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Commissioning Report of a High Field MR-Linac System

N Tyagi1*, E Subashi2, P Godoy Scripes3, J Mechalakos4, X Gonzalez Souto5, S Lim6, (1) Memorial Sloan-Kettering Cancer Center, New York, NY, (2) Memorial Sloan Kettering Cancer Center, New York, NY, (3) Memorial Sloan-Kettering Cancer Center, New York, NY, (4) Memorial Sloan-Kettering Cancer Center, New York, NY, (5) Elekta, Atlanta, GA, AF, (6) Memorial Sloan-Kettering Cancer Center, New York, NY

Presentations

(Sunday, 7/12/2020)   [Eastern Time (GMT-4)]

Room: AAPM ePoster Library

Purpose: 1.5T Unity MR-linac provides unique challenges in commissioning the linac and treatment planning system (TPS) component due to electron return effect (ERE), asymmetric penumbra and detector response. We report on a comprehensive list of mechanical, dosimetric and TPS validation tests performed during commissioning of the system.


Methods: Mechanical tests included MR-MV coincidence, MR and MV image quality, MV isocenter size, and MLC/diaphragm positioning accuracy with gantry angles. Dosimetric tests included MU linearity, beam holdoff, MLC leaf-gap and transmission, beam quality and reference dosimetry with TRS-398 and TRS-483. TPS validation were performed by comparing output factor (Sp), off-center ratios (OCR) and depth (PDD) profile water-tank measurements with the calculations from Monte-Carlo based Monaco TPS. Inhomogeneity validation was performed in a 25x25x18cm³ water-lung-water phantom using a 5x5cm² beam. Measurements were taken with an ion-chamber (IC) at 3cm above and below heterogeneity. Multiple IMRT plans were delivered on an ArcCheck®-MR. An end-to-end test (E2E) was performed with clinical IMRT fields delivered on radiochromic films and IC.


Results: The maximum MLC/diaphragm positioning uncertainty, calculated using root-mean-squared error was 0.62/0.96 mm, at cardinal gantry angles and three nominal positions (0.0, 40.0, 100.0 mm). Calculated PDDs, OCRs and Sp were within 2% and 3% of measurement in high dose and low dose region. The largest discrepancy was found to be in crossline profiles (maximum direction of ERE) for small fields. Reference dosimetry and inhomogeneity measurements were with 0.5% and 2.2% of calculations. MLC leaf gap analysis showed a 0.2 mm higher gap compared to measurement beyond 6 mm leaf-gap. E2E measurements were within 2% of calculations. Gamma analysis for all IMRT plans were > 95% using 3%/3mm passing criteria.


Conclusion: Unity system is commissioned to within TG-142 specs and TPS is modeled within MPPG5a guidelines. Discrepancies in crossline profiles for small fields need further investigation.

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Keywords

Commissioning, Magnetic Fields, Quality Assurance

Taxonomy

TH- External Beam- Photons: Quality Assurance - Linear accelerator

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