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Rapid Commissioning of An MR-Linac with Hydrostatic and Cherenkov Imaging Techniques

R Zhang*, D Alexander, B Williams, L Gates, F Rafie, N Nelson, P Bruza, B Pogue, B Zaki, D Gladstone, Dartmouth-Hitchcock Med. Ctr., Lebanon, NH

Presentations

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

Room: AAPM ePoster Library

Purpose: share data and experience of commissioning a ViewRay MRIdian MR-linac. To demonstrate novel and efficient methods of beam data collection and TPS validation with optical and hydrostatic techniques where conventional motor-driven scanning hardware is precluded.

Methods: MR image-guidance system was commissioned by imaging various phantoms including the field homogeneity spherical phantom, the large ACR MR accreditation phantom and a large field spatial fidelity phantom. Tissue-Maximum ratio (TMR) curves were collected for a range of field sizes (~1.6 cm to ~24 cm squares) by connecting an MRI compatible water tank hydrostatically to a conventional scanning system with TMR control. PDDs were calculated from TMRs and compared to spot measurements made manually with a 1D hand operated tank and treatment planning system (TPS) calculations. Beam projections and projected depth dose images were measured optically by imaging Cherenkov emission in water. Projected PDDs and profiles extracted from the optical images were compared to those extracted from the dose clouds calculated by the Monte Carlo based TPS. Selected profiles were also compared to conventional measurements with the Sun Nuclear IC profiler-MR.

Results: results on MR field homogeneity, SNR, artifacts, spatial/contrast resolution, slice position/accuracy, large field spatial fidelity, imaging isocenter drift and imaging-radiation isocenter coincidence were within the tolerance set by AAPM TG-142 guidelines. A complete set of baseline beam characterization data was acquired and analyzed in only two days. Agreement with spot checks against conventionally measured and calculated data validate the beam model and provide a wealth of beam characterization data that would be impractical to acquire with manual measurements.

Conclusion: MR-linac was commissioned to meet specifications. Hydrostatic and optical imaging techniques can be used to dramatically speed data acquisition and analysis in machines where the magnetic field and bore size restrict the use of conventional scanning technologies.

Download ePoster [PDF]

Keywords

Commissioning, MRI, Optical Imaging

Taxonomy

IM/TH- MRI in Radiation Therapy: MRI/Linear accelerator combined (general)

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