Y Niu*, Y Yuan, A Rashid, D Pang, Department of Radiation Medicine, MedStar Georgetown University Hospital, Washington, DC

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  Y Niu

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TH-A-TRACK 6-6 (Thursday, 7/29/2021) 10:30 AM - 11:30 AM [Eastern Time (GMT-4)]

Purpose: The ZAP-X is a novel self-shielded radiosurgery system providing precise and highly conformal treatment to intracranial lesions. Here, we aimed to verify the delivery accuracy of ZAP-X and validate the dosimeric accuracy of its dedicated treatment planning system (TPS).

Methods: ZAP-X is characterized by a 2.7-MV linac mounted on a gyroscope-like gantry rotating around isocenter. A tungsten wheel collimator provides circular collimation with diameters from 4 to 25 mm. A patient table enables translational head movement resulting in multi-iso treatment. Non-coplanar beams are optimized from the beam-geometry space with 2π steradians solid angles to achieve desired plan quality. Dose calculation uses ray-tracing algorithm with 0.5 or 1 mm grid sizes. Beam data were acquired using water tank, including the TPR converted from PDD, off-axis profiles and collimator output factors. For checking the iso-positioning accuracy, end-to-end test was performed using an anthropomorphic head phantom with skull structure. An integrated kV imaging system rotating around axial axis provides skull/phantom alignment. The head phantom accommodates the inserts holding thermo-/optically stimulated luminescence detectors (TLD/OSLD) and Gafchromic films to verify point absolute doses and relative dose profiles. Measurements of absolute dose profiles were performed using a polymethyl methacrylate cylindrical phantom with SRS MapCheck, i.e., a 2D diode array with 0.5 mm resolution.

Results: Routinely performed end‐to‐end test for iso-positioning accuracy resulted in deviations of less than 0.7 mm. Absolute point dose measurements of TLD and OSLD agreed with TPS calculation within 3.5% with average difference -1.9±1.8% and 2.3±0.6%, respectively. Relative and absolute dose profiles measured by Gafchromic film and SRS MapCheck showed an average gamma pass rates of 98.06% and 98.93% when using 3%/1mm criteria.

Conclusion: The positional and dosimetric accuracy of ZAP-X system were evaluated. The TPS and its commissioning beam data can be used to predict the delivered dose precisely and accurately.

Handouts

Keywords

Absolute Dosimetry, Film, Radiation Dosimetry

Taxonomy

TH- External Beam- Photons: intracranial stereotactic/SBRT

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