Simulation study of real-time tumor tracking by OpenPET using the 4D XCAT phantom with a realistic 18F-FDG distribution

We are developing the OpenPET which can provide an open space observable and accessible to the patient during PET measurements. In addition, we have proposed the real-time imaging system for the OpenPET which is expected to be used in PET-guided tumor tracking radiation therapy and demonstrated its tracking ability using a point source and a small OpenPET prototype. However, tumor tracking in the human body still remains as a challenging task when we use ¹⁸F_FDG which is the best available tracer for tumors because of its background activity, scatter and attenuation in the body. In this study, we assess conditions under which tumor tracking is feasible in the human body by using the 4D XCAT phantom which is a realistic 4D human whole body phantom. To simulate realistic ¹⁸F_FDG distributions, we assigned standardized uptake values (SUVs) to normal organs based on the literature. We conducted Monte Carlo simulation of a human-sized OpenPET geometry by using Geant4 Application for Tomographic Emission (GATE) ver. 6.1 assuming a measurement at 100 minutes after the ¹⁸F_FDG injection of 370 MBq and a spherical tumor with the diameter of 10 to 30 mm and SUV of 3 to 10. List-mode data were generated for each 0.5 s time frame of a respiratory cycle of 5 s. Image reconstruction was done in a frame-by-frame manner and tumor position was automatically extracted for each time frame by a pattern matching technique. Tumor movement in the 4D XCAT phantom was about 17 mm at the maximum. The mean error of the tumor positions extracted from the reconstructed images was similar to the PET image resolution when the tumor size was 20 mm or more and SUV was 5 or more. We showed that tumor tracking by the OpenPET is feasible even in the human body scale and for realistic conditions.