Liver SBRT Without Fiducial or Retained Ethiodol Guidance Warrants Greater than 5mm Setup Margins

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For patients who undergo liver stereotactic body radiation therapy (SBRT), the placement of fiducial markers or retention of ethiodol delivered by trans-arterial chemoembolization (TACE) provides an accurate landmark for consistent target localization using cone-beam CT (CBCT). However, TACE and fiducial marker placement are invasive procedures that may harbor additional risk. We hypothesize that the delivery of liver SBRT can be safe and accurate without the use of these surrogate markers. In this study, we aim to assess the accuracy of this approach by comparing daily alignment using retained ethiodol within the treated lesion vs. a liver contour surrogate. We retrospectively identified 7 consecutive patients that received TACE prior to 5 fraction SBRT for a single peripheral liver lesion (>2 cm from left and right portal veins). All lesions exhibited retained ethiodol on simulation CT and each CBCT. All patients underwent respiratory motion management using either end-expiratory Active Breathing Coordination (ABC, n = 3) or abdominal compression (n = 4). Liver and retained ethiodol contours were determined on the CT simulation and each daily CBCT. All images and structure sets were exported to a commercial software where rigid image fusion was performed between the planning CT and daily CBCTs. To approximate the positioning of our clinical treatment couch, no roll or yaw was applied during registration. For each fraction, two manual rigid image registrations were performed by the treating physician – once using the liver contour as a surrogate for the target (concentrating on the liver edge(s) closest to the treated liver segment) and once aligning only to the radio-opaque retained ethiodol of the treated lesion. As a figure of merit, the magnitude of the change in vector (i.e., the distance) between the two registration methods was used to assess the target localization if ethiodol were not present. Mean PTV volume was 23.5 cc (range: 5.4cc – 44.5cc). Across all patients, the average change in target localization throughout 5 fraction liver SBRT was 5.8 mm (range: 1 – 15.4 mm) when using the liver contour as surrogate for the target instead of the retained ethiodol region. For patients treated with end-expiratory ABC, average change was 5.7 mm (range: 1.1 – 10.9 mm). For patients treated with abdominal compression, average change was 5.9 mm (range: 1 – 15.4 mm). Across all patients, the average change in target position exceeded 5 mm, for image registration methods based solely on the liver contour versus concentration on retained ethiodol only. Given this, we suggest that greater than standard 5mm PTV margins should be applied when delivering respiratory-managed liver SBRT to peripheral lesions without the assistance of fiducial markers or retained ethiodol.