By: Dan Sperling, MD

This is the last in a series of five articles about PET scans and their application in prostate cancer.

In today’s imaging practice, PET (positron emission tomography) is usually done in combination with CT (computed tomography) scans. CT depicts the anatomy, while PET highlights abnormal function of tissues within the anatomic structures. When radiotracers concentrate in prostate cancer tumors, they are registered by PET and superimposed on CT, so the exact tumor location shows up on the combination scan. The combination gives a higher degree of accuracy while avoiding the inconvenience of having to undergo two separate scanning sessions. As discussed in earlier articles, the two most commonly used radiotracers for detecting prostate cancer by PET/CT scans are choline C-11 (also called C11-choline) and carbon-11 acetate (C-11 acetate). There are additional tracers either used less commonly, or being explored for their potential to reveal prostate cancer in a more advantageous way. In fact, the potential for PET to track specific biological processes anywhere in the body is virtually limitless, so new radiopharmaceuticals are constantly being synthesized and tested.

A consideration in choosing one tracer over another has to do with its half-life, or rate of decay. The shorter the half-life, the faster the radioactivity decays. With isotopes such as carbon-11 (C-11) bonded to either acetate or choline, the half-life is about 20 minutes, meaning it loses half of its radioactivity roughly every 20 minutes. This does not give much time to get the newly produced radiotracer from the cyclotron into a patient’s body by intravenous injection before it begins to lose its maximum effectiveness as an imaging agent. Therefore, a tracer with a longer half-life offers more time to transport it from one location to another, making it more available to facilities not directly on-site with the cyclotron.

Among tracers with longer half-life that most demonstrate affinity with prostate cancer cells are:

1. ¹⁸F-choline (or F-18 choline), with a half-life of approximately 110 minutes, appears able to detect metastatic prostate cancer best in patients with higher PSA value (>10).[i] However, others have found that even with PSA < 5 F-18 choline picks up metastatic lesions.[ii] One study demonstrated value in locating early metastasis of prostate cancer to the bone.[iii]
2. ¹⁸F-fluoroacetate, which also has a half-life of about 110 minutes, preferentially concentrates in tumor cells; it appears to be more excretable from the liver and bladder than C-11 acetate so there is less risk of radioactivity lingering in the body. More human studies need to be done.

Novel radiotracers such as C-11 and F-18 1-aminocyclobutane-1-carboxylic acid and many others are undergoing testing.

The range of prostate cancer applications for PET or PET/CT is justified by the growing body of literature in these areas:

• Detecting and staging primary PCa
• Detecting metastatic PCa locally (prostate bed), regionally (lymph nodes and pelvis) and distantly (liver, lung, skeletal, etc.)
• Detecting recurrence following treatment, even when PSA is low
• Assessing the effects of systemic treatment (hormone ablation, chemotherapy).

PET/CT for prostate cancer offers numerous benefits. It can demonstrate abnormal tissue processes within specific anatomic structures, even identifying changes at the cellular level, something that conventional ultrasound, CT and MRI (with the exception of multiparametric MRI) cannot do as well. Such information aids in diagnosis and treatment planning. When weighed against the benefits, risks appear small, but because of the low-dose exposure to radiation, patients should be informed of any possibility of long-term adverse effects. Allergic reactions to radiopharmaceuticals (tracers) may occur, but are very rare and usually mild. While certain types of PET imaging for other cancers can be time consuming, this is typically not so for prostate cancer. In the case of C-11 choline or C-11 acetate, the process is efficient and the scan complete in less than a half hour. However, patients must arrive on time because of the short half-life of these tracers.

There is a growing and evolving role for PET/CT detection of prostate cancer. Work is now being done to develop PET/MRI technology, as the magnet-based detection of MRI has no ionizing radiation, thus reducing a patient’s exposure to radioactivity. In short, PET/CT has a very health future in the detection, diagnosis, treatment planning and monitoring of prostate cancer.

[i] Calabria F, Chiaravalloti A, Tavolozza M, Ragano-Caracciolo C, Schillaci O. Evaluation of extraprostatic disease in the staging of prostate cancer by F-18 choline PET/CT: can PSA and PSA density help in patient selection? Nucl Med Commun. 2013 Aug;34(8):733-40.

[ii] Heinisch M, Dirisamer A, Loidl W, Stoiber F et al. Positron emission tomography/computed tomography with F-18-fluorocholine for restaging of prostate cancer patients: meaningful at PSA < 5 ng/ml? Mol Imaging Biol. 2006 Jan-Feb;8(1):43-8.

[iii] Beheshti M, Vali R, Waldenberger P, Fitz F et al. The use of F-18 choline PET in the assessment of bone metastases in prostate cancer: correlation with morphological changes on CT. Mol Imaging Biol. 2010 Jan-Feb;12(1):98-107.