Room: Exhibit Hall | Forum 5
Purpose: This work describes our initial experience using the TrueBeam fiducial-based gating system utilizing kV-triggered images for prostate SBRT treatments.
Methods: Eight prostate SBRT patients (40 fractions) were treated using this system. 7.25-7.5 Gy/fx was prescribed to the prostate and 8-9 Gy/fx to a simultaneous integrated boot volume. Four fiducials were tracked using a 2 mm tolerance radius and a 3-5 sec kV-triggering interval. The beam was paused when any fiducial was detected as out of tolerance or if any two fiducials were undetected. If fiducials did not move back within tolerance after a few images, then the alignment was reassessed using additional on-board imaging and shifted if needed.
Results: Treatment was paused several times due to the patient shifting on the table, clenching the external sphincter muscles due to the full bladder, and bowel gas movement. The average(min,max) total number of triggered kV images was 51(27,88), 247(134,442), and 510(336,772) for an arc, fraction, and total treatment. The average(min,max) total collimated air kerma from kV-triggered images was estimated to be approximately 1.3(0.9,2) Gy. On average, 1 CBCT, 6 kV-MV pairs, 1 MV, and 50 non-triggered kV images were also acquired over the treatment course for realignment. The average(min,max) total number of time the patient was repositioned was 2(0,6), 4(2,9), and 19(9,43) for an arc, fraction, and total treatment. The average vertical, longitudinal, lateral, pitch, roll, rotation shifts applied to reposition the patient during treatment were 1.0mm, 1.2mm, 0.8mm, 0.3Â°, 0.1Â°, and 0.2Â°, respectively. The maximum translation and rotation was 12.6mm and 4.8Â°. The average(min,max) time to complete a single arc, delivery, and total appointment time was 10.5(1.3,35) min, 20.6(7.6, 46) min, and 31.3(15.3, 64.9) min respectively.
Conclusion: Despite the increase in imaging dose and treatment time, the system successfully detected and corrected for numerous positional offsets caused by intrafraction motion.