Patient Setup Correction Research Articles

SU‐GG‐J‐88: Evaluation of Patient Setup Accuracy for Targeted Marrow Irradiation with Image‐Guided Helical Tomotherapy

Purpose: Targeted marrow irradiation (TMI) using intensity‐modulated fields could allow dose escalation while limiting radiation toxicities to critical organs for patients who traditionally would receive total body irradiation. In this study, we evaluated patient setup uncertainties and the efficacy of our image‐guided setup techniques for TMI treatments based on helical tomotherapy megavoltage CT image registrations. Method and materials: We analyzed image registrations from 206 treatment fractions for 26 TMI patients. Prior to each treatment fraction, two MVCT scans were performed encompassing the head & neck region and the pelvic region, respectively. image registrations were performed for each scan. The two image registrations were averaged to correct patient setup errors. Results: In the head & neck region, the mean and standard deviation (SD) of setup corrections was 3.5±6.4 mm in the RL direction, −2.4±5.3 mm in the SI direction, −0.7±5.8 mm in the AP direction, and 0±1.2° in roll. In the pelvic region, it was 1.5±7.7 mm in the RL direction, −3.7±5.6 mm in the SI direction, 2.1±5.7 mm in the AP direction, and 0±1.1° in roll. After the two registrations were averaged, it was 2.5±6.6 mm in the RL direction, −3.1±5.1 mm in the SI direction, 0.7±5.1 mm in the AP direction, and 0±0.8° in roll. The residual setup errors in the head & neck region had mean and SD of 1.0±2.5 mm in the RL direction, 0.7±1.9 mm in the SI direction, −1.4±2.7 mm in the AP direction, and 0±0.8° in roll. Conclusion: The two‐scan technique ensured that the superior and inferior portions of the patient bony structure were properly set up in most cases. In cases when the residual setup errors were significant, the superior and inferior segments should be treated separately with separate setup corrections to ensure adequate target volume coverage.

Benefits of Six Degrees of Freedom for Optically Driven Patient Set-up Correction in SBRT

To quantify the advantages of a 6 degrees of freedom (dof) versus the conventional 3- or 4-dof correction modality for stereotactic body radiation therapy (SBRT) treatments. Eighty-five patients were fitted with 5-7 infra-red passive markers for optical localization. Data, acquired during the treatment, were analyzed retrospectively to simulate and evaluate the best approach for correcting patient misalignments. After the implementation of each correction, the new position of the target (tumor's center of mass) was estimated by means of a dedicated stereotactic algorithm. The Euclidean distance between the corrected and the planned location of target point was calculated and compared to the initial mismatching. Initial and after correction median+/-quartile displacements affecting external control points were 3.74+/-2.55 mm (initial), 2.45+/-0.91 mm (3-dof), 2.37+/-0.95 mm (4-dof), and 2.03+/-1.47 mm (6-dof). The benefit of a six-parameter adjustment was particularly evident when evaluating the results relative to the target position before and after the re-alignment. In this context, the Euclidean distance between the planned and the current target point turned to 0.82+/-1.12 mm (median+/-quartile values) after the roto-translation versus the initial displacement of 2.98+/-2.32 mm. No statistical improvements were found after 3- and 4-dof correction (2.73+/-1.22 mm and 2.60+/-1.31 mm, respectively). Angular errors were 0.09+/-0.93 degrees (mean+/-std). Pitch rotation in abdomen site showed the most relevant deviation, being -0.46+/-1.27 degrees with a peak value of 5.46 degrees . Translational misalignments were -0.68+/-2.60 mm (mean+/-std) with the maximum value of 12 mm along the cranio-caudal direction. We conclude that positioning system platforms featuring 6-dof are preferred for high precision radiation therapy. Data are in line with previous results relative to other sites and represent a relevant record in the framework of SBRT.

