The impact of stool and gas volume on intrafraction prostate motion in patients undergoing radiotherapy with daily endorectal balloon

Purpose: The aim of this study was to quantify the impact of rectal stool/gas volumes on intrafraction prostate motion for patients undergoing prostate radiotherapy with daily endorectal balloon (ERB). Methods: Total and anterior stool/gas rectal volumes were quantified in 30 patients treated with daily ERB. Real-time intrafraction prostate motion from 494 treatment sessions, at most 6 min in length, was evaluated using Calypso® tracking system. Results: The deviation of prostate intrafraction motion distribution was a function of stool/gas volume, especially when stool/gas is located in the anterior part of the rectum. Compared to patients with small anterior stool/gas volumes (<10 cm³), those with large volume (10-60 cm³) had a twofold increase in 3D prostate motion and interquartile data range within the 6th minute of treatment time. The 10% of the overall CBCT session where large anterior rectal volumes were observed demonstrated larger percentage of time at displacement greater than our proposed internal margin 3 mm. Conclusion: Volume and location of stool/gas can directly impact the ERB's intrafraction immobilization ability. Although our patient preparation protocol and the 100 cm³ daily ERB effectively stabilized prostate motion for 90% of the fractions, a larger-sized ERB may improve prostate fixation for patients with greater and/or variable daily rectal volume.