Intrafractional Target Motion Research Articles

Four‐dimensional cone‐beam computed tomography and digital tomosynthesis reconstructions using respiratory signals extracted from transcutaneously inserted metal markers for liver SBRT

Respiration-induced intrafraction target motion is a concern in liver cancer radiotherapy, especially in stereotactic body radiotherapy (SBRT), and therefore, verification of its motion is necessary. An effective means to localize the liver cancer is to insert metal fiducial markers to or near the tumor with simultaneous imaging using cone-beam computed tomography (CBCT). Utilizing the fiducial markers, the authors have demonstrated a method to generate breath-induced motion signal of liver for reconstructing 4D digital tomosynthesis (4DDTS) and 4DCBCT images based on phasewise and/or amplitudewise sorting of projection data. The marker extraction algorithm is based on template matching of a prior known marker image and has been coded to optimally extract marker positions in CBCT projections from the On-Board Imager (Varian Medical Systems, Palo Alto, CA). To validate the algorithm, multiple projection images of moving thorax phantom and five patient cases were examined. Upon extraction of the motion signals from the markers, 4D image sorting and image reconstructions were subsequently performed. In the case of incomplete signals due to projections with missing markers, the authors have implemented signal profiling to replace the missing portion. The proposed marker extraction algorithm was shown to be very robust and accurate in the phantom and patient cases examined. The maximum discrepancy of the algorithm predicted marker location versus operator selected location was < 1.2 mm, with the overall average of 0.51 +/- 0.15 mm, for 500 projections. The resulting 4DDTS and 4DCBCT images showed clear reduction in motion-induced blur of the markers and the anatomy for an effective image guidance. The signal profiling method was useful in replacing missing signals. The authors have successfully demonstrated that motion tracking of fiducial markers and the subsequent 4D reconstruction of CBCT and DTS are possible. Due to the significant reduction in motion-induced image blur, it is anticipated that such technology will be useful in image-guided liver SBRT treatments.

Analysis of Intra-fractional Variability of the Internal Target Volume during Stereotactic Radiotherapy in Peripheral Lung Tumors

For stereotactic treatment of lung tumors the use of on-line image guidance based on soft tissue matching using CBCT implies that intrafraction target motion is largely responsible for setup margins. In order to try to reduce PTV size we investigated the components of variance in intrafraction motion and their impact on reducing setup margins for the Internal Target Volume (ITV).

Experimental Investigation of Dosimetric Impact of Intrafraction Target Motion during Proton Uniform Scanning Treatment using Polymer Gel Dosimeters and a 4D Phantom

Experimental Investigation of Dosimetric Impact of Intrafraction Target Motion during Proton Uniform Scanning Treatment using Polymer Gel Dosimeters and a 4D Phantom

TU‐E‐204B‐01: First Demonstration of Image‐Based Dynamic MLC Tracking of a Moving Target during Intensity Modulated Arc Therapy

Purpose:Intensity modulated arc therapy (IMAT) enables efficient and highly conformai dose delivery. However, the dose distribution can be compromised by intrafraction target motion. Dynamic MLC (DMLC) tracking can potentially maintain the planned target dose distribution. The purpose of this study was to use a single kV imager for DMLC tracking during IMAT and to investigate the ability of this tracking to maintain the planned dose distribution. Method and Materials:A motion phantom carrying a 2D ion chamber array and build‐up material with an embedded gold marker was programmed to reproduce eight representative tumor trajectories (four prostate, four lung tumors). For each trajectory, a low and high intensity modulated IMAT plan were delivered with and without DMLC tracking, and the resulting 2D dose distributions were measured. The 3D real‐time target position signal for tracking was provided by fluoroscopic kV images acquired immediately before and during treatment. Each kV image was used together with all previous images to estimate the 3D target position and then the MLC leaves were refitted to this estimated target position. For lung, a prediction algorithm was used to compensate for the tracking latency. A 3%/3mm γ‐test was used to quantify the discrepancy between the measured dose distributions and a reference dose distribution delivered without target motion. Results:For the low modulated plans, tracking reduced the mean 3%/3mm γ‐failure rate from 32% to 0.2% for prostate and from 14% to 1% for lung. For the high modulated plans, the γ‐failure rate reduction was from 17% to 6% (prostate) and from 36% to 11% (lung). The finite MLC adaptation speed was a major cause for the residual γ‐failures during tracking. Conclusion:Image‐based tracking with IMAT was demonstrated for the first time. The tracking greatly improved the dose distributions to moving targets. Conflict of Interest: Supported by NTH/NCI R01‐93626 and Varian Medical Systems.