SU‐GG‐J‐81: Efficacy of Patient Setup Correction Procedure in Prostate Radiotherapy

Purpose: Using an on‐board cone‐beam CT (CBCT) based statistical margin model to determine whether a prostate center‐of‐mass (COM) based translation correction procedure can significantly reduce the CTV‐to‐PTV margin required for prostate patients who were first setup based on skin/alpha‐cradle markers (SAM). Method and Materials: A total 110 CBCT scans were performed, daily in the first week of radiotherapy and once weekly afterward, for 10 prostate patients immediately after their clinical setup based on SAM. Soft tissue matching was used to estimate the required translation corrections by matching the COM of the observed prostate volumes on treatment planning CT and CBCTs. Based on the CBCT prostate volumes, a statistical model (Bayesian margin expectation model) provided patient specific optimal margins before and after a prostate COM based translation correction was added to the patient SAM based setup. We compared these margins to determine the efficacy of prostate COM based translation correction when it is added to the SAM based setup. Results: After the 5 daily CBCTs from the first week, all 10 patient prostate uniform margins for 95% tumor geometric coverage were reduced after applying the prostate COM based translation correction to their SAM based setup. However, the percentage reduction varied from patient to patient, ranging from 1% to 60% with a median value of 22%. In 4 of 10 patients, margin reduction greater than 4mm was obtained. In general, patients with more deformable prostates benefited less from COM translation. Beyond the first 5 CBCTs, very little further margin change was obtained. Conclusions: The addition of prostate COM based translation correction to SAM based setup did reduce the uniform margin required to achieve 95% tumor geometric coverage, but the extent of reduction was patient specific. In general, incorporating COM translation in prostate margin determination can help reduce bladder/rectal doses.

A fast, accurate, and automatic 2D–3D image registration for image‐guided cranial radiosurgery

The authors developed a fast and accurate two-dimensional (2D)-three-dimensional (3D) image registration method to perform precise initial patient setup and frequent detection and correction for patient movement during image-guided cranial radiosurgery treatment. In this method, an approximate geometric relationship is first established to decompose a 3D rigid transformation in the 3D patient coordinate into in-plane transformations and out-of-plane rotations in two orthogonal 2D projections. Digitally reconstructed radiographs are generated offline from a preoperative computed tomography volume prior to treatment and used as the reference for patient position. A multiphase framework is designed to register the digitally reconstructed radiographs with the x-ray images periodically acquired during patient setup and treatment. The registration in each projection is performed independently; the results in the two projections are then combined and converted to a 3D rigid transformation by 2D-3D geometric backprojection. The in-plane transformation and the out-of-plane rotation are estimated using different search methods, including multiresolution matching, steepest descent minimization, and one-dimensional search. Two similarity measures, optimized pattern intensity and sum of squared difference, are applied at different registration phases to optimize accuracy and computation speed. Various experiments on an anthropomorphic head-and-neck phantom showed that, using fiducial registration as a gold standard, the registration errors were 0.33 +/- 0.16 mm (s.d.) in overall translation and 0.29 degrees +/- 0.11 degrees (s.d.) in overall rotation. The total targeting errors were 0.34 +/- 0.16 mm (s.d.), 0.40 +/- 0.2 mm (s.d.), and 0.51 +/- 0.26 mm (s.d.) for the targets at the distances of 2, 6, and 10 cm from the rotation center, respectively. The computation time was less than 3 s on a computer with an Intel Pentium 3.0 GHz dual processor.

Actual Dose Variation of Parotid Glands and Spinal Cord for Nasopharyngeal Cancer Patients During Radiotherapy

For intensity-modulated radiotherapy of nasopharyngeal cancer, accurate dose delivery is crucial to the success of treatment. This study aimed to evaluate the significance of daily image-guided patient setup corrections and to quantify the parotid gland volume and dose variations for nasopharyngeal cancer patients using helical tomotherapy megavoltage computed tomography (CT). Five nasopharyngeal cancer patients who underwent helical tomotherapy were selected retrospectively. Each patient had received 70 Gy in 35 fractions. Daily megavoltage CT scans were registered with the planning CT images to correct the patient setup errors. Contours of the spinal cord and parotid glands were drawn on the megavoltage CT images at fixed treatment intervals. The actual doses delivered to the critical structures were calculated using the helical tomotherapy Planned Adaptive application. The maximal dose to the spinal cord showed a significant increase and greater variation without daily setup corrections. The significant decrease in the parotid gland volume led to a greater median dose in the later phase of treatment. The average parotid gland volume had decreased from 20.5 to 13.2 cm3 by the end of treatment. On average, the median dose to the parotid glands was 83 cGy and 145 cGy for the first and the last treatment fractions, respectively. Daily image-guided setup corrections can eliminate significant dose variations to critical structures. Constant monitoring of patient anatomic changes and selective replanning should be used during radiotherapy to avoid critical structure complications.

