Page 98 - Quantitative Imaging of Small Tumours with Positron Emission Tomography

Discussion We investigated how accuracy, precision, and lesion detectability of analogue whole body [18F]FDHT PET-CT are affected by image count statistics and reconstruction protocol, to optimize imaging protocols for research and clinical use. Reducing counts by 50% introduced <20% SUV intrascan variability for EARL1 images, which only increased test-retest variability to a small extent. Improving image spatial resolution by adhering to EARL2 guidelines might reduce the size- dependent bias in SUV, but it hampers repeatability and increases sensitivity to count statistics. Lesion detectability is only slightly affected by reduced counts and 4 only marginally increased by resolution modelling. SUVs of 50% count scans correlated highly with SUVs of 100% count scans, indicating accuracy is preserved at lower count statistics. However, when comparing split scans directly a variability in SUV ranging 8.5% (SUVmean EARL1) to 22.2% (SUVmax EARL2) was observed. Hence, while SUV accuracy is maintained at low counts, its precision might be hampered. Still, test-retest variability only increased to a small and non-significant extent, which indicates that the statistical Poisson image noise is a minor determinant of SUV repeatability for [18F]FDHT. SUV repeatability of oncological 18F-tracers (ie. [18F]FDG, [18F]- fluorothymidine, [18F]-fluoromethylcholine, [18F]FDHT) ranges between 10-30%, yielding 30% as the preferred upper threshold for SUV variability for use in e.g. response monitoring studies (27-30). As expected, repeatability of SUVmax was most affected by count reduction and EARL2 reconstruction, yielding RCs >30%. In contrast, SUVpeak seemed to be robust to both count statistics and reconstruction protocol, yielding an RC of approximately 30% after count reduction, which was even lower (27.9%) when only lesions >4.2mL were considered. The improved repeatability of SUV when excluding small lesions seems a direct consequence of the size-dependency of intrascan variability at reduced counts (Supplemental Figure 1). Note that test-retest variability of [18F]FDHT can be even lower when evaluating only selected target lesions, or analysing on a patient- instead of lesion- basis (12). In the current study, all avid lesions were primarily included to avoid selection bias and also evaluate the effect of count reduction on smaller and less avid lesions. Between SUV normalizations, differences in test-retest variability were observed, with larger variability in SUVauc-pp (>30%) compared SUVbw. While [18F]FDHT PET repeatability 97