Page 163 - Quantitative Imaging of Small Tumours with Positron Emission Tomography

Chapter 7 and increased sensitivity (Table 1). However, when applying cut-offs of 2.55 and 2.8 (as derived from ROC analysis) for uncorrected and PVC data, respectively, PVC increased sensitivity from 72% to 94% while specificity remained 91%. This provides further demonstration that PVC may indeed increase diagnostic accuracy when SUV cutoffs are adequately adapted for this correction. Obviously, each proposed threshold requires external validation. Another explanation of the limited impact of PVC on diagnostic accuracy as published in the literature may relate to the size spectra of included lesions, with the distribution of benign and malignant lesions therein. When performing PVC analysis on all lesions, both large and small simultaneously, the overall impact of PVC on diagnostic accuracy will be diluted. Indeed, several studies demonstrated high impact of PVC on accuracy for small lesions (when stratifying for lesion size), but less so when including all lesions regardless of size (15,29). Therefore, we suggest that investigators stratify diagnostic performance results for lesion size in secondary analyses. However, since partial-volume effects are not merely size-dependent, but are also affected by lesion contrast and shape, reliable classification of lesions that are (most) affected by partial-volume effects will be difficult. In our previous simulation study we observed that for high contrast spherical lesions partial-volume effects started to occur below 3cm diameter (8). A practical approach for stratification would thus be to stratify results using a 3cm lesion diameter or a 14mL metabolic volume cutoff (corresponding to a 3cm diameter sphere). Even though larger lesions may also be somewhat affected by partial-volume effects depending on their shape and contrast, such a size cutoff will ensure that lesions that are most affected by partial-volume effects are separated. Another approach would be to plot the percentage increases in SUV after PVC as function of metabolic tumour volume to determine an appropriate size cutoff for stratification of results within studies (not possible when applying the RC method). Regarding visual nodal staging, PSF reconstruction did not significantly alter accuracy, but tended to increase sensitivity in lung, breast, and colorectal cancer (Table 7.2) (31-33). This may be attributed to improved qualitative reads, improved (small) lesion detection, and higher diagnostic confidence (31-33). Therefore, it may be worthwhile to validate these higher-resolution reconstruction algorithms for use in clinical practice, especially for detection of small lymph node metastases and lesions embedded in high background activity such as in liver or mediastinum. However, PFS reconstructions may suffer from Gibbs artefacts 162