PURPOSE: To determine the planning target volume (PTV) margin for an adaptive radiotherapy procedure that uses five computed tomography (CT) scans to calculate an average prostate position and rectum shape. To evaluate alternative methods to determine an average rectum based on a single delineation. METHODS AND MATERIALS: Repeat CT scans (8-13) of 19 patients were used. The contoured prostates of the first four scans were matched on the planning CT (pCT) prostate contours. With the resulting translations and rotations the average prostate position was determined. An average rectum was obtained by either averaging the coordinates of corresponding points on the rectal walls or by selecting the "best" rectum or transforming the pCT rectum. Dose distributions were calculated for various expanded average prostates. The remaining CT scans were used to determine the dose received by prostate and rectum during treatment. RESULTS: For the prostate of the pCT scan and a 10-mm margin, all patients received more than 95% of the prescribed dose to 95% of the prostate. For the average prostate, a margin of 7 mm was needed to obtain a similar result (average PTV reduction 30%). The average rectum overestimated the mean dose to the rectum by 0.4 +/- 1.6 Gy, which was better than the pCT rectum (2.1 +/- 3.0 Gy) and the alternative average rectums (1.0 +/- 2.6 Gy and 1.4 +/- 3.2 Gy). CONCLUSIONS: Our adaptive procedure allows for reduction of the PTV margin to 7 mm without decreasing prostate coverage during treatment. For accurate estimation of the rectum dose, rectums need to be delineated and averaged over multiple scans.