Suitability of Hypersight CBCT for Online Adaptive Radiotherapy of Locally-Advanced Lung Cancer
Abstract
Purpose
To characterize how well voluntary breath hold during imaging reduces motion-induced artifacts in HyperSight cone-beam CT (CBCT) images that make it challenging to provide adaptive radiotherapy of moving targets in the lung on the Varian Ethos platform.
Methods
Locally-advanced non-small cell lung cancer patients were screened such that they could voluntarily hold their breath for 10 seconds. Six screened patients were imaged on the Ethos HyperSight platform using 6 s CBCT scans under voluntary exhale breath hold at the beginning of each radiotherapy fraction. CBCT images acquired at the first fraction were compared with 4DCT planning images using lung tissue Hounsfield Units (HU) histograms and image similarity metrics.
Results
HU values in first-fraction CBCT images demonstrated good agreement with the planning CT images. The lung volumes, excluding the target, had slightly lower mean and mode HU values in the CBCT images (average difference in mean 31.9 HU [range: 4.3-71.9]; average difference in mode 24.3 HU [range: 6.0-41.0]), but the standard deviation and skew of the HU distributions were not significantly different (p>0.05). Histograms of the lung HU values were similar between images, as demonstrated by small Bhattacharyya distances (average 0.098 [range: 0.070-0.174]) and large histogram intersections (average 0.73 [range: 0.57-0.81]). The structural similarity index measures (SSIM) of the overlapping volumes following rigid registration of the images were moderately high (average 0.608 [range: 0.505-0.675]), consistent with similar images acquired at different phases of respiration.
Conclusion
The results demonstrate that HU values and image characteristics of CBCT images acquired with the breath-hold technique are similar to those in images acquired during CT simulation. This indicates the breath-hold technique overcomes the issue of HU accuracy degradation caused by respiratory motion. Future investigation will extend this analysis to additional patients and relevant volumes, as well as to moving targets in the liver.