SU‐E‐J‐132: Inter‐Fractional Setup Accuracy of the Body Pro‐Lok™ System for Hypo‐Fractionated Liver Radiotherapy Using Novalis Tx™

Purpose: To assess the inter‐fractional set‐up uncertainty of the Body Pro‐Lok™ System used in combination with the cone beam kVCT (CBCT) onboard imaging of the Novalis Tx™ during hypo‐fractionated liver radiotherapy. Methods: Thirty‐two patients (n=32) treated with 3 to 10 fractions were prospectively analyzed. The Body Pro‐Lok™ system was used in both CT simulation and during patient treatment setup. Patients were immobilized with a vacuum cushion placed posteriorly over the thoracic region, an abdominal compression plate, and a knee and foot sponge. Pretreatment CBCT scans of the Novalis Tx™ unit were fused with the planning kVCT prior to delivery of each fraction to determine the inter‐ fractional set‐up error. A total of 229 shifts were analyzed to assess the inter‐fractional set‐up accuracy. Results: Results showed that the mean inter‐fractional set‐up errors and standard deviations were 0.1 ± 4.5 mm, −1.3 ± 4.3mm, and 0.5 ± 3.6mm for lateral (IEC‐X), longitudinal (IEC—Y), and vertical (IEC—Z) variations, respectively. Maximum motion was 14.0, 15.0 and 17.0mm in the lateral, longitudinal, and vertical direction, respectively. When all three translational coordinates were analyzed, a mean composite displacement vector of 4.8 ± 2.1mm (range: 1.7–9.4mm) was obtained for all patients. Additionally, no differences were noted in setup accuracy between the first and second fractions (mean vector displacement 5.7 vs. 5.9mm, p=0.49) and between the first and last fraction (mean vector displacement 5.7 vs. 6.7mm, p=0.24). Conclusions: Based on the findings, image‐guided hypo‐fractionated radiotherapy to the liver using the Body Pro‐Lok™ system in conjunction with the CBCT of Novalis Tx™ is capable of minimizing inter‐fractional set‐up error.