External Respiratory Surrogates Research Articles

Continuous generation of volumetric images during stereotactic body radiation therapy using periodic kV imaging and an external respiratory surrogate

We present a technique for continuous generation of volumetric images during SBRT using periodic kV imaging and an external respiratory surrogate signal to drive a patient-specific PCA motion model. Using the on-board imager, kV radiographs are acquired every 3 s and used to fit the parameters of a motion model so that it matches observed changes in internal patient anatomy. A multi-dimensional correlation model is established between the motion model parameters and the external surrogate position and velocity, enabling volumetric image reconstruction between kV imaging time points. Performance of the algorithm was evaluated using 10 realistic eXtended CArdiac-Torso (XCAT) digital phantoms including 3D anatomical respiratory deformation programmed with 3D tumor positions measured with orthogonal kV imaging of implanted fiducial gold markers. The clinically measured ground truth 3D tumor positions provided a dataset with realistic breathing irregularities, and the combination of periodic on-board kV imaging with recorded external respiratory surrogate signal was used for correlation modeling to account for any changes in internal-external correlation. The three-dimensional tumor positions are reconstructed with an average root mean square error (RMSE) of 1.47 mm, and an average 95th percentile 3D positional error of 2.80 mm compared with the clinically measured ground truth 3D tumor positions. This technique enables continuous 3D anatomical image generation based on periodic kV imaging of internal anatomy without the additional dose of continuous kV imaging. The 3D anatomical images produced using this method can be used for treatment verification and delivered dose computation in the presence of irregular respiratory motion.

Optimization of training periods for the estimation model of three-dimensional target positions using an external respiratory surrogate

BackgroundDuring therapeutic beam irradiation, an unvisualized three-dimensional (3D) target position should be estimated using an external surrogate with an estimation model. Training periods for the developed model with no additional imaging during beam irradiation were optimized using clinical data.MethodsDual-source 4D-CBCT projection data for 20 lung cancer patients were used for validation. Each patient underwent one to three scans. The actual target positions of each scan were equally divided into two equal parts: one for the modeling and the other for the validating session. A quadratic target position estimation equation was constructed during the modeling session. Various training periods for the session—i.e., modeling periods (TM)—were employed: TM ∈ {5,10,15,25,35} [s]. First, the equation was used to estimate target positions in the validating session of the same scan (intra-scan estimations). Second, the equation was then used to estimate target positions in the validating session of another temporally different scan (inter-scan estimations). The baseline drift of the surrogate and target between scans was corrected. Various training periods for the baseline drift correction—i.e., correction periods (TCs)—were employed: TC ∈ {5,10,15; TC ≤ TM} [s]. Evaluations were conducted with and without the correction. The difference between the actual and estimated target positions was evaluated by the root-mean-square error (RMSE).ResultsThe range of mean respiratory period and 3D motion amplitude of the target was 2.4–13.0 s and 2.8–34.2 mm, respectively. On intra-scan estimation, the median 3D RMSE was within 1.5–2.1 mm, supported by previous studies. On inter-scan estimation, median elapsed time between scans was 10.1 min. All TMs exhibited 75th percentile 3D RMSEs of 5.0–6.4 mm due to baseline drift of the surrogate and the target. After the correction, those for each TMs fell by 1.4–2.3 mm. The median 3D RMSE for both the 10-s TM and the TC period was 2.4 mm, which plateaued when the two training periods exceeded 10 s.ConclusionsA widely-applicable estimation model for the 3D target positions during beam irradiation was developed. The optimal TM and TC for the model were both 10 s, to allow for more than one respiratory cycle.Trial registrationUMIN000014825. Registered: 11 August 2014.

SU‐E‐J‐235: Audiovisual Biofeedback Improves the Correlation Between Internal and External Respiratory Motion