SU-GG-T-08: A Novel Treatment Couch for Real-Time Tracking of Respiration Induced Target Motion: Evaluating Its Geometric Accuracy

Purpose: To characterize the performance of a novel treatment couch designed and developed for respiration-induced real-time tumor motion tracking and to investigate its behavior with real tumor trajectories. Method and Materials: A new treatment couch has been developed to track targets in real-time. The system is capable of tracking targets with less than 0.5 mm accuracy. The max physical velocity for the couch is 13 cm/sec along SI and ML directions and 10 cm/sec along AP direction. It can reach a maximum acceleration of 100 cm/sec2 along all the three axes. To quantify the performance of the couch we used 25 tumor trajectories showing a peak-to-peak displacement in the range of 20 – 39 mm and sampled at 38 ms. These trajectories were fed to 3D time-varying trajectories derived from patient data. The motion of the phantom measured in real-time via a camera system was feed into the couch control system. Tracking errors were analyzed. Results: We found that the mean error in tracking for 20 tumor trajectories was 0.37 mm with a standard deviation of 0.29 mm. The system dead time was approximately 40ms. For individual trajectories, the median error varied from 0.15 mm to 0.70 mm whereas the individual mean values varied from 0.17 mm to 0.74 mm. Couch speed and acceleration were acceptable for real-time target tracking. Conclusion: We have developed a new couch for use with a linear accelerator that can track and correct for intrafraction (including respiration-induced) target motion. Mean and median tracking errors are less than 0.5 mm even for peak-to-peak displacements up to 4 cm. The couch shows a fairly stable and vibration free behavior over a wide range of tumor displacements studied. Conflict of Interest: NIH grant CA124766

TU-B-201B-04: Four-Dimensional Cone-Beam Computed Tomography and Digital Tomosynthesis Using Motion Signals Extracted from Fiducial Marker Inserted for Liver Cancer Radiation Therapy

Purpose: To reconstruct both phase‐wise and amplitude‐wise sorted 4D digital tomosynthesis (DTS) and 4D cone‐beam CT(CBCT)images based on breathing signals extracted from transcutaneously inserted metal markers for livercancer IGRT. Methods and Materials: The reconstruction process consists of 4 stages where 1) features of metal marker from selected region of interest (ROI) of its position is extracted, followed by 2) generating breath induced marker motion signal based on its position and predicting the missing signal through “profiling” of pre‐acquired breathing signal, 3) undergo amplitude‐wise and phase‐wise sorting with acquired projection data, and finally 4) reconstruct 4DCBCT and 4DDTS images.Results: In half‐fan geometry, the metal markers may be absent in up to 18.5% of the total X‐ray projections taken, which can be successfully replaced by signal “profiling” method that we propose here. With this, 4DDTS and 4DCBCT images were reconstructed. Comparing the reconstruction results between phase‐wise and amplitude‐wise sorting of 4DDTS and 4DCBCT images, motion artifacts of fiducial marker were less in amplitude‐wise than phase‐wise reconstructed images due to lesser amount of residual motion at each state of sorting process, thus more ideal for use in 4D image guidance. Conclusions: We have proposed the use of fiducial markers imaged at CBCT projections to acquire breath induced motion signal of liver to generate 4DDTS as well as 4DCBCT images. To the best of our knowledge, utilizing breath induced motion signal using metal markers for 4D imaging applications has never been attempted. Proposed method is advantageous compared to other methods in ways that 1) it does not require external gating system and 2) amplitude as well as phases‐wise sorting is selectively achievable. The proposed method can be utilized as intrafractional target motion verification process to improve precision of on‐line image guided radiotherapy for livercancer patients.

