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Characterization and Longitudinal Assessment of Daily Quality Assurance On An MR-Guided Radiotherapy (MRgRT) Linac

D Dunkerley*, P Yadav , J Bayouth , K Mittauer , School of Medicine and Public Health UW-Madison, Madison, WI


(Monday, 7/15/2019) 3:45 PM - 4:15 PM

Room: Exhibit Hall | Forum 3

Purpose: Describe and characterize daily machine quality assurance (QA) for an MR-guided radiotherapy (MRgRT) linac program and longitudinal assessment of the dosimetric and mechanical accuracy of a commercial MRgRT linac over a seven-month period of clinical operation.

Methods: QA procedures were developed to evaluate MR imaging/radiation isocenter alignment, imaging and patient positioning system accuracy, and linear accelerator stability. A longitudinal assessment was characterized for: safety interlocks, laser and imaging isocenter coincidence, imaging and radiation (RT) isocentricity, radiation dose rate and output, couch motion, and MLC positioning. A cylindrical water phantom and MR-compatible A1SL detector (4.4mm length, 4mm diameter) were utilized for measurements. MR/RT isocentricity and MLC positional accuracy was quantified through dose measured with a 0.4x0.83cm² field at each cardinal angle. The relationship between detector response to MR/RT isocentricity and MLC positioning was established by introducing known errors in phantom position.

Results: The relationship between dose difference and introduced positional error was sensitive at 6.6%/mm for the 0.4x0.83cm2 field. Correlation between detector response and positional error introduced (N=27) was r²=0.9996 (IEC-X error), r²=0.9967 (IEC-Y error), r²=0.9968 (IEC-Z error). Over the longitudinal assessment(N=7 months), average positional errors (MR/RT isocentricity+MLC) were < 0.2mm. For the temporal assessment (N=7months), safety interlocks were functional. Laser coincidence to MR was within ±0.2cm and ±0.1cm for 99.6% and 86.8% of measurements, respectively. IGRT position-reposition shifts were within ±0.2cm and ± 0.1cm for 99.4% and 92.4% of measurements, respectively. Output was within ±3% for 99.4% of all measurements. A drift in dose rate was observed of -0.58 MU/min/day(p=0.0001).

Conclusion: The linac and IGRT accuracy of an MRgRT linac has been validated and monitored over seven months for routine QA. Longitudinal assessment demonstrated a drift in dose rate, but temporal assessment of output, MLC position, and isocentricity has been stable and within TG-142 criteria for 95% of measurements.

Funding Support, Disclosures, and Conflict of Interest: Dr Mittauer reports personal fees and consulting fees from ViewRay Inc. during conduct of this study. Dr. Bayouth reports membership on scientific advisory board of ViewRay Inc. during conduct of this study.


MR, Quality Assurance, Radiation Therapy


TH- External beam- photons: Quality Assurance - Linear accelerator

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