Page 99 - Quantitative Imaging of Small Tumours with Positron Emission Tomography

Chapter 4 SUV normalized to AUC-PP correlates better with reference pharmacokinetic parameters than SUV normalized to bodyweight (19), deriving it is more technically demanding and less precise compared to more simple factors such as dose per bodyweight, making it less suitable for multicentre studies. Hence, a trade- off between accuracy, precision, and ease of use has to be made when selecting the preferred SUV normalization. For example, while SUVpeak normalized to bodyweight had a RC of 30% at half of counts, it exceeded 30% when normalizing to AUC-PP rendering it unfit for response assessment. Partial-volume effects generally result in volume-dependent underestimations of tumour SUV and possibly hamper lesion detectability (31). Correcting for PVE in the reconstruction algorithms might be particularly important in [18F]FDHT due to the high frequency of small (e.g. <4.2 ml) detected lesions. Novel reconstruction algorithms incorporating the PSF either within or after reconstruction have been proposed to improve image resolution (17). The EARL2 standards have adopted these algorithms as a step forward in scanner calibration harmonization between centres (15). However, PSF reconstructions are known to suffer from noise propagation and image artefacts (e.g. Gibbs phenomenon resulting in edge overshoot), which might lead to misinterpretation regarding treatment effects (17,18,32). Indeed, we observed that repeatability was worse for the EARL2 reconstruction with higher sensitivity to count statistics, resulting in a higher minimal detectable change for response assessment. Previous reports argued that PSF reconstructions should be used for qualitative purposes (i.e. lesion detection), and that non-PSF images (such as EARL1) should be used for tumour quantification (18,33). However, Quak et al. found that with additional image filtering the higher lesion detection and image resolution of PSF images do not need to be impaired in order to meet the EARL criteria (34). In the present study we observed a very small increase in lesion CNR when PSF was applied. This will not likely result in clinically relevant different conclusions regarding the extent of disease or intrapatient heterogeneity (Figure 1) due to the vast amount of detected lesions (336 lesions in 12 patients). The small reduction in CNR by <5% after count reduction is also not likely to have clinical consequences (Figure 1). This corresponds to [18F]FDG PET-CT data in several cancer types, where reducing acquisition time from 3 to 1.5min per bed position reduced image quality, but did not impair lesion detection rates (13). Another factor affecting image count statistics is the injected tracer dosage. In the present cohort, patients received a relatively low dosage compared to other 98