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Small Field MLC SRS DCA Plan Verification with Brainlab Elements

C Geraghty1*, (1) Anne Arundel Medical Center, Annapolis, MD

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PO-GePV-T-401 (Sunday, 7/10/2022)   [Eastern Time (GMT-4)]

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Purpose: To measure SRS DCA plans with target size from 4mm-15mm planned on Brainlab Elements using SNC SRSMapcheck and micro ion chamber.

Methods: SRS plans were created in Brainlab Multiple Brain Mets SRS 1.5.0 for PTVs ranging from 4mm to 15mm delivering 10Gy to 98.5% of PTV in one fraction. Plans utilized four noncoplanar dynamic conformal arcs (DCAs). Plans were mapped to an SRSMapcheck phantom with the detector array and also with a CC01 ion chamber. Verification plans were calculated using Monte Carlo with 1mm spatial resolution and 2% dose resolution. The plan and dose files were exported for diode array comparison. For ion chamber verification, the mean dose to the chamber volume from the verification plan was recorded. Next, the plans were delivered, using planned table angles, to the SRSMapcheck with the diode array inserted. Measured and planned fluence were compared using gamma, absolute dose criterion of 3%, 1mm. Finally, the plans were delivered with the ion chamber inserted. Electrometer readings and delivery times were recorded. The electrometer readings were corrected for chamber leakage. Both diode array and ion chamber readings were normalized to a 5x5 open field.

Results: : Diode array passing rates ranged from 93.4% to 99.7% for all field sizes with no correlation of passing rate to field size. CAX diode readings ranged from 2.5% to 16.2% different from expected values with percent difference increasing with decreasing field size. CAX ion chamber readings were within 1.5% of expected for all field sizes.

Conclusion: SRS DCA plans created in Brainlab Elements for targets ranging from 4mm-15mm showed diode array passing rates greater than 93% for all target sizes. Central axis diode readings for targets 5mm and below were greater than 7.5% different than planned values. CAX ion chamber readings for all field agreed well with expected values.

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