Time Dependence of Intrafraction Patient Motion Assessed by Repeat Stereoscopic Imaging

To quantify intrafraction patient motion and its time dependence in immobilized intracranial and extracranial patients. The data can be used to optimize the intrafraction imaging frequency and consequent patient setup correction with an image guidance and tracking system, and to establish the required safety margins in the absence of such a system. The intrafraction motion of 32 intracranial patients, immobilized with a thermoplastic mask, and 11 supine- and 14 prone-treated extracranial spine patients, immobilized with a vacuum bag, were analyzed. The motion was recorded by an X-ray, stereoscopic, image-guidance system. For each group, we calculated separately the systematic (overall mean and SD) and the random displacement as a function of elapsed intrafraction time. The SD of the systematic intrafraction displacements increased linearly over time for all three patient groups. For intracranial-, supine-, and prone-treated patients, the SD increased to 0.8, 1.2, and 2.2 mm, respectively, in a period of 15 min. The random displacements for the prone-treated patients were significantly higher than for the other groups, namely 1.6 mm (1 SD), probably caused by respiratory motion. Despite the applied immobilization devices, patients drift away from their initial position during a treatment fraction. These drifts are in general small if compared with conventional treatment margins, but will significantly contribute to the margin for high-precision radiation treatments with treatment times of 15 min or longer.

SU-FF-J-108: Dose Variation of Critical Structures in Daily Treatment of Nasopharyngeal Cancer Patients Using Helical Tomotherapy Megavoltage CT System

Purpose: To study daily spinal cord and parotid gland dose variation for nasopharyngeal cancer patients undergoing helical tomotherapy treatments, as a result of patient weight loss during the course of treatments.Method and Materials: Five nasopharygeal cancer patients who received helical tomotherapy treatments were selected. At the end of the treatments, patients lost an average weight of 6.3 kg, or 8.4% relative to the weight at the beginning of the treatments. Daily megavoltage CT (MVCT) scans were performed prior to each treatment fraction, and MVCT images were registered with planning CTimages for patient setup corrections. Contours for spinal cord and parotid glands were drawn on MVCT images throughout the treatment course. Based on daily MVCT images, actual dose delivered to the critical structures were calculated using the Planned Adaptive application in the helical tomotherapy treatment planning system. The maximum dose to the spinal cord and the median doses to the parotid glands were analyzed. Results: The maximum dose to the spinal cord, after setup corrections, did not show significant variation as a result of patient weight loss. However, the median doses to the parotid glands showed significant increase due to facial tissue shrinkage during treatments. The average median dose to the parotid glands for the five patients were 83 cGy for the first treatment fraction, and it was 143 cGy per fraction for the last fraction. The average parotid gland volume decreased from 20.5 cc to 13.2 cc during treatments.Conclusion: Daily patient setup corrections based on MVCT image registrations prevented significant dose variation to the spinal cord due to setup uncertainty. During the course of radiotherapy, as a result of body weight loss, the parotid glands typically receive significantly higher dose compared to the dose dictated in the treatment plan.

SU-FF-J-42: Analytical Image Guided Radiotherapy for Head and Neck

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.

Assessment of secondary patient motion induced by automated couch movement during on-line 6 dimensional repositioning in prostate cancer treatment

Background and purpose The purpose of this study is to assess retrospectively secondary patient motion induced by 6D patient setup correction. Materials and methods For 104 patients, treated with Novalis ®, 6D setup correction prior to treatment was performed by ExacTrac5.0/NovalisBody ® in combination with the Robotic Tilt Module™ mounted underneath the Exact Couch ® top. This 6D correction might induce additional setup errors due to patient reaction against the rotations. To evaluate induced secondary motion, the 6D setup correction is verified and evaluated with respect to the tolerance limits. Results The majority of measured secondary motions are found within the tolerance limits. Detected secondary motions are mostly found in longitudinal shifts and lateral rotations, and mainly found in only 1 dimension during the same verification. The verifications indicate that the patient population can be divided into a group that hardly moves and a group that moves throughout all 6D setup corrections. The patient’s behavior can be predicted by the evaluation of the first five fractions as none of the patients demonstrate a learning curve during the treatment. Conclusions 6D setup correction does not induce secondary motion for the majority of the patients and can therefore be applied for all treatment indications.

