Byungchul Cho

An analysis of thoracic and abdominal tumour motion for stereotactic body radiotherapy patients

An analysis of thoracic and abdominal tumour motion for stereotactic body radiotherapy patients was performed using more than 70 h of tumour motion estimated from the correlation between the external and internal motion for 143 treatment fractions in 42 patients. The tumour sites included lungs (30 patients) and retroperitoneum (12 patients). The overall mean respiratory-induced peak-to-trough distance was 0.48 cm, with individual treatment fraction means ranging from 0.02 to 1.44 cm. The overall mean respiratory period was 3.8 s, with individual treatment fraction means ranging from 2.2 to 6.4 s. In 57 treatment fractions (40%), the mean respiratory-induced peak-to-trough distance was greater than 0.5 cm. In general, tumour motion was predominantly superior-inferior (60% of all the treatment fractions), while anterior-posterior and left-right motion were 22% and 18%, respectively. The motion was predominantly linear, and the overall mean of the first principal component was 94%. However, for motion magnitude, direction and linearity, large variations were observed from patient to patient, fraction to fraction and cycle to cycle.

First Demonstration of Combined kV/MV Image-Guided Real-Time Dynamic Multileaf-Collimator Target Tracking

PURPOSE For intrafraction motion management, a real-time tracking system was developed by combining fiducial marker-based tracking via simultaneous kilovoltage (kV) and megavoltage (MV) imaging and a dynamic multileaf collimator (DMLC) beam-tracking system. METHODS AND MATERIALS The integrated tracking system employed a Varian Trilogy system equipped with kV/MV imaging systems and a Millennium 120-leaf MLC. A gold marker in elliptical motion (2-cm superior-inferior, 1-cm left-right, 10 cycles/min) was simultaneously imaged by the kV and MV imagers at 6.7 Hz and segmented in real time. With these two-dimensional projections, the tracking software triangulated the three-dimensional marker position and repositioned the MLC leaves to follow the motion. Phantom studies were performed to evaluate time delay from image acquisition to MLC adjustment, tracking error, and dosimetric impact of target motion with and without tracking. RESULTS The time delay of the integrated tracking system was approximately 450 ms. The tracking error using a prediction algorithm was 0.9 +/- 0.5 mm for the elliptical motion. The dose distribution with tracking showed better target coverage and less dose to surrounding region over no tracking. The failure rate of the gamma test (3%/3-mm criteria) was 22.5% without tracking but was reduced to 0.2% with tracking. CONCLUSION For the first time, a complete tracking system combining kV/MV image-guided target tracking and DMLC beam tracking was demonstrated. The average geometric error was less than 1 mm, and the dosimetric error was negligible. This system is a promising method for intrafraction motion management.

Toward submillimeter accuracy in the management of intrafraction motion: the integration of real-time internal position monitoring and multileaf collimator target tracking.

PURPOSE We report on an integrated system for real-time adaptive radiation delivery to moving tumors. The system combines two promising technologies-three-dimensional internal position monitoring using implanted electromagnetically excitable transponders and corresponding real-time beam adaptation using a dynamic multileaf collimator (DMLC). METHODS AND MATERIALS In a multi-institutional academic and industrial collaboration, a research version of the Calypso position monitoring system was integrated with a DMLC-based four-dimensional intensity-modulated radiotherapy delivery system using a Varian 120-leaf multileaf collimator (MLC). Two important determinants of system performance-latency (i.e., elapsed time between target motion and MLC response) and geometric accuracy-were investigated. Latency was quantified by acquiring continuous megavoltage X-ray images of a moving phantom (with embedded transponders) that was tracked in real time by a circular MLC field. The latency value was input into a motion prediction algorithm within the DMLC tracking system. Geometric accuracy was calculated as the root-mean-square positional error between the target and the centroid of the MLC aperture for patient-derived three-dimensional motion trajectories comprising two lung tumor traces and one prostate trace. RESULTS System latency was determined to be approximately 220 milliseconds. Tracking accuracy was observed to be sub-2 mm for the respiratory motion traces and sub-1 mm for prostate motion. CONCLUSION We have developed and characterized a research version of a novel four-dimensional delivery system that integrates nonionizing radiation-based internal position monitoring and accurate real-time DMLC-based beam adaptation. This system represents a significant step toward achieving the eventual goal of geometrically ideal dose delivery to moving tumors.

Stereotactic body radiation therapy as an alternative treatment for small hepatocellular carcinoma.

