Page 62 - Quantitative Imaging of Small Tumours with Positron Emission Tomography

Background In clinical oncology positron-emission tomography (PET) is a valuable tool allowing guidance of treatment on a per-patient basis (1). Clinical decision- making using PET-CT is commonly limited to visual analysis, where local disease and the presence of nodal or distant metastases is evaluated (2,3). However, since PET is an inherently quantitative technique, it may also be used for quantitative 3 assessment of tumour metabolic, proliferative, or drug targeting characteristics (1,4,5). For quantitative PET-CT to be of practical clinical utility, metrics need to be easily extracted from static whole-body PET-CT images as performed in routine clinical practice. To this end, standardized uptake values (SUV) are typically used as simplified semi-quantitative measures of tracer uptake (6). However, pharmacokinetic modeling using dynamic PET-CT acquisitions with arterial or venous blood sampling is an essential first step to technically validate the clinical use of these simplified metrics as biomarkers of, e.g., response to treatment (4,5,7,8). As is well known, quantification of tracer distribution on PET-CT scans is hampered by several sources of error. Amongst these are attenuation, Compton scatter, random coincidences, and decay, all accounted for by contemporary image reconstruction algorithms. However, due to the inherently limited spatial resolution of PET-CT, acquired images still suffer from partial-volume effects (9). Partial-volume effects lead to spill-in and spill-out of measured activity distributions, generally resulting in net underestimations of tracer uptake, the extent of which depend on tumour size, shape, and contrast (9). Hence, partial- volume correction (PVC) is needed for accurate quantification, especially for small and/or heterogeneous lesions (9-12). In oncological studies, PVC has been predominantly applied to static PET-CT images (in contrast with brain (13-22) or cardiac (23,24) PET imaging). However, in dynamic acquisitions the activity spill-over in and from tumours due to partial-volume effects may vary over time. The impact of PVC on tumour kinetic parameter estimates could therefore differ from its impact on simplified measures of uptake. Consequently, it may not only affect absolute quantitative reads, but also validation of simplified parameters for clinical implementation. The present study aims to evaluate the impact of frame-wise parametric PVC in dynamic PET-CT studies on tumour kinetic micro- and macroparameter PVC in dynamic PET-CT 61