TU‐C‐ValB‐09: Setup Error Analysis of HN‐IMRT Patients Using Electronic Portal Images and Cone‐Beam CTs

Purpose: It is important to monitor and correct patient setup during treatment course for head and neck IMRT because highly conformal dose distribution is sensitive to setup uncertainties. Setup for HN region is unstable because patient is usually uncomfortable under the mask and the flexible bony structures in the neck region. The purpose of this study is to analyze the setup errors during entire treatment course. These findings will help make appropriate corrective decisions. Method and Materials: Patients enrolled in our IMRT protocol are immobilized with a large thermoplastic mask attached to the MedTec IPPS. 2D analysis is accomplished by comparing electronic portal images to DRRs using in‐house software. Systematic setup error exceeding 3mm is corrected. 3D analysis is performed by registering cone‐beam CT to planning CT. Data from 21 patients with total 185 sessions were used. Correlation between 2D and 3D were analyzed. Time trend was analyzed for patients with daily CBCTs (4 patients with 131 scans total). Results: Good correlations were observed between 2D and 3D analyses with mean difference less than 1mm. Both methods showed that the mean of setup errors is under 1mm in all directions. The systematic and random errors were about 2mm. Margin of 5mm used in the planning seemed to be adequate based on empirical recipes. Time trend analysis shows that changes occurring during treatment course are significant for 3 (out of 4) patients. Conclusion: 2D and 3D analyses agree with each other, but 3D should be used whenever possible because it has the advantage of better image quality, lower imaging dose, and better software to interpret information. The difference is caused mainly from image quality and non‐rigid bony motion. It may be necessary to redo the mask in the middle of treatment course to reduce overall setup error.

SU‐DD‐A3‐02: Evaluation of Helical Tomotherapy Megavoltage CT System for Daily Automatic Patient Setup Correction and Manual Prostate Gland Motion Correction

Purpose: To evaluate the efficacy of helical tomotherapy megavoltage CT system (MVCT), and to study patient setup uncertainty and inter‐fractional internal organ motion for prostate cancer patients during the course of external beam treatment. Method and Materials: 34 prostate cancer patients that received definitive helical tomotherapy treatments were included in this study. MVCT images were registered with planning CT images using automatic bone registration followed by manual registration based on soft tissue match. Patient setup corrections and internal organ motion corrections in the medial‐lateral (ML), superior‐inferior (SI), anterior‐posterior (AP) directions, and rotations around the longitudinal axis were obtained from 1345 daily MVCT image registrations. Results: The mean and standard deviation of patient setup corrections were 3.1±7.3 mm in the ML direction, −0.8±4.9 in the SI direction, −0.2±6.4 in the AP direction, and 0.8±1.3° for rotations around the longitudinal axis. The mean and standard deviation of internal organ motion corrections were −0.1±0.8 mm in the ML direction, −0.1±0.7 mm in the SI direction, and 0.0±1.9 mm in the AP direction. The fraction of manual registrations that did not have adjustment in the ML, SI, or AP direction was 84%, 95%, and 71%, respectively. The prostate motion variability did not change during the course of treatment. Conclusion: Patient setup uncertainty dominated target position uncertainty. Helical tomotherapy MVCT system was effective in correcting patient setup errors and internal organ motions in the ML and AP directions, but provided limited soft tissue resolution in the SI direction.

Dynamic targeting image-guided radiotherapy

Volumetric imaging and planning for 3-dimensional (3D) conformal radiotherapy and intensity-modulated radiotherapy (IMRT) have highlighted the need to the oncology community to better understand the geometric uncertainties inherent in the radiotherapy delivery process, including setup error (interfraction) as well as organ motion during treatment (intrafraction). This has ushered in the development of emerging technologies and clinical processes, collectively referred to as image-guided radiotherapy (IGRT). The goal of IGRT is to provide the tools needed to manage both inter- and intrafraction motion to improve the accuracy of treatment delivery. Like IMRT, IGRT is a process involving all steps in the radiotherapy treatment process, including patient immobilization, computed tomogaphy (CT) simulation, treatment planning, plan verification, patient setup verification and correction, delivery, and quality assurance. The technology and capability of the Dynamic Targeting ™ IGRT system developed by Varian Medical Systems is presented. The core of this system is a Clinac® or Trilogy ™ accelerator equipped with a gantry-mounted imaging system known as the On-Board Imager ™ (OBI). This includes a kilovoltage (kV) x-ray source, an amorphous silicon kV digital image detector, and 2 robotic arms that independently position the kV source and imager orthogonal to the treatment beam. A similar robotic arm positions the PortalVision ™ megavoltage (MV) portal digital image detector, allowing both to be used in concert. The system is designed to support a variety of imaging modalities. The following applications and how they fit in the overall clinical process are described: kV and MV planar radiographic imaging for patient repositioning, kV volumetric cone beam CT imaging for patient repositioning, and kV planar fluoroscopic imaging for gating verification. Achieving image-guided motion management throughout the radiation oncology process requires not just a single product, but a suite of integrated products to manipulate all patient data, including images, efficiently and effectively.