Background Even with early stage hepatocellular carcinoma (HCC), patients are often ineligible for surgical resection, transplantation, or local ablation due to advanced cirrhosis, donor shortage, or difficult location. Stereotactic body radiation therapy (SBRT) has been established as a standard treatment option for patients with stage I lung cancer, who are not eligible for surgery, and may be a promising alternative treatment for patients with small HCC who are not eligible for curative treatment. Materials and Methods A registry database of 93 patients who were treated with SBRT for HCC between 2007 and 2009 was analyzed. A dose of 10-20 Gy per fraction was given over 3-4 consecutive days, resulting in a total dose of 30-60 Gy. The tumor response was determined using dynamic computed tomography or magnetic resonance imaging, which was performed 3 months after completion of SBRT. Results The median follow-up period was 25.6 months. Median size of tumors was 2 cm (range: 1-6 cm). Overall patients’ survival rates at 1 and 3 years were 86.0% and 53.8%, respectively. Complete and partial tumor response were achieved in 15.5% and 45.7% of patients, respectively. Local recurrence-free survival rate was 92.1% at 3 years. Most local failures were found in patients with HCCs > 3 cm, and local control rate at 3 years was 76.3% in patients with HCC > 3 cm, 93.3% in patients with tumors between 2.1-3 cm, and 100% in patients with tumors ≤ 2 cm, respectively. Out-of-field intrahepatic recurrence-free survival rates at 1 and 3 years were 51.9% and 32.4%, respectively. Grade ≥ 3 hepatic toxicity was observed in 6 (6.5%). Conclusions SBRT was effective in local control of small HCC. SBRT may be a promising alternative treatment for patients with small HCC which is unsuitable for other curative therapy.

Three-dimensional prostate position estimation with a single x-ray imager utilizing the spatial probability density

In radiotherapy, target motion during treatment delivery can be managed either by motion inclusive margins or by gating or tracking based on intrafraction target position monitoring. If radio-opaque fiducial markers are used the required three-dimensional (3D) target position signal for gating or tracking can be obtained by simultaneous acquisition of two x-ray images from different angles. However, most treatment machines do not have such stereoscopic imaging capability. Alternatively, the 3D target position may be estimated with a single imager (monoscopic imaging) although it only provides the projected target position in the two dimensions of the imager plane. In this study, we developed a probability-based method to estimate the unresolved motion component parallel to the imager axis from the projected motion. A 3D Gaussian probability density was assumed for the target position. Projection of the target into a certain point on the imager means that it is located on the ray line that connects this point with the focus point of the x-ray source. The 1D probability density along this line was calculated from the 3D probability density and its expectation value was used as the estimate for the unresolved position. The mathematical framework of the method was developed including analytical expressions for the estimated unresolved component as a function of resolved components and for the estimation uncertainty. Use of the method was demonstrated for prostate in a simulation study of monoscopic imaging. First, the required 3D probability density was constructed as a population average from a data set consisting of 536 continuous prostate position tracks from 17 patients recorded at 10 Hz. Next, monoscopic imaging at a fixed imaging angle and imaging frequency was simulated for each prostate track. Estimated 3D prostate tracks were constructed from the simulated projection images by the proposed method and compared with the actual tracks in order to determine the root-mean-square (rms) error. The simulations were performed with imaging angles in the range from 0 degrees to 180 degrees (relative to vertical) and imaging frequencies in the range from 0.1 s (corresponding to continuous imaging) to 600 s (corresponding to no intrafraction imaging). For comparison, simulations were also performed with stereoscopic imaging, where perfect position determination in all three directions was assumed, and with monoscopic imaging without estimation of the unresolved motion, where the motion component along the imager axis was assumed to be zero. For continuous imaging, the accuracy of monoscopic imaging was limited by the uncertainty in the unresolved position estimation. The resulting vector rms error for the population corresponded closely to the theoretically derived estimation uncertainty. The estimation did not improve the accuracy of lateral monoscopic imaging, but it reduced the population rms error from 1.59 mm to 1.11 mm for vertical imaging. This improvement was most prominent for outlying tracks with large unresolved motion. Stereoscopic imaging was clearly superior to monoscopic imaging for high frequency imaging. For less frequent imaging, the accuracy of both monoscopic and stereoscopic imaging decreased due to target motion between images. Since this was most prominent for stereoscopic imaging, the difference in accuracy between monoscopic and stereoscopic imaging decreased with increasing imaging period. In conclusion, a method for estimation of the 3D target position from 2D projections has been developed and its use has been demonstrated in a simulation study of monoscopic prostate tracking.

