Utility of respiratory-gated small-animal PET/CT in the chronologic evaluation of an orthotopic lung cancer transplantation mouse model

Our aim in this study was to clarify the effects of respiratory-gated PET in the evaluation of lung cancer according to the 18 F-FDG uptake in an orthotopic transplantation mouse model. We created such a model, and we performed PET/CT. The mice were divided into two groups according to tumor volume: a small-tumor group (<20 mm 3 ) and a large-tumor group (>20 mm 3 ). We reconstructed the following conditions based on list-mode data: non-gated (3D) images and gated (4D) images, divided based on the respiratory cycle (expiration phase, stable phase, and inspiration phase). We calculated the maximum standardized uptake values (SUV max ) in each phase. We used the % difference [= (4D SUV max − 3D SUV max )/3D PET SUV max × 100 (%)] to evaluate the differences in the 4D SUV max and 3D SUV max . The 4D SUV max values were significantly higher than the 3D SUV max , regardless of the tumor size. The % difference for the small tumors was greater than that for the large tumors, and it was highest in the stable phase. We conclude that the SUV max in the stable phase under respiratory-gated PET are the most reliable. The SUV max observed under non-gated PET are considered to be more frequently underestimated in cases involving small tumors than in those involving large tumors. In the chronologic study evaluating the time course of tumor development, the size of the tumor is small in early stage, and respiratory-gated PET is effective in reducing the underestimation of such tumors caused by respiratory motion.