Purpose:External respiratory surrogates are often used to predict internal lung tumor motion for beam gating but the assumption of correlation between external and internal surrogates is not always verified resulting in amplitude mismatch and time shift. To test the hypothesis that audiovisual (AV) biofeedback improves the correlation between internal and external respiratory motion, in order to improve the accuracy of respiratory‐gated treatments for lung cancer radiotherapy.Methods:In nine lung cancer patients, 2D coronal and sagittal cine‐MR images were acquired across two MRI sessions (pre‐ and mid‐treatment) with (1) free breathing (FB) and (2) AV biofeedback. External anterior‐posterior (AP) respiratory motions of (a) chest and (b) abdomen were simultaneously acquired with physiological measurement unit (PMU, 3T Skyra, Siemens Healthcare Erlangen, Germany) and real‐time position management (RPM) system (Varian, Palo Alto, USA), respectively. Internal superior‐inferior (SI) respiratory motions of (c) lung tumor (i.e. centroid of auto‐segmented lung tumor) and (d) diaphragm (i.e. upper liver dome) were measured from individual cine‐MR images across 32 dataset. The four respiratory motions were then synchronized with the cine‐MR image acquisition time. Correlation coefficients were calculated in the time variation of two nominated respiratory motions: (1) chest‐abdomen, (2) abdomen‐diaphragm and (3) diaphragm‐lung tumor. The three combinations were compared between FB and AV biofeedback.Results:Compared to FB, AV biofeedback improved chest‐abdomen correlation by 17% (p=0.005) from 0.75±0.23 to 0.90±0.05 and abdomen‐diaphragm correlation by 4% (p=0.058) from 0.91±0.11 to 0.95±0.05. Compared to FB, AV biofeedback improved diaphragm‐lung tumor correlation by 12% (p=0.023) from 0.65±0.21 to 0.74±0.16.Conclusions:Our results demonstrated that AV biofeedback significantly improved the correlation of internal and external respiratory motion, thus suggesting the need of AV biofeedback in respiratory‐gated treatments.

TH-CD-207-09: Retrospective 4D-MRI with a Novel Image-Based Surrogate: A Sagittal-Coronal-Diaphragm Point of Intersection (SCD-PoI) Motion Tracking Method

Purpose: The unreliable stability of internal respiratory surrogates and inconvenience of external respiratory surrogates for current retrospective 4D-MRI techniques largely affects the image quality of 4D-MRI. This study aims at developing image-based surrogate, a sagittal-coronal-diaphragm point of intersection (SCD-PoI) motion tracking method for retrospective 4D-MRI reconstruction.Methods/materials: As a pre-estimate of respiratory motion pattern, single-slice sagittal cines (FIESTA) were acquired at a location near the dome of the diaphragm. Subsequently, multi-slice coronal cines (FIESTA) were acquired and used for 4D-MRI reconstruction with phase sorting. Diaphragm motion trajectories were measured from the point of intersection between sagittal MRI cine plane, coronal MRI cine plain and the diaphragm dome surface. This point is defined as sagittal-coronal-diaphragm point of intersection (SCD-PoI). We pre-estimate respiraotyr motion by tracking SCD-PoI on sagitall cine. Then coronal images were then re-binned to different phased bins according to SCD-PoI motion tracking on coronal cine. This 4D-MRI technique was evaluated on a 4D Digital Extended Cardiac-Torso (XCAT) human phantom with a hypothesized moving tumor, six healthy voluneteers and two cancer patients under an IRB-approved study. Region of interest (ROI: tumor for XCAT and patients, dome of left kidney for healthy volunteers) trajectories on 4D-MRI were measured and compared with the reference (input respiratory curve for XCAT and ROI trajectories extracted from reference single-slice MRI cine (FIESTA) for human subjects). Superior-inferior (SI) mean absolute amplitude difference (D) and cross-correlation coefficient (CC) were calculated. Results: 4D-MRI on XCAT demonstrated highly accurate motion information with a low D (1.13mm) and a high CC (0.98) in the SI direction. Minimal artifacts were observed in human participants’ 4D-MRI, and images were adequate to reveal the respiratory motion of organs and tumor (D=1.08±1.03mm; CC=0.96). Conclusion: A novel 4D-MRI technique with image-based respiratory surrogate has been developed and tested on a digital phantom and human subjects.

