Page 17 - Quantitative Imaging of Small Tumours with Positron Emission Tomography
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                                Chapter 1 metastases at biochemical recurrence, but may also be useful in assessing response to chemotherapy (45). A limitation of [18F]FCH is that it is less suitable for detection of prostate cancer metastases at low PSA levels (46). [18F]FDHT Prostate cancer progression is driven by the androgen receptor (AR) through several mechanisms, designating the AR as an attractive target for molecular imaging (20). Hence, [18F]-fluorodihydrotestosterone ([18F]FDHT) was developed as a radiotracer allowing for visualization and quantification of tumour AR expression and its heterogeneity in-vivo. [18F]FDHT was successfully used in early phase clinical trials to demonstrate AR-specific drug binding (47,48). Also, interlesional [18F]FDHT heterogeneity seems to be predictive for survival after AR-targeted treatment (15). [18F]DCFPyL PSMA is a type II transmembrane glycoprotein that is overexpressed in prostate cancer cells in all stages of the disease. Using [68Ga] or [18F]-labelled PSMA- ligands, such as [18F]DCFPyL, this characteristic has been exploited with high success for prostate cancer PET imaging (49). Mainly in the setting of biochemical recurrence, many reports have shown that PSMA-ligand PET has superior lesion detection rates than conventional modalities, with a resulting high impact on clinical management (27). Nonetheless, its place in primary staging of the disease has yet to be established. As performance of visual image assessment seems to be lacking, quantitative approaches of image analysis could be of high benefit in this setting (50,51). Quantification on PET The gold standard for quantification of radiotracer uptake in tumours is full pharmacokinetic analysis (52). Such analysis requires dynamic PET acquisitions over a certain time frame, with blood sampling from an arterial line or venous samples to derive a tracer input function (17,44,53,54). Full quantitative analysis on PET yields kinetic rate constants that can be used to calculate macroparameters, such as the volume of distribution (VT), binding potential (BP), or Ki, which are assumed to represent the biological behavior of radiotracers (52). Unfortunately, dynamic PET acquisitions are not feasible in clinical practice due to the limited field of view (FOV), the long duration of imaging, and the need for blood 16 


































































































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