Clinical evaluation of the impact of rectal and bladder filling on prostate position during hypofractionated radiotherapy for prostate cancer

Objective: To evaluate the impact of rectal and bladder filling on prostate position during hypofractionated radiotherapy for prostate cancer. Methods: Three gold fiducials were implanted into the prostate in 25 patients. Each patient underwent CT scanning under four different conditions: full rectum and bladder, full bladder and empty rectum, full rectum and empty bladder, and empty rectum and bladder (primary image). The other three scans were aligned with the primary image by pelvic bone, then by the implanted fiducials. Prostate displacements were determined by computing the difference between these two alignments. The magnitude and directions of displacement were analyzed. Results: A full rectum shifted the prostate most significantly in the anterior (0.93 ± 0.31 cm) and superior directions (0.48 ± 0.30 cm). A full rectum was associated with anterior shifts ≥0.5 cm in 92.0% of the patients, ≥1 cm in 44.0%, while superior shifts ≥0.5 cm were observed in 48.0% and ≥1 cm in 8.0%. A full bladder shifted the prostate mildly in the superior direction (0.19 ± 0.30 cm). A full rectum and full bladder shifted the prostate more in the anterior (1.19 ± 0.37 cm) and superior directions (0.49 ± 0.50 cm). 100% and 56.0% had ≥0.5 cm of anterior and superior displacements. 68.0% and 16.0% had ≥1 cm of anterior and superior shift. Conclusions: Our study demonstrated that a full rectum shifted the prostate mainly anterosuperiorly. The rectal volume’s impact on prostate movement is much larger than that of the bladder.