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Characterization of the 6 MV FFF CBCT in Halcyon Treatment Delivery System: Image Quality and Potential for Dose Calculation

I Malajovich*, H Petroccia , L Dong , B Teo , T Li , University of Pennsylvania, Philadelphia, PA


(Tuesday, 7/31/2018) 7:30 AM - 9:30 AM

Room: Room 205

Purpose: The Varian Halcyonâ„¢ includes an ultrafast (15-second) 6MV-FFF-CBCT with potential advantages for on-line image guidance and adaptive radiotherapy. MV imaging should improve HU for dose calculation and reduce artifacts caused by metal-implants. We characterized the reproducibility and linearity of HU with electron density relative to water (Ï?ₑᵂ) and image field-of-view (FOV), the detectability of soft-tissue in phantoms and patients, and the effect of metals on image quality.

Methods: A Gammex phantom was scanned using a Halcyonâ„¢ 6MV-CBCT (MV-CBCT) and a Siemens Definition Edge CT scanner using 120kVp (kVCT). The resulting images were used to extract the average-HU and HU standard deviation for materials of different Ï?ₑᵂ. Voxel-statistics were calculated either in a single-slice or 10-slices. For metal-induced artifact comparisons, kVCT images were reconstructed with and without metal artifact reduction (MAR).

Results: The daily variation of average-HUs was less than 12HU for MV-CBCT. Increasing dose from 5MU to 10MU did not change the average-HUs but decreased the standard deviation by 30%. A near-linear (R²=0.991) HU-to-Ï?ₑᵂ curve was obtained. The slope of this curve varies with axial CBCT FOV: 1200HU/Ï?ₑᵂ at 10cm vs. 900HU/Ï?ₑᵂ at 20cm. Although the image quality of MV-CBCT is inferior to kVCT, it is possible to distinguish all soft tissue inserts from the Gammex phantom. Contrast-to-noise ratio (CNR) detectability improves by 40% on average from 5MU to 10MU. For clinical cases, soft-tissue contrast was sufficient for online image-guidance. Metal rods did not affect HU values or introduce noticeable artifacts, presenting an advantage over regular kVCT and—in some circumstances—over kVCT with MAR.

Conclusion: Halcyonâ„¢ MV-CBCT images are capable of discerning different soft-tissues and metal-induced artifacts were not observed. HU-to-Ï?ₑᵂ conversions were linear and stable day-to-day, but there is FOV dependence. MV-CBCT showed clinical suitability for daily image guidance and promise for use in dose calculation for adaptive radiotherapy.


Image-guided Therapy, Cone-beam CT, Inhomogeneity Corrections


TH- External beam- photons: Development (new technology and techniques)

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