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SU-E-J-16: Prostate Bed Motion during Post-Prostatectomy Radiotherapy
Purpose:
To report the inter-fractional prostate bed motion (PBM) during post-prostatectomy radiotherapy using cone-beam CT (CBCT). The correlations between PBM, anterior rectal wall and posterior bladder wall shifts were also investigated.
Methods:
Seventy CBCT and 8 planning CT scans from 8 patients treated with prostate bed radiotherapy were retrospectively analyzed. For each CBCT-CT pair, two rigid image registrations were performed: one based on surgical clips and the other based on pelvic bony anatomy. Each registration gives a displacement vector. The difference between the two registration displacements represents the PBM. In addition, rectum/bladder contours on CT and CBCT scans were compared to identify the organ wall motion. Shifts of the anterior rectal wall and posterior bladder wall were assessed by averaging the slice-by-slice distances between contours on two image sets, measured along an anterior-posterior line passing through the symphysis pubis.
Results:
The prostate bed motion in the left-right (LR), anterior-posterior (AP) and superior-inferior (SI) directions was (0.1±0.9)mm, (0.9±1.6)mm, and (-0.4±1.9)mm, respectively. The derived PTV-CTV margin for LR, AP and SI motion was 3mm, 5mm, and 6mm, respectively. Pearson's correlation coefficients between PBM and anterior rectal wall (whole length) shifts, PBM and the cranial half of anterior rectal wall shifts, and PBM and posterior bladder wall shifts, were 0.43, 0.47, and 0.67, respectively.
Conclusions:
The magnitude of PBM relative to pelvic bony anatomy in all three directions was small. The correlation between average anterior rectal wall shifts and PBM was weak-to-moderate, which may be due to rectum contouring inconsistency. Possible sources for this inconsistency include: non-uniform rectal wall motion through its length, low CBCT image contrast, and artifacts due to filling. Significant correlation between average posterior bladder wall shifts and PBM suggests bladder wall motion may also be a suitable surrogate for PBM in the AP direction.