Effectiveness of external respiratory surrogates for in vivo liver motion estimation

Due to low frame rate of MRI and high radiation damage from fluoroscopy and CT, liver motion estimation using external respiratory surrogate signals seems to be a better approach to track liver motion in real-time for liver tumor treatments in radiotherapy and thermotherapy. This work proposes a liver motion estimation method based on external respiratory surrogate signals. Animal experiments are also conducted to investigate related issues, such as the sensor arrangement, multisensor fusion, and the effective time period. Liver motion and abdominal motion are both induced by respiration and are proved to be highly correlated. Contrary to the difficult direct measurement of the liver motion, the abdominal motion can be easily accessed. Based on this idea, our study is split into the model-fitting stage and the motion estimation stage. In the first stage, the correlation between the surrogates and the liver motion is studied and established via linear regression method. In the second stage, the liver motion is estimated by the surrogate signals with the correlation model. Animal experiments on cases of single surrogate signal, multisurrogate signals, and long-term surrogate signals are conducted and discussed to verify the practical use of this approach. The results show that the best single sensor location is at the middle of the upper abdomen, while multisurrogate models are generally better than the single ones. The estimation error is reduced from 0.6 mm for the single surrogate models to 0.4 mm for the multisurrogate models. The long-term validity of the estimation models is quite satisfactory within the period of 10 min with the estimation error less than 1.4 mm. External respiratory surrogate signals from the abdomen motion produces good performance for liver motion estimation in real-time. Multisurrogate signals enhance estimation accuracy, and the estimation model can maintain its accuracy for at least 10 min. This approach can be used in practical applications such as the liver tumor treatment in radiotherapy and thermotherapy.

SU‐E‐J‐159: Correlation of Respiration‐Induced Motion of an External Surrogate and Implanted Internal Markers

Purpose: To investigate the correlation between the respiratory motion of implanted internal markers (IM) and external RPM traces for lung cancer patients in dynamic imaging. Methods: Under an IRB‐approved research protocol, 3 patients with VISICOIL marker (10 mm long) implants near/in their lung tumor were scanned with a daily single‐slow gantry rotation arc 4D‐CBCT acquisition. The Varian Real‐time Position Management system (RPM) was used as an external respiratory motion surrogate, with the marker block placed on the patient abdominal surface near the diaphragm. The IM motion was segmented in all 2D projections with an in‐house built Matlab program. The positions in the 3D spatial domain of the markers were extracted to (LR: left‐right, CC: cranial‐caudal, AP: anterior‐posterior). A Pearson‐type auto‐correlation function of the RPM AP trace and cross‐ correlation functions between RPM AP motion and the IM motions in three orthogonal directions was used to analyze the time series. Results: Our data shows that there is a strong positive correlation between the RPM AP motion and the IM motion in the CC direction. The correlation between the external motion and the IM motions in LR and AP directions varied from fraction to fraction. In addition, phase shift is observed in LR and AP directions. Different implanted markers in the same patient presented the very similar correlations, especially in the CC direction. This result suggests that multiple implanted markers can be used to provide supplementary information of the tumor motion. When the patient coughs during their respiratory cycle, internal motion and external motion may become uncorrelated. Conclusions: The RPM respiratory trace provides a good surrogate of the internal respiratory motion in the CC direction when the patient maintains a normal breathing pattern.Supported by NIH grant P01 CA 116602.

WE‐D‐220‐02: Relation of External Surface to Diaphragm Motion Based on Automatic Analysis of Ultrasound Images

Purpose: The accuracy of delivering gated‐radiation therapy to lung tumors using an external respiratory surrogate relies on the degree of correlation between the marker serving as a surrogate from breathing and motion of the anatomic structures. The irregular breathing during an imaging and treatment session and from one session to another may cause the dose distribution actually delivered to be different from the intended dose. The purpose of this work was to use the diaphragm images acquired from Ultrasound system and abdominal motion signal from Real‐time Position Management (RPM) system, to study the relation between diaphragm and external surface motion during uncontrolled breathing cycles. Methods: The ultrasound images of diaphragm were captured at the same time as RPM signal acquisition for three volunteers from our staffs. After capture, one point at the image of diaphragm was selected and tracked for the whole breathing cycles using a proposed automatic tracking method which combines moving average strategy with Gaussian filter. Then, the diaphragm motion was compared to the respiratory signal from RPM system. Results: On three normal volunteer data sets: (1) the phase shifts were measured between the RPM signal in the abdominal region and diaphragm motion. Three volunteers showed the diaphragm motion either leading or delay abdominal motion with the mean time around 0.2–0.4 second; (2) the phase shifts at difference breathing cycle for the same people were different; (3) there is no significant difference between the phases at the end of inhale and exhale. Conclusions: We conclude that the external monitor can be used to predict internal respiratory motion. However, it may be important to check with Ultrasound images or fluoroscopy of diaphragm for possible time delays. We found that the Ultrasound can be a more reliable non‐ionizing method than RPM system used for respiratory gating in radiation therapy.

