By: Dan Sperling, MD

This is the second in a series of five articles about PET scans and their use in detecting prostate cancer.

Choline C-11 (also called C11-choline) is a radiotracer that is injected into a patient prior to positron emission tomography (PET) scanning. As explained in the first article in this series, low-dose, manmade radioactive isotopes are bonded with biologically active molecules. This is done by replacing a few of their normal carbon atoms with radioactive carbon atoms. These molecules are now “labeled” or tagged with a tiny amount of radioactivity, and are called radiotracers. They are drawn to certain tissues in the body, where they become concentrated.

Because prostate cancer cells are hungry for anything that will fuel their rapid growth, it is this feature that allows the tumor to be imaged. Prostate cancer eagerly takes up choline, which is a naturally occurring part of the B-vitamin complex. The tumor cells need nutrients to multiply quickly, and they use choline as a kind of building block. It collects in any prostate cancer tumors, whether located in the prostate, lymph nodes, or more remote locations. When choline is labeled with a type of radioactivity called C-11, the PET scanner picks up the exact location of the tracer concentrations. As 3-dimensional images of the target regions are processed, the tumors are shown as brightly lit spots or areas.

Choline C-11 has a rapid rate of decay: it loses half of its radioactivity every 20 minutes, so the tiny amount of original radioactivity is soon gone from the patient’s body. As the radiation fades, the body continues to process the choline just as it would if it had come from a natural source such as broccoli, eventually eliminating any waste.

Choline C-11 PET is FDA-approved for the detection of primary, recurrent and metastatic prostate cancer. The most common use of choline C-11 PET is, in fact, identifying cancer that has come back after treatment, including any spread to the lymph nodes or beyond. It is particularly useful in monitoring patients who are on androgen deprivation therapy (ADT, or hormone blockade) because of recurrent cancer; as tumors shrink in size, that reduction can be assessed by the choline C-11 PET images.

A recent study out of Bologna, Italy evaluated what impact choline C-11 PET might have on treatment planning for men diagnosed with recurrence.[i] 150 men with biochemical recurrence (rising PSA) after radical prostatectomy were intended for either salvage radiation to the prostate bed (95 patients) or ADT (55 patients). All were scanned with this type of imaging, and as a result of the findings, 70 of the patients (46.7%) had their treatment plans changed. In terms of exact numbers, the scan was positive for cancer in 109 patients (72.7%). Of those, 64 had local relapse (prostate bed/nearby lymph nodes); 31 patients had distant relapse (more distant lymph nodes/bone lesions); and 14 had both local and distant relapse. Image findings were confirmed either by lab analysis of tissue (17.3% or cases) or by correlative imaging/clinical follow-up (82.7% of cases).

With prostate cancer recurrence, early detection allows the greatest range of appropriate treatment options. Work done at the Mayo Clinic (Rochester, MN) showed that choline C-11 PET detected metastatic prostate cancer lesions in about 1/3 of men in whom other types of imaging missed the cancer; in addition, the false positive rate (registering the presence of cancer when none is there) is about 5%.[ii]

Subsequent articles will discuss other PET radiotracers that are being studied with regard to prostate cancer, as well as “fusion” or co-registration imaging such as PET-MRI and PET-CT.

[i] Ceci F, Herrmann K, Castellucci P, Graziani T, Bluemel C et al. Impact of 11C-choline PET/CT on clinical decision making in recurrent prostate cancer: results from a retrospective two-centre trial. Eur J Nucl Med Mol Imaging. 2014 Aug 15. [Epub ahead of print]

[ii] http://www.mayo.edu/research/discoverys-edge/early-warning-system-recurrent-prostate-cancer