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Innovative Point Based Analytical 4D IGRT for Lung SBRT with Fiducial Markers

J Barbiere*, B Lewis , A Ndlovu , HUMC, Hackensack, NJ


(Tuesday, 7/16/2019) 10:30 AM - 11:00 AM

Room: Exhibit Hall | Forum 5

Purpose: Contours of fiducial markers for stereotactic body radiation therapy (SBRT) image guided radiation therapy (IGRT) of lung tumors on the Maximum Intensity Projection (MIP) four dimensional treatment planning CT (4DCT) are often used to visually verify treatment setup. This process is user and display dependent, not quantitative, subject to imaging artifacts, and not suitable for automation. This work describes an innovative analytical process.

Methods: A 4DCT is acquired at simulation and binned into ten phases. The center of each fiducial is assigned a position coordinate at each phase to form the point set P. Orthogonal fluoroscopic images are obtained over many breathing cycles. The center of each fiducial is assigned a position coordinate at a preset frequency of 0.5 seconds on each view forming the point set F. Since the goal is to treat the target continuously all points must be considered. A center of mass approach will bias the registration towards the most common position. The Axis-Aligned Bounding Box (AABB) algorithm is an expression of the maximum extent of a data set. If we consider a lateral fluoroscopic fiducial image, then AABB produces a box with sides at the maximum/minimum Sup-Inf positions and maximum/minimum Ant-Post positions. The next step is to compute the center of the bounding box for each fiducial both for plan point set P (Pc) and fluoroscopic images F (Fc). The translation registration (TR) is then computed such that TR(Fc) = Pc for each fiducial. The mean TR is then applied to the patient.

Results: 4DCT tends to underestimate both the amount and variability of the fiducial motion. The analytical process results in improved registration compared to visual estimates.

Conclusion: An innovative analytical process has been developed which can improve 4D IGRT accuracy for lung SBRT.


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