SU-E-J-177: Understanding the Seminal Vesicle's Motion Relative to the Prostate: Range and Pattern.

PURPOSE: To investigate inter-fractional (inter-fx) and intra-fractional (intra-fx) seminal vesicle (SV) motion to determine motion management strategies for prostate SBRT. METHODS: 9 prostate SBRT patients (37Gy/5fx) were studied. All patients were implanted with fiducial markers in the prostate for soft tissue tracking. SVs were contoured on 5 pairs of preand post-treatment CBCTs and the planning CT. All CT/CBCT images were registered according to fiducial markers. Inter-fx motion was analyzed by comparing pre-treatment CBCT with planning CT; intra-fx motion was analyzed by comparing pre- and post-treatment CBCTs. Both the center of mass (COM) and boarders of SV were assessed to determine motion ranges. SV were segmented into left SV (LSV) and right SV (RSV) to analyze independent motion patterns. RESULTS: Inter-fx SV motion is significant in all directions. The 95% range of COM is 9.4, 14.3 and 10.3 mm for LR, AP and SI direction. Larger motion variation is observed with SV boarder. A 5 mm margin can provide 60% and 58% CTV coverage for COM and border. Intra-fx SV motion is smaller than inter-fx. A 5 mm margin can provide 93% and 84% CTV coverage for COM and boarder. LSV and RSV show different motion patterns in LR direction with correlation of -0.194 for COM while AP motion is strongly correlated (0.837). SV COM shows weak correlation with RSV COM, LSV COM, right boarder of RSV and left boarder of LSV (coefficient of 0.326, 0.450, 0.087, -0.192). SV COM's AP motion is strongly correlated with both LSV and RSV. CONCLUSION: SV moves substantially both inter- and intra-fractionally, indicating the difficulty of covering CTV with fixed 5mm margin. Additional considerations/strategies in margin design are required. SV boarder motion shows independence from COM, indicating potential underdose to CTV if tracking with COM. Soft tissue imaging is highly desired for SV localization.

Duke Scholars

Author Qingrong Wu Radiation Oncology