SU‐GG‐J‐28: Evaluation of Various MIP CT Images for Target Volume Delineation

Purpose: To compare iGTV delineations of NSCLC with four maximum intensity projection (MIP) CTs with and without respiratory gating. Method and Materials: We identified 26 NSCLC patients whose tumors exhibited respiratory‐induced motion of ≥ 1cm. Each patient had a four‐dimensional computed tomography (4D‐CT) therapy simulation scan on a GE PET/CT scanner with Varian RPM respiratory surrogate. For each patient, we used an in‐house 4D‐CT sorting software to create MIP images using three different binning methods: 1) raw RPM phase‐binned 2) phase‐corrected (PC, force 0% to end inspiration and 50% to end expiration) phase‐binned, and 3) amplitude‐binned. Additionally, we created a MIP from the entire unsorted cine CT image set. All four MIP data sets were sent to Pinnacle 8.1w treatment planning software and visible tumor motion envelope (iGTV, internal gross target volume) was auto‐contoured using a lower threshold of 600. Results: In all cases, the cine iGTV without respiratory gating was the largest. Expressed as a percent of cine iGTV, our 4D‐CT iGTV ranged from 83.8% to 99.1%; the largest average 4D‐CT iGTV was from the PC dataset, followed by the phase‐binned dataset. Expressed as a percentage of cine PTV, the 4D‐CT PTV ranged from 90.0% to 99.9%. In regions of the respiratory waveform corresponding to high time rate of change in phase or amplitude, duplicate image selection can occur; duplication was most frequent with amplitude and PC binning methods. Contrary to previous studies, the image corresponding to the extreme respiratory amplitude did not inherently exhibit the full tumor motion extent, suggesting a phase delay between the internal tumor motion and the external respiratory surrogate. Conclusion: This work identifies quantitative differences and probable causes of the incomplete capture of tumor motion extent when using 4D‐CT based MIPs, and demonstrated a potential application of cine CT MIP without respiratory gating.

WE‐A‐BRD‐03: Gated Radiotherapy for Lung Cancer

Respiratory gated radiotherapy holds promise to reduce the incidence and severity of normal tissue complications and to increase local control through dose escalation for lung cancer patients. In this lecture, we discuss the current status, existing problems, and potential solutions for applying gating techniques to lung cancer treatment. First, the motion artifacts in CT simulation are discussed and the 4D CT scan technique is recommended for treatment simulation of lung cancer patients under gated radiotherapy. Second, we discuss two currently available forms of gated radiotherapy: internal (fluoroscopic) gating and external (optical) gating. Internal gating utilizes internal tumor motion surrogates such as implanted fiducial markers while external gating uses external respiratory surrogates such as markers placed on the surface of the patient's abdomen. The major strengths of external gating are that it is non‐invasive, is relatively easy and does not require any radiation dose for imaging. However, the relationship between the tumor motion and the surrogate signal may change over time, inter‐ and intra‐fractionally. The major strength of the internal gating systems is the precise and real‐time localization of the tumor position during the treatment. The two major weaknesses of internal gating are the risk of pneumothorax for implantation of markers in lung and the high imaging dose required for fluoroscopic tracking. Third, we discuss the potential solutions and future development of gated radiotherapy for lung treatment. We propose the combination of external and internal surrogates (hybrid gating) to solve the imaging dos problem and the direct fluoroscopic tracking of lung mass to avoid seed implantation. Other related issues are also discussed.

A fully automated non-external marker 4D-CT sorting algorithm using a serial cine scanning protocol

Current 4D-CT methods require external marker data to retrospectively sort image data and generate CT volumes. In this work we develop an automated 4D-CT sorting algorithm that performs without the aid of data collected from an external respiratory surrogate. The sorting algorithm requires an overlapping cine scan protocol. The overlapping protocol provides a spatial link between couch positions. Beginning with a starting scan position, images from the adjacent scan position (which spatial match the starting scan position) are selected by maximizing the normalized cross correlation (NCC) of the images at the overlapping slice position. The process was continued by ‘daisy chaining’ all couch positions using the selected images until an entire 3D volume was produced. The algorithm produced 16 phase volumes to complete a 4D-CT dataset. Additional 4D-CT datasets were also produced using external marker amplitude and phase angle sorting methods. The image quality of the volumes produced by the different methods was quantified by calculating the mean difference of the sorted overlapping slices from adjacent couch positions. The NCC sorted images showed a significant decrease in the mean difference (p < 0.01) for the five patients.

