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Statistical Process Control Analysis of Adaptive Patient Specific QA On the Elekta Unity MRI-Linac

S Strand*, A Boczkowski, J Snyder, D Hyer, S Yaddanapudi, D Dunkerley, J St-Aubin, University of Iowa, Iowa City, IA

Presentations

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

Room: AAPM ePoster Library

Purpose:

MRI-guided Adaptive Radiotherapy (MRIgRT) machines, such as the Elekta Unity, are revolutionizing patient tailored radiation therapy by allowing day-to-day treatment plan adjustments. The Unity utilizes a daily magnetic resonance (MR) image which allows plan adaptation. In the Unity workflow, Adapt to Position (ATP) and Adapt to Shape (ATS) options are available which represent a virtual shift (ATP) or full re-plan with contour adjustments (ATS). With a new intensity modulated radiation therapy (IMRT) plan being generated daily while the patient lies on the treatment table, an investigation into the quality assurance (QA) pass rates of ATP and ATS plans compared to the initial reference plan is needed.

Methods:

Statistical process control methodology was used to compare ATP and ATS IMRT QA measurements performed on the MR compatible SunNuclear ArcCheck with a gamma criterion of 3%/2 mm using global normalization and a 10% low dose threshold. The Lower Control Limit (LCL) was determined using 56 IMRT measurements on reference plans, and a one-sided process capability ratio (C0) was used to assess both the ATP and ATS results. A C0 value above 1 would indicate that the variability of the test data (ATP and ATS pass rates) is within the inherent variability of the process (reference plan pass rates).

Results:

The LCL of the reference plans was determined to be a gamma pass rate of 0.963, and the C0 was determined using 424 measured ATP plans and 54 measured ATS plans after removing outliers due to known machine issues. The C0 was calculated to be 1.177 and 1.151 for ATP and ATS plans respectively.

Conclusion:

Through the statistical process control method, the process capability ratio confirms that the gamma pass rates of the ATP and ATS plans are within the statistical variability of the reference plans.

Download ePoster [PDF]

Keywords

Intensity Modulation, Quality Assurance, Treatment Verification

Taxonomy

IM/TH- MRI in Radiation Therapy: MRI/Linear accelerator combined Quality Assurance

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