Room: Track 1
Purpose: Conventional pre-treatment thoracic 4D CBCT scans take 1320 projections over 4 minutes. We investigate whether scan time and dose can be reduced by implementing Respiratory-Motion-Guided (RMG) scans with motion compensated reconstruction, while maintaining image quality.
Methods: We present data from 5 lung cancer radiotherapy patients. Each patient has a planning 4DCT, and for their first 2 treatment fractions, 3 CBCT scans. The CBCT scans are a conventional 1320 projection scan, a 600 projection “slow” RMG and a 200 projection “quick” RMG scan. All are reconstructed with Feldkamp-Davis-Kress (FDK), and the RMG scans are also reconstructed with motion compensated FDK (MCFDK). The quality of the resulting 10 reconstructions is quantified by Signal-to-Noise Ratio (SNR), Tissue Interface Sharpness (TIS) and Structural Similarity (SSIM) to the conventional scan FDK.
Results: The quick scan delivers 80% less dose and is 75% faster than conventional 4D CBCT. The conventional FDK had average SNR of 15.1 and TIS of 0.766. The quick scan MCFDK had average SNR of 19.7, TIS of 1.09 and SSIM of 0.992. Patient 5 showed significant deviation between plan CT and CBCT, compromising the motion compensation and lowering average SNR to 13.6, TIS to 0.327 and SSIM to 0.972. The slow RMG scan delivers 55% less dose and is 25% faster than conventional 4D CBCT, and non-motion-compensated FDK reconstructions had average SNR of 14.3, TIS of 0.630 and SSIM of 0.990.
Conclusion: Image quality can be maintained while reducing scan time 25% and dose 55% via RMG acquisition. For 4/5 patients anatomical variation from planning to treating was low enough to use motion compensated reconstruction, allowing 75% faster and 80% lower dose RMG scans while maintaining image quality.
Funding Support, Disclosures, and Conflict of Interest: This work was supported by grant #1123068 awarded through the Priority-driven Collaborative Cancer Research Scheme and funded by Cancer Australia and NHMRC project grant #1138899. RO would like to acknowledge the support of a Cancer Institute of NSW Career Development Fellowship.