The Impact of Daily Urinary Volume Variations and Prostate Volume on Prostate Position During Radiotherapy

Abstract

BackgroundDaily variations in bladder volumes are thought to be a major cause of prostate position changes during radiotherapy, however, the impact of such changes on prostate position is not fully established. We hypothesized that prostate volume (PV) could also be related to the susceptibility of prostate movement to changes in urinary volume (UV). The purpose of this study is to investigate the impact of daily UV variations and PV on the prostate position during radiotherapy.MethodsA total of 268 treatment sessions in 12 consecutive prostate cancer patients who were treated with proton beam therapy in our hospital were retrospectively analyzed. All patients were instructed to void urine and feces one hour before the treatment and the UV was measured by a portable ultrasound device just before the treatment. After the positional registration based on bone structure, the prostate positional difference from the planned position was measured using fiducial markers in the prostate and a real-time tumor tracking system. The measurements were for the differences in the left-right (LR), cranial-caudal (CC), and anterior-posterior (AP) directions. The PV was measured by ultrasound imaging before every treatment. The impact of the UV variations on the prostate position was evaluated by the linear mixed effect model, and including PV (<20ml and ≥20ml) as covariates. ResultsThere was significant movement of the prostate in the posterior direction with increasing UV in patients with PV ≥ 20 ml (p=0.04, estimated coefficient of regression -0.436 mm/100ml, 95% CI from -0.854 to -0.018). However, there was no significant correlation between UV changes and movement of the prostate in either of the LR, CC, or AP directions in all cases and patients with PV < 20 ml.ConclusionsOur results suggest that the increase of UV could result in posterior prostate movement in patients with prostatic hyperplasia, however, the clinical impact is thought to be small. There is however a need for further research to identify other factors that could affect the daily changes in prostate position as well as a need for a more ideal preparation protocol to improve the reproducibility of the treatment-planning CT.