SU‐F‐T‐652: Verification of Inter‐Fractional Tumor Motion Using Daily Fiducial Length Measurement for Pancreatic SBRT

Abstract

Purpose:To evaluate possible inter‐fractional tumor motion variability for pancreatic SBRT with length measurement of the fiducial depiction on the scan and to develop an empirical correlation using a moving phantom and fiducials for adjusting gating window on a daily basisMethods:In this retrospective study, motion data for fifty pancreatic SBRT patients with fiducials were selected to find the tumor motion statistics. Three gold fiducials of 5mm were placed around a solid target inside the Quasar Phantom and the 4D CT images of the Phantom were acquired with tumor motion of 1cm in superior‐inferior direction. The breathing period was set to 4 seconds with a sinusoidal breathing form. After 4D‐CT acquisition, CT50% images were imported into the Eclipse and setup fields were created for acquiring CBCT images. CBCT images were acquired by using the TrueBeam‐STx and Pelvis technique. For sinusoidal tumor motions, CBCT images were acquired with tumor motion from zero to 2cm with 0.5cm increment along SI direction. The breathing period was varied from 2∼6 seconds with 2‐second increment. Also, real patients’ breathing files acquired from Varian‐RPM system were used to simulate real patients’ breathing patterns. Then length of fiducial depiction was measured with a HU threshold of zero and was used to find its correlation with real tumor motion.Results:Tumor motions in SI, AP, and LR directions were 0.7±0.5cm, 0.2±0.3cm, and 0.1±0.2cm, respectively. Breathing period was 4.3±0.8seconds. For a sinusoidal breathing form, there was a significant correlation between length of fiducial and real tumor motions with R2 of 0.98. However, the length of fiducial was independent of varying breathing periods. For a real patient's breathing file, the correlation between fiducial length(Y) and tumor motion(X) was Y=1.1268X+0.2696 with R2 of 0.99.Conclusion:With daily fiducial length measurement, we can evaluate possible changes in the inter‐fractional tumor motion in pancreatic SBRT.