1. Derivation of the tumor position from external respiratory surrogates with periodical updating of external/internal correlation E Kanoulas 1, JA Aslam 1, GC Sharp 2, RI Berbeco 3, S Nishioka 4, H Shirato 5, SB Jiang 2 (1) Northeastern University, Boston, MA, (2) Mass General Hospital and Harvard Medical School, Boston, MA, (3) Brigham and Women's Hospital and Harvard Medical School, Boston, MA, (4) Department of Radiology, NTT Hospital, Sapporo, Japan, (5) Hokkaido University, School of Medicine, Sapporo, Japan

2. Motivation Beam tracking or gating of moving tumors requires precise real-time tumor localization Fluoroscopic marker tracking is accurate however requires large imaging dose Deriving tumor position from external surrogates is dose free however inaccurate due to the uncertainties in internal/external correlation This work  Derive tumor position using external surrogate  Periodically image to update internal/external surrogate correlation  Study the minimum updating rate and optimal updating approach

3. Data used for the study X-ray imagers Laser housing NTT Hospital (Dr. Seiko Nishioka) Mitsubishi RTRT system (track 3D tumor position) AZ-733 “Resp-gate” system Monitors abdominal surface 3-D internal tumor motion + 1-D abdominal motion Berbeco et al., “Residual motion of lung tumours in gated radiotherapy with external respiratory surrogates”, PMB, (2005 Aug 21), 50(16), p. 3655-67

4. Updating the correlation function During patient setup  both external and internal surrogate position at 30Hz During treatment  external surrogate position at 30Hz  periodically (at low frequency) internal marker position

5. Update Methods 1. Aggressive update (through the update point) a. Shift line through update point b. Re-fit line and force it through update point 2. Conservative update (balance between update and training points) a. Re-fit line with extra weight to update point b. Minimize the distances to update point and previous line

6. Method 1a. Shift line through point

7. Method 1b. Re-fit line through point

8. Method 2a. Re-fit line with extra weight to point

9. Method 2b. Minimize the distances to update point and previous line

10. Results 5 patients 25 data sets Large SI motion only (>20 mm)

11. Conclusions Tumor localization using external surrogates requires periodical update of the internal/external correlation Update frequency down to 2Hz gives 2 mm motion error (95% confidence level) The aggressive update methods outperform the conservative ones at high update frequencies while the opposite is true for low update frequencies