Purpose: The aim of this study was to evaluate the extent of the variation in the position of the prostate bed with respect to the bony anatomy.
Methods and materials: Four patients were treated to 70 Gy in 35 fractions. Before each fraction, a megavoltage computed tomography (CT) of the prostate bed was obtained, resulting in a total of 140 CT studies. Retrospectively, each CT scan was aligned to the simulation kilovoltage scan based on bony anatomy and the prostate bed. The difference between the 2 alignments was calculated for each scan.
Results: The average differences (+/-1 SD) between the two alignments were 0.06+/-0.37, 0.10+/-0.86, and 0.39+/-1.27 mm in the lateral, longitudinal (SI), and vertical (AP) directions, respectively. Laterally, there was no difference>or=3 mm. The cumulative frequency of SI differences were as follows; >or=3 mm: 3%, >or=4 mm: 1%, and >or=5 mm: 1% (maximum: 5 mm). The cumulative frequency of AP differences were as follows; >or=3 mm: 7%, and >or=4 mm: 3% (maximum: 4 mm).
Conclusion: In patients with prostate cancer receiving postoperative radiotherapy, the prostate bed motion relative to the pelvic bony anatomy is of a relatively small magnitude. Significant motion (>or=3 mm) is infrequent. However, small differences between the prostate bed and the bony anatomy still exist. This might have implications on treatment margins when daily alignment on bony anatomy is performed.