Quantitative prediction of respiratory tidal volume based on the external torso volume change: a potential volumetric surrogate

An external respiratory surrogate that not only highly correlates with but also quantitatively predicts internal tidal volume should be useful in guiding four-dimensional computed tomography (4DCT), as well as 4D radiation therapy (4DRT). A volumetric surrogate should have advantages over external fiducial point(s) for monitoring respiration-induced motion of the torso, which deforms in synchronization with a patient-specific breathing pattern. This study establishes a linear relationship between the external torso volume change (TVC) and lung air volume change (AVC) by validating a proposed volume conservation hypothesis (TVC = AVC) throughout the respiratory cycle using 4DCT and spirometry. Fourteen patients' torso 4DCT images and corresponding spirometric tidal volumes were acquired to examine this hypothesis. The 4DCT images were acquired using dual surrogates in ciné mode and amplitude-based binning in 12 respiratory stages, minimizing residual motion artifacts. Torso and lung volumes were calculated using threshold-based segmentation algorithms and volume changes were calculated relative to the full-exhalation stage. The TVC and AVC, as functions of respiratory stages, were compared, showing a high correlation (r = 0.992 ± 0.005, p < 0.0001) as well as a linear relationship (slope = 1.027 ± 0.061, R2 = 0.980) without phase shift. The AVC was also compared to the spirometric tidal volumes, showing a similar linearity (slope = 1.030 ± 0.092, R2 = 0.947). In contrast, the thoracic and abdominal heights measured from 4DCT showed relatively low correlation (0.28 ± 0.44 and 0.82 ± 0.30, respectively) and location-dependent phase shifts. This novel approach establishes the foundation for developing an external volumetric respiratory surrogate.

Fluoroscopic gating without implanted fiducial markers for lung cancer radiotherapy based on support vector machines

Various problems with the current state-of-the-art techniques for gated radiotherapy have prevented this new treatment modality from being widely implemented in clinical routine. These problems are caused mainly by applying various external respiratory surrogates. There might be large uncertainties in deriving the tumor position from external respiratory surrogates. While tracking implanted fiducial markers has sufficient accuracy, this procedure may not be widely accepted due to the risk of pneumothorax. Previously, we have developed a technique to generate gating signals from fluoroscopic images without implanted fiducial markers using template matching methods (Berbeco et al 2005 Phys. Med. Biol. 50 4481–90, Cui et al 2007b Phys. Med. Biol. 52 741–55). In this note, our main contribution is to provide a totally different new view of the gating problem by recasting it as a classification problem. Then, we solve this classification problem by a well-studied powerful classification method called a support vector machine (SVM). Note that the goal of an automated gating tool is to decide when to turn the beam ON or OFF. We treat ON and OFF as the two classes in our classification problem. We create our labeled training data during the patient setup session by utilizing the reference gating signal, manually determined by a radiation oncologist. We then pre-process these labeled training images and build our SVM prediction model. During treatment delivery, fluoroscopic images are continuously acquired, pre-processed and sent as an input to the SVM. Finally, our SVM model will output the predicted labels as gating signals. We test the proposed technique on five sequences of fluoroscopic images from five lung cancer patients against the reference gating signal as ground truth. We compare the performance of the SVM to our previous template matching method (Cui et al 2007b Phys. Med. Biol. 52 741–55). We find that the SVM is slightly more accurate on average (1–3%) than the template matching method, when delivering the target dose. And the average duty cycle is 4–6% longer. Given the very limited patient dataset, we cannot conclude that the SVM is more accurate and efficient than the template matching method. However, our preliminary results show that the SVM is a potentially precise and efficient algorithm for generating gating signals for radiotherapy. This work demonstrates that the gating problem can be considered as a classification problem and solved accordingly.

