DOTATATE PET-CT scan: what it is, how to prepare and what to expect

PET-CT combines two types of picture in one session. The CT part shows structure, in other words what an organ looks like. The PET part shows function, meaning how tissues behave, using a small amount of a radioactive tracer. The Christie NHS Foundation Trust explains that this lets doctors look at both the structure and the function of organs and tissues, giving different information from a CT or MRI scan.

In a DOTATATE scan the tracer is gallium-68 attached to a molecule called DOTATATE. Many neuroendocrine tumours display somatostatin receptors on their surface. The tracer binds to those receptors, and the PET camera detects where the signal collects. The UK protocol we read describes neuroendocrine tumours as arising from cells in endocrine organs, or from endocrine islets in the thyroid, pancreas, respiratory or gastrointestinal tract.

You may also see the name DOTATOC. This is a closely related gallium-68 tracer. The Christie leaflet we cite describes a DOTATOC scan, and the practical steps it sets out are very similar to those in the DOTATATE protocol. Which tracer a hospital uses is a local decision, so it is worth checking your own appointment letter.

The UK protocol lists the main indications. They are localising and characterising neuroendocrine tumours, finding the primary tumour and sites of spread when diagnosing and staging, re-staging during follow-up to detect residual, recurrent or progressive disease, and selecting patients with metastatic disease who may be suitable for lutetium-177 DOTATATE (Lutathera) treatment, also called peptide receptor radionuclide therapy. It can also be used to monitor how a tumour responds to surgery, radiotherapy, chemotherapy or that radionuclide treatment.

The scan is therefore usually requested by a specialist team that already suspects or has confirmed a neuroendocrine tumour. It is not a general screening test for people without symptoms or a known diagnosis.

Local instructions vary, so always follow the appointment letter first. The two documents we read agree on the following points.

• Octreotide and lanreotide. The Christie leaflet asks you to tell the department as soon as possible if you have monthly octreotide LAR (Sandostatin LAR), monthly lanreotide (Lanreotide Autogel) or short-acting octreotide injections, usually 2 to 3 times a day. A short interruption of the drug may be needed, and any change is discussed with your consultant. Do not stop these injections on your own.

• Timing after injections. The protocol suggests the scan is not done until at least 3 weeks after a long-acting somatostatin analogue injection, and ideally as near as possible before the next scheduled injection, to avoid the drug blocking the receptors that the tracer needs.

• Food and drink. There are no fasting requirements. You can eat as normal, and the protocol encourages drinking water for 2 hours before.

• Other medicines. The Christie leaflet says to continue your other medicines as usual.

• Confirm your attendance. The tracer is individually ordered and has a very short shelf life, so scans cannot normally be delayed. The Christie asks you to phone at least 48 hours ahead to confirm you will attend.

• Clothes. Leave jewellery at home and remove metal, including metal on clothing. The leaflet suggests loose-fitting, comfortable clothes so that you may not need to change into a hospital gown.

• Pregnancy and breastfeeding. Pregnancy is a contraindication in the protocol, and strict guidance would be given to anyone breastfeeding after a DOTATATE injection. Tell the department in advance if you are or might be pregnant, or are breastfeeding.

On arrival, staff explain the procedure and confirm that you are not pregnant. The tracer is prepared on the day, so the Christie leaflet says you may be given a time to return, usually about 60 to 90 minutes after arrival, and can go to the cafe in the meantime. When you return, a cannula is placed in a vein in the hand or arm and the tracer is injected. If you have a central venous line, the team may be able to use it.

You then sit and relax quietly, alone, for around 60 minutes while the tracer travels around the body. This is called the uptake period. For radiation protection you cannot have anyone sitting with you, but you can read, listen to music and carry on eating and drinking. Before the scan you empty your bladder.

You lie on the scan bed with your arms raised above your head or by your sides. It is important to stay as still as possible. The Christie says most scans last 30 to 45 minutes. The protocol describes imaging from the mid-thigh to the vertex of the head about 60 minutes after the injection.

The Christie leaflet states that there are no side effects from the tracer injection, and that it will not make you sleepy or affect your ability to drive. If you are in a lot of pain or find it hard to keep still, tell the team beforehand, because movement can blur the pictures.

You are asked to wait about 15 minutes while the team checks that the pictures are technically good and that they have all the information they need. Drinking plenty of fluid for the rest of the day and emptying your bladder regularly helps clear the tracer.

The protocol gives an effective dose of 4 mSv for the gallium-68 DOTATATE scan, and the Christie leaflet describes the radiation risk as small and outweighed by the benefit of the information gained. The leaflet also asks that you do not bring anyone who is pregnant or under 18 to the appointment.

A radiologist, or specialist reporting the nuclear medicine images, writes a report that goes to the consultant who referred you. Your care team then discusses it with you, usually in clinic or at a multidisciplinary meeting. The scan result is one input. The team will also consider your symptoms, blood tests, CT or MRI pictures and, where relevant, tissue samples.

If a scan shows strong tracer uptake in a known tumour, that can be relevant when your team considers a receptor-targeted treatment. If uptake is weak, it does not necessarily mean the disease is absent or that no treatment is possible, so questions about what a particular result means are best taken to the specialist team.