Target activity repeatability in a clinically relevant phantom: 18F vs 68Ga

1442 Objectives: Quantification of tumor uptake in PET imaging is commonly used to evaluate therapy response. For 18F FDG PET scans, a threshold of approximately 25% for relatively large lesions is commonly required to consider changes in uptake significant. For PET scans using radiopharmaceuticals such as 68Ga DOTATATE, the threshold for a significant difference is unknown. For novel 68Ga radiotracers, physical factors may influence the repeatability of these measurements. This study evaluates the effects of imaging time, target activity concentration, and target size on the repeatability of maximum SUV (SUVmax) and mean SUV (SUVmean) in a clinically relevant phantom for both 18F and 68Ga. The purpose is to characterize these effects and directly compare the repeatability of these radionuclides. Methods: An Esser PET phantom (Data Spectrum, Durham NC) was scanned with data acquired and reconstructed on a Biograph Horizon PET/CT scanner (Siemens Medical Solutions, Malvern PA) using both 18F and 68Ga. Data were acquired for 5 minutes in list mode using a single bed position centered over the hot cylinders. To quantify the effect of scan duration, each data set was reconstructed with scan durations of 0.5, 1, 2, 3, 4, and 5 minutes, centered temporally during the 5-minute acquisition. The scans were performed with the background activity concentration selected to mimic clinical scans with the target cylinder concentrations adjusted to result in target-to-background (T/B) ratios ranging from 1.7 to 19.8. The SUVmax and SUVmean were measured for 5 adjacent slices centered within each target cylinder. The mean, standard deviation, and coefficient of variation (COV) of the 5 slices were calculated. The effects of radionuclide type, imaging time, activity concentration, and lesion size were evaluated with respect to the COV using multivariate gamma regressions with a random intercept for each cylinder to account for repeated measurements across the different imaging times. Results: The COV for comparable scans was 41% higher and 53% higher on average for 68Ga compared to 18F when measuring SUVmax and SUVmean, respectively. For both radionuclides, increases in target size, imaging time, and activity concentration were significantly and independently associated with decreases in COV for both SUVmax and SUVmean (p<0.001). It should be noted that even for longer imaging times, COV for SUVmean and SUVmax were substantially greater for the small (8 mm) target than for larger targets. Despite the trend of higher COV found in 68Ga scans, the COV was significantly reduced at high T/B. At a clinically relevant T/B of 19.8, the COV for 68Ga for both SUVmax and SUVmean was well within the typical range seen for 18F, with regression estimated mean values of 0.026 for SUVmax and 0.028 for SUVmean for the small 8 mm targets with 3 minutes of imaging time. Conclusions: The physical properties of 68Ga are suboptimal when compared to 18F, resulting in higher variability of uptake metrics. Target size, imaging time (counts), and T/B are independent factors contributing to COV and increasing these significantly lowers the COV. However, this modest difference may be overcome if radiotracer target uptake is very high, as commonly seen in 68Ga DOTATATE studies. This may have clinical implications for reproducibility of uptake metrics for novel 68Ga radiotracers with high tumor uptake.