Real-time Position Management System Research Articles

Assessment of Respiratory Tumor Movement using 4D Computed Tomography for Stereotactic Radiosurgery in Lung Tumor

Purpose: To assess the respiratory tumor movement using 4D-CT (4-dimensional computed tomography) for minimizing setup and target volume uncertainty of body-frame based stereotactic radiosurgery (SRS) in lung tumor. Materials and Methods: Fifty-seven stereotactic radiation therapies with respiratory gating system in 44 patients (two targets in seven patients and three in three patients) were executed in Asan Medical Center from May 2005 to June 2006. We used respiratory gating system consisted of RPM (Real-time Positioning Management system, Varian, USA) and 4D-CT (GE healthcare, USA), if tumor movement was exceeding 5 mm by respiration on fluoroscopy. Accurate tumor movement on reconstructed 4D-CT image was determined for respiratory gated therapy. Respiratory gated therapy was done if tumor movement was exceeding 5 mm, and non-gated therapy was done if it was below 5 mm. Results: Forty-five tumors were treated with supine position, and the other twelve were with prone position. Median tumor movement (3-dimensional) by respiration was 8.78±5.30 mm, and it was mostly affected by superior-inferior movement (8.53±5.23 mm). Tumor movements were different by tumor location, whether upper (5.38±2.85 mm) or lower (10.12±5.08 mm) lobe (p=0.015). Tumor movement was exceeding 5 mm in 27 (47.3%) tumors, and below 5 mm in 30 tumors in 4D-CT evaluation. Tumor movements on adopted respiratory gated phase were wholly below 5 mm, and its median value was 3.70±1.13 mm. Conclusion: Assessment of respiratory tumor movement using 4D-CT and gating system was helpful for minimizing target volume uncertainty. As a result, image-guided radiation therapy could improve the treatment accuracy of high precision stereotactic radiosurgery. (J Lung Cancer 2007; 6(1):24 �� 28)

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.

SU-FF-J-90: Investigating Factors Affecting Weight Selection for Safe Delivery of Four Dimensional Weighted Radiotherapy (4D-WRT)

Purpose: We have developed a 4D weighted radiotherapy (4D-WRT) method based on 4D-CT imaging to reduce respiratory motion margins. Here we discuss the issues related to weighting factor selection for this method to incorporate both patient breathing variability and machine performance limitations. Method and Materials: The Varian Real Time Position Management System (RPM) was used to measure patient respiratory motion. Free breathing was assessed based on extent of motion, breathing period, and drift of the mean breathing amplitude. Motion was also evaluated based on average marker trajectory as a function of phase for the entire breathing session. Measurements of machine limitations were conducted using a moving phantom to emulate tumor motion. The MLC motion was synchronized with the phantom motion to simulate 4D-WRT. By examining log files and film dosimetry we assess MLC motion accuracy, dose rate fluctuations, synchronization errors, and tolerance selection. Results: The analysis of the average marker trajectory shows that the end-inhale position is more variable than the end-exhale position, and patients spend more time at end-exhale. The average extent of motion, period, and drift for these patients was 1.01cm (0.14–1.15cm), 3.80sec (2.30–7.42sec), and 0.50cm (0.14–1.15cm), respectively. There was a correlation between the standard deviation of the period and that of the extent of motion. In the log file it was found that there is a phase delay between the actual and planned MLC position of 50msec, the period of motion was greater by 1% and the amplitude was greater by 0.05cm. It is estimated that a total phase difference of about 0.2sec exists due to delays in the communication, MLC motion, and dose rate stabilization. An appropriate tolerance should be selected to prevent beam hold-off due to MLC velocity limitations. Conclusions: Quantification of patient breathing variability and machine limitations is necessary for safe delivery of 4D-WRT.

