1. Gating Latency Of Soft Tissue Tracking Beam Control For An Onboard MR-IGRT System R.Kashani, L.Santanam, O.Green, C.Noel, J.Victoria, O.Wooten,J.Michalski, S.Mutic, H.Gay, K.Tanderup

2. MR guided Gating using ViewRay 0.35T MRI 3 Co-60 heads – (~550 cGy/min @ iso) Max: 15,000 Ci per head Large imaging FOV (50 cm) and Tx volume (27cmx27cm) Resolution for volumetric images: 1.5x1.5x1.5 mm Resolution for real-time treatment-monitoring images: 3.5x3.5x5.0 mm Real-time imaging at 4 frames per second (one plane) or 2 frames per second (3 parallel planes) Automatic gating based on the target outside user generated boundary

3. The phantom was configured to move in a repetitive sinusoidal pattern with peak-to-peak amplitudes of 1 cm at 10 and 15 cycles/min, crossing 3, 5and 8 mm gating boundaries. Phantom was positioned by aligning 3D MR localization with a planning CT data set. Cine MR was acquired during irradiation at 4 FPS. The intersection of the 3D target contour was automatically registered in a cine keyframe via deformable registration. When the MR target moved outside the gating boundary, the beam off signal and 60Co source retraction (aperture sensor) was triggered. Beams from each of the three 60Co heads at 90, 180 and 270 individually and in combination were tested. A total of 19 runs of around 270 sec each were tested. To establish a ground-truth temporal measurement of phantom motion, sensors were placed at superior and inferior motion extents of the phantom and were set to trigger when motion exceeded 5, or 8 mm. An oscilloscope was connected directly to the beam delivery system to determine the time delay between the sensor trigger and the start of the source moving to beam off position (T1) and the sensor trigger and the aperture sensor signal (T2). Methods and Materials

4. CIRS Phantom Oscilloscope Computers with CIRS software Ancillary equipment

5. Results and Conclusion Direction For 8 MM Margin ( mSec) For 5MM Margin ( mSec) 15 BPM10 BPM15BPM10 BPM SUP402438356466 INF369382314374 MIN 248 MAX527 MEAN387.8 mSec The system in its current form is clinically acceptable for treatment of thoracic tumors provided there is clear visibility in the MR cine imaging. Average latency measurements: