technical issues related to kV systems. 2. Understand the impact of kV systems on clinical processes. 3. Present a comprehensive commissioning protocol and QA program for kV systems. This work is supported, in part, by a research grant from Elekta Oncology Systems.

SU-FF-J-48: Three-Dimensional MRI and Cone-Beam CT Matching for Localization of Prostate Cancer Treatment

PURPOSES: There is a growing interest using MRI only for prostate cancertreatment 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 planningCT, planning MRIimages were acquired for each of these patients for better soft tissue delineation. The MRIimages were fused to the planning CT during the clinical planning process. Nine sets of on‐board CBCTimages 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.

Sci‐Sat AM (2) Therapy‐06: Clinical experience with adaptive radiation therapy for lung cancer with tomotherapy

Helical tomotherapy (TomoTherapy Inc., Madison, WI) is a new form of image‐guided radiation therapy that combines features of a linear accelerator and a helical CT scanner. Helical tomotherapy allows megavoltage computer tomography (MVCT) imaging of the patient immediately prior to treatment. The MVCT images are compared to the treatment planning (kilovoltage) CT images at the operator station in order to ensure correct alignment to the planning target volume and critical structures. We report clinical experience with our technique for adaptive therapy in two lung cancer cases where the delivery fluence modifications were considered imperative due to concerns regarding changes in patient anatomy. Daily MVCT imaging with helical tomotherapy co‐registered with the planning kVCT study can provide feedback allowing correction of patient position due to systematic or random error or organ motion. In the case of lung cancer, the gross tumour volume is clearly visible on MVCT due to high tissue contrast in the lung facilitating daily positioning correction for small setup or organ location variations. If the target has dramatically changed in its shape or location and/or other major anatomy changes occur, the MVCT study can be used for re‐calculation of the expected dose distribution which is then compared to the originally planned one. If the difference is considered clinically significant, a repeat kVCT study can be performed and a new plan is developed for further treatment, thus allowing the patient specific adaptive radiotherapy to account for anatomic changes during treatment.

TH‐C‐224A‐02: QA for Linac KV Imaging Systems

The recent commercialization of linac‐mounted kilovoltage imaging systems (i.e., kV systems) has allowed high‐precision verification and correction of patient position immediately prior to the delivery of radiation therapy using fluoroscopic, radiographic, or cone‐beam tomographic modes. This novel technology challenges the radiotherapy community to redefine their patient positioning practice. One can now correct patient position using image information displaying not only bony anatomy and airways, like portal imaging, but also markers and soft tissue structures within the patient, including the target to be irradiated. On‐line kilovoltage imaging further allows, using daily imaging sessions, on‐line correction of patient translations and rotations, and the comparison for successive daily volumetric images permits to track changes of anatomy through the course of therapy. Introducing kV systems within busy radiation therapy clinics requires thoughtful testing and quality assurance protocols (QA) of the device, and judicious modification of existing radiation therapy processes and protocols.The individual components making up kV systems are well‐established technologies, and the quality assurance for each component follows accepted standards. However, the specific features of the integrated systems require particular attention. First, the kilovoltage beam may not share a common central axis with the megavoltage treatment beam; therefore, the relation of the kilovoltage imaging matrix to the megavoltage treatment beam must be monitored to ensure adequate localization, scaling, and geometric accuracy. Second, cone‐beam tomography differs from conventional or helical CT imaging, and thus encounters specific image quality issues. A well‐planned QA program integrates closely the kV system procedures with linac procedures described in accepted QA standards such as the AAPM task group 40 report.This lecture will present a brief review of commercial systems, but will focus on a suitable QA program, with special emphasis on issues germane to the system and the clinical processes relying on its use. Examples of clinical use in a number of sites will also be presented.Educational Objectives:1. Understand the technical issues related to kV systems.2. Understand the impact of kV systems on clinical processes.3. Present a comprehensive commissioning protocol and QA program for kV systems.This work is supported, in part, by a research grant from Elekta Oncology Systems.

A Neural Network Based Method for Optical Patient Set-up Registration in Breast Radiotherapy

Patient set-up optimization is required in breast-cancer radiotherapy to fill the accuracy gap between personalized treatment planning and uncertainties in the irradiation set-up. Opto-electronic systems allow implementing automatic procedures to minimize the positional mismatches of light-reflecting markers located on the patient surface with respect to a corresponding reference configuration. The same systems are used to detect the position of the irradiated body surface by means of laser spots; patient set-up is then corrected by matching the control points onto a CT based reference model through surface registration algorithms. In this paper, a non-deterministic approach based on Artificial Neural Networks is proposed for the automatic, real-time verification of geometrical set-up of breast irradiation. Unlike iterative surface registration methods, no passive fiducials are used and true real-time performance is obtained. Moreover, the non-deterministic modeling performed by the neural algorithm minimizes sensitivity to intra-fractional and inter-fractional non-rigid motion of the breast. The technique was validated through simulated activities by using reference CT data acquired on four subjects. Results show that the procedure is able to detect and reduce simulated set-up errors and revealed high reliability in patient position correction, even when the surface deformation is included in testing conditions.