Impact of Translational and Rotational Target Motion during Hypofractionated Prostate Radiotherapy

Using tracking data acquired with a 4D electromagnetic localization system, we have quantified the dosimetric impact of both translational and rotational intrafraction target motion in the delivery of hypofractionated prostate radiotherapy.

SU-FF-J-102: Determining Residual Target Motion and Gating Window Width From Gated CT for Routine Gated Delivery

Purpose: Respiratory gating is widely used to reduce errors in target volume in planning CT and errors in treatment delivery. In selecting a patient-specific gating window width for gated treatment, the planner needs to correlate the magnitude of external motion signal to that of the 3D target motion in order to determine the magnitude of residual motion. We report our experience in determining patient-specific residual variation in routine clinics. Material and Methods: Amplitude-gated CT scans were performed on 96 patients with lung cancer and 32 patients with liver cancer using Varian RPM gating system. Two gated CT scans with gating windows at the end of expiration and end of inspiration were acquired. The two gated CTs were registered to determine the two extreme positions of the target. The amplitude of the external motion signals between end of expiration and inspiration was correlated to these two extreme positions. The magnitude of residual external motion during CT beam-on time was converted to 3D target residual motion. Results: In 96 lung cancer patients, significant target motions were found in inferior (84 patients, 3–18 mm, median 7), anterior (57 patients, 3–20 mm, median 5 mm), posterior (23 patients, 3–15 mm, median 6 mm), left (30 patients, 3–15 mm, median 5 mm), right (28 patients, 3–15 mm, median 5 mm) direction. In 32 liver cancer patients, significant target motion were found in inferior (32 patients, 5–20 mm, median 10), anterior (20 patients, 3–13 mm, median 7 mm), right (13 patients, 3–10 mm, median 7 mm) directions. Residual motion during CT beam-on time ranged 14–32% of the excursion. This information was used to determine gating window width for delivery and motion margin. Conclusion: Intrafractional residual target motion is not negligible and can be routinely addressed with this method.

Real-time motion-adaptive delivery (MAD) using binary MLC: I. Static beam (topotherapy) delivery

Intra-fraction target motion hits the fundamental basis of IMRT where precise target positions are assumed. Real-time motion compensation is necessary to ensure that the same dose is delivered as planned. Strategies for conventional IMRT delivery for moving targets by dynamic multi-leaf collimators (MLC) tracking are well published. Binary MLC-based IMRT, such as TomoTherapy®, requires synchronized motion of MLC, the couch and the gantry, which suggests a unique motion management strategy. Thanks to its ultra-fast leaf response and fast projection rate, real-time motion compensation for binary MLC-based IMRT is feasible. Topotherapy is a new IMRT delivery technique, which can be implemented in commercial helical TomoTherapy® machines using only fixed gantry positions. In this paper, we present a novel approach for TopoTherapy delivery that adjusts for moving targets without additional hardware and control requirement. This technique uses the planned leaf sequence but rearranges the projection and leaf indices. It does not involve time-consuming operations, such as reoptimization. Unlike gating or breath-hold-based methods, this technique can achieve nearly a 100% duty cycle with little breath control. Unlike dynamic MLC-based tracking methods, this technique requires neither the whole target motion trajectory nor the velocity of target motion. Instead, it only requires instantaneous target positions, which greatly simplifies the system implementation. Extensive simulations, including the worst-case scenarios, validated the presented technique to be applicable to relatively regular or mild irregular respirations. The delivered dose conforms well to the target, and significant margin reduction can be achieved provided that accurate, real-time tumor localization is available.

