Kristen M. Lechleiter

Development of the 4D Phantom for patient-specific end-to-end radiation therapy QA

In many patients respiratory motion causes motion artifacts in CT images, thereby inhibiting precise treatment planning and lowering the ability to target radiation to tumors. The 4D Phantom, which includes a 3D stage and a 1D stage that each are capable of arbitrary motion and timing, was developed to serve as an end-to-end radiation therapy QA device that could be used throughout CT imaging, radiation therapy treatment planning, and radiation therapy delivery. The dynamic accuracy of the system was measured with a camera system. The positional error was found to be equally likely to occur in the positive and negative directions for each axis, and the stage was within 0.1 mm of the desired position 85% of the time. In an experiment designed to use the 4D Phantoms encoders to measure trial-to-trial precision of the system, the 4D Phantom reproduced the motion during variable bag ventilation of a transponder that had been bronchoscopically implanted in a canine lung. In this case, the encoder readout indicated that the stage was within 10 microns of the sent position 94% of the time and that the RMS error was 7 microns. Motion artifacts were clearly visible in 3D and respiratory-correlated (4D) CT scans of phantoms reproducing tissue motion. In 4D CT scans, apparent volume was found to be directly correlated to instantaneous velocity. The system is capable of reproducing individual patient-specific tissue trajectories with a high degree of accuracy and precision and will be useful for end-to-end radiation therapy QA.

Evaluation of linear accelerator gating with real-time electromagnetic tracking.

PURPOSE Intrafraction organ motion can produce dosimetric errors in radiotherapy. Commonly, the linear accelerator is gated using real-time breathing phase obtained by way of external sensors. However, the external anatomy does not always correlate well with the internal position. We examined a beam gating technique using signals from implanted wireless transponders that provided real-time feedback on the tumor location without an imaging dose to the patient. METHODS AND MATERIALS An interface was developed between Calypso Medicals four-dimensional electromagnetic tracking system and a Varian Trilogy linear accelerator. A film phantom was mounted on a motion platform programmed with lung motion trajectories. Deliveries were performed when the beam was gated according to the signal from the wireless transponders. The dosimetric advantages of beam gating and the system latencies were quantified. RESULTS Beam gating using on internal position monitoring provided up to a twofold increase in the dose gradients. The percentage of points failing to be within +/-10 cGy of the planned dose (maximal dose, approximately 200 cGy) was 3.4% for gating and 32.1% for no intervention in the presence of motion. The mean latencies between the transponder position and linear accelerator modulation were 75.0 +/-12.7 ms for beam on and 65.1 +/- 12.9 ms for beam off. CONCLUSION We have presented the results from a novel method for gating the linear accelerator using trackable wireless internal fiducial markers without the use of ionizing radiation for imaging. The latencies observed were suitable for gating using electromagnetic fiducial markers, which results in dosimetric improvements for irradiation in the presence of motion.

Bronchoscopic Implantation of a Novel Wireless Electromagnetic Transponder in the Canine Lung: A Feasibility Study

PURPOSE The success of targeted radiation therapy for lung cancer treatment is limited by tumor motion during breathing. A real-time, objective, nonionizing, electromagnetic localization system using implanted electromagnetic transponders has been developed (Beacon electromagnetic transponder, Calypso Medical Technologies, Inc., Seattle, WA). We evaluated the feasibility and fixation of electromagnetic transponders bronchoscopically implanted in small airways of canine lungs and compared to results using gold markers. METHODS AND MATERIALS After approval of the Animal Studies Committee, five mongrel dogs were anesthetized, intubated, and ventilated. Three transponders were inserted into the tip of a plastic catheter, passed through the working channel of a flexible bronchoscope, and implanted into small airways of a single lobe using fluoroscopic guidance. This procedure was repeated for three spherical gold markers in the opposite lung. One, 7, 14, 28, and 60 days postimplantation imaging was used to assess implant fixation. RESULTS Successful bronchoscopic implantation was possible for 15 of 15 transponders and 12 of 15 gold markers; 3 markers were deposited in the pleural space. Fixation at 1 day was 15 of 15 for transponders and 12 of 12 for gold markers. Fixation at 60 days was 6 of 15 for transponders and 7 of 12 for gold markers, p value = 0.45. CONCLUSIONS Bronchoscopic implantation of both transponders and gold markers into the canine lung is feasible, but fixation rates are low. If fixation rates can be improved, implantable electromagnetic transponders may allow improved radiation therapy for lung cancer by providing real-time continuous target tracking. Developmental work is under way to improve the fixation rates and to reduce sensitivity to implantation technique.

