2.2.1 Choline. Choline is a precursor for phosphatidylcholine, a phospholipid that is integrated in the cell membrane making it a marker for cell membrane turnover and metabolism . 11C-Choline was first used as a PET tracer in 1997 . Since then many studies have been performed with 11C-Choline and later 18F-Choline (FCH) having approximately the same properties as 11C-Choline, but with a longer half-life (110 min vs. 20 min) making it more versatile in every-day use. A recent study from our department tested the use of FCH-PET/CT in detecting lymph node metastasis in 210 patients with PCa . The study concluded that FCH-PET/CT was not ideally suited for lymph node staging but could give additional information about bone metastasis. Several other investigators have also implied this , . A likely reason for the better detection of metastasis to the bone is that bone metastasis in an early stage is actually bone-marrow metastases rather than bone matrix metastasis and that it most likely that bone-marrow metastases proceed to bone matrix metastases. These will not be detectable by WBS (and probably not by CT and MRI either). Larger prospective series on the use of FCH-PET/CT for imaging of bone metastasis in PCa-patients have not yet been produced.