Daily organ tracking in intensity-modulated radiotherapy of prostate cancer using an electronic portal imaging device with a dose saving acquisition mode

Background and purposeDaily use of conventional electronic portal imaging devices (EPID) for organ tracking is limited due to the relatively high dose required for high quality image acquisition. We studied the use of a novel dose saving acquisition mode (RadMode) allowing to take images with one monitor unit per image in prostate cancer patients undergoing intensity-modulated radiotherapy (IMRT) and tracking of implanted fiducial gold markers. Patients and methodsTwenty five patients underwent implantation of three fiducial gold markers prior to the planning CT. Before each treatment of a course of 37 fractions, orthogonal localization images from the antero-posterior and from the lateral direction were acquired. Portal images of both the setup procedure and the five IMRT treatment beams were analyzed. ResultsOn average, four localization images were needed for a correct patient setup, resulting in four monitor units extra dose per fraction. The mean extra dose delivered to the patient was thereby increased by 1.2%. The procedure was precise enough to reduce the mean displacements prior to treatment to ≤0.3mm. ConclusionsThe use of a new dose saving acquisition mode enables to perform daily EPID-based prostate tracking with a cumulative extra dose of below 1Gy. This concept is efficiently used in IMRT-treated patients, where separation of setup beams from treatment beams is mandatory.

515 Systematic error reduction using a novel patient set-up correction strategy

515 Systematic error reduction using a novel patient set-up correction strategy

TU‐EE‐A4‐02: Impact of Ultrasound‐Guided Patient Setup On OAR Dose in Conformal Radiation Therapy for Prostate Carcinoma

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

SU‐FF‐J‐80: Clinical Procedures for Daily Patient Setup Verification Using On‐Board Imager (OBI)

Purpose: The newly installed OBI system consists of a kilo‐voltage (KV) X‐ray source and a flat‐ panel detector mounted orthogonally to the megavoltage portal imager (MV) in Varian 21EX machine. These devices are linked to the OBI workstation which provides a platform for acquisition/analysis of images and auto couch motion for repositioning the patient. This paper presents our clinical procedures for patient setup verification using OBI. Method and Materials: In phase I, two sets of orthogonal MV/MV and KV/KV images were acquired to confirm the accuracy of the KV system by comparing daily KV images to traditional MV images and corresponding Digital Reconstructed Radiographs (DRRs). The phase II study acquired only one set of orthogonal MV/KV portal images for daily patient setup verification. An anterior MV and lateral KV images were obtained without gantry rotation. The OBI workstation provided the tools to match the images with DRRs and to analyze the shifts to be made for patient setup correction based on either implanted and/or anatomical markers. The treatment couch was moved automatically based on this analysis. Post‐shift MV/KV images were acquired to reconfirm patient positioning. All images and positioning data were electronically saved and reviewed. Results: Four prostate IMRT patients were included in phase I study. The isocenter alignment of the two sets of images (MV/MV and KV/KV) was found within 2mm relative to DRRs during the entire treatment courses. The phase II study included 30 prostate IMRT patients. The review of post‐shift MV/KV images showed an average of 2 mm deviation relative to DRRs. All procedures could be completed within 3–5 minutes. Conclusion: Phase I study proves that KV images are as reliable as MV portal images with better image quality. Phase II study verifies that matching/analysis tools and auto‐couch motion provide fast and accurate patient positioning.

