SU‐FF‐I‐142: Quantifying the 4D PET/CT Volumetric Distortions: A 4D Dynamic Phantom Study Based On Pre‐Recorded Patient Data

Abstract

Introduction 4DPET/CT imaging is a powerful technique to assist clinicians in accurate definition of gross‐tumor‐volumes (GTV) in the lung and abdomen. We present the results of phantom studies designed to evaluate the accuracy of 4DPET/CT at reconstructing the true GTV when tumor motion and abdominal motion are reproduced based on pre‐recorded data from real patients. Methods Measurements were conducted using a 4D dynamic phantom control system capable of reproducing a time‐dependent 3D motion (GTV) and a synchronous, decoupled 1D motion (abdominal surface). The tumor and the abdominal motion trajectories programmed into the phantom are based on pre‐recorded patient data (tumor and abdominal motion). A 4ml spherical vial of FDG, representing the tumor, was used within a water‐FDG mixture adjusted to produce a target‐to‐background activity‐ratio of 8:1. The image reconstruction was done using 10 phase bins over the respiratory cycle. The patient selection was based on the amplitude of the internal marker motion (> 1cm). Results and Discussion Based on a 3DPET scan with no target motion, two methodologies were used to determine the activity threshold that recovers the active volume of the target (4ml) relative to: (1) the maximum intensity voxel; (2) the average intensity obtained from the five highest intensity voxels;. The percentage difference between the true and calculated volume was determined for each phase bin of the 4D studies. Errors as large as 45% were observed for specific patients and phases with a statistical bias towards overestimation of the target volume. However, the activity threshold based on the average intensity has shown to reduce the inter‐phase volumetric distortions. Conclusion Our results have quantified volumetric distortions that occur during 4D PET/CT imaging based on previously recorded internal target and abdominal motions in patients. These results are relevant to the accurate application of 4D PET/CT to radiotherapy target definition.