European Nuclear Medicine Guide
[18F]fluorocholine and [methyl-11C]choline: radiolabelled choline integrates to phospholipidic membranes. The uptake depends on choline kinases, which are upregulated in proliferating cells, including in hyper-functioning parathyroid glands. Moreover, the increased PTH may upregulate phospholipid-dependent choline kinases [152,153].
L-[methyl-11C]methionine: radiolabelled methionine enters into the cells by passive diffusion and through several neutral amino acid transporters, mainly the LAT1 and LAT2, which are located on cell surface. Several processes contribute to induce an over-expression of LATs and increase amino acid transport, probably including increased PTH synthesis [154,155].
To date there is no clear consensus on the optimal imaging modality and sequencing for the localization of hyper-functioning parathyroid glands in primary hyperthyroidism (HPT).
Radiolabelled choline derivates and methionine PET/CT are indicated for the identification of hyper-functioning parathyroid glands in the pre-surgical setting, in order to facilitate surgery and to allow a mini-invasive approach.
These imaging techniques may be particularly indicated as a second-line modality in case of negative or discordant standard techniques (([99mTc]Tc-sestamibi scintigraphy and neck US). Moreover, they may be useful in addition to standard imaging when a multiple gland hyperplasia is suspected, in case of persistent or recurrent HPT after surgery, and in case of normo-calcaemic HPT [155–159].
Pregnancy is a relative contra-indication.
[18F]fluorocholine in a metanalysis analysing 14 studies, sensitivity ranged from 85% to 100% and from 71% to 100% on a per-patient and per-lesion analysis respectively, with pooled sensitivities of 95% (95% CI: 92–97%) and 92% (95% CI: 88–96%) respectively [24].In another metanalysis, specificity was evaluated in 7 studies and varied between 95% and 100% [156].
[methyl-11C]choline: few data are available. A study found a concordance between PET/CT and surgical findings in 24 of 27 patients who underwent surgery [160].
L-[methyl-11C]methionine: in a metanalysis analysing 24 studies, sensitivity for the detection of a lesion in the correct quadrant ranged from 44% to 91%, with a pooled sensitivity of 77% (95% CI 71–84%). In three studies that included patients with negative or inconclusive conventional imaging pooled sensitivity was 81% (95 % CI 70–91 %) [161].
There is no consensus about the activities to administer, image acquisition time and number of acquisitions. In available studies, administered activities and image acquisition times were:
[18F]fluorocholine: 100-370 MBq or 1.5-3.2 MBq/kg. Several authors performed two image acquisitions: an early image (0-15 minutes after injection), followed by a late image (30-120 minutes after injection). Other authors performed a single image acquisition few minutes to 1 hour after tracer administration [159].
[methyl-11C]choline: 370 MBq, with immediate TC acquisition followed by PET acquisition [160].
L-[methyl-11C]methionine: 370-1100 MBq. In available studies, authors performed a single acquisition 0-40 minutes after tracer injection (mostly 10-20) [155–161].
[18F]fluorocholine: the effective dose is 20 µSv/MBq. The organ with the highest absorbed dose is the kidney: 97 µGy/MBq [162]. The range in effective dose per procedure is 2.0-7.4 mSv.
[methyl-11C]choline: the effective dose is 4.4 µSv/MBq. The organ with the highest absorbed dose is the pancreas: 29 µGy/MBq [163]. The effective dose per procedure is around 1.6 mSv.
L-[methyl-11C]methionine: the effective dose is 8.2 µSv/MBq. The organ with the highest absorbed dose is the bladder wall:92 µGy/MBq [164]. The range in effective dose per procedure is 1.9-5.2 mSv.
The radiation exposure related to the CT scan carried out as part of a PET/CT study depends on the intended use of the CT study and may differ from patient to patient.
Caveat
“Effective Dose” is a protection quantity that provides a dose value related to the probability of health detriment to an adult reference person due to stochastic effects from exposure to low doses of ionizing radiation. It should not be used to quantify the radiation risk for a single individual associated with a particular nuclear medicine examination. It is used to characterize a certain examination in comparison to alternatives, but it should be emphasized that if the actual risk to a certain patient population is to be assessed, it is mandatory to apply risk factors (per mSv) that are appropriate for the gender, the age distribution and the disease state of that population."
Any focal uptake posterior to the thyroid lobes or in the upper mediastinum should be considered as suspect. CT images should be evaluated in search for corresponding nodule(s).
[18F]fluorocholine and [methyl-11C]choline: major pitfalls include false positive results in case of inflammatory lesions, benign tumours and malignant tumours (including thyroid cancer, parathyroid carcinoma, and lymph node metastases). False negative results may occur in case of relapse of hyperparathyroidism after parathyroid surgery, multiple gland hyperplasia (for example in MEN I), ectopic glands, and intra-thyroid parathyroid adenomas [156].
L-[methyl-11C]methionine: false positive results can occur in case of benign and malignant tumours (including head and neck tumours and thyroid adenoma). False negative results may occur in case of multiple gland hyperplasia, small adenomas, and diffuse thyroid disease [161,165].