Four-Dimensional Computed Tomography–Based Interfractional Reproducibility Study of Lung Tumor Intrafractional Motion

To evaluate the interfractional reproducibility of respiration-induced lung tumors motion, defined by their centroids and the intrafractional target motion range. Twentythree pairs of four-dimensional/computed tomography scans were acquired for 22 patients. Gross tumor volumes were contoured, Clinical target volumes (CTVs) were generated. Geometric data for CTVs and lung volumes were extracted. The motion tracks of CTV centroids, and CTV edges along the cranio-caudal, anterior-posterior, and lateral directions were evaluated. The Pearson correlation coefficient for motion tracks along the cranio-caudal direction was determined for the entire respiratory cycle and for five phases about the end of expiration. The largest motion extent was along the cranio-caudal direction. The intrafractional motion extent for five CTVs was <0.5 cm, the largest motion range was 3.59 cm. Three CTVs with respiration-induced displacement >0.5 cm did not exhibit the similarity of motion, and for 16 CTVs with motion >0.5 cm the correlation coefficient was >0.8. The lung volumes in corresponding phases for cases that demonstrated CTVs motion similarity were reproducible. No correlation between tumor size and mobility was found. Target motion reproducibility seems to be present in 87% of cases in our dataset. Three cases with dissimilar motion indicate that it is advisable to verify target motion during treatment. The adaptive adjustment to compensate the possible interfractional shifts in a target position should be incorporated as a routine policy for lung cancer radiotherapy.

Real-time tracking of tumor motions and deformations along the leaf travel direction with the aid of a synchronized dynamic MLC leaf sequencer

Advanced radiotherapeutical techniques like intensity-modulated radiation therapy (IMRT) are based on an accurate knowledge of the location of the radiation target. An accurate dose delivery, therefore, requires a method to account for the inter- and intrafractional target motion and the target deformation occurring during the course of treatment. A method to compensate in real time for changes in the position and shape of the target is the use of a dynamic multileaf collimator (MLC) technique which can be devised to automatically arrange the treatment field according to real-time image information. So far, various approaches proposed for leaf sequencers have had to rely on a priori known target motion data and have aimed to optimize the overall treatment time. Since for a real-time dose delivery the target motion is not known a priori, the velocity range of the leading leaves is restricted by a safety margin to c × vmax while the following leaves can travel with an additional maximum speed to compensate for the respective target movements. Another aspect to be considered is the tongue and groove effect. A uniform radiation field can only be achieved if the leaf movements are synchronized. The method presented in this note is the first to combine a synchronizing sequencer and real-time tracking with a dynamic MLC. The newly developed algorithm is capable of online optimizing the leaf velocities by minimizing the overall treatment time while at the same time it synchronizes the leaf trajectories in order to avoid the tongue and groove effect. The simultaneous synchronization is performed with the help of an online-calculated mid-time leaf trajectory which is common for all leaf pairs and which takes into account the real-time target motion and deformation information.

E14-03: Image guided (4D) radiation therapy

E14-03: Image guided (4D) radiation therapy

MO‐E‐M100J‐06: Intra‐Fraction Target‐Beam Positioning

The problem of intrafraction tumor‐target positioning can be divided into two separate processes: (1) estimating the real‐time target position and (2) repositioning the beam to account for intrafraction target motion. Several solutions exist for both target position estimation and beam repositioning, however in principal any method of position estimation could be integrated with any method of beam repositioning. Note that beam repositioning can include moving the target to align with the beam (e.g. couch motion), which in the target reference is the same as moving the beam to align with the target.There are several common features in target position estimation systems. First, that in the absence of a real‐time volumetric imaging system, the target position is estimated from surrogates and thus there is uncertainty in the position of the target with respect to the surrogates. Second, for all but EPID‐based methods target position estimation is independent of the treatment beam and therefore careful quality assurance is required to ensure the accurate position estimation with respect to the treatment beam.There are also several common features of the beam repositioning systems. First, control systems with secondary feedback are required to ensure accurate and precise real time mechanical motion. Second, that there is a delay between the target motion and the beam repositioning. This delay causes delay‐time dependent errors which can be reduced with prediction algorithms.The various target position estimation and beam repositioning processes either in development or available for clinical use will be described and contrasted.Educational Objectives:1. Understand the separation of the target position estimation and beam repositioning processes.2. Understand the advantages and disadvantages of several real‐time target position estimation systems.3. Understand the advantages and disadvantages of several beam repositioning systems.Some research in this area at Stanford is supported by Varian Medical stems.

