Abstract # 2124 Intrafraction Reproducibility of Liver Position in Patients Undergoing Cone Beam CT Image Guided Breath Hold and Free Breathing Stereotactic Body Radiotherapy

Presenter: Case, Robert

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Table 1 displays the random (s) and systematic () intra-fraction setup errors in liver position, according to motion management strategy. Ninety percent of all intra-fraction change in liver position was < 2.3, 3.4 and 2.6 [mm] in the medial-lateral (ML), cranial-caudal (CC), and anterior-posterior (AP) directions respectively. In the ML, CC and AP directions, the percent intra-fraction change > 3mm was 7%, 15%, and 7%, and change > 2mm was 13%, 24%, and 16% respectively. The largest intra-fraction change in liver position was 7mm CC measured once in a patient who was experiencing pain during therapy. Displays of the difference in CT numbers of pre and post CBCTs were used to visualize regions of variability. The intraobserver reproducibility (s, sampled from 23 fractions) was 1.0, 1.4, and 1.6 [mm] in the ML, CC, and AP directions. There was no significant correlation between treatment time and magnitude of intra-fraction error (R2 = 0.07).