Digital Twin–Enabled Virtual Imaging Framework for Scalable Development of Upright Radiotherapy
Abstract
Purpose
Upright patient positioners coupled with diagnostic-quality vertical CTs at treatment isocenter introduce significant opportunities for online adaptive particle therapy. However, limited upright data constrains our ability to rigorously test and optimize this new paradigm. To address this, we developed the first upright digital twin (DT) framework and virtual upright CT system to enable flexible and scalable evaluation.
Methods
Matched pair supine-upright 0.6T pelvis 3D T1-weighted gradient echo MRI and derived synthetic CT for a healthy volunteer and supine-upright 10-phase 4DCT/average CTs (120 kVp) for a lung cancer patient served as representative patient models to develop upright DTs. Organs were segmented using a pre-trained nnU-Net-based ensemble and refined to match underlying anatomy. Posture-dependent geometric and excursion differences were characterized. A GPU-based upright CT simulation platform was developed using technical specifications and a tissue-mimicking calibration phantom scanned on a novel upright CT scanner (120 kVp, 250 mAs). Simulated calibration phantom images were benchmarked to upright CT using mean absolute error (MAE) and coefficient of determination (R2).
Results
Our framework successfully modeled subject-specific DTs for upright pelvis MRI and thoracic 4DCTs. In pelvis, the upright DT demonstrated 4.6% bladder volume increase with 3.2 cm elongation anterior-posterior compared to supine. Prostate and rectal volumes decreased in the upright DT and displaced inferior (~2.8-8.2 mm) compared to supine. For thoracic CT, lung volumes for the upright DT at end-inhalation and exhalation were ~23.3-25.8% greater, exhibited > 2.5 cm increase in the long axis (superior/inferior), and demonstrated reduced respiratory motion relative to supine. The upright simulation platform yielded excellent agreement between virtual and real calibration phantoms (MAE of 32.5±31.8 HU, R2 = 0.99).
Conclusion
We validated the first upright DT framework with a virtual upright CT to facilitate future simulations, support rigorous evaluation, and provide a critical platform for accelerating innovation in upright radiation therapy.