**6. Choline**

**Figure 6.** FET PET/CT (*left-sided panels*) and pre/post intravenous gadolinium MRI (*right-sided panels*) in a patient with

FDOPA was originally developed for imaging the DOPA-decarboxylase pathway in Parkinson's disease and other neurodegenerative diseases. It is a fluorinated form of L-DOPA (**Figure 7**) which is used to increased dopamine concentrations in the treatment of Parkinson's disease. FDOPA has since been shown to be a marker of amino acid transport in

FDOPA uptake has been shown to correlate with tumour proliferation and grade [42], and more accurate than FDG for evaluating low-grade tumours and distinguishing tumour

It also accumulates in neuroendocrine tumours (NETs) such as phaeochromocytomas and

Thymidine (T) is the pyrimidine deoxynucleoside in DNA that pairs with deoxyadenosine (A). F-18 fluorothymidine (FLT) is a thymidine analogue (**Figure 8**) and a substrate for thymi‐

two small cerebral melanoma metastases (*red and blue arrows*).

**Figure 7.** Chemical structure of F-18 fluorodihydroxyphenylalanine.

recurrence from radiation necrosis [43].

paragangliomas.

**5. Fluorothymidine**

**4.3. Fluorodihydroxyphenylalanine**

190 Neurooncology - Newer Developments

brain tumours and metastases.

Choline is a water-soluble B-complex vitamin (**Figure 9**), normally found in blood, which is phosphorylated and subsequently integrated into lecithin, a component of cell membrane phospholipids. Malignant tumour cells demonstrate increased proliferation which results in increased cell membrane turnover and a greater demand for cell membrane components such as choline.

**Figure 9.** Chemical structure of choline.

Like the amino acid radiotracers, PET imaging with choline offers excellent delineation of tumour from brain with a 10:1 contrast ratio achievable within 5 min of radiotracer injection [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 differentiating radiation necrosis and tumour recurrence [47].

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

It has also been investigated extensively forimaging in prostate cancer, with some studies also suggesting a potential role in oesophageal and lung cancer [46, 48–50].
