Four-dimensional on-beam computed tomography reconstruction using projection-difference images

Clinical Snapshot

Key Findings

• P Value: Not reported

• Effect Size: RMSE: 4D OBCT 58.46 HU vs 4D CBCT 74.37 HU (lung); 4D OBCT 23.78 HU vs 4D CBCT 66.84 HU (chest wall). Gamma pass rate at 2%/2 mm: 4D OBCT 99.76% vs 4D CBCT 72.50%; at 1%/1 mm: 4D OBCT 99.08% vs 4D CBCT 57.77%

• Primary Outcome: Image reconstruction accuracy assessed by root mean square error (RMSE) in Hounsfield units, and dosimetric accuracy assessed by gamma pass rates

• Nnt Or Sensitivity: Not applicable (phantom technical study); no NNT, sensitivity, or specificity calculable. Gamma pass rate improvement of approximately 27 percentage points at 2%/2 mm and 41 percentage points at 1%/1 mm criteria compared to 4D CBCT

• Confidence Interval: Not reported

Clinical Application

Clinical feasibility is undemonstrated. Key barriers include: (1) computational time for Monte Carlo simulation must be compatible with clinical workflow; (2) EPID-based transmission imaging requires calibration and quality assurance protocols not yet standardised for this application; (3) integration with treatment planning systems and record-and-verify software is unaddressed; (4) regulatory approval pathways have not been discussed In Australia, lung SBRT and VMAT are well-established within the public and private radiation oncology sectors, with EPID-based dosimetry increasingly used for pre-treatment and transit dose verification. The Australian Clinical Dosimetry Service (ACDS) and RANZCR provide oversight of dosimetric quality assurance. Should 4D OBCT demonstrate clinical validity in future patient studies, it would require TGA evaluation as a software medical device (SaMD) under the TGA's Digital Health regulatory framework. PBS listing is not directly relevant to imaging reconstruction software, but Medicare Benefits Schedule (MBS) implications for adaptive radiotherapy workflows would need consideration. RACGP involvement is minimal; this technology sits within the domain of radiation oncology physicists and radiation oncologists. Australian centres with MR-linac capability (e.g., Peter MacCallum Cancer Centre) may find this technology complementary or competitive with existing intrafractional imaging solutions. Not yet applicable to any clinical population. The framework is theoretically relevant to patients with thoracic malignancies (lung, oesophageal, breast) undergoing external beam radiotherapy where intrafractional tumour motion is clinically significant, particularly those treated with stereotactic body radiotherapy (SBRT) or volumetric modulated arc therapy (VMAT)

Abstract

Purpose.Accurate verification of delivered dose requires anatomical information during the treatment beam delivery. However, conventional cone-beam computed tomography (CBCT) is typically acquired prior to beam delivery and fails to account for intrafractional tumor shift/drift. This study aims to develop a 4D on-beam computed tomography (4D OBCT) framework to capture anatomical states during irradiation.Materials and methods.A thoracic phantom with rigid tumor motion was used to develop a conformal arc treatment plan. During delivery, the target was displaced by 1 cm superior to its planned position to simulate a baseline shift, and megavoltage transmission data were acquired using an electronic portal imaging device. Transmission data corresponding to the planned tumor position were obtained from the planning CT (pCT) via Monte Carlo simulation. Volumetric difference images were reconstructed from the differences between the measured and simulated transmission data for each phase. Subsequently, the images were superimposed onto the corresponding phases of the 4D pCT to obtain 4D OBCT. For evaluation, we compared 4D OBCT with 4D pCT (ground-truth) and 4D CBCT which were acquired after the target displacement. A dosimetric performance was evaluated by recalculating 4D dose distributions on the three images and comparing in dose-volume histograms and gamma indices.Results.4D OBCT achieved lower root mean square error than 4D CBCT in both the lung (58.46 HU vs 74.37 HU) and chest wall regions (23.78 HU vs 66.84 HU). This quantitative accuracy was also reflected in the dosimetric comparison. Under 2%/2 mm and 1%/1 mm criteria, 4D OBCT achieved gamma pass rates of 99.76% and 99.08%, respectively, whereas 4D CBCT yielded 72.50% and 57.77%, respectively.Conclusions.The proposed 4D OBCT framework successfully reconstructed accurate phase-resolved images of the target during irradiation. By providing high-resolution volumetric images that reflect intrafractional motion, 4D OBCT could provide a robust solution for precise post-delivery dosimetric verification and adaptation.

References

1. 1.Hwang, J., Moon, R. H., Yoon, J., Cheong, K.-H., Jung, J. W., Cho, S., Yeo, I., & Lee, H. (2026). Four-dimensional on-beam computed tomography reconstruction using projection-difference images. Physics in Medicine and Biology. https://doi.org/10.1088/1361-6560/ae8355