SU‐FF‐J‐99: Automation of Image Registration and Verification for Image‐Guided Stereotactic Body Radiotherapy

Purpose: To facilitate image‐guided stereotactic body radiotherapy (SBRT) with high precision and fully automated target localization and patient position verification. Method and Materials: A mutual information based image registration algorithm was developed to register the patient's daily 3D computed tomography (CT) images with those taken for treatment planning to assess and correct patient position shifts in all three translational and three rotational directions. A digitally reconstructed radiograph (DRR) algorithm generates and displays the corresponding DRRs in one frame in a split screen format along with their intensity differences to enhance verification. In the same way, the portal image verification algorithm processes the portal image and displays it with the corresponding planning DRR in the same frame as final verification of the patient's position. The algorithms were programmed using MatLab and were assembled seamlessly in one package and the operation was fully automated. Results: Target localization and patient position verification process in our image‐guided SBRT is significantly simplified and the time needed for this process is reduced from 30 minutes or more to 3–4 minutes. The image registration algorithm was found to be accurate to within 0.1 mm with a head and neck phantom. The DRR generation algorithm generates high quality DRRs with a spatial resolution of 0.289×0.289 mm2/pixel. It removes unrelated anatomical structures for a cleaner background in the DRR to help identify the target isocenter more easily and more accurately. The DRR comparison and portal image verification algorithms readily detect misalignments of less than 0.2 mm. Conclusion: Application of the algorithms in determining patient position shift in actual SBRT cases demonstrated that they were accurate, fast, and reliable. They serve as a useful tool for image‐guided radiotherapy.

Adaptive radiation therapy for compensation of errors in patient setup and treatment delivery

In this paper, an adaptive radiation therapy algorithm is derived and evaluated using numerical simulations. Patient setup errors are considered and an off-line adaptive method to compensate for the effect of these is provided. The method consists of two parts, one for correction of patient position to account for the systematic error, and one for modulation of the fluence profiles to account for the random errors. The method is based on standard control theory for linear systems. It is investigated if this adaptive method can replace the use of a planning target volume (PTV) and therefore increase the possibilities to escalate the dose. Numerical simulations of treatments of a prostate patient indicate that this is the case. The simulations show that better organ-at-risk protection can be achieved when using the adaptation algorithm to correct for the geometrical uncertainties than when using a PTV.

A structure-based method for on-line matching of portal images for an optimal patient set-up in radiotherapy

In radiotherapy, portal images are used to ensure a correct patient position during every radiation session. A reliable on-line verification is of clinical interest to interrupt the radiation in time in case the patient is not at the right position. A great problem for successful image registration is the poor image quality of portal images. They are corrupted by noise and of very low contrast. A method directly based on the grey levels is not sufficient. Therefore a structure-based method was developed which is almost insensitive to distrubances (air bubbles, noise, slowly varying grey levels). In most cases the selection of a region of interest (ROI) can be omitted. Besides the automatical segmentation of the radiation field, only the structures relevant for matching the anatomy are enhanced by using a bandpass filter. It is possible to detect the maximum correlation between different image modalities reliably (simulator image, digitally reconstructed radiograph, portal image). By using Fast Fourier Transformation (FFT), the calculation time is smaller than five seconds, which enables a clinical on-line verification. We have matched 1139 pairs of images of different modalities and various regions of the body (pelvis, nasopharyngeal space, head, lung). The success rate is greater than 95%.

Automated determination of patient setup errors in radiation therapy using spherical radio-opaque markers.

Patient positioning accuracy can be quantified by the three-dimensional (3-D) translations and rotations required to transform the patient back to the desired position. Results of the current study show that the translations and rotations could be obtained from two projection images obtained radiographically on a linear accelerator when spherical radio-opaque markers were implanted inside or affixed to the surface of a skull phantom. In this study, film used to record the images were converted into digital gray scale images using a laser film digitizer. The marker images were located automatically by the computer using image processing techniques. By combining information from both projections, the 3-D locations of the markers were determined to submillimeter precision. Pairs of films were also analyzed for the phantom displaced from its original location by known amounts. The accuracy of the computed translations and rotations required for realignment of the phantom were found to be better than 1 mm and 0.3 degrees, respectively; comparable to the accuracy of the readout system of the equipment used. The general methodology could be coupled with an electronic portal imaging device for use in computer aided or automated correction of patient position in radiotherapy.

Breathing circuit occlusion due to defect in swivel port connector

participate in medical education and publications. However, her assumption that the patient photographed for this article was unaware of the consequences of the consent, is erroneous. Informed written consent was obtained; and the published photograph is quite appropriate for the topic of presentation. Any further framing of the face or torso would have resulted in less than optimal illustration of the necessity for correct patient positioning. Any suggestion of impropriety and breech of confidentiality in this case is unwarranted as consent was obtained and editorial and peer review determined the photograph educational. As Slue suggests, the practice of placing a bar across the eyes in a photograph does not ensure patient anonymity, is distracting, and probably should be abandoned in the medical literature.