A Method to Estimate Mean Position, Motion Magnitude, Motion Correlation, and Trajectory of a Tumor From Cone-Beam CT Projections for Image-Guided Radiotherapy

PURPOSE To develop a probability-based method for estimating the mean position, motion magnitude, and trajectory of a tumor using cone-beam CT (CBCT) projections. METHOD AND MATERIALS CBCT acquisition was simulated for more than 80 hours of patient-measured trajectories for thoracic/abdominal tumors and prostate. The trajectories were divided into 60-second segments for which CBCT was simulated by projecting the tumor position onto a rotating imager. Tumor (surrogate) visibility on all projections was assumed. The mean and standard deviation of the tumor position and motion correlation along the three axes were determined with maximum likelihood estimation based on the projection data, assuming a Gaussian spatial distribution. The unknown position component along the imager axis was approximated by its expectation value, determined by the Gaussian distribution. Transformation of the resulting three-dimensional position to patient coordinates provided the estimated trajectory. Two trajectories were experimentally investigated by CBCT acquisition of a phantom. RESULTS The root-mean-square error of the estimated mean position was 0.05 mm. The root-mean-square error of the trajectories was <1 mm in 99.1% of the thorax/abdomen cases and in 99.7% of the prostate cases. The experimental trajectory estimation agreed with the actual phantom trajectory within 0.44 mm in any direction. Clinical applicability was demonstrated by estimating the tumor trajectory for a pancreas cancer case. CONCLUSIONS A method for estimation of mean position, motion magnitude, and trajectory of a tumor from CBCT projections has been developed. The accuracy was typically much better than 1 mm. The method is applicable to motion-inclusive, respiratory-gated, and tumor-tracking radiotherapy.

ELECTROMAGNETIC-GUIDED DYNAMIC MULTILEAF COLLIMATOR TRACKING ENABLES MOTION MANAGEMENT FOR INTENSITY-MODULATED ARC THERAPY

PURPOSE Intensity-modulated arc therapy (IMAT) is attractive because of high-dose conformality and efficient delivery. However, managing intrafraction motion is challenging for IMAT. The purpose of this research was to develop and investigate electromagnetically guided dynamic multileaf collimator (DMLC) tracking as an enabling technology to treat moving targets during IMAT. METHODS AND MATERIALS A real-time three-dimensional DMLC-based target tracking system was developed and integrated with a linear accelerator. The DMLC tracking software inputs a real-time electromagnetically measured target position and the IMAT plan, and dynamically creates new leaf positions directed at the moving target. Low- and high-modulation IMAT plans were created for lung and prostate cancer cases. The IMAT plans were delivered to a three-axis motion platform programmed with measured patient motion. Dosimetric measurements were acquired by placing an ion chamber array on the moving platform. Measurements were acquired with tracking, without tracking (current clinical practice), and with the phantom in a static position (reference). Analysis of dose distribution differences from the static reference used a γ-test. RESULTS On average, 1.6% of dose points for the lung plans and 1.2% of points for the prostate plans failed the 3-mm/3% γ-test with tracking; without tracking, 34% and 14% (respectively) of points failed the γ-test. The delivery time was the same with and without tracking. CONCLUSIONS Electromagnetic-guided DMLC target tracking with IMAT has been investigated for the first time. Dose distributions to moving targets with DMLC tracking were significantly superior to those without tracking. There was no loss of treatment efficiency with DMLC tracking.

Dynamic Multileaf Collimator Tracking of Respiratory Target Motion Based on a Single Kilovoltage Imager During Arc Radiotherapy

PURPOSE To demonstrate and characterize dynamic multileaf collimator (DMLC) tracking of respiratory moving targets that are spatially localized with a single kV X-ray imager during arc radiotherapy. METHODS AND MATERIALS During delivery of an arc field (358 degrees gantry rotation, 72-sec duration, circular field shape), the three-dimensional (3D) position of a fiducial marker in a phantom was estimated in real time from fluoroscopic kV X-ray images acquired orthogonally to the treatment beam axis. A prediction algorithm was applied to account for system latency (570 ms) before the estimated marker position was used for DMLC aperture adaptation. Experiments were performed with 12 patient-measured tumor trajectories that were selected from 160 trajectories (46 patients) and reproduced by a programmable phantom. Offline, the 3D deviation of the estimated phantom position from the actual position was quantified. The two-dimensional (2D) beam-target deviation was quantified as the positional difference between the MLC aperture center and the marker in portal images acquired continuously during experiments. Simulations of imaging and treatment delivery extended the study to all 160 tumor trajectories and to arc treatments of 3-min and 5-min duration. RESULTS In the experiments, the mean root-mean-square deviation was 1.8 mm for the 3D target position and 1.5 mm for the 2D aperture position. Simulations agreed with this to within 0.1 mm and resulted in mean 2D root-mean-square beam-target deviations of 1.1 mm for all 160 trajectories for all treatment durations. The deviations were mainly caused by system latency (570 ms). CONCLUSIONS Single-imager DMLC tracking of respiratory target motion during arc radiotherapy was implemented, providing less than 2-mm geometric uncertainty for most trajectories.

