The imaging of the hypoxic microenvironment in tumorous tissue using PET/CT and PET/MRI

Tissue hypoxia becomes the driver of the micro-environmental transformation of the malignant tumorous tissue in majority of solid tumors. Even more, during the development of solid tumors, a large amount of nutrients is consumed due to rapid proliferation of tumor cells. The way of use of nutrients could be assessed using the probes during positron emission tomography (PET), the most frequent is the use of ¹⁸F-fluorodeoxyglucose (¹⁸F-FDG) demonstrating the turn-over of glucose exploitation in a manner of Warburg phenomenon. Even tumor cells can undergo metabolic reprogramming by changing the expression of glycolysis-related proteins, development of hypoxic condition increases glucose uptake to promote their growth. The hypoxic conditions are stimulation also the development of drug and/or radiation resistance, thus, the identification of the hypoxic tissue could play an important role in therapy planning. The process that enables use of hypoxia specific PET radiopharmaceutical starts when it enters cell by passive diffusion. Intracellularly, a model substance - ¹⁸F-fluoromisonidazol (¹⁸F-FMISO) is reduced by nitroreductase enzymes to become trapped in cells within reduced tissue oxygen partial pressure. The identification of the hypoxic tissue aids to plan the target to radiotherapy in squamous cell carcinomas of head and neck or cervical carcinoma, when ¹⁸F-FMISO-PET/CT is used as a planning procedure for a target volume, while ¹⁸F-FDG-PET/MRI is being the local and whole body staging procedure. Hypoxia imaging helps to identify the high-grade component of an astrocytic tumor before stereotactic biopsy.