Towards a shuttle-based workflow for daily online magnetic resonance imaging-guided particle therapy: Anatomical robustness and organ variability in the female pelvis

Background and Purpose: Online adaptive particle therapy can take into account anatomical changes during the course of treatment. In a shuttle-based daily online magnetic resonance imaging (MRI)-guided strategy, patients remain immobilized in the treatment position during transfer between MRI and irradiation. This study investigated positional stability and organ variability in the female pelvic region during shuttle transfers. Material and Methods: = 1.5 T) according to the following protocol: volunteer positioning with an indexed knee and foot support, MRI 1, 10-minute time control phase, MRI 2, short transport, MRI 3, long transport, and MRI 4. In each MRI set, the external, cervix, uterus, bladder, intestine, and rectum were contoured. Dice similarity coefficients (DSC) and mean distances to agreement (MDA) were calculated for shifts between consecutive image sets and throughout the study. Results: Median DSC (MDA) of the external was 0.99 (0.5 mm) for MRI 1-2, 0.99 (0.9 mm) for MRI 2-3, and 0.99 (0.9 mm) for MRI 3-4. Median MDA values observed for MRI 1-2/MRI 2-3/MRI 3-4 were 2.2/2.8/1.8 mm (cervix), 3.4/3.7/3.6 mm (uterus), 3.6/4.2/3.7 mm (bladder), 2.0/2.3/2.3 mm (intestine), and 1.6/1.4/1.7 mm (rectum). Statistically significant Pearson correlations were found between DSC/MDA values of gynecological structures and bladder volume differences. Conclusions: High stability in the external outline of the female pelvis can be maintained in a shuttle-based workflow for MRI-guided adaptive particle therapy. Physiological organ motion remained the main source of variability and was only marginally influenced by the shuttle transport.