Author: Ben Durkee, Renata Farrell, Carri K. Glide-Hurst, Colin Harari, Alex Singleton Kuo, Chase Ruff, Jordan M. Slagowski, Yuhao Yan 👨🔬
Affiliation: University of Wisconsin, Departments of Human Oncology and Medical Physics, University of Wisconsin-Madison, Department of Human Oncology, University of Wisconsin-Madison 🌍
Purpose: Advances in upright CT and patient positioning system now enables high quality daily CT imaging and treatment delivery in the upright position, providing benefits including reduced internal motion, improved normal tissue sparing and greater patient comfort. We investigated variations in pelvic organ geometry between supine and upright positioning in preparation for clinical implementation of upright CT-guided proton therapy
Methods: Matched-pair supine and upright spoiled gradient echo MR of the pelvis region were acquired within 40 minutes for six healthy volunteers (3 males, 3 females) on a 0.6-Tesla MR. Organ-at-risk contours were generated using a commercialized AI software and verified by an institutional expert. Upright images were rigidly registered to supine focusing on lumbar spine alignment to establish consistent frames of reference. Organ shape changes and displacement between the two orientations were analyzed using bounding box (BB) and centroid measurements, respectively.
Results: In the upright orientation, the bladder dimension decreased significantly in the superior-inferior dimension (mean bounding box extent differences ΔBBmean=-1.2/-1.0 cm for male/female, respectively) and exhibited anterior-posterior elongation (ΔBBmean=2.5/1.0 cm for male/female) except for one female outlier with substantially inconsistent bladder filling. External anatomy extended anteriorly (ΔBBmean=4.1/2.3 cm for male/female) in the upright orientation. The rectum demonstrated subject-specific differences with no apparent pattern. Femoral heads exhibited rotations with different postures. In males, prostate exhibited notable but inconsistent shape change across the subjects while seminal vesicles showed minimal changes. Centroid analysis revealed shifts in the bladder (0.7/0.6 cm inferior for male/female) and seminal vesicles (0.6 cm in the inferior and posterior directions). With upright orientation, the prostate and seminal vesicles displayed minimal volumetric change, with ΔVmean < 5% and ΔVmean < 1% respectively.
Conclusion: Considerable variations in pelvic anatomy were observed between supine and upright positions which will be evaluated in future dosimetric studies.