Movements of the prostate due to rectal and bladder distension: implications for radiotherapy.

Precision radiotherapy depends upon the accurate definition of a target volume. Conformal therapy of prostate cancer requires an understanding of the prostate volume and its potential movements. This study was performed to quantify movement of the prostate due to rectal or bladder distention. Eighteen men with localized prostate cancer (clinical stages A, B, and C) were included in this study. Following simulation, treatment planning computed tomography (CT) scans were performed. The scans were first obtained with a small amount of contrast in the rectum and the bladder. Identical CT cuts were then obtained after filling the rectum with contrast. In 11 patients, the bladder was then filled with contrast and the same CT cuts were obtained again. Movement of the prostate and seminal vesicles secondary to distension of the rectum and bladder was measured. Distension of the rectum shifted the posterior margin of the prostate anteriorly from 0 to 0.9 cm. (median = 0.1 cm). The posterior border of the prostate was displaced anteriorly by more than 0.5 cm in 3 of the 18 (17%) patients with rectal distension. Distension of the bladder shifted the posterior border of the prostate posteriorly from 0 to 0.8 cm. (median = 0.2 cm). The posterior border of the prostate was shifted more than 0.5 cm. in 1 of 11 (9%) patients with bladder distension. Movements of other margins of the prostate and seminal vesicles were measured. The available literature is reviewed and suggestions are given regarding the movement of the prostate and treatment planning.