The initial step of such a radiotheranostic approach is to employ a targeting molecule labeled with a diagnostic radionuclide (e.g., gallium-68, technetium-99m), which is used for quantitative imaging of a tumor-related biomarker, either with PET or single photon emission computed tomography (SPECT). Radiotheranostics thus represents the epitome of personalized (precision) nuclear medicine. The PET tracer Ga-DOTA-TOC is not only suitable for imaging, but also for a so-called theranostic approach, which has gained high importance in recent years. β +: positron, e −: electron from surrounding tissue. Principle of positron emission tomography (PET). For proper diagnostics, the radiotracer should demonstrate high target-to-background ratios, whereas the absolute uptake into the targeted tissue does not necessarily have to be high. These properties enable the imaging of the patient shortly after radiotracer administration and minimize the dose to the patient, who is free to go home right after the examination is finished. Other important properties of such a PET tracer are demonstrated by a rapid uptake in targeted tissue (high specificity), a rapid clearance from non-targeted tissue, a high stability in vivo, and by an absence of host-immune response. An optimal PET tracer should be easy to produce and to radioactively label. This imaging technique is ideal to monitor molecular events in early disease stages as well as treatment response. Only if two detectors opposite to each other detect a signal within a very short coincidence time (nanoseconds), a positron decay is registered. PET systems use multiple detectors connected by a coincidence circuit that are distributed in opposite directions and encircle the patient. During this process two annihilation photons with an energy of 511 keV each are emitted in an angle of 180° ± 0.25° (see Figure 1). After a certain reach in tissue (depending on the nature of both the radionuclide and the tissue), the positron will interact with an electron and this interaction will result in an annihilation event. These radioactive nuclides decay by positron emission. For this imaging method so-called PET tracers, which are biological molecules or sometimes artificial building blocks for specific targets labeled with positron emitters (e.g., gallium-68, fluorine-18), are intravenously injected into the patient. It allows a non-invasive and quantitative imaging of cellular and molecular events in patients, giving functional information in contrast to morphological information obtained from conventional imaging techniques like computed tomography (CT) or magnetic resonance imaging (MRI). PET is a commonly used imaging technique in nuclear medicine. These kits allow decentralized tracer production and therefore enable the application of the radiotracer to patients who do not live in the vicinity of a centralized production site. a kit preparation for 68Ga-labeling of DOTA-TATE (NETSPOT TM, AAA, a Novartis Company, Saint-Genis-Pouilly, France) was approved by the FDA on 1 June 2016. Use of this kit along with an authorized 68Ge/ 68Ga-generator enables on-site preparation of Ga-DOTA-TOC even in small facilities. Also in Europe, a kit preparation for 68Ga-labeling of DOTA-TOC (SomaKit TOC ®, AAA, a Novartis company, Saint-Genis-Pouilly, France) was approved by the European Medicines Agency (EMA) on 8 December 2016. The ready-to-use 68Ga-labeled peptide was already approved in some European countries (Austria, Germany, and France) in 2016 (IASOtoc ®, IASON GmbH, Graz, Austria) and in 2018 (TOCscan ®, ITM AG, München, Germany). Holder of the marketing authorization is the UIHC–PET Imaging Center (University of Iowa Health Care (UIHC)), in Iowa, USA. On 21 August 2019, Ga-DOTA-TOC was approved by the Food and Drug Administration (FDA) for positron emission tomography (PET) imaging of somatostatin receptor (SSTR)-positive gastroenteropancreatic neuroendocrine tumors.
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