WE-SAMS-AUD B-03: Gated Radiotherapy for Lung Cancer

Respiratory gated radiotherapy holds promise to reduce the incidence and severity of normal tissue complications and to increase local control through dose escalation for lungcancer patients. In this lecture, we discuss the current status, existing problems, and potential solutions for applying gating techniques to lungcancertreatment. First, the motion artifacts in CT simulation are discussed and the 4D CT scan technique is recommended for treatment simulation of lungcancer patients under gated radiotherapy. Second, we discuss two currently available forms of gated radiotherapy: internal (fluoroscopic) gating and external (optical) gating. Internal gating utilizes internal tumor motion surrogates such as implanted fiducial markers while external gating uses external respiratory surrogates such as markers placed on the surface of the patient's abdomen. The major strengths of external gating are that it is non‐invasive, is relatively easy and does not require any radiationdose for imaging. However, the relationship between the tumor motion and the surrogate signal may change over time, inter‐ and intra‐fractionally. The major strength of the internal gating systems is the precise and real‐time localization of the tumor position during the treatment. The two major weaknesses of internal gating are the risk of pneumothorax for implantation of markers in lung and the high imagingdose required for fluoroscopic tracking. Third, we discuss the potential solutions and future development of gated radiotherapy for lungtreatment. We propose the combination of external and internal surrogates (hybrid gating) to solve the imaging dos problem and the direct fluoroscopic tracking of lung mass to avoid seed implantation. Other related issues are also discussed.

TH-D-332-08: A Fully Automated 4D-CT Registration Algorithm for Lung Studies

Purpose: To develop an automated 4D‐CT registration algorithm that performs without the aid of data collected from an external respiratory surrogate. Method and Materials: 5 patients with lungcancer were scanned with a GE Healthcare 4‐Slice CT scanner using an overlapping cine protocol: the thorax was scanned with 4×2.5 mm cine scans. The couch was translated 7.5 mm's between adjacent cine scans, resulting in a common 2.5 mm overlapping slice linking the two adjacent cine scans. A 4D‐CT dataset was produced by aligning entire 3D volumes at each phase of the breathing cycle: a reference couch position was selected, images at that position were truncated to 1 breathing cycle and then interpolated into 16 evenly spaced phases. To begin, the images of the reference couch position were matched to images from the adjacent couch position by maximizing the Normalized Cross Correlation of the overlapping slice. The process continued in a ‘daisy chain’ fashion through all couch positions using the selected images until an entire 3D volume was selected. The algorithm was repeated for all 16 phases to complete a 4D‐CT dataset. The data were also registered using a reference external marker 4D‐CT amplitude registration method. Image quality was quantified by calculating the mean difference of the registered overlapping slices from adjacent couch positions. Banding artifacts, as the result of image misalignment, cause higher mean differences. The values obtained from each registration method were compared using t‐statistics. Results: The NCC volumes showed a decrease of banding artifacts. Artifact correction was accompanied by a significant decrease in mean difference for 3 of the 5 patients (p < 0.01). The other patient data, that showed no initial artifacts, showed no significant differences. Conclusion: An automatic 4D‐CT registration method was developed and shown to perform as well or better than an external marker method.

Multiple template-based fluoroscopic tracking of lung tumor mass without implanted fiducial markers

Precise lung tumor localization in real time is particularly important for some motion management techniques, such as respiratory gating or beam tracking with a dynamic multi-leaf collimator, due to the reduced clinical tumor volume (CTV) to planning target volume (PTV) margin and/or the escalated dose. There might be large uncertainties in deriving tumor position from external respiratory surrogates. While tracking implanted fiducial markers has sufficient accuracy, this procedure may not be widely accepted due to the risk of pneumothorax. Previously, we have developed a technique to generate gating signals from fluoroscopic images without implanted fiducial markers using a template matching method (Berbeco et al 2005 Phys. Med. Biol. 50 4481–90, Cui et al 2007 Phys. Med. Biol. 52 741–55). In this paper, we present an extension of this method to multiple-template matching for directly tracking the lung tumor mass in fluoroscopy video. The basic idea is as follows: (i) during the patient setup session, a pair of orthogonal fluoroscopic image sequences are taken and processed off-line to generate a set of reference templates that correspond to different breathing phases and tumor positions; (ii) during treatment delivery, fluoroscopic images are continuously acquired and processed; (iii) the similarity between each reference template and the processed incoming image is calculated; (iv) the tumor position in the incoming image is then estimated by combining the tumor centroid coordinates in reference templates with proper weights based on the measured similarities. With different handling of image processing and similarity calculation, two such multiple-template tracking techniques have been developed: one based on motion-enhanced templates and Pearson's correlation score while the other based on eigen templates and mean-squared error. The developed techniques have been tested on six sequences of fluoroscopic images from six lung cancer patients against the reference tumor positions manually determined by a radiation oncologist. The tumor centroid coordinates automatically detected using both methods agree well with the manually marked reference locations. The eigenspace tracking method performs slightly better than the motion-enhanced method, with average localization errors less than 2 pixels (1 mm) and the error at a 95% confidence level of about 2–4 pixels (1–2 mm). This work demonstrates the feasibility of direct tracking of a lung tumor mass in fluoroscopic images without implanted fiducial markers using multiple reference templates.

