AbstractID: 9073 Title: Daily Alignment for Prostate Radiotherapy: Bony Anatomy versus Intraprostatic
Markers
The purpose of this study was to use a dual KV x-ray system (Novalis Body, BrainLab) to determine the difference in prostate
position from pelvic bone anatomy on a daily basis. Patients undergoing radiotherapy for prostate cancer were implanted with three
gold marker seeds in the prostate: two at the base and one at the apex. Each day dual x-ray images were obtained for each patient. Two
modes of image registration were performed on each pair of x-rays: automatic mutual information registration between DRRs and xrays, and manual registration of the intraprostatic markers from CT to x-rays. The difference between these registrations was used to
determine the prostate location relative to the bony anatomy. Data were collected on twenty patients and a total of 287 separate
alignments. The average shift (± SD) in the three orthogonal directions were -1.60 ± 3.40 mm vertical, -0.50 ± 2.72 mm longitudinal,
and -1.25 ± 1.43 mm lateral. The directions indicate a prostate displacement posteriorly, inferiorly, and to the right. The maximum
shifts were 15.66 mm vertical, 14.94 mm longitudinal, and 9.07 mm lateral. Although the software can indicate rotations, these were
not recorded. The difference between the bony anatomy and prostate position is relatively minimal; however, these differences can be
occasionally significant. Daily alignment using intraprostatic markers is therefore more desirable for accurately targeting the prostate
for radiotherapy.