The effect of pressure from the table top and patient position on pelvic organ location in patients with prostate cancer.

Purpose: To assess the impact of pressure from the table top and patient position on the relationship of the prostate, rectum, and bladder to the bony pelvis.

Methods And Materials: In 9 patients with prostate cancer (3 status postprostatectomy), computed tomography (CT) scans were obtained in four positions: supine with and without false table top under the buttocks, prone with and without false table top under the lower abdomen. In four patients, a fifth scan was obtained in the first position (supine with table top in place) to assess the impact of changes in bladder/rectal fullness over time. Urination and defecation were not permitted between scans. For each patient, the four (or five) CT scans were registered to each other.

Results: The anal canal and the rectum caudal to the coccyx shifted posteriorly in 7/9 patients when the support under the buttocks was removed in the supine position. When pressure from the table top was removed in the prone position, the anterior bladder extension increased. The superior rectum was adjacent to the prostate in all scans and the prostate/superior rectum/bladder generally moved together. Rectal fullness changed with time and rectal gas position was gravity-dependent and shifted with patient position. Bladder volume increased with time. Organs had shifted and/or changed fullness between the first and fifth scan obtained in the same patient position approximately 90 min apart, mostly due to increase in bladder volume. All patients found the supine position most comfortable.

Conclusions: The bladder and rectal fullness vary with time, confounding the ability to attribute changes in organ location to positional factors. Pressure from the table top affects the relative location of pelvic organs and, in part, is responsible for changes previously attributed to position/gravity.