Purpose: To evaluate the imaging performance of a novel helical kVCT system, the Radixact ClearRT, for use in image-guided radiotherapy and dose calculation in adaptive radiotherapy and to compare this performance to existing systems used for these purposes.
Methods: A commonly used CT image quality phantom was scanned across clinically-relevant acquisition modes for image quality characteristics including noise, uniformity, contrast, spatial resolution, and mean CT number and CT number linearity. These metrics were analyzed using ImageJ software and compared to those measured for the Radixact MVCT, TomoTherapy MVCT, Varian TrueBeam OBI CBCT, and the Siemens SOMATOM Definition Edge. Individual metric performance was compared relative to the image noise for each acquisition.
Results: Similar noise levels were seen for ClearRT and Siemens, while noise for the other systems was 2-5 times higher on average. Uniformity was highest for Siemens. On average, ClearRT performed best for contrast, followed closely by Siemens and Varian. Spatial resolution was best for Varian, while ClearRT showed slightly worse resolution than Siemens and TomoTherapy. For CT number vs. electron density calibration curves for analogous protocols, ClearRT showed a similar linear fit to the Siemens system commonly used for dose calculation in planning images. Mean CT numbers were also similar for ClearRT and Siemens, especially for inserts mimicking fat and soft tissue. Scan times for Siemens were less than 20s and approximately 30-40s for ClearRT. Times for the other systems were 2-10 times higher on average.
Conclusion: ClearRT performed well for metrics most important to IGRT – mainly contrast. For image characteristics most important to dose calculation - mean CT number and CT number linearity - similar values were measured for ClearRT and Siemens. Thus, ClearRT appears capable of providing accurate images for adaptive treatments, which will be further evaluated with measurements on dose calculation accuracy.