Page 110 - Quantitative Imaging of Small Tumours with Positron Emission Tomography

Introduction Prostate cancer (PCa) is the second most common cancer in men worldwide, with an estimated annual number of deaths over 350.000 (1). Prostate-Specific Membrane Antigen (PSMA) Positron Emission Tomography (PET) is increasingly used for PCa diagnostics (2). PSMA is a class II trans-membrane glycoprotein that provides a valuable target for radiolabelled imaging, as its expression is upregulated in malignant prostate cells and associated with aggressive disease characteristics (3). Due to larger availability, 68Gallium-labeled PSMA tracers have been studied most frequently to date, demonstrating high detection rates for metastatic disease (2,4). Alternatively, 18Fluorine-labeled tracers have been developed, including [18F]DCFPyL (2-(3-(1-carboxy-5-[(6-18F-fluoro-pyridine-3-carbonyl)-amino]- pentyl)-ureido)-pentanedioic acid), a second-generation small-molecule ligand 5 that strongly binds to PSMA (5,6). The 18F-radionuclide provides for PET-images with a higher resolution compared to 68Ga, due to a shorter positron range and higher positron yield (2). Quantitative analysis of PSMA uptake may be used to predict or evaluate treatment response, as changes in PSMA uptake over time may indicate response to treatment or progression of disease (7-9). Recently, we performed a full pharmacokinetic analysis of [18F]DCFPyL to validate simplified methods for tumour uptake quantification. Tumour-to-Blood Ratios (TBR; tracer activity concentration in the tumour normalized to the whole blood activity concentration on PET) were found to best describe the tumour tracer uptake (10). For reliable use of quantitative PSMA PET metrics in clinical practice, it is important to determine their repeatability. Only changes that exceed random variability should be interpreted as treatment response or disease progression. To the best of our knowledge, this is the first study reporting on the day-to-day variability of [18F]DCFPyL uptake in PCa lesions. The aim of this study was to evaluate the repeatability of quantitative [18F]DCFPyL PET/CT measurements in patients with metastatic PCa. [18F]DCFPyL PET repeatability 109