J Liang¹*, S Lim², N Tyagi³, X Gonzalez Souto⁴, J Moran⁵, E Subashi⁶, (1, 2, 3, 5, 6) Memorial Sloan-Kettering Cancer Center, New York, NY, (4) Elekta, Atlanta, GA

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  J Liang

SU-H330-IePD-F6-3 (Sunday, 7/10/2022) 3:30 PM - 4:00 PM [Eastern Time (GMT-4)]

Exhibit Hall | Forum 6

Purpose: To investigate the feasibility of automating longitudinal mechanical and dosimetric quality assurance (QA) measurements with the electronic portal imaging device (EPID) in a 1.5T MR-Linac (MRL).

Methods: The isocenter location, symmetric/asymmetric field size, output from static/dynamic fields, flatness, and symmetry, were measured bi-weekly over the course of one month. An automated QA plan was delivered on the EPID, and the acquired portal images were analyzed using an in-house built Python script. Isocenter location and field size were estimated based on edge-detection methods and validated using EBT-XD radiochromic film. Flatness and symmetry were calculated by considering equidistant points across the central axis. The output of a 10x10cm² open field and of a 1cm dynamic MLC were determined by integrating the signal over the exposed area. The dynamic MLC output was defined by the ratio between two measurement sets. Baseline values were established by cross-checking measurements using a Farmer-type and ion-chamber array.

Results: The longitudinal assessment of daily measurements reveals a high reproducibility of QA metrics estimated from the analysis of EPID images. The isocenter position was within one voxel of baseline. Median bias for X- and Y-jaw size was 0.1mm and 0.5mm, agreeing with radiochromic film. Incremental changes in jaw size by 1mm were consistently measured to within 0.1mm for both X- and Y-jaw. The position of the central 28 MLC leaves was within 0.1±0.07mm from expected value. Baseline output change of a static and dynamic field were 0.19±0.20% and 0.23±0.28%, respectively. Radial and transverse symmetry were within 0.30±0.24% and 0.21±0.21% from baseline, respectively. Similarly, radial and transverse flatness were within 0.20±0.16% and 0.15±0.15% from baseline.

Conclusion: Longitudinal measurements with EPID demonstrated a feasible solution to routine QA testing in a high-field MR-linac. Future work will aim to evaluate and establish the long-term stability of baseline values in our institution.

Funding Support, Disclosures, and Conflict of Interest: MSKCC has a Master Research Agreement with Philips and Elekta healthcare

Keywords

Quality Assurance, MR, Linear Accelerator

Taxonomy

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

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