SU-E-T-640: Development of Liver SBRT Planning and Delivery at the Mean Respiratory Position Using Deformable Image Registration.

Abstract

Deformable image registration (DIR) allows modeling of liver tumors on respiratory correlated (4D) imaging. The mid-position CT was reconstructed for liver SBRT plans using DIR, and the potential for dose-escalation was investigated. Thirty patients were planned clinically with IMRT for 27-48 Gy in 6 fractions on static exhale 4DCT with PTVs encompassing the breathing amplitude. For research, exhale 4DCT was deformed to the inhale 4DCT using biomechanical DIR. The mid-position CT was created by applying a percentage (the time-averaged normalized position between exhale and inhale calculated from daily 4D cone-beam CT) to this deformation map, assuming a linear trajectory. A probability-based PTV margin, using patient-specific breathing amplitude from DIR of 4DCT, was created around the GTV on the mid-position CT where IMRT was re-optimized. Dose was maximally escalated according to clinical protocol (e.g. liver NTCP <5%). The 4D predicted breathing dose was accumulated by interpolating the elements' positions at exhale, mid-position and inhale onto the respective dose matrices (weighted by time spent nearest each matrix) then summed. Compared the exhale plans, the GTV-to-PTV volume decreased on the mid-position plans by a mean of 31% (p<0.01, range: 24-38%). Static re-planning on the mid-position CT decreased the mean effective liver volume by 7% (p=0.032), enabling escalation of the nominal prescribed dose in 80% of patients of 6-12 Gy. Reconstruction of the 4D predicted breathing dose resulted in a mean increase of 6.7 Gy (p<0.01, maximum increase of 15.0 Gy) in mean GTV dose for the mid-position versus the exhale plan. For the mid-position plan, the minimum 0.5 cm3 GTV dose received 100% of the prescription in the 4D distribution. Liver SBRT Planning at the mean respiratory position enables PTV reduction and a mean dose escalation of 6.7 Gy, potentially improving local control. Dr. Brock has financial interest in deformable registration technology through the licensing of Morfeus to RaySearch Laboratories. Research is funded by NIH 1R01CA124714.