SU‐FF‐J‐86: Intra‐ and Inter‐Breath‐Hold Position Variations for OBI Guided Amplitude Gating Treatment with Breath Hold

Purpose: We have applied OBI guidance together with a respiratory gating system to effectively control patient positioning for amplitude gating treatment with breath hold. The deep inspiration breath‐hold treatment has been proven effective in reducing the heart dose for left‐breast treatment. Breath‐hold technique is also useful for other sites where respiratory motion exists. In this study, we quantified the degree of intra‐fraction motion detected via continuous monitoring of patient position throughout treatment delivered during a single breath hold or multiple breath holds. Method and Materials: A real‐time position management (RPM) system (Varian, CA) was used to monitor radiation treatment for patients with tumors located in the thoracic through upper‐abdominal regions and the left breast. Simulation CT scans were acquired during breath holds when the breathing level was between the upper and lower thresholds that were preset in the RPM system. OBI kV radiographs and CBCT images were used to verify the patient position and breath‐hold level during the treatment sessions. To assure accurate positioning, the OBI/CBCT images were acquired only when the breathing level was within the same thresholds as those used for the simulation CT. Cine mode portal vision images were acquired every second during treatment. The intra‐ and inter‐breath‐hold position variations were analyzed based on the cine mode portal vision and kV radiographic images. Results: The intra‐/inter‐ breath‐hold position variations were analyzed for a total of ninety two fractions in six patients undergoing OBI guided breath‐hold treatments. The breath hold length ranged from 10 to 50 seconds. The intra‐breath‐hold position variation was within 2 mm. The inter‐breath‐hold position variation, within the same fraction, was within 4 mm. Conclusion: With proper OBI guidance and respiratory gating, the breath‐hold technique can effectively control the target position for treatment of cancer sites where respiratory motion exists.

WE‐C‐ValA‐03: The Use of CT Density Changes at Internal Tissue Interfaces to Monitor Respiratory Induced Lung Tumor Motion

Purpose: To describe a non‐invasive method to monitor the motion of internal organs affected by respiration without using external markers or spirometry, to apply the method to construct 4D‐CT datasets, to test the correlation with external markers, and to calculate any time shift between the datasets.Method and Materials: Ten lung cancer patients were CT scanned with a General Electric Fast 4‐Slice CT scanner operating in ciné mode. An external signal was also acquired simultaneously using the Real‐Time Position Management (RPM) Respiratory Gating System (Varian Medical Systems). We retrospectively reconstructed the raw CT data to obtain consecutive 0.5s reconstructions at 0.1s intervals to increase image sampling. We defined regions of interest containing tissue interfaces that move due to breathing on each axial slice and measured the mean CT number as a function of respiratory phase. We constructed 4D‐CT data sets by retrospectively sorting each image set based on the respiratory phase determined by the mean CT number curve. The external marker and tumor motion were directly correlated using the sample coefficient of determination, r2. Any time shift between the two data sets was calculated by shifting the tumor motion curve until r2 was maximized. Results: Only three of the ten patients showed correlation higher than r2=0.80 between tumor motion and external marker position. However, after taking into account time shifts (ranging between 0s and 0.4s) between the two data sets, all ten patients showed correlation better than r2=0.8. Conclusions: 4D‐CT acquisition using an internal method improves the temporal registration of CT images affected by respiratory motion without the need for external markers or spirometry. A non‐invasive method to directly correlate the motion of external markers and internal organs can be used to help guide decisions regarding the validity of the RPM system for respiratory gated radiotherapy on a patient‐specific basis.

SU‐FF‐T‐431: Time Delay Study of a CT Simulator in Respiratory Gated CT Scanning

Purpose: To evaluate the time delay in respiratory gated CT scanning and its effect to motion synchronized radiotherapy.Materials and Methods: In this study, we presented a technique to measure the time delay of AcQSim CT simulator (Philips Medical Systems) using Varian's Real‐Time Positioning Management (RPM) system (Varian Medical Systems). A respiratory gating platform (REF 91150, Standard Imaging, Inc.) was first set at the position of amplitude maximum (Phase 0). Then a ball of 1.3 cm diameter was put on the platform and set at the CT laser. A single axial‐scan was acquired across the center of the ball without motion. Then the motion was turned on and single axial‐scans gated at different phases were acquired with a very narrow gating window. The time between the phase giving a good estimate of the ball and Phase 0 is the overall delay time. The delays were also verified using metal balls of 1.5 mm diameter initially set at the amplitude minimum (Phase 180).Results: We found that for AcQSim CT, the overall delay for a single axial‐scan (with 1s‐scan time) is 1.75s. For multiple axial‐scans, the overall delay is 1.75s for the first scan and 0.75s for the subsequent ones. This demonstrated that the CT mechanical startup delay is 1s. After the first axial‐scan, the overall delay per scan is less because CT gantry continuously spins and no mechanical delay exists. We call the overall delay without the mechanical part the scanning delay, which basically equals to half the scan time (0.5s for 1s‐scan time) plus the gating pulse triggering delay (250 ms). Conclusion: It is the scanning delay rather than the triggering delay that should be compensated when doing motion synchronized radiotherapy. The current interface between the RPM system and the AcQSim CT does not compensate for this delay.

