2430 - Advancing Adaptive Radiotherapy with High-Fidelity CBCT-Based In-Situ Dose Calculation

Purpose/Objective(s): In adaptive radiotherapy (ART), accurate dose calculation based on daily imaging is crucial for treatment precision. Integrating CBCT-based dose calculations enables precise adaptation by improving anatomical fidelity and dose recalculation accuracy. However, the major limitation of CBCT in ART has been its inferior image quality and Hounsfield Unit (HU) uncertainty to the planning CT, hindering its reliability for precise dose calculations. This study aims to evaluate the feasibility and accuracy of CBCT-based dose calculation without dependence on conventional CT scans.

Materials/Methods: The high-fidelity iterative cone-beam CT (iCBCT) with both Acuros and Feldkamp-Davis-Kress (FDK) reconstruction algorithm images of ten cancer patients were acquired during the adaptive simulation. Three treatment plans were generated based on planning CT (reference), iCBCT-Acuros, and FDK CBCT. The primary CT structures were anatomical references for all plans after rigid registration. Dose comparisons were performed by assessing deviations in dose distributions in planning target volume (PTV) and organs at risk (OARs) between the iCBCT Acuros and FDK CBCT plans relative to the CT-based plan.

Results: The absolute dose distributions for seven anatomical structures were quantitatively compared between iCBCT Acuros and FDK CBCT, using the primary CT-based treatment planning system (TPS) as the reference standard. The mean dose variation in PTV of iCBCT Acuros and FDK plan from the CT-based TPS plan is 0.44 ± 0.3 Gy and 0.38 ± 0.39 Gy respectively. The maximum dose difference in PTV is 0.79 ± 0.82 Gy and 0.56 ± 0.57 Gy for iCBCT Acuros and FDK plan compared to CT-based TPS plan. The mean dose difference between iCBCT Acuros and FDK structures is 0.42 ± 0.26 Gy. This may be attributed to the low image quality of FDK. The D 0.5cc variation in OARS of iCBCT Acuros and FDK plan from CT-based plan were compared. The average dose variations are as follows; Duodenum 0.55 ± 0.16 Gy (3.29 ± 0.16 %), Heart 0.28 ± 0.0 Gy (5.10 ± 0.44%), Esophagus 0.21 ± 0.05 Gy (5.84 ± 2.27 %), Large bowel 0.26 ± 0.11 Gy (1.55 ± 0.78 %), Small bowel 0.37 ± 0.0 Gy (2.74± 1.42 %), Stomach 0.77 ± 0.25 Gy (4.91 ± 0.77%). The maximum dose variation observed in the Heart, Stomach and Esophagus is primarily attributed to anatomical changes between the planning CT and CBCT, as these structures are located near body contour and are more susceptible to positional variations. In contrast, dose discrepancies in Duodenum are predominantly influenced by HU uncertainty because of air-tissue artifacts. The differences in HU values between CT and CBCT images are within 35-400 based on structure positioning.

Conclusion: Through this study, we demonstrated that CBCT-based dose calculation for ART is clinically practical. However, additional caution is needed with HU uncertainty considering the variation in image quality. This finding highlights the potential of CBCT-derived dose calculations to enhance treatment precision and workflow efficiency in ART.