The Applications and Pitfalls of Cone-Beam Computed Tomography-Based Synthetic Computed Tomography for Adaptive Evaluation in Pencil-Beam Scanning Proton Therapy

SIMPLE SUMMARY: For proton therapy, verification CT (vCT) scans are routinely acquired to ensure the accuracy and precision of treatment delivery, as this approach allows clinicians to monitor and adapt treatment plans based on potential changes to the patient’s anatomy and tumor. This study investigates synthetic CT’s (sCT) potential as an alternative to vCT scans, focusing on its reliability across various treatment sites. Synthetic CT could offer a more efficient approach and possibly enhance patient experience by reducing the necessity for frequent vCT scans. The consistency between sCT and vCT in terms of image quality and dosimetric impact is crucial. It will allow clinicians to monitor and adjust treatment plans promptly and more accurately based on patients’ anatomical changes, potentially optimizing treatment processes and outcomes. The insights from this study are pivotal for refining clinical practices and fostering advancements in treatment strategies, ultimately aiming at precision and accuracy in adaptive treatment. ABSTRACT: Purpose: The study evaluates the efficacy of cone-beam computed tomography (CBCT)-based synthetic CTs (sCT) as a potential alternative to verification CT (vCT) for enhanced treatment monitoring and early adaptation in proton therapy. Methods: Seven common treatment sites were studied. Two sets of sCT per case were generated: direct-deformed (DD) sCT and image-correction (IC) sCT. The image qualities and dosimetric impact of the sCT were compared to the same-day vCT. Results: The sCT agreed with vCT in regions of homogeneous tissues such as the brain and breast; however, notable discrepancies were observed in the thorax and abdomen. The sCT outliers existed for DD sCT when there was an anatomy change and for IC sCT in low-density regions. The target coverage exhibited less than a 5% variance in most DD and IC sCT cases when compared to vCT. The D(max) of serial organ-at-risk (OAR) in sCT plans shows greater deviation from vCT than small-volume dose metrics (D0.1cc). The parallel OAR volumetric and mean doses remained consistent, with average deviations below 1.5%. Conclusion: The use of sCT enables precise treatment and prompt early adaptation for proton therapy. The quality assurance of sCT is mandatory in the early stage of clinical implementation.