Intra-respiratory variation in tumour-diaphragm geometric association as a potential surrogate for dynamic tumour-tracking radiotherapy.

Background and Purpose: For thoracic and upper abdominal tumours, respiratory motion management is essential; dynamic tumour-tracking radiotherapy (DTT-RT) is one such approach, but it typically requires fiducial marker (FM) insertion. We evaluated intra-respiratory variation in the tumour-diaphragm geometric association and its potential as a surrogate for DTT-RT. Materials and Methods: Data from 22 patients (11 lung cancer [LC]; 11 locally advanced pancreatic cancer [LAPC]), treated with FM-based DTT-RT, were analysed. Ten-phase, four-dimensional computed tomography was used to quantify the 3D gross tumour volume (GTV)-FM displacement, 3D GTV-diaphragm displacement, and 3D GTV centroid displacement under free-breathing (FB). Phase-wise relative displacements for GTV-FM, GTV-diaphragm, and the GTV centroid under FB were calculated along the left-right (LR), anterior-posterior (AP), and superior-inferior (SI) axes as differences from the end-expiratory phase. Results: In LC, the mean 3D GTV-FM displacement, 3D GTV-diaphragm displacement, and 3D GTV centroid displacement under FB across all phases were 1.0 (range, 0-6.9), 4.6 (0.1-19.4), and 5.8 (0.1-31.8) mm, respectively (p < 0.001). In LAPC, corresponding values were 1.6 (0.2-4.7), 4.5 (0.3-21.8), and 3.2 (0.4-17.0) mm (p < 0.001). Phase-wise relative GTV-diaphragm displacement was largest in the SI direction in LC and in the AP direction in LAPC, with patterns primarily driven by inspiratory-phase variations. Conclusions: As a surrogate, the diaphragm exhibited larger intra-respiratory variation than FMs, primarily driven by inspiratory displacements. Incorporating phase-dependent tumour-diaphragm geometry may support markerless, diaphragm-based DTT for LC and LAPC.