Simulation Study of Upright Prostate Radiation Therapy: Comparison of Upright and Supine Anatomy and Dose-Volumetric Factors.

[PURPOSE] This study aimed to assess the feasibility of upright radiation therapy for prostate cancer, compared to conventional supine radiation therapy, by quantitatively evaluating anatomic changes and their dosimetric impact through treatment planning simulations based on paired upright and supine positioning computed tomography (CT) images.

[METHODS AND MATERIALS] This study compared pelvic organ anatomies and dose distributions, in hypothetical prostate cancer patients, between upright and supine prostate radiation therapy using paired CT images from asymptomatic volunteers. Anatomic changes in pelvic organ position, shape, volume, and rectal gas were quantitatively evaluated. Prostate radiation therapy was simulated on these CT images for volumetric modulated arc therapy with a prescribed dose of 60 Gy in 20 fractions. Dosimetric comparisons were performed using dose-volume histogram parameters based on a clinical protocol.

[RESULTS] Fifty-one asymptomatic volunteers were registered. Compared to the supine position, pelvic organs in the upright position moved inferiorly (prostate: 2.8 mm ± 5.2, rectum: 2.9 mm ± 5.8, bladder: 3.7 mm ± 6.9, bowel: 25.2 mm ± 18.6; all P < .001). Rectal gas moved superiorly (4.8 mm ± 11.9; P = .004) and posteriorly (10.7 mm ± 23.5; P = .008). All dose-volume histogram parameters met the dose constraints, and the dose distributions of the upright and supine plans were clinically comparable in terms of organ-at-risk sparing and target coverage (P > .05). In some cases, the rectum curved more posteriorly in the upright position, allowing better sparing of high-dose areas, and the bladder moved inferiorly while expanding in the anterior-posterior direction in the upright position, preventing the small bowel from dropping into the radiation field.

[CONCLUSIONS] Our treatment planning simulation showed that upright radiation therapy for prostate cancer is feasible with a dose distribution comparable to that of conventional supine radiation therapy, despite significant anatomic changes in pelvic organ positioning between the upright and supine patient positions.