Derivation of the tumor position from external respiratory surrogates with periodical updating of the internal/external correlation

In this work we develop techniques that can derive the tumor position from external respiratory surrogates (abdominal surface motion) through periodically updated internal/external correlation. A simple linear function is used to express the correlation between the tumor and surrogate motion. The function parameters are established during a patient setup session with the tumor and surrogate positions simultaneously measured at a 30 Hz rate. During treatment, the surrogate position, constantly acquired at 30 Hz, is used to derive the tumor position. Occasionally, a pair of radiographic images is acquired to enable the updating of the linear correlation function. Four update methods, two aggressive and two conservative, are investigated: (A1) shift line through the update point; (A2) re-fit line through the update point; (C1) re-fit line with extra weight to the update point; (C2) minimize the distances to the update point and previous line fit point. In the present study of eight lung cancer patients, tumor and external surrogate motion demonstrate a high degree of linear correlation which changes dynamically over time. It was found that occasionally updating the correlation function leads to more accurate predictions than using external surrogates alone. In the case of high imaging rates during treatment (greater than 2 Hz) the aggressive update methods (A1 and A2) are more accurate than the conservative ones (C1 and C2). The opposite is observed in the case of low imaging rates.

TU‐D‐L100J‐03: Initial Phantom Evaluation of a Surface Imaging System (GateCT®) for 4‐D CT

Purpose: Many external surrogates exist for 4‐D CT, but all require some device attached to the patient. We evaluated a markerless optical surface imaging system for use with 4‐D CT. Method and Materials: A prototype of the GateCT optical surface imaging system (VisionRT, London, UK) was installed with a Philips Brilliance 64 CT simulator (Philips Medical, Cleveland, OH). The system is similar to the AlignRT product, but only contains one camera pod which is focused on the bore of the CT scanner. It also monitors the x‐ray on signal and sends regular pulses to the scanner similar to other external respiratory surrogates. First, a static plate was imaged to detect any drift in the measured motion. Next, the 4D phantom was used to create sinusoidal motion with simultaneous longitudinal motion to model breathing whilst the patient was scanned, and the motion measured by GateCT was compared with the phantom motion. Next, the 4‐D phantom moved a 2 cm marble in a sinusoidal pattern, and the scanner was operated in retrospective helical mode. The respiratory waveform was exported from the GateCT program to the scanner, and the inspiration and exhalation image sets were reconstructed and compared. Results: When the phantom was still, the GateCT point track gave a stable reading with a noise level of 0.5 mm. When the phantom was moving in one or two dimensions, the GateCT point track accurately recorded the amplitude, phase and frequency of the motion. For the 4‐D CT scans, the reconstructed CT images indicated that the amplitude, phase, and frequency of the marble motion were recorded accurately by GateCT. Conclusion: GateCT hold potential as a markerless respiratory surrogate. More work is necessary to determine the optimal patient selection for this technique.

