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Non-Uniform PTV Margins for Prostate Bed Based On Daily CBCT Interfraction Motion of Surgical Clips
H Song, J Salama, W Lee, Q Wu, Duke University Medical Center, Durham, NC
Presentations
SU-H3-GePD-J(B)-5 (Sunday, July 30, 2017) 4:00 PM - 4:30 PM Room: Joint Imaging-Therapy ePoster Lounge - B
Purpose: We hypothesized that the interfraction motion of the superior and inferior portion of the prostate bed differs, therefore different margins should be applied. In this study, we evaluated the motion of post-prostatectomy surgical clips (separated to superior and inferior portions) within PTV using CBCTs acquired for daily setup and recommend data-driven PTV margins.
Methods: Among patients undergoing prostate bed irradiation (2.5 Gyx26 fractions) with daily CBCT image guidance, eleven patients with identifiable surgical clips on all daily CBCT (265 image sets total) formed our study cohort. All identifiable clips within the PTV were manually contoured on the planning CTs and CBCTs. All CBCTs were registered to the planning CT based on pelvic bony structures. The superior border of the pubic symphysis was used to mark the division between the superior and inferior portions.
Results: A total of 28 superior clips and 56 inferior clips were identified (ranging 3 to 11 clips per patient). In the left-right direction, the global mean M, systematic error Σ, residue error σ are 0.02, 0.03, 0.16 cm for superior clips respectively, and 0.00, 0.03, 0.03 cm for inferior clips. In the AP direction, the corresponding values are M=0.01, Σ =0.25, σ=0.37 for superior, and M=0.08, Σ=0.13, σ=0.15 for inferior. In superior-inferior direction, M=-0.06, Σ=0.23, σ=0.27 superior, and M=-0.01, Σ=0.21, σ=0.20 inferior. Using the margin formula 2.5 Σ+0.7σ, margins (from interfraction motion alone) are: 0.19 cm LR, 0.89 cm AP, 0.76 cm SI for superior portion of the prostate bed, and 0.10 cm LR, 0.44 cm AP, and 0.67 cm SI for inferior prostate bed.
Conclusion: The observed motion in the superior portion is larger than the inferior portion suggesting that non-uniform margins are justified. The PTV margin of 0.4 to 0.6 cm may not be sufficient for superior portion of prostate bed.
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