WE-G-213CD-02: 4D-PET Maximum Intensity Projections Improve Accuracy of Mobile Tumor Volume Delineation

To examine the efficacy and accuracy of respiratory-gated positron emission tomography (4D-PET) maximum intensity projections (MIPs) in segmenting mobile tumor internal target volumes (ITVs) through a phantom study.

An acrylic phantom was used for PET list-mode acquisition, consisting of 11C-filled spheres affixed inside a cylinder containing an 18-fluorodeoxyglucose solution. The phantom was attached to a robotic arm that underwent 1D motion based on clinically-derived patient breathing trajectories. An amplitude-based gating was performed on sequential list-mode sub-files of varying signal-to-background ratios, and PET-MIPs were generated from the gated images. ITVs were segmented by first denoising and deblurring images using a custom post-processing module and then applying an absolute SUV threshold. ITV accuracy was assessed using the volume recovery fraction (VRF) - the ratio of measured ITV to true volume occupied by the moving phantom spheres - which was used to compare PET-MIP, ungated PET, and static sphere images. In addition, the effects of tumor trajectories, number of gating windows, and margin additions were investigated.

VRFs of ITVs generated from PET-MIPs were consistently higher than those from ungated PET. They also demonstrated a closer agreement to VRFs of static spheres (up to 99% similarity vs. 72% for ungated PET), suggesting that tumor motion had very little effect on the accuracy of PET-MIP measurements. Trajectories with higher amplitude and baseline drift decreased VRF by up to 14% for both PET-MIP and ungated PET. Increasing the number of gating windows (up to eight windows) resulted in higher VRFs for PET-MIPs without producing excessive image noise. PET-MIPs required a smaller margin addition than ungated PET to achieve a better overlap with the ground truth ITV.

Compared to ungated PET, PET-MIPs are not significantly affected by tumor motion. 4D-PET imaging is a promising and viable methodology to better delineate mobile tumor volumes.