SU‐E‐T‐450: Dosimetric Impact of Rotational Error On Multiple‐Target Intensity‐Modulated Radiosurgery (IMRS) with Single‐Isocenter

Abstract

Purpose:Evaluating the dosimetric‐impact on multiple‐targets placed away from the isocenter‐target with varying rotational‐error introduced by initial setup uncertainty and/or intrafractional‐movementMethods:CyberKnife‐Phantom was scanned with the Intracranial SRS‐protocol of 1.25mm slice‐thickness and the multiple‐targets(GTV) of 1mm and 10mm in diameter were contoured on the Eclipse. PTV for distal‐target only was drawn with 1mm expansion around the GTV to find out how much margin is needed to compensate for the rotational‐error. The separation between the isocenter‐target and distal‐target was varied from 3cm to 7cm. RapidArc‐based IMRS plans of 16Gy single‐fraction were generated with five non‐coplanar arcs by using Varian TrueBeam‐STx equipped with high resolution MLC leaves of 2.5mm at center and with dose‐rate of 1400MU/min at 6MV for flatteringfilter‐ free(FFF). An identical CT image with intentionally introduced 1° rotational‐error was registered with the planning CT image, and the isodose distribution and Dose‐Volume‐Histogram(DVH) were compared with the original plans. Additionally, the dosimetric‐impact of rotational error was evaluated with that of 6X photon energy which was generated with the same target‐coverage.Results:For the 1mm‐target with 6X‐FFF, PTV‐coverage(D100) of the distal‐target with 1° rotational‐error decreased from 1.00 to 0.35 as the separation between isocenter‐target and distal‐target increased from 3cm to 7cm. However, GTV‐coverage(D100) was 1.0 except that of 7cm‐separation(0.55), which resulted from the 1mm‐margin around the distal‐target. For 6X photon, GTV‐coverage remained at 1.0 regardless of the separation of targets, showing that the dosimetric‐impact of rotational error depends on the degree of rotational‐error, separation of targets, and dose distribution around targets. For 10mm‐target, PTV‐coverage of distaltarget located 3cm‐away was better than that of 1mm‐target(0.93 versus 0.7) and GTV‐coverage was 1.0 regardless of 6X‐FFF/6X photon beams.Conclusion:RapidArc‐based Multiple‐target IMRS may Resultin a compromised outcome due to the setup‐error for distal‐targets. With 1mm‐margin, GTV‐coverage of distal‐target was 1.0 although the separation of targets was up to 5cm.