Initial setup and subsequent temporal position monitoring using implanted RF transponders

Transcription

1 Initial setup and subsequent temporal position monitoring using implanted RF transponders James Balter, Ph.D. University of Michigan Has financial interest in Calypso Medical Technologies

2 Acknowledgements Edward Vertatschitsch 3, J Nelson Wright 3, Steven Dimmer 3, Barry Friemel, PhD 3, Laurence J. Newell 3, Andrew Silber, PhD 3, Y Cheng 3, Lisa Levine, PhD 3, *Timothy Mate 1, Daniel Low 4, 1 Swedish Medical Center, Seattle WA 3 Calypso Medical Technologies, Inc., Seattle, WA 4 Washington University, St. Louis MO * Has financial interest in Calypso

3 Objectives Most current volumetric imaging paradigms require significant time to acquire and process information prior to making a decision Radiographic and fluoroscopic methods have shown the feasibility of localization and rapid fiducial position monitoring, although dose may be an issue for fractionated treatments A wireless electromagnetic localization is under development for positioning and tracking during radiotherapy

4 Implantable Sensor - Beacon TM Transponder Wireless AC electromagnetic resonant circuit No external lead wires No internal power supply Designed for permanent implantation Implanted prior to therapy Positioned in soft tissue in or near treatment target Remains inactive until energized by system console 1.85 mm x 8 mm for initial prostate application

5 Basic Principle of Operation Magnetic Source Coils Magnetic Sensor Coils Preamplifiers f1 f2 f3 f1 f2 f3 Excitation Response

6

7 Step 1: Implant Beacon Transponders

8 Step 2: Treatment Planning CT Reference coordinates of transponders with respect to isocenter are established from a treatment planning CT scan Transponders X Isocenter Z Y x T,y T,z T Relative Position (xt,yt,zt) = transponder location (x,y,z) isocenter (x,y,z)

9 Step 3: Localization Localization system components at the linac: 1. Wireless Transponders 2. Array 3. Console 4. Infrared Cameras 5. Tracking Station - Cameras determine array position relative to isocenter - Transponders are continuously localized relative to the array - The relative offset between expected and actual transponder positions is reported for feedback in positioning

10 Flintstone, Fred

11 Accuracy and linac compatibility Evaluate the system in the radiation therapy environment for: Accuracy of setup localization Stability over time Effect on localization accuracy during linac operation (IMRT) Effect on localization accuracy with displacement of transponders (to simulate minor deformations/volume changes)

12 Test Assembly Prior to linac testing, accuracy was established using a calibrated benchtop system Custom designed stand with precision mounts at fixed offsets (- 8, -4, 0, 4, 8 cm) from center and distances from the array selected by the length of precision-machined mounting posts Constructed using Ultem 1000 (GE Thermoplastics) high rigidity, low thermal expansion Machined positions and inserts ( <0.1 mm precision) Calibrated by reference alignment marks and validated by a null position transponder

13 Test assembly Source Array Transponder

14 Experimental setup Transponder

15 Localization experiments Comparison of actual versus predicted position at distances of 80 and 270 mm from the source array (minimum and maximum distances determined from retrospective evaluation of prostate position from clinical sites). Tests performed in air and separately in a tank of 0.9% saline solution to mimic properties of human tissue Tests of tracking rate Reporting position over multiple readings to report stability and precision at 10 Hz sampling at 20s versus 20 minutes Tracking of linear trajectories at varying speed

16 Stability 20 s (10 Hz) (80 mm from source array) 0.03 e rro r (c m ) x axis y axis z axis time (s) σ x = mm, σ y = 0.01 mm, σ z = mm

17 Stability 20 s (10 Hz) (270 mm from source array) Error (cm ) time (s) σ x = 0.27mm, σ y = 0.36 mm, σ z = 0.48 mm x axis y axis z axis

18 Stability over 20 minutes Stability does not change measurably over 20 minutes 80 mm from source Change in average position from 20 s: mm Change in standard deviation: ~0 (10^-4) mm 270 mm from source Change in average position from 20 s: mm Change in standard deviation: mm

19 Accuracy 80 mm distance to array E rro r ( cm ). dx dy dz Offset (cm)

20 Accuracy 270 mm distance to array 0.3 Error (cm) ofset (cm) dx dy dz

21 Accuracy in saline Tested under conditions of maximum likely error (270 mm distance to array) in 0.9% saline (140 mm depth to transponder) Centered position Accuracy: dx 0.11 mm, dy 0.06 mm, dz 0.32 mm Precision: σ x 0.27 mm, σ y 0.36 mm, σ z 0.69 mm Offset 8 cm laterally Accuracy: dx 0.29 mm, dy 0.43 mm, dz 0.27 mm Precision: σ x 0.54 mm, σ y 0.41 mm, σ z 0.43 mm

22 Accuracy with 3 transponders To demonstrate lack of cross talk, a mount with three transponders was tested (separation of 1 cm between pairs of transponders) Accuracy of localization: 80 mm from source array: dx 0.17 mm, dy 0.03 mm, dz 0.05 mm σ x 0.01 mm, σ y 0.01 mm, σ z 0.01 mm 270 mm from source array: dx 0.16 mm, dy 0.18 mm, dz 0.12 mm σ x 0.26 mm, σ y 0.49 mm, σ z 0.62 mm.