TH‐C‐M100J‐08: Dosimetric Effects of a 4D Magnetic Localization System for LINAC Beam Gating On Prostate and Lung Radiation Therapy

Purpose: The Calypso® Medical 4D Localization System is capable of tracking continuous dynamic motion, and has been FDA cleared for use in the prostate. To date, automatic intervention for the measured motion is not a function of the system. We investigated use of the system to gate radiation therapydelivery on a motion phantom for both a lung and prostate fraction Materials and Methods: A Calypso gating prototype system with an optically isolated relay was connected to the BEAM_HOLD interface of a Varian Trilogy linac. A sample 3D lung plan and SMLC prostate plan were randomly selected from the active patient list. The Washington University 4D phantom was programmed to move a film box through 2 patient‐measured prostate and lung trajectories. Radiation therapy was delivered to the static phantom, to the phantom undergoing each motion trajectory without gating, and to the phantom undergoing motion trajectories while the Calypso System was gating the linac. A 4×4×4 mm gating window centered on isocenter was used for the prostate, while a linear gating window corresponding to exhalation (ie 2 mm<y<5 mm) was used for the lung.Results: Without gating beam delivery, prostate motion during treatment delivery caused approximately 10% underdosing and overdosing due to the superior/inferior and anterior/posterior prostate motion. The Calypso System triggering gated therapy delivery reduced these areas significantly, as seen in difference images using film dosimetry. Likewise, lungtumor motion caused a geometric mismatch between static and motion delivery. The Calypso System with prototype gating showed a decrease in this mismatch. Conclusions: A wireless electromagnetic implanted transponder system for linac gating was effective in reducing dosimetric errors caused by prostate and lung motion. More work is needed to define optimal gating windows to provide maximal clinical efficiency with minimal delivery error. This work was supported by Calypso Medical Technologies.

MO-D-ValB-01: Characterization of Cardiac Motion in the Lung Using a Novel Electromagnetic System in An Animal Model

Purpose: Previous studies have examined the accuracy of the use of three internal ACelectromagnetic transponders and wireless tracking system (Calypso® Medical) for tumor localization in prostate cancer. This study focuses on the use of the system to investigate and characterize cardiac induced lungtissue motion to better predict three‐dimensional lungtumor position in real‐time. Method and Materials: Under an institutional approved animal study, three 1.8 mm ACelectromagnetic transponders are bronchoscopically implanted in the periphery of the lungs of five hounds. The transponders are positioned in a triangle, each spaced 1–3 cm apart. The transponder positions are sequentially measured every 50 ms at five time points. During each measurement, the subject is stressed with several respiratory patterns. Signal processing of the data involves the design and application of a Butterworth highpass filter to obtain the component of transponder movement due to cardiac motion. Results: The data for the 1st three time points of the first animal are presented. FFT spectrum analysis indicated signal frequency components of 13.05 and 123.8 cycles/minute, due to respiration and cardiac motion respectively. Cardiac‐induced lungtissue motion was detected in vivo, ranging from 0.0007cm – 0.3592cm, by applying the highpass filter to the data. The motion was smaller on the implant day compared with the other two time points. Moreover, transponder position and distance from the heart had an effect on calculated motion. Finally, breathing patterns also affected the observed motion at a statistically significant 0.1% level. Conclusion:Cardiac contractions cause quantifiable motion in surrounding lungtissues that cannot be measured with existing onboard imaging capabilities. The motion varies depending on transponder position, distance from the heart, breathing pattern, and day of measurement. Though the motion maximum was 3.6mm, this motion could cause imaging artifacts when using respiratory correlates. Research sponsored by Calypso® Medical Technologies.

