Experimental Verification of the Electron Return Effect in Lung Heterogeneity Measurements On Elekta Unity MR-Linac
N Tyagi, J Liang, P Godoy Scripes, E Subashi, J Moran, M Chan, S Lim*, Memorial Sloan Kettering Cancer Center, NY
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PO-GePV-M-262 (Sunday, 7/10/2022) [Eastern Time (GMT-4)]
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Purpose: To verify Monte-Carlo-based dose calculations with the Monaco treatment planning system (TPS) calculations within and at the tissue-lung interface for clinical use. This interface is challenging in the presence of a B-field MR-linac system due to the electron return effect (ERE) resulting from the Lorentz force on the scattered secondary electrons.
Methods: Dose was measured and calculated for slab and anthropomorphic phantoms for a 7X-FFF beam on a 1.5Tesla MR-linac system. Regions of interest included measurements at multiple locations from 0.0 to 5.0cm downstream of a solid-water(SW)-lung-SW phantom (ρ_lung = 0.45g/cm³) with a diamond detector (CVD); (b) SW-balsawood-SW phantom (ρ_balsawood = 0.1g/cm³) with an embedded EBT-XD film placed parallel to the beam direction and (c) a CIRS spine-lung phantom with a centrally located lung tumor and CC04 ion-chamber (IC) insert. The geometry for the slab phantom was at SAD 143.5cm using a single AP beam (G0) with field sizes of 2x2, 3x3, 5x5, and 10x10 cm². Two 10-beam conventional fractionation lung IMRT plans were delivered on the spine-lung phantom to the tumor. Measurements were compared with TPS doses calculated with an isotropic grid of 1.0mm³ and statistical uncertainty of 0.5%.
Results: Calculations beyond 0.5 cm downstream from the lung-tissue interface were ≤ 4.2% of the CVD measurements for 2x2 to 10x10 cm². Percent depth-dose (PDDs) measured with films were within 12.4% at the interface and 6% beyond the balsawood slab. The average differences within balsawood were 5.6% and 0.7% for 2x2 and 10x10 cm² respectively. IMRT plans measured with IC were within 3% of TPS.
Conclusion: Single beams showed the largest difference between measurement and calculation at the interface due to ERE. The dosimetric impact is minimized with delivery from multiple beam angles. Calculations and measurements met clinical acceptance criteria of <5% for distances >5.0mm from the lung-tissue interfaces
Funding Support, Disclosures, and Conflict of Interest: MSKCC has a Master Research Agreement with Philips and Elekta healthcare
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