Time delay study of a CT simulator in respiratory gated CT scanning

In respiratory-gated radiotherapy (RGRT), if the time delay in a computed tomography (CT) simulator and that in a linear accelerator (Linac) are different, the simulation and the treatment cannot be synchronized. In this study, we presented a technique to measure the time delay of the AcQSim CT simulator (Philips Medical Systems, Cleveland, OH) using Varian's Real-Time Positioning Management (RPM) system (Varian Medical Systems, Palo Alto, CA). A respiratory gating platform (REF 91150, Standard Imaging, Inc., Middleton, MI) was first set at the position of amplitude maximum (phase 0). Then a ball of 1.3 cm diameter was put on the platform and set at the CT laser. A single axial scan was acquired across the center of the ball without motion. Then the motion was turned on and single axial scans gated at different phases were acquired with a very narrow gating window. The time between the phase giving a good estimate of the ball and phase 0 is the overall delay time. We found that for AcQSim CT, the overall delay for a single axial scan (with 1 s scan time) is 1.75 s. For multiple axial scans, the overall delay is 1.75 s for the first scan and 0.75 s for the subsequent ones. This demonstrated that the CT mechanical startup delay is 1 s. After the first axial scan, the overall delay per scan is less because CT gantry continuously spins and no mechanical delay exists. We call the overall delay without mechanical part the scanning delay, which basically equals half the scan time (0.5 s for 1 s scan time) plus the gating pulse triggering delay (250 ms). The delays were also verified using metal balls of 1.5 mm diameter set at the amplitude minimum (phase 180, initially). We note that it is the scanning delay rather than the triggering delay that should be compensated when doing motion-synchronized radiotherapy. The current interface between the RPM system and the AcQSim CT does not compensate for this scanning delay.

Determining Margin for Target Deformation and Rotation in Respiratory Motion-Tracked Stereotactic Radiosurgery of Pancreatic Cancer

Purpose/Objective: Respiratory motion tracking during radiotherapy may allow reducing the margin required for targets that move with breathing such as tumors in the upper abdomen. However simply tracking translational shifts, as might be indicated by implanted fiducials, may fail to correct for more complex changes in target volume such as deformation and rotation. We describe a method of analyzing these higher order breathing-induced volume changes using 4-dimensional computed tomography (4-D CT), and of determining appropriate margin expansion to compensate. Materials/Methods: As part of an ongoing institutional protocol involving stereotactic radiosurgery for pancreatic adenocarcinoma, 5 patients with pancreatic cancer received biphasic contrast CT scans in breath hold as well as 4-D CT scans in free breathing for treatment planning between March and April 2005. All patients had 4 gold fiducials implanted under CT guidance in or near the tumor. 4-D scans were acquired on a GE Discovery ST multislice PET-CT scanner with the Varian Real-time Position Management (RPM) system, and 4-D image reconstruction was performed using a GE Advantage Workstation. Treatment was delivered using the Accuray CyberKnife stereotactic radiosurgery system with the Synchrony respiratory tracking system. Treatment volumes were defined using the Varian Eclipse treatment planning system. The primary tumor was contoured on both the end-expiratory and end-inspiratory phases (GTVe and GTVi) from the 4-D scan. The internal target volume (ITV) that would need to be treated without compensating for respiratory motion was defined as the union of GTVe and GTVi. A modified ITV for fiducial-tracked treatment (tITV) was defined as the union of GTVe and GTVi after registering the expiratory and inspiratory scans such that the implanted fiducials were superimposed between them. Only translations in 3 dimensions were used for the registration. Thus residual differences between the tITV and GTV were due to target/organ rotation and/or deformation. Results: Table 1 summarizes the mean fiducial displacement between expiration and inspiration, mean residual fiducial displacement after registration, GTVe/GTVi, ITV, and tITV for the 5 patients. The ITV for uncompensated treatment was substantially larger than the GTV in all cases. Fiducial tracking allowed reduction of the tITV versus ITV in 3 of 5 cases. However, in all cases tITV was still larger than GTV, reflecting the presence of target deformation and/or rotation.Table 1 Conclusions: Breathing compensated stereotactic radiosurgery for pancreatic cancer using respiratory tracking permits reducing the target volume in most cases. Target deformation and/or rotation exists, can be analyzed using 4-D imaging, and can be compensated for by appropriate margin expansion.

