P Dupre¹*, K Wilson¹, S Benhabib², M Joo¹, Q Xu¹, K Huynh¹, W Nie¹, J Fan¹, (1) Inova Health System, Fairfax, VA, (2) UVA Health/Novant Health Cancer Center, Gainesville, VA

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  P Dupre

(Saturday, 3/26/2022)   [Central Time (GMT-5)]

Purpose: To validate the small field relative output for a 6FFF beam at and far from the central axis (CAX) for the purposes of single isocenter, multiple metastasis stereotactic radiosurgery (SRS).

Methods: A stereotactic diode was used to measure the output factors for field sizes down to a 1 cm square at CAX at a depth of 10 cm Solid Water. A plan was created with 1 cm square MLC openings at distances of at least 7 cm from the CAX for 12 locations. Output factors for the plan were measured in Solid Water by driving the couch to place the detector at the center of the planned fields. The output factors for CAX measurements were normalized relative to a 10cm square field and output factors for off-axis locations were normalized to the 1 cm CAX output. Comparison with the treatment planning system (TPS) was conducted to evaluate the accuracy of the MLC modeling within the beam model compared to the measured output factors. Finally, portal dosimetry was run on six plans with 1 to 10 targets (1, 3, 5, 7, 9, 10) to test the relative output far from the CAX in a VMAT setting.

Results: Output factors at CAX were within 2% of the TPS for 1 cm square fields shaped with MLC and within 5% of the TPS for fields shaped with the jaws. At 12 locations at least 7 cm from the central axis, the output relative to CAX ranged from -0.3-2.2% from the TPS values. Portal Dosimetry, using a 3%/1mm gamma criteria for analysis, resulted in an average pass rate of 98.6% per field (95.9-99.8% range).

Conclusion: The combination of the TPS beam model and the MLC parameters at our institution was validated and determined as appropriate for single isocenter multi-met SRS planning.

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• 174-60674-16011654-182616-770914536.pdf (P Dupre)

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