C Ainsley*, Thomas Jefferson University, Philadelphia, PA

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  C Ainsley

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PO-GePV-M-268 (Sunday, 7/10/2022)   [Eastern Time (GMT-4)]

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Purpose: To investigate appropriate gating thresholds for the real-time motion tracking of a surrogate for the treatment target during MR-guided radiotherapy.

Methods: In MR-guided radiotherapy, ideally one is able to track directly the motion of the intended treatment target, and then gate the beam on only when that target lies within the PTV’s boundary margin. In practice, excursions of a small fraction (e.g., 5%) of the target beyond the boundary are typically permitted; the permissible level (the “ROI%”) is user-configurable. When the target is insufficiently well differentiated on imaging for direct tracking to be viable, tracking of a surrogate structure becomes necessary. In this study, a simple mathematical model was developed to determine how to set the ROI% for a surrogate structure, such that the delivered target coverage would be equivalent to that were the target to have been tracked directly using standard ROI% settings. As an example, the model is applied to a patient’s whole liver (10.5cm dimension), used as a surrogate for a difficult-to-track liver met (1.8 cm dimension), targeted with 3mm PTV margin.

Results: The relationships between ROI%, the physical distance of a tracking structure’s excursion beyond the boundary margin, and its dimension were derived and charted. In the example patient scenario, to achieve the equivalent of 5% for ROI% as applied to the liver met would correspond to setting 0.8% if tracking the whole liver. By contrast, naively applying the standard ROI% of 5% for the whole liver is seen to correspond to a setting of 32% if tracking the target directly—likely compromising the dosimetry.

Conclusion: When tracking on a surrogate structure whose dimension along the axis of motion differs from the dimension of the treatment target, appropriate allowances must be made on the ROI% to ensure the accuracy of the dose delivered to the target.

Keywords

MRI, Gating, Image-guided Therapy

Taxonomy

IM/TH- MRI in Radiation Therapy: MRI/Linear accelerator combined- IGRT and tracking

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