Relation of external surface to internal tumor motion studied with cine CT

The accuracy of delivering gated-radiation therapy to lung tumors using an external respiratory surrogate relies on not only interfractional and intrafractional reproducibility, but also a strong correlation between external motion and internal tumor motion. The purpose of this work was to use the cine images acquired by four-dimensional computed tomography acquisition protocol to study the relation between external surface motion and internal tumor motion. The respiratory phase information of tumor motion and chest wall motion was measured on the cine images using a proposed region-of-interest (ROI) method and compared to measurement of an external respiratory monitoring device. On eight lung patient data sets, the phase shifts were measured between (1) the signal of a real-time positioning-management (RPM) respiratory monitoring device placed in the abdominal region and four surface locations on the chest wall, (2) the RPM signal in the abdominal region and tumor motions, and (3) chest wall surface motions and tumor motions. Respiratory waveforms measured at different surface locations during the same respiratory cycle often varied and had significant phase shifts. Seven of the 8 patients showed the abdominal motion leading chest wall motion. The best correlation (smallest phase shift) was found between the abdominal motion and the superior-inferior (S-I) tumor motion. A wide range of phase shifts was observed between external surface motion and tumor anterior-posterior (A-P)/lateral motion. The result supported the placement of the RPM block in the abdominal region and suggested that during a gated therapy utilizing the RPM system, it is necessary to place the RPM block at the same location as it is during treatment simulation in order to reduce potential errors introduced by the position of the RPM block. Correlations between external motions and lateral/A-P tumor motions were inconclusive due to a combination of patient selection and the limitation of the ROI method.

WE‐C‐ValA‐04: Derivation of the Tumor Position From External Respiratory Surrogates with Periodical Updating of External/internal Correlation

Purpose: To develop techniques that can derive the tumor position from external respiratory surrogates through periodically updated internal/external correlation. Method and Materials: A simple linear function is used to express the correlation between tumor and surrogate motion. The function parameters are established during patient setup session with both tumor and surrogate positions measured at 30Hz rate. During treatment, the surrogate position, constantly acquired at 30Hz, is used to derive the tumor position. Occasionally, a tumor image is acquired to enable the updating of the correlation function. Four update methods are investigated. (a) Line shift. (b) Fit model — through point. (c) Fit model — extra weight. (d) Function difference — fit point. Results: Tumor and external surrogate motion demonstrates a high degree of correlation however it dynamically changes over time. Occasionally updating the correlation function leads to more accurate predictions than using external surrogates alone. At the lowest tumor imaging rate tested in this work (0.1Hz) an accuracy improvement of 10% over the prediction by the mere use of external surrogate was observed for the best update method. Update methods (a) and (b) derive the tumor position with larger accuracy than (c) and (d) in case of high imaging rates. The opposite is observed in case of low imaging rates. Conclusion: Occasional calibration of the tumor/external surrogate correlation during treatment substantially increases the accuracy of the tumor localization compared to tumor position derivation by using the external surrogate alone.This work is partially supported by CenSSIS.

Residual motion of lung tumours in gated radiotherapy with external respiratory surrogates

Due to respiration, many tumours in the thorax and abdomen may move as much as 3 cm peak-to-peak during radiation treatment. To mitigate motion-induced irradiation of normal lung tissue, clinics have employed external markers to gate the treatment beam. This technique assumes that the correlation between the external surface and the internal tumour position remains constant inter-fractionally and intra-fractionally. In this work, a study has been performed to assess the validity of this correlation assumption for external surface based gated radiotherapy, by measuring the residual tumour motion within a gating window. Eight lung patients with implanted fiducial markers were studied at the NTT Hospital in Sapporo, Japan. Synchronized internal marker positions and external abdominal surface positions were measured during the entire course of treatment. Stereoscopic imaging was used to find the internal markers in four dimensions. The data were used retrospectively to assess conventional external surrogate respiratory-gated treatment. Both amplitude- and phase-based gating methods were investigated. For each method, three gating windows were investigated, each giving 40%, 30% and 20% duty cycle, respectively. The residual motion of the internal marker within these six gating windows was calculated. The beam-to-beam variation and day-to-day variation in the residual motion were calculated for both gating modalities. We found that the residual motion (95th percentile) was between 0.7 and 5.8 mm, 0.8 and 6.0 mm, and 0.9 and 6.2 mm for 20%, 30% and 40% duty cycle windows, respectively. Five of the eight patients showed less residual motion with amplitude-based gating than with phase-based gating. Large fluctuations (>300%) were seen in the residual motion between some beams. Overall, the mean beam-to-beam variation was 37% and 42% from the previous treatment beam for amplitude- and phase-based gating, respectively. The day-to-day variation was 29% and 34% from the previous day for amplitude- and phase-based gating, respectively. Although gating reduced the total tumour motion, the residual motion behaved unpredictably. Residual motion during treatment could exceed that which might have been considered in the treatment plan. Treatment margins that account for motion should be individualized and daily imaging should be performed to ensure that the residual motion is not exceeding the planned motion on a given day.