Incorporating electromagnetic tracking into respiratory correlated imaging for high precision radiation therapy

It is well established that respiratory motion has significant effects on lung tumor position, and incorporation of this uncertainty increases the normal lung tissue irradiated. Respiratory correlated CT, which provides three dimensional image sets for different phases of the breathing cycle, is increasingly being used for radiation therapy planning. Cone beam CT is being used to obtain cross sectional imaging at the time of therapy for accurate patient set-up. However, it is not possible to obtain cross sectional respiratory correlated imaging throughout the course of radiation, leaving residual uncertainties. Recently, implantable passive transponders (Calypso Medical Technologies) have been developed which are currently FDA-cleared for prostate use only and can be tracked via an external electromagnetic array in real-time, without the use of ionizing radiation. A visualization system needs to be developed to quickly and efficiently utilize both the dynamic real-time point measurements with the previously acquired volumetric data. We have created such a visualization system by incorporating the respiratory correlated imaging and the individual transponder locations into the Image Guided Surgery Toolkit (IGSTK.org). The tool already allows quick, qualitative verification of the differences between the measured transponder position and the imaged position at planning and will support quantitative measurements displaying uncertainty in positioning.

SU-FF-J-75: The Effect of Time On Inter-Transponder Distance Implanted in Lung: An Initial Study in a Canine Model

Purpose: The Calypso 4D Localization® system uses non‐ionizing AC electromagnetic technology to localize implanted Beacon® transponders. The system is capable of real‐time measurement of internal motion. Effective use of this technology in the lung requires placing the transponders in fixed positions that will not change over time. This study compares inter‐transponder distance over an implantation time period of 0–57 days in canine lung.Method and Materials: A pulmonologist bronchoscopically implanted three transponders in a single lung lobe of five canines under an institutionally approved protocol. Distances between transponder pairs were measured over 0–57 days using the Calypso system. The positions were measured both when the dogs were breathing freely and during varied ventilatory amplitudes and frequencies, variable ventilation. In animals with at least two transponders, inter‐transponder distances were calculated. Results: The mean inter‐transponder distances during breathing patterns were stable on the same day, at 2.32cm (free breathing) and 2.40cm (variable ventilation). One animal retained all 3 transponders at 57 days and exhibited significant change in inter‐transponder distance from day 1–9. Changes in mean inter‐transponder distance from day 9–29 ranged from 0.2–1.9mm. For reasons not understood, transponder distances on day 57 for one animal were larger at 2.0–6.5mm. Conclusion: The inter‐fiducial distance was stable regardless of breathing pattern on the same day. Measurements taken on the first day post‐implant varied significantly from later measurements, probably due to local tissuetrauma. Up to 30 days post‐implant, the inter‐transponder distances were stable. However, in one animal after 30 days, the relationship between transponder positions changed. More work is required to improve implant retention and to understand optimal transponder placement relative to a lungtumor target. Future studies will acquire more frequent transponder positions to be a more representative model of clinical patient data. This work was supported by Calypso Medical Technologies.

A comparison of lung motion measured using implanted electromagnetic transponders and motion algorithmically predicted using external surrogates as an alternative to respiratory correlated CT imaging

Three-dimensional volumetric imaging correlated with respiration (4DCT) typically utilizes external breathing surrogates and phase-based models to determine lung tissue motion. However, 4DCT requires time consuming post-processing and the relationship between external breathing surrogates and lung tissue motion is not clearly defined. This study compares algorithms using external respiratory motion surrogates as predictors of internal lung motion tracked in real-time by electromagnetic transponders (Calypso® Medical Technologies) implanted in a canine model. Simultaneous spirometry, bellows, and transponder positions measurements were acquired during free breathing and variable ventilation respiratory patterns. Functions of phase, amplitude, tidal volume, and airflow were examined by least-squares regression analysis to determine which algorithm provided the best estimate of internal motion. The cosine phase model performed the worst of all models analyzed (R2 = 31.6%, free breathing, and R2 = 14.9%, variable ventilation). All algorithms performed better during free breathing than during variable ventilation measurements. The 5D model of tidal volume and airflow predicted transponder location better than amplitude or either of the two phasebased models analyzed, with correlation coefficients of 66.1% and 64.4% for free breathing and variable ventilation respectively. Real-time implanted transponder based measurements provide a direct method for determining lung tissue location. Current phase-based or amplitude-based respiratory motion algorithms cannot as accurately predict lung tissue motion in an irregularly breathing subject as a model including tidal volume and airflow. Further work is necessary to quantify the long term stability of prediction capabilities using amplitude and phase based algorithms for multiple lung tumor positions over time.