Lung volume assessment for a cross-comparison of two breathing-adapted techniques in radiotherapy

To assess the validity of gated radiotherapy of lung by using a cross-check methodology based on four-dimensional (4D)-computed tomography (CT) exams. Variations of volume of a breathing phantom was used as an indicator. A balloon was periodically inflated and deflated by a medical ventilator. The volume variation (DeltaV) of the balloon was measured simultaneously by a spirometer, taken as reference, and by contouring 4D-CT series (10 phases) acquired by the real-time position management system (RPM). Similar cross-comparison was performed for 2 lung patients, 1 with free breathing (FB), the other with deep-inspiration breath-hold (DIBH) technique. During FB, DeltaV measured by the spirometer and from 4D-CT were in good agreement: the mean differences for all phases were 8.1 mL for the balloon and 10.5 mL for a patient-test. End-inspiration lung volume has been shown to be slightly underestimated by the 4D-CT. The discrepancy for DeltaV between DIBH and end-expiration, measured from CT and from spirometer, respectively, was less than 3%. Provided that each slice series is correctly associated with the proper breathing phase, 4D-CT allows an accurate assessment of lung volume during the whole breathing cycle (DeltaV error <3% compared with the spirometer signal). Taking the lung volume variation into account is a central issue in the evaluation and control of the toxicity for lung radiation treatments.

Breathing adapted radiotherapy for breast cancer: Comparison of free breathing gating with the breath-hold technique

Background and purposeAdjuvant radiotherapy after breast-conserving surgery for breast cancer implies a risk of late cardiac and pulmonary toxicity. This is the first study to evaluate cardiopulmonary dose sparing of breathing adapted radiotherapy (BART) using free breathing gating, and to compare this respiratory technique with voluntary breath-hold. Patients and methods17 patients were CT-scanned during non-coached breathing manoeuvre including free breathing (FB), end-inspiration gating (IG), end-expiration gating (EG), deep inspiration breath-hold (DIBH) and end-expiration breath-hold (EBH). The Varian Real-time Position Management system (RPM™) was used to monitor respiratory movement and to gate the scanner. For each breathing phase, a population based internal margin (IM) was estimated based on average chest wall excursion, and incorporated into an individually optimised three-field mono-isocentric wide tangential photon field treatment plan for each scan. The target included the remaining breast, internal mammary nodes and periclavicular nodes. ResultsThe mean anteroposterior chest wall excursion during FB was 2.5mm. For IG and EG, the mean excursions within gating windows were 1.1 and 0.7mm, respectively, whereas for DIBH and EBH the excursions were 4.1 and 2.6mm, respectively.For patients with left-sided cancer, the median heart volume receiving more than 50% of the prescription dose was reduced from 19.2% for FB to 2.8% for IG and 1.9% for DIBH, and the median left anterior descending (LAD) coronary artery volume was reduced from 88.9% to 22.4% for IG and 3.6% for DIBH. Simultaneously, the median ipsilateral relative lung volume irradiated to >50% of the prescribed target dose for both right- and left-sided cancers was reduced from 45.6% for FB to 29.5% for IG and 27.7% for DIBH. For EBH and EG, both the irradiated heart, LAD and lung volumes increased compared to FB. ConclusionsThis is the first study to demonstrate the dosimetric benefits of free breathing gated breast cancer radiotherapy. IG compared favourably with DIBH, substantially reducing cardiac doses simultaneous with significant pulmonary tissue sparing.

