Purpose: Megavoltage cone-beam CT (MV-CBCT) offers an opportunity to expand IGRT capabilities in under-resourced environments. MV-only treatment machines have advantages of decreased infrastructure and maintenance requirements alongside lower capital costs. MV-CBCT imaging advantages include reduced metal artifacts and accurate electron density mapping for adaptive or emergent situations, or if a CT is otherwise not available. However, MV-CBCT imaging is limited by the generally poor efficiency of current detectors. Here we examine a new MV imager and compare CBCT reconstructions under clinically relevant scenarios.
Methods: A multilayer imager (MLI), consisting of four identical vertically stacked standard flat-panel imagers, was mounted to a clinical linear accelerator. A custom anthropomorphic pelvis phantom (CIRS, Inc) with replaceable 5cm diameter femoral heads was imaged using MV-CBCT and kV-CBCT. Femoral head inserts used were trabecular bone and titanium. A Developer Mode script was written to acquire 8MU 2.5MV 360-degree full-fan scans for all four layers and for the top layer only, as reference. Prostate and bladder were contoured on a reference kV-CT scan and transferred to the other scans after rigid registration, from which the structural similarity index measure (SSIM) was calculated. Prostate and bladder were also contoured on CBCT scans without guidance, and Dice coefficients compared to CT contours computed.
Results: kV-CBCT demonstrated the highest SSIMs with bone inserts (prostate:0.86, bladder:0.94) and lowest with metal inserts (0.32, 0.37). 4-layer MV-CBCT SSIMs were preserved with bone (0.75, 0.80) compared to titanium (0.67, 0.74). 1-layer MLI MV-CBCT consistently underperformed 4-layer results across all modalities. Dice coefficients trended similarly to SSIM.
Conclusion: MV-CBCT with a 4-layer MLI showed improvement over single-layer MV scans, approaching kV-CBCT quality for soft-tissue contrast. In the presence of artifact-producing metal implants, four-layer MV-CBCT scans outperformed kV-CBCT. MV-CBCT with a novel imager has promise as an alternative to kV-CBCT without compromising quality of care.
Funding Support, Disclosures, and Conflict of Interest: NIH/NCI R01CA188446