**7.2. Fluoroazomycin**

[46]. Higher choline uptake generally corresponds with more malignant tumours, and choline PET also appears promising for radiotherapy planning because of more precise delineation of biological target volume [47]. It also has higher accuracy than FDG PET and MRI for the

Choline can be radiolabelled with C-11 or F-18, permitting its use in centres without an on-site

It has also been investigated extensively forimaging in prostate cancer, with some studies also

Hypoxia is an important factor in the malignant progression of tumour and its resistance to therapy. The majority of hypoxia PET radiotracers belong to a group of compounds known as nitroimidazoles that freely cross the blood–brain barrier, enter cells by diffusion and are subsequently reduced by nitroreductases at a rate inversely proportional to oxygen tension. Thus, in an oxygen-rich environment, they are able to diffuse back out ofthe cell again, whereas under hypoxic conditions, they are reduced and become irreversibly trapped in the cell. Anotherfavourable property ofthe nitroimidazoles is theirrapid equilibration within the brain

F-18 fluoromisonidazole (FMISO) was the first of the nitroimidazole radiotracers (**Figure 10**) to be developed for imaging with PET and has been widely used in preclinical and clinical

**Figure 10.** Nitroimidazoles are the most commonly used hypoxia PET radiotracers. FMISO: fluoromisonidazole; FA‐

differentiating radiation necrosis and tumour recurrence [47].

suggesting a potential role in oesophageal and lung cancer [46, 48–50].

cyclotron.

**7. Hypoxia radiotracers**

192 Neurooncology - Newer Developments

**7.1. Fluoromisonidazole**

studies.

parenchyma independently of perfusion.

ZA: fluoroazomycin; FETNIM: fluoroerythronitroimidazole.

F-18 fluoroazomycin (FAZA) is a second-generation nitroimidazole derivative (Figure 10) with more favourable pharmacokinetics than FMISO. FAZA demonstrates faster clearance of unbound radiotracer from non-hypoxic areas (thereby resulting in shorter waiting time for imaging) and improved biodistribution (does not cross the intact blood–brain barrier due to its increased hydrophilicity). Consequently, there is improved hypoxia-to-normoxia contrast, and FAZA shows considerable promise and is expected to overcome the disadvantages of FMISO for imaging hypoxia in brain tumours.