SU‐FF‐J‐11: A Novel Use of a Real‐Time Tumor Positioning System in Reducing Cone Beam CT Artifacts

Purpose: The Calypso® Medical 4D Localization system is capable of tracking real‐time dynamic motion without ionizing radiation. A limitation of any fiducial based system is the inability to visualize surrounding tissues. Cone beam CT(CBCT) of moving objects results in image blurring due to long acquisition times. We investigated the use of the Calypso® 4D localization system to improve motion artifacts obtained from the Varian Trilogy CBCT.Materials and Methods:. A research Calypso® 4D tracking system was installed in a Varian Trilogy vault. A rectangular phantom with implanted transponders was attached to an internally‐developed 4D stage. A CBCT was obtained while moving the phantom under the Calypso® measurement array using a patient tumor derived trajectory. The projection images were obtained and shifted using the corresponding Calypso® transponder positioning information and then reconstructed into CBCTimages. This process was repeated for a dog with transponders implanted in the lung as part of an IRB‐approved study. Results: The Calypso® based image shifts caused the radiographic projection of the transponders remained stable in sinogram space. CBCTimages from the shifted sinogram exhibited reduced image motion artifacts. Without artifact reduction, the transponders were visualized as multiple streaks and the surface of the phantom was heavily deformed. With artifact reduction, the transponders were accurately localized, and the deformation was removed. The dogs breathing cycle made qualitative image motion artifact reduction review difficult. Quantitative analysis of the reconstructedCT numbers showed sharper gradients through the transponders, indicating that the shifting process had improved the image quality. Conclusions: Use of a wireless electromagnetic implanted transponder system for motion correction of CBCT is possible. This preliminary sinogram shifting technique was very effective for non‐deforming objects. Further work will increase the synergy between real‐time tracking systems and volumetric imaging. This work was supported by Calypso® Medical Technologies.

MO‐D‐ValB‐06: Concurrent Tracking and Fluoroscopic Imaging of Implantable Wireless Electromagnetic Transponders

Purpose: Multiple technologies are being utilized to improve real‐time tumor tracking. To date, there have not been methods to prospectively compare different technologies with realistic tumor trajectories. We evaluated the capabilities of the Calypso® Medical 4D Localization System (Calypso Medical, Seattle, WA) and Varian Trilogy System (Varian Medical Systems, Palo Alto, CA) fluoroscopy in tracking dynamic objects. Method and Materials: Initially, a quality assurance fixture containing three implantable transponders was moved by an in‐house developed 4D phantom through an ellipse and a non‐uniform human lungtumor path modeled with CTimaging and spirometry. Subsequently, three transponders that had been implanted in a canine lung were tracked. In both experiments, the transponders were fluoroscopically imaged on a Trilogy system while simultaneously being tracked by the Calypso® 4D localization system. The fluoroscopic images were recorded and later analyzed using a custom‐written (MATLAB) image processing program to determine the transponder projection positions with respect to time. The trajectories derived from the fluoroscopic images were synchronized with and compared to the Calypso System position data. Results: The root mean square (RMS) position differences were less than 0.03 mm for all tested measurement system combinations. While both were small, the Calypso System RMS error was slightly lower than that of the fluoroscopy when compared against the 4D phantom positions. Of the three trajectories, the RMS error between imaging modalities was largest for the patient trajectory and smallest for the ellipses. Conclusion: This work indicates that both tracking methods provide excellent positioning accuracy. Although the accuracy discrepancy between the two systems is negligible, the Calypso® System also offers the ability to localize in three dimensions and has the advantage of being able to track a target continuously without the use of ionizing radiation.Conflict of Interest: Supported in part by Calypso Medical Technologies, Inc.