Types of Pet Tracers

Types of Pet Tracers

Fluorine-18 Fluorodeoxyglucose (F-18 FDG)

F-18 FDG was the first PET radiopharmaceutical on the market, and is still the most commonly used PET/CT tracer worldwide. Fluorodeoxyglucose (FDG) is a molecule very similar to normal glucose -the main sugar molecule of the body.  It localises in tissues with altered glucose metabolism and does not have a therapeutic use. Tissues with altered glucose metabolism include cancers, infections and areas of inflammation. 

F18-FDG does not remain in the body permanently, as soon as the cell starts to break down the radioactive sugar, it realises that the sugar is not “real” and then pushes it out of the cell. It is then carried to the kidneys and eventually reaches the bladder, where it leaves the body in urine. Another way of it being eliminated from the body is due to its natural process of decaying/breaking down, by the time you leave our practise there is minimal radioactive FDG left in your body. 

Uses of F-18 FDG:

Oncology

It is mostly used in oncology because cancer cells are metabolically active. They have a high rate of glucose consumption due to the rapid division of these cells, as well as expressing high levels of glucose transporters. An advantage of using PET/CT for imaging is because this is a type of functional imaging; we are looking at metabolic activity. Therefore, lesions (metastases) may be seen before there are any anatomical changes. The vast majority of cancers show an increased rate of glucose metabolism making imaging with F-18 FDG possible. There are a few types of cancers that do not take up FDG, your referring doctor will be aware of these cancers and will not send you unnecessarily. 

Infection and inflammation

The accumulation of F-18 FDG in inflammatory and infectious diseases is based on the high uptake in activated white blood cells which are typically released during an infection. These white bloods cells use glucose as an energy source and express a high amount of the glucose transporter on their cell membranes.

Indications for imaging in infection includes in fever of unknown origin i.e. when your doctor is unable to find the source of your infection, suspected infection of a prosthetic heart valve or vascular graft, to assess for infection of the spine, to determine the extent of Tuberculosis infection and response to therapy amongst many other indications.

Indications for imaging inflammation includes in assessment of sarcoidosis (growth of tiny collections of inflammatory cells in any part of your body) specifically of your heart but also elsewhere or vasculitis (inflammation of the wall of your blood vessels).

Preparation:

The injection is completely safe, it does not interact with your blood sugar level and will not cause a rise in sugar in your body even in diabetic patients. Please refer to our website ”Patient corner”  for further details regarding how to prepare, what to expect and specific preparations for diabetics. There is also a separate tab for imaging in patients with cardiac sarcoidosis, a special diet needs to be followed as well as a fasting period that needs to be adhered to for optimal scan quality.  It is very important to follow these instruction as these will influence the interpretation of the scan.

 

Gallium-68 DOTA-TATE/TOC/NOC (Ga-68 DOTA)

This radiopharmaceutical is used for imaging of well-differentiated (tumour made up of cells that look very similar to the normal cells) neuroendocrine tumours (NETs). NETs are a rare form of cancer that start in specialised cells called neuroendocrine cells. These cells have features similar to those of nerve cells and hormone-producing cells (endocrine cells). NETs express receptors called somatostatin receptors (SSTR), there are 5 different types of SSTR. It is this feature of the tumour that we use for imaging. Ga-68 DOTA-TATE/TOC/NOC are radioactive somatostatin receptor analogues which bind to SSTR on the tumour cells. 

NETs are broadly divided into two large groups, well-differentiated and poorly differentiated. This is determined by the Ki-67 % of the tumour, which is a proliferation index reported by the pathologist when evaluating a sample taken from the tumour. If the tumour is classified as well-differentiated, then this study may be used for imaging including for staging, to assess response to therapy, in suspected recurrence and even at times for diagnosis. 

The general cut-off value used in nuclear medicine to determine if you are a candidate for this scan is a Ki-67 of 20 %. If the Ki-67 % is higher than 20 % a F-18 FDG PET/CT may be more appropriate. Due to the diversity of these tumours, they do not always behave in the same way, and in certain instances we may perform both a Ga-68 DOTA-TATE/TOC/NOC and F-18 FDG PET/CT scan to accurately stage the disease. 

Preparation:

  • Ideally we would like to perform this scan at least 1 month after your last long-acting somatostatin injection (e.g. Lantreotide/Somatuline) or just prior to your next dose. 
  • If you are using short-acting somatisation (octreotide) we like to wait 24 hours after the last dose to perform imaging.
  • No further preparation is needed, you may eat and drink as normal prior to the study.

 

Gallium-68 Prostate specific membrane antigen (Ga-68 PSMA)

Prostate cancer is the most common cancer in men. Nearly all (± 90 %) adenocarcinomas of the prostate demonstrate increased expression of prostate-specific membrane antigen (PSMA). The PSMA of the Ga-68 PSMA binds to these receptors and Ga-68, the radioactive component, enables us to image the prostate cancer. Ga-68 PSMA imaging plays many different roles in prostate cancer, most notably after you have had your initial therapy with a good response in your PSA blood level however there is now an increase in the PSA (also known as biochemical recurrence) and the cause (such as a lymph node, bone lesion etc.) is not detected on conventional imaging using MRI or CT scans. It may also be used for staging, restaging and evaluating response to therapy. The most recent role is determining whether you may be a suitable candidate for treatment using Lutetium 177 (Lu-177) – PSMA peptide receptor therapy (PRRT).

Preparation:

There is no preparation required for this study, you may eat and drink as normal prior to the study.

 

Fluorine-18 Fluorodopa (F-18 FDOPA): 

This radiopharmaceutical has two main uses including in oncology and in the work-up of movement disorders, also known as Parkinsonism. In oncology it may be used in the evaluation of gliomas, a type of brain tumour, including in planning surgery or radiotherapy or to distinguish post-therapy changes seen on MRI from residual viable brain tumour. It may also be used in certain well-differentiated neuroendocrine tumours such as a phaechromocytoma or an insulinoma, which is a tumour of the pancreas. Regarding work up of movement disorders, it is used to confirm or exclude presynaptic dopaminergic (nerves using or involving dopamine as a neurotransmitter) degeneration, most commonly seen in Parkinson’s Disease.  

F-18 FDOPA is produced only once a month in South Africa, so please contact the Cape PET/CT centre for information on when the next available date is. 

Imaging procedure:

We will inject you with the radiopharmaceutical and then wait a certain duration of time prior to imaging. 

    • Oncology: Brain tumours we image at ± 10 minutes after injection and phaechromocytoma at ± 60 minutes. For insulinomas, imaging is acquired at 5-minutes and sometimes 30-minutes after injection.
  • Movement disorders:  We image at ± 90 minutes after injection.

Preparation:

Oncology including brain tumours, phaechromocytoma and insulinoma:

  • Please fast for at least 4-hours prior to this study, preferably not eating or drinking anything other than plain unflavoured water from midnight the previous evening. 
  • Diabetic patients, please ensure that your blood sugar level is not too low, too high is not a contra-indication to this study.

Movement disorders:

  • Please fast for at least 4-hours prior to this study, preferably not eating or drinking anything other than plain unflavoured water from midnight the previous evening. 
  • Diabetic patients, please ensure that your blood sugar level is not too low, too high is not a contra-indication to this study.
  • Certain medications need to be withheld for a duration of time, these are only neurological/psychiatric medications and include:
  • Talcopone (Tasmar®) at least 15 hours, Entacapone (Comtan®Stalevo®) at least 15 hours and Reserpine (Reserpine®Rinerdin®) 20 days.

You may continue normally with any other medication. Please contact our practice if further clarification is needed.