C Teng*, E Sudentas, L Fu, M Chao, Y Lo, Mount Sinai Health System, New York, NY

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  C Teng

PO-GePV-T-385 (Sunday, 7/10/2022)   [Eastern Time (GMT-4)]

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Purpose: While portal dosimetry (PD) is commonly used for IMRT patient specific plan verification, it is challenging to be used for multiple-lesion radiosurgery. This study evaluated diode arrays and PD for SRS plans of single isocenter with 1-5 lesions.

Methods: Two 2D diode array devices, Sun Nuclear srsMapCHECK, MapCHECK3 and Varian PD were used for pre-treatment plan verifications. Gamma Index passing rate was computed for a combination of criteria from 1-3mm distance-to-agreement and 1-3% dose-difference with >10% threshold. VMAT plans consisted of 5-8 arcs of 6MV FFF beams were delivered by TrueBeam V2.7 and 120 MLC. Dose was calculated by Eclipse AAA (Version 15.6.03) in 1mm grid.

Results: For single-lesion plans, all the three QA devices showed good agreement. For example, an 8-arc SRS plan for a single lesion of 1.9cm had >97% passing rates (2%2mm, 10% threshold) in all devices measured. For multiple-lesion plans, PD overestimated predicted dose, but the same plans passed QA when measured by diode arrays. For example, an 8-arc SRS single-isocenter 3-target plan (PTV sizes = 1.4cm, 0.9cm, and 0.7cm) failed PD QA at 61.4% passing rate (2%2mm, 10% threshold), but had 98.0% and 99.3% passing rates with srsMapCHECK and MapCHECK3, respectively. Area of failing in PD was concentrated in low-to-mid dose regions, i.e., <60% dose level; passing rate increased to 90% if threshold for dose analysis was up to 50%. The data indicated PD might not accurately calculate dose from scatter, off-axial and/or small lesion size for multiple-lesion SRS.

Conclusion: Portal dosimetry failed to verify single-isocenter multiple-lesion SRS plans. Its algorithm might be too simple to accurately model the actual dose in these regions. Diode array device is more appropriate for multiple-lesion plan verification.

Keywords

Stereotactic Radiosurgery, Commissioning, Portal Imaging

Taxonomy

Not Applicable / None Entered.

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