Many men with prostate cancer at some point after their diagnosis get a PET scan to see where, if at all, cancer has spread.

However, it seems like most people don’t know how PET scans work.

Let’s talk about that…

What is a PET scan?

A PET scan (positron emission tomography (PET)) is an imaging medical exam to determine where the disease is in your body.

In the case of cancer, where cells have a high metabolic rate, a radiotracer is injected into the vein or by swallowing where then the diseased cells sort of gobble up the tracers which causes the cancer cells (which typically have a higher metabolic rate than normal cells) to “light up” on imaging.

The lit up areas indicates where cancer cells are located, if there is a recurrence after treatment, and if treatment is working.

How are PET scans Different than CT scans and MRI?

PET scans show metabolic changes at a cellular level where CT scans (computed tomography)  and magnetic resonance (MRI) don’t, therefore, PET scans show if there is a recurrence of cancer earlier than other scans.

The fears many people have with Xrays and CT scans of exposing the body to excess radiation ( which can also be associated with cancer) do not apply to PET scans.

However, it is possible that you can be allergic to a radiotracer.

What Radiotracers are used in Prostate Cancer?

There are numerous radioactive tracers used during a PET scan that helps locate problem areas in the body after prostate cancer.

The most common is a sugar tracer called F-fluoro-2-deoxy-D-glucose (FDG) as a result of the Warburg effect

Prostate cancer cells vary greatly between slow moving and fast moving cancer cells.

PET Scans using FDG is not useful in detecting primary organ-confined prostate cancer, detecting local recurrence after radical prostatectomy, or in differentiating between post-operative scar and local recurrence for a few reasons.

Prostate cancer is slow growing and may not have a high metabolic rate, which results in low FDG uptake. In addition, FDG-PET cannot reliably differentiate between benign prostate hypertrophy and cancer, and the uptake of the tracer does not correlate with the tumor grade or stage

This glucose (FDG) tracer is useful in detecting bone and soft-tissue prostate cancer metastases, although it is less sensitive, therefore not as good as a bone scan for prostate cancer

Lastly, FDG tracer are now less favorable in use for prostate cancer compared to others discussed below.

Choline

Choline is a nutrient important for human life. It is also used as a radioactive tracer in prostate cancer in the form of 11C-choline and 18F-fluorocholine. Numerous studies have reported choline based PET scans to be up to 85% sensitive to recurrent prostate cancer in several areas of the body including lymph nodes.

Choline radiotracer PET scans are not widely available in the United States.

ProstaScint (indium 111 capromab pendetide)

Prostascint is a radiolabeled monoclonal antibody that binds to prostate-specific membrane antigen(PSMA.)

The positive predictive value ( ability of this test to show a positive response) of ProstaScint is only 25 to 50%, as inflammation and other anatomical changes from surgery may falsely read as a positive scan

Additionally, men with a positive scan after RT have no difference in progression-free survival compared with those with a negative scan.

Axumin

Fluciclovine is a synthetic amino acid (l-leucine) as a radiotracer and was approved in May 2016 for PET imaging in men with suspected prostate cancer recurrence.

Fluciclovine is preferentially taken up by prostate cancer cells and gliomas via specialized amino acid transporters and is commonly known by its trade name Axumin.

Amino acid transporters such as ASCT2 play a critical role in amino acid metabolism in prostate cancer cells. ASCT2 is an important transporter of glutamine, which is known to be an essential tumor nutrient and has been implicated in cancer signaling pathways

Fluciclovine is predominantly transported by ASCT2 and transports in a manner similar to glutamine

Unlike glutamine, however, 18-F fluciclovine does not undergo additional metabolism in the cell, which lends to its intracellular accumulation particularly in prostate cancer cells and at major sites of amino acid metabolism such as the liver and pancreas.

Axumin PET Scans are available in the United States.

PSMA

Prostate-specific antigen (PSA) and PSMA (prostate speficific membrance antigen) are different in several ways.

PSMA is a type II glycoprotein,  found to be overexpressed in prostate cancer cells, and its expression level has been correlated with disease aggressiveness

Bostwick and colleagues described PSMA immunohistochemical expression in 184 prostate specimens examined, all of which had PSMA expression and demonstrated a correlation between this expression and severity of cancer. There was an increase in the percentage of PSMA staining from benign epithelial tissue (69.5% of cells positive) to high-grade prostatic intraepithelial neoplasia (77.9% of cells positive) to malignant cells (80.2% of cells positive)

Prostate-specific membrane antigen (PSMA) ligands, have shown greater cancer sensitivity and specificity.

PSMA PETs are only available in the United States for research purposes at this time (March 2019).

Why are PET scans important for prostate cancer?

After definitive treatment for prostate cancer, there can be a recurrence up to 50% of the times but the patient may not know where the recurrence is occurring until the PSA is very high, 20ng/ml or higher. Again, this is after either radiation treatment or surgery. PET scans, especially those with improved radiotracers can detect cancer cells much earlier after prostate cancer treatment.

Which PET scans work best for Prostate Cancer and When should they be considered?

Numerous studies have compared Choline radiotracers with PSMA and Axumin showing better imaging in men with biochemical recurrence from PSMA and Auximin compared to choline radiotracers.

At this point the types of PET scans I recommend after prostate cancer recurrence is either PSMA or Axumin. PSMA PET is not yet commercially available in the United States but it is throughout Europe.

 

 

Reference: Practical Radiation Oncology, Feb 2018

Note: New York University (NYU) Langone Health offers Axumin (fluciclovine) but it is not on the list.