23 Phantom design A tissue equivalent phantom was developed for accuracy studies containing external marks for laser-based alignment and a precision slot for test inserts Inserts were machined containing transponders in known configurations The configuration of transponders for a given insert generates an average coordinate with a known (nonzero) offset relative to isocenter (when the phantom is aligned to lasers using the external marks)

24 Accuracy Demonstration - Method Laser accuracy was established using a stereotactic isocenter standard (~0.25 mm) Target position was accuracy was defined by absolute error from baseline position established by laser alignment and phantom design ( truth ) Comparison to existing standard for localization was accomplished by diagnostic radiographic localization at UM using inroom X-Ray system Perform both a kv and Calypso isocenter localization and compare each to the measured ( absolute ) isocenter offset by CT

25 Static Accuracy Results Absolute Isocenter Offset: mm lat, 4.70mm long, 0.01 mm vert Reported Position (mm) Reported & truth difference (mm) Localization Method Lat Long Vert Lat Long Vert 3-D error Radiographic Localization Calypso Localization

26 System Stability in RT Environment - Beam Off Continuous localization (gantry angle 180 degrees), no radiation beam Initial positioning using manual couch adjustment guided by Calypso interface Average offsets: (-0.13,0.13,0.26) mm Variation (σ): Location (mm) Target Position - Beam Off Lat Long Vert (0.04,0.02,0.05) mm Time (Minutes)

27 System Stability in RT Environment - Beam On Same setup process as beam off experiment Continuous monitoring during delivery of 5 field IMRT (step and shoot) treatment over 15 minutes Slight (<0.2 mm) dependence of gantry angle on average offset measurements Variation (σ): (0.8,0.6,0.6) mm Location (mm) Target Position during IMRT 225 o gantry 290 o gantry 330 o gantry Lat Vert 20 o gantry Long 65 o gantry Time (minutes)

28 Static Accuracy Table Shift Localizations Reported Calypso Position (mm) Calypso & Radiographic Difference (mm) 10 mm Table Shifts Lat Long Vert Lat Long Vert 3D difference (mm) At isocenter align Posterior shift Anterior shift Right shift Left shift Inferior shift Superior shift When 10 mm table shifts were made (assuming a precise shift), The system could detect these shifts to within 0.4 mm, well within the uncertainty of radiographic verification (mean error 1.5 mm)

29 Study - Conclusions The Calypso System was able to localize the isocenter with sub-millimeter (< 1mm) accuracy After initial setup, the system was capable of performing stable continuous localization (drift <0.2 mm) Minimal localization error (<0.2mm) induced by linac operation Accuracy of tracking offsets due to table shifts up to 10 mm was within the accuracy of reference standard (radiographic) localization technique

30 Wash U Phantom (Low) Validation Digitizer Beacon Phantom 3X 1D Stages

31 Comparison Along Longitudinal Axis: Patient Motion File Low, Wash. U.

32 Parikh and Low Accuracy vs speed 0.9 2x4x2 cm ellipse - 20 bpm - 1 transponder 33ms error (mm) speed (mm/sec)

33 Initial clinical trials - prostate First stage assess marker position stability over time UC-02: Beacon Transponder Positional Stability Inter-Transponder Distance (mm) CT images are denoted w ith an "open symbol" # Days Post-Implant A-L L-R A-R Treatment planning CT 2 weeks post implant

34 Initial trials - prostate Standard Deviations of Inter-Beacon Distances All Beacon Geometry Data Day 14 - Fraction 20 A-L L-R A-R Avg A-L L-R A-R Avg UC MD MD MD UM MD UC MD MD UM UM UM MD MD UM UM MD UC UM MD Mean N/A N/A

35 2 nd phase Beacon-based positioning Initial positioning using Beacons (35 seconds for an untrained operator) Radiographic verification of position Starting 2-5 minutes after initial positioning, 8 minutes of position data is collected 1 P a tie n t A lig n m e n t U s in g th e C a lyp s o S ys te m MD Target Position (cm) Longitudinal, lateral, and vertical table shifts m ade sequentially (~35 seconds) -2 L a te r a l Longitudinal V e r tic a l Time (sec)

36 8 Minutes of Continuous Calypso Monitoring MD Target Position (cm) Lateral X Y Z -0.2 Longitudinal -0.4 Vertical Time (sec)

37 0.5 8 Minutes of Continuous Calypso Monitoring MD Target Position (cm) Longitudinal Lateral Vertical X Y Z Time (sec)

38 8 Minutes of Continuous Calypso Monitoring 0.2 UM Lateral Target Position (cm) Longitudinal Vertical X Y Z Time (sec)

39 Table 2. Maximum motion during 8 minute tracking sessions (meas in mm) Willoughby (MDA Orlando) X (Lateral) Y (Ant/Post) Z (Sup/Inf) md md md md md md um um um um um

40 Summary RF localization using implanted transponders is feasible This system has shown the potential to provide rapid positioning based on transponder location Intratreatment monitoring is possible, and early studies show the potential value for detecting large transient shifts, as well as slower trends in position variation Developments are underway for other body sites