Sci‐PM Sat ‐ 08: Feasibility of gated helical tomotherapy using the real‐time position management (RPM) system

We propose a new method of gating helical tomotherapy (HT) delivery for lung lesions that does not require breath holding. Without interrupting the gantry rotation and couch translation, the complete closure of the bMLC leaves would be triggered by the Real Time Position Management (RPM) system (Varian Oncology Systems, Palo Alto, CA), whenever the associated chest (and implied tumour) position moves outside the gating window. Once the initial radiation delivery is complete, additional iterations of the helical treatment are repeated until &gt;95 % of the planned beam projections are delivered. Each iteration commences at a different phase of the respiratory cycle to “fill in” the previously undelivered beam projections. In this study, a gated HT delivery was simulated on the HT unit by delivering a series of “modified” leaf sinograms — altered according to the RPM curve of a real lung patient. The original leaf sinogram created for a stationary target (non‐gated) was modified such that all beam projections whose delivery times fall outside the gating window were nulled. The resulting dose distribution was measured by film dosimetry, and compared to the dose distribution of the non‐gated delivery. The gated HT delivery showed excellent agreement with the non‐gated HT delivery, with a mean dose difference of less than 1% observed in the central plane. In conclusion, the feasibility of gated HT delivery using the RPM system has been demonstrated. Future work will address different breathing patterns and more complicated target shapes treated with intensity modulation.

SU-FF-J-02: A Comparison of Amplitude- and Phase-Based 4D CT

Purpose: Four‐dimensional (4D) CT depends on accurate correlation between temporally acquired CT slices and the patient's respiratory cycle. One approach is to record the position of an external marker placed on the abdomen or chest during the scan, and retrospectively match the CT data with the phase of the marker motion. While very effective for regular breathing patterns, the phase‐based approach can lead to significant mismatch between adjacent image segments when the respiratory motion exhibits irregularities. We propose a method of extracting amplitude‐based 4D CT from cine‐acquired CTdata sets, and compare the amplitude‐based 4D CT with the phase‐based 4D CT for both phantom and patient data. Method and Materials:CTdata sets were acquired in cine mode on the GE Discovery ST, and motion of an infrared reflecting block was recorded using Varian's Real‐time Position Management (RPM) camera. Rather than use the phase‐based calculations of the RPM system, we replaced the phase field with pseudo‐amplitude values spanning the full respiratory cycle (i.e., differentiating inspiration from expiration). The modified respiratory trace file was then sent, along with the cine CT data, to the GE Advantage Workstation for processing. The method was applied to a thoracic phantom moving irregularly in the longitudinal direction, and to an abdominal 4D scan of a lungcancer patient. Results: For both phantom and patient data, the phase‐based 4D CTimages showed boundary mismatches of up to 1 cm between couch positions. The mismatch on the amplitude‐based sets, however, was less than 2 mm throughout the field of view. Conclusion: Phase‐based 4D CT can lead to mismatched slices when the respiratory cycle involves irregularities. In such situations, by replacing phase with a modified definition of amplitude that distinguishes inspiration from expiration, a substantially improved 4D CTimage can be generated.

Radiofrequency ablation of cardiac arrhythmias using a three-dimensional real-time position management and mapping system.

A recently developed three-dimensional real-time position management system (RPM) uses an ultrasound ranging technique that enables multiple distance measurements between two reference catheters and a mapping catheter each equipped with ultrasound transducers. In addition to three-dimensional representation of the catheters and ablation sites it displays real-time movements of catheters (including the tip and shaft). A recently released version of the system enables additional geometry reconstruction of the heart chamber and activation mapping. This study included 21 patients (mean age 59 +/- 14.5 years) referred for radiofrequency catheter ablation of various arrhythmias. Geometry was reconstructed by tracing the endocardial contour of the respective heart chambers. Global and local color coded activation maps were constructed to confirm the nature of arrhythmia and to guide ablation. Spontaneous or induced arrhythmias were typical atrial flutter (n = 8), atypical atrial flutter (n = 3), atrioventricular nodal reentrant tachycardia (n = 3), atrial tachycardia (n = 2), atrial fibrillation (n = 2), ventricular tachycardia (n = 2), and Wolff-Parkinson-White syndrome (n = 1). Geometry reconstruction and mapping of arrhythmias were possible in 20 of 21 patients. RPM-guided radiofrequency ablation was successful in 19 (95%) of 20 patients. Due to difficulties in steering the RPM mapping/ablation catheter, in 6 (28%) successfully mapped patients, radiofrequency ablation was performed using another catheter. In one patient, the RPM-guided map was inconclusive and in another patient, ablation failed due to multiple reentrant circuits. No complications were observed. In conclusion, the new RPM system enables geometry reconstruction and three-dimensional positioning of the ablation catheters, reconstruction of the activation maps, marking of anatomic structures and reproducible tracking of multiple ablation sites. The system could be used to guide radiofrequency ablation of atrial and ventricular arrhythmias.