SU-FF-J-13: Simulated Real Time Image Guided Intra-Fraction Tracking-Delivery Method for Prostate IMRT

Purpose: The magnitude and frequency of intrafraction target motion (due to anatomical, physiological and physical changes) has an important role in delivery accuracy for image-guided radiotherapy. This motion could show significant variations even in few-second timeframe. However, there are no standard characteristics of target motion that can be assumed for a particular patient. This work aims to study the feasibility of tracking real time prostate motion with subsequent irradiation using Step-and-Shoot IMRT. Method and Materials: The prostate motion for actual patient was recorded using image-tracked Cyberknife system during a 150-minute single fraction treatment, and subsequently analyzed. Single fraction IMRT plan was planned for the same patient. The plan was normalized such that 95.1% of the CTV received 100% of the prescribed dose. To study the effect of intrafraction prostate motion and how the dose delivered was affected, two groups of IMRT plans were generated, one for 30 (group1) and one for 15 (group2) minutes total delivery time. Group 1 and 2 has 5 and 10 plans, respectively, to reflect the 150 minutes intrafraction prostate movement recorded. In these plans, for each segment, time averaged displacements of the target (AP, LR, and SI movements) were introduced and the MLC positions were shifted accordingly. Results: The dose coverage of the CTV was reduced from the original 95.1% to 92.5 ± 2.6 % in group1, and to 90.4 ± 7.2 % in group2. Plan 5 of group2 shows the lowest dose coverage of 71.2 % due to significant prostate movement (0.5 – 2 cm) within 3601–4500 second period. Dose coverage without plan 5 increased to 92.6 ± 2.7% for group2. Transformed plans receive lower dose to the selected organs-at-risk (OAR) compared to original plans. Conclusion: This result indicates the necessity of real time image-guided compensated IMRT delivery for moving target.

TH-C-J-6B-06: Using Portal Imaging and a Model-Based Estimation Algorithm to Correct for Intrafraction Prostate Motion

Purpose: It is investigated how portal images from IMRT beams can be used to estimate intrafraction prostate motion when fiducial markers are implanted in the prostate and how these estimates can be used in adaptive radiation therapy.Method and Materials: The software framework ORBIT4ART has been used to simulate intrafraction motion, marker positions and portal images (PI). The target motion is governed by a rigid body transformation. The following procedure estimates the position of the target before the delivery of each treatment beam. An image of the largest segment is recorded. The projection of transformed reference marker positions known from the planning CT are compared with the true marker positions in the PI. By minimizing the distance in the image plane between corresponding markers, the 3D rotation as well as the in‐plane translation is estimated. ORBIT4ART has also been used to perform dose registration in deformable tissue and to evaluate the effect of patient positioning on the delivered dose. In the tests the patient was repositioned based on the target position estimate acquired before the delivery of each beam. Results: The target registration error (TRE) is approximately as large as the unknown out‐of‐plane translation. If the target does not move between images the TRE is improved further since more information is added for every new PI. If couch corrections are made according to the estimates provided by the method, the delivered dose is in better agreement with the planned dose than if no such adjustments are made. Conclusion: The algorithm can be used to monitor intrafraction target motion. Although the estimated target translation lacks the out‐of‐plane component, online couch translations reduces the discrepancy between delivered and planned dose.Conflict of Interest: All authors are employees of the submitting company and own stocks or stock options.

Intrafractional tumor motion: lung and liver

Three-dimensional (3D) dose distribution has been improved by 3D conformation and intensity modulation in external radiotherapy. Interfractional uncertainty has been reduced by image-guided setup techniques. Reduction of ambiguity because of intrafractional target motion is the next step forward. Respiratory organ motion is known to be the largest intrafractional organ motion. Radiotherapy techniques controlling, gating, or tracking respiratory motion are under investigation to use smaller safety margins and higher doses for moving tumors. However, data on intrafractional tumor motion are sparse. We developed a fluoroscopic real-time tracking system and implantation techniques of fiducial markers for moving organs and have been accumulating knowledge about internal tumor motion. We also found the importance of 4-dimensional treatment planning to account for tumor motion in precision radiotherapy. This article reviews the current basic knowledge on respiratory physiology and summarizes the accumulating knowledge on internal motion of lung and liver tumors.