Implementation of a New Method for Dynamic Multileaf Collimator Tracking of Prostate Motion in Arc Radiotherapy Using a Single kV Imager

PURPOSE To implement a method for real-time prostate motion estimation with a single kV imager during arc radiotherapy and to integrate it with dynamic multileaf collimator (DMLC) target tracking. METHODS AND MATERIALS An arc field with a circular aperture and 358 degrees gantry rotation was delivered to a motion phantom with a fiducial marker under continuous kV X-ray imaging at 5 Hz, perpendicular to the treatment beam. A pretreatment gantry rotation of 120 degrees in 20 sec with continuous imaging preceded the treatment. During treatment, each kV image was first used together with all previous images to estimate the three-dimensional (3D) target probability density function and then used together with this probability density function to estimate the 3D target position. The MLC aperture was then adapted to the estimated 3D target position. Tracking was performed with five patient-measured prostate trajectories that represented characteristic prostate motion patterns. Two data sets were recorded during tracking: (1) the estimated 3D target positions, for off-line comparison with the actual phantom motion; and (2) continuous portal images, for independent off-line calculation of the 2D tracking error as the positional difference between the marker and the MLC aperture center in each portal image. All experiments were also made with 1- Hz kV imaging. RESULTS The mean 3D root-mean-square error of the trajectory estimation was 0.6 mm. The mean root-mean-square tracking error was 0.7 mm, both parallel and perpendicular to the MLC. The accuracy degraded slightly for 1- Hz imaging. CONCLUSIONS Single-imager DMLC prostate tracking that allows arbitrary beam modulation during arc radiotherapy was implemented. It has submillimeter accuracy for most prostate motion types.

Detailed analysis of latencies in image-based dynamic MLC tracking

PURPOSE Previous measurements of the accuracy of image-based real-time dynamic multileaf collimator (DMLC) tracking show that the major contributor to errors is latency, i.e., the delay between target motion and MLC response. Therefore the purpose of this work was to develop a method for detailed analysis of latency contributions during image-based DMLC tracking. METHODS A prototype DMLC tracking system integrated with a linear accelerator was used for tracking a phantom with an embedded fiducial marker during treatment delivery. The phantom performed a sinusoidal motion. Real-time target localization was based on x-ray images acquired either with a portal imager or a kV imager mounted orthogonal to the treatment beam. Each image was stored in a file on the imaging workstation. A marker segmentation program opened the image file, determined the marker position in the image, and transferred it to the DMLC tracking program. This program estimated the three-dimensional target position by a single-imager method and adjusted the MLC aperture to the target position. Imaging intervalsΔTimage from 150 to 1000 ms were investigated for both kV and MV imaging. After the experiments, the recorded images were synchronized with MLC log files generated by the MLC controller and tracking log files generated by the tracking program. This synchronization allowed temporal analysis of the information flow for each individual image from acquisition to completed MLC adjustment. The synchronization also allowed investigation of the MLC adjustment dynamics on a considerably finer time scale than the 50 ms time resolution of the MLC log files. RESULTS ForΔTimage=150ms, the total time from image acquisition to completed MLC adjustment was 380±9ms for MV and 420±12ms for kV images. The main part of this time was from image acquisition to completed image file writing (272 ms for MV and 309 ms for kV). Image file opening (38 ms), marker segmentation (4 ms), MLC position calculation (16 ms), and MLC adjustment (52 ms) were considerably faster. For ΔTimage=1000ms, the total time from image acquisition to completed MLC adjustment increased to 1030±62ms(MV) and 1330±52ms(kV) mainly because of delayed image file writing. The MLC adjustment duration was constant 52 ms (±3 ms) for MLC adjustments below 1.1 mm and increased linearly for larger MLC adjustments. CONCLUSIONS A method for detailed time analysis of each individual real-time position signal for DMLC tracking has been developed and applied to image-based tracking. The method allows identification of the major contributors to latency and therefore a focus for reducing this latency. The method could be an important tool for the reconstruction of the delivered target dose during DMLC tracking as it provides synchronization between target motion and MLC motion.