D3 PET and nuclear medicine (PET/SPECT)

Topic

Positron emission tomography detects positron emissions from radionuclides administered to the patient that bind to molecules of biological interest within the body.

The radionuclide most commonly used for this purpose in oncology is fluorodeoxyglucose labeled with radioactive fluorine—a molecule resembling glucose. Cells with a high rate of glycolysis—a characteristic of tumor cells known as the Warburg effect—take up this labeled fluorodeoxyglucose just as they would normal glucose; however, they cannot fully metabolize it, causing the labeled fluorodeoxyglucose to become trapped inside the cells.

The amount of labeled fluorodeoxyglucose trapped in a specific area of ​​the body—measured as the standardized uptake value—is proportional to that area's metabolic activity. This metabolic activity, in turn, serves as a proxy for both the tumor's growth rate and its cell count in that region: the faster the tumor grows and the more tumor cells it contains, the more labeled fluorodeoxyglucose becomes trapped and the higher the standardized uptake value.

However, positron emission tomography does not clearly visualize the patient's anatomy; it only reveals areas of elevated metabolic activity. Consequently, it is combined with computed tomography in a single scan, integrating the functional image of metabolism with the morphological image of anatomy. This integration has become the current standard in oncology because it allows for the precise anatomical localization of the areas of high metabolic activity detected by positron emission tomography. There is also another functional imaging technique based on the same principle of detecting radionuclide emissions, but using conventional gamma emitters instead of positrons: single-photon emission computed tomography (SPECT), which typically employs a radioactive isotope known as technetium-99m. While SPECT offers lower resolution than positron emission tomography (PET), it is available at far more facilities; these two functional imaging techniques share the same underlying principle of radionuclide detection but differ in the trade-off between resolution and availability. In either form, nuclear medicine provides oncology with something that neither computed tomography (CT) nor magnetic resonance imaging (MRI) can offer on its own: a direct image of tumor metabolism, rather than just its shape.

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