2316 - Variation of Liver Dome Position vs. Targets Distances and Locations in Liver Segments during Exhale Breath-Hold Liver SBRT

R. Jacob, R. Sullivan, R. A. Popple, and S. Shen; University of Alabama at Birmingham, Birmingham, AL

Purpose/Objective(s): Liver SBRT with implanted fiducials improves confidence in delivering target doses, however, fiducial implants are unavailable for many clinics. Liver dome position can be visualized using triggered imaging, and may be used as a surrogate for monitoring target position during treatment. We evaluated impact of target distance to the liver dome, and target locations in various liver segments to the magnitude of interfractional variation of liver dome position during exhale breath-hold in liver SBRT. Materials/

Methods: 30 liver SBRT patients, having implanted fiducials, treated in exhale breath-hold (Ex-BH) using triggered imaging of fiducials and amplitude gating. All treated either 3 x 15 Gy or 5 x 10 Gy. Patients underwent Ex-BH CBCTs, monitored with external RPM marker block. Prior to treatments, the implanted fiducials were registered with planning CTs, as part of image guidance. For this analysis, we studied 92 fiducials-registered CBCTs. The inter-fractional shifts of the liver dome in the superior/inferior directions (compared to planning CT) were recorded for each treatment. In addition, the distance from individual targets to liver dome (center of most superior liver slice) were measured on the planning CTs.
Results: There was no clear linear correlation between target location and the liver dome position during treatments. However, positional deviation of liver dome (compared to that in planning CT) seems to be categorized in 2 distinct groups. For target located <7.5 cm from the liver dome, the median positional deviation was +0.15 cm (range = -0.40 cm to +0.55 cm; SD =0.23 cm; n=47). For targets located > 7.5 cm from liver dome, the median positional deviation was +0.30 cm (range = -0.65 cm to +1.1 cm; SD = 0.48 cm; n=45). For targets in segment VII or VIII, median positional deviation was +0.1 cm (range = -0.40 cm to +0.55 cm; SD = 0.25 cm; n=44). For targets in segment I and IVa, IVb, V, V or VII, the median positional deviation was +0.3 cm (range = -0.65 cm to +1.10 cm; SD = 0.45 cm; n=48).
Conclusion: In this group of patients treated with exhale breath-hold with implanted fiducials, liver dome seems more suitable as a surrogate for monitoring liver targets in segment VII & VIII, or targets with distance to liver dome within 7.5 cm.