Cone-beam-CT guided radiation therapy: A model for on-line application

Background and PurposeThis paper presents efficient and generalized processes for the clinical application of on-line X-ray volumetric cone-beam CT imaging (XVI) to improve the accuracy of patient set-up in radiation therapy. XVI image-guided therapy is illustrated by application to two contrasting sites, intra-cranial radiosurgery and prostate radiation therapy, with very different characteristics regarding organ motion, treatment precision, and imaging conditions. Patients and methodsOn-line set-up errors are determined in a two-step process. First the XVI data is registered to the planning data by matching the machine-isocenter with the planning-isocenter, respectively. The machine isocenter is defined in the XVI data during the reconstruction. The planning-isocenter is defined during the planning process in the planning CT data. Set-up errors are then determined from a second registration to remove residual displacements. The accuracy of the entire procedure for on-line set-up error correction was investigated in precision radiosurgery phantom studies. ResultsThe phantom studies showed that sub-pixel size set-up errors (down to 0.5mm) can be correctly determined and implemented in the radiosurgery environment. XVI is demonstrated to provide quality skull detail enabling precise skull based on-line alignment in radiosurgery. A ‘local XVI’ technique was found to give encouraging soft-tissue detail in the high-scatter pelvic environment, enabling on-line soft-tissue based set-up for prostate treatment. The two-step process for determination of set-up errors was found to be efficient and effective when implemented with a dedicated six panel interface enabling simultaneous visualization on the XVI and planning CT data sets. ConclusionsXVI has potential to significantly improve the accuracy of radiation treatments. Present image quality is highly encouraging and can enable bony and soft-tissue patient set-up error determination and correction. As with all image guided treatment techniques the development of efficient procedures to utilize on-line data are of paramount importance.

Development of Computerized Patient Setup Verification and Correction System in Radiotherapy

Visual comparison of a reference image with a verification image is commonly used for setup verification in external beam radiation therapy. However, it sometimes lacks reproducibility and provides insufficient quantitative evidence. The present study was performed to develop computerized methods for determining landmarks to verify a portal image with digital reconstruction radiograph (DRR), and to investigate the clinical effectiveness of our method. Our computer algorithm consisted of three main procedures--preprocessing, determination of landmarks, and verification--none of which required manual operation. Finally, our system indicated the distance for setup correction. We evaluated the accuracy of our system using pelvic phantom images, and the maximum magnitude of error was shown to be 1.12 (n=9). The results indicated that the error range of our system was sufficiently small to examine patient positioning error, which should be less than 5 mm, as described in AAPM report TG40. Our system will aid operators in positioning patients accurately for external radiation therapy.

Constrained anatomical surface registration for patient positioning in breast cancer radiotherapy

Constrained anatomical surface registration for patient positioning in breast cancer radiotherapy

Online ultrasound image guidance for radiotherapy of prostate cancer: impact of image acquisition on prostate displacement

Aim Numerous studies reported the use of ultrasound image-guidance system to assess and correct patient setup during radiotherapy for prostate cancer. We conducted a study to demonstrate and quantify prostate displacement resulting from pressure of the probe on the abdomen during transabdominal ultrasound image acquisition for prostate localization. Material and methods Ten healthy volunteers were asked to undergo one imaging procedure. The procedure was performed in a condition that simulates the localization of prostate during online ultrasound guidance. A 3D ultrasound machine was used. The procedure started with the placement of the probe on the abdomen above the pubis symphysis. The probe was tilted in a caudal and posterior direction until the prostate and seminal vesicle were visualized. The probe was then fixed with a rigid arm, which maintained the probe in a static position during image acquisition. The probe was then moved, in a short time, stepwise toward the prostate, acquiring images at each step. The prostate and seminal vesicles were identified and selected in all planes. The first 3D volume was used as reference 1, to which all other scans were matched using a gray value matching algorithm. Results Prostate motion was quantified as a 3D translation relative to the patient coordinate system. The resulting translations represented the amount of prostate movement as a function of probe displacement. Between 7 and 11 images were obtained per volunteer, with a maximal probe displacement ranging between 3 and 6 cm. Prostate displacement was measured in all volunteers for all the probe steps and in all directions. The largest displacements occurred in the posterior direction in all volunteers. The absolute prostate motion was less than 5 mm in 100% of the volunteers after 1 cm of probe displacement, in 80% after 1.5 cm, in 40% after 2 cm, in 10% after 2.5 cm, and 0% after 3 cm. To achieved a good-quality ultrasound images, the probe requires an average displacement of 1.2 cm, and this results in an average prostate displacement of 3.1 mm. No correlations were observed between prostate motion and prostate-probe distance or bladder size. Conclusion Probe pressure during ultrasound image acquisition causes prostate displacement, which is correlated to the amount of probe displacement from initial contact. The induced uncertainty associated with this process needs to be carefully evaluated to determine a safe margin to be employed during online ultrasound image-guided radiotherapy of the prostate.