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Dosimetric Impact of Rotational Error On Multiple-Target Intensity-Modulated Radiosurgery (IMRS) with Single-Isocenter

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S Jang

S Jang*, M Huq , University of Pittsburgh Medical Center, Pittsburgh, PA


SU-E-T-450 Sunday 3:00PM - 6:00PM Room: Exhibit Hall

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-movement

Methods: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 flattering-filter-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 distal-target 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 result in 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.

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