KEY POINTS
- This prospective imaging study enrolled 10 prostate cancer patients undergoing proton therapy. Each patient received standard gantry-mounted CBCT followed by CBCT using a prototype tungsten two-dimensional antiscatter grid (2D ASG), with identical acquisition parameters and imaging dose.
- Both scans used 120 kVp, 1,872 mAs and 471 projections, with a CTDI of 10.9 mGy. Standard CBCT was evaluated both without and with the system’s software scatter correction, allowing hardware-based suppression to be compared with the existing clinical approach.
- Conventional algorithmic scatter correction reduced median soft-tissue CT-number error from 394 to 169 HU. Adding the 2D ASG reduced it further to 38 HU (p=0.002), representing a roughly 78% reduction versus the scatter-corrected clinical CBCT.
- Scatter-induced artifact amplitude similarly fell from 510 HU without correction to 377 HU with software correction and 144 HU with the 2D grid. The improvement was visible across all ten participants and brought the reconstructed anatomy substantially closer to the reference planning CT.
- Soft-tissue contrast increased from 62 to 125 HU, while contrast-to-noise ratio increased from 0.99 to 1.57, a 59% improvement over scatter-corrected CBCT. Structural similarity to planning CT increased from 0.53 to 0.87, and peak signal-to-noise ratio improved by approximately 45%; all comparisons were significant at p=0.002.
- Bone CT-number accuracy improved particularly strongly, which is relevant to proton range calculations: median bone error was 628 HU without correction, 333 HU with software correction and 75 HU with the 2D ASG.
- Hardware scatter rejection was not cost-free: image noise was approximately 21% higher than with software-corrected CBCT because less scattered fluence reached the detector. The study also evaluated image quality rather than actual proton dose recalculation or online adaptation, used only ten pelvic patients, and employed a research prototype with a restricted reconstructed imaging volume.
CLINICAL TAKEAWAY
If proton CBCT is expected to support direct dose calculation and adaptive replanning, CT-number accuracy becomes as important as anatomical visibility. Hardware scatter suppression produced a large improvement in both here, but the crucial next step is demonstrating that those image-quality gains translate into sufficiently accurate proton dose calculations and a practical online workflow.
SOURCE
International Journal of Radiation Oncology, Biology, Physics