PET Scans

A PET scan is an imaging test used to look for cancer throughout the body. PET (positron emission tomography) scans use a radioactive substance called a tracer that is taken up by your body’s organs and tissues after it’s injected into your vein. The more tracer that the organs and tissues take up, the brighter they show up on the PET scanner image. A PET scan can show areas of cancer that other imaging tests, such as MRI or CAT/CT scan, may not show.

A PET scan isn’t used to screen for a new breast cancer. This is because PET scans often can’t find very small cancers.

If you’ve already been diagnosed with breast cancer, your care team may order a PET scan to see if the cancer has metastasized — spread to areas of the body away from the breast — or has come back after your main treatments are complete (known as a recurrence).

Depending on the characteristics of the metastatic cancer, your doctor may order a PET scan to figure out if the cancer is responding to  treatment.

Both a PET scan and a CT scan create images of the body, but each scan has a different focus.

A CT scan images structures and creates an image of organs, bones, and tissues. Doctors can use CT scans to detect cancer sites based on their size, shape, and density, and to determine the exact location of a breast cancer.

A PET scan shows how the cells in the tissues in your body are working, meaning their biological features. Cells have biology that is different from most normal tissues, such as rapidly dividing. A PET scan targets these processes to find cancer sites.

In almost all cases, a PET scan is done with a CT scan, combining the benefits of both types of scans. This is commonly called a PET/CT scan by doctors.

There are different types of PET scans, named for the tracers they use. For breast cancer, there are currently two main types, though researchers are always looking for new tracers that may be more effective in detecting breast cancer. 

FDG stands for fluorodeoxyglucose, which is the tracer used in FDG PET scans. FDG has a chemical structure that is similar to glucose. It’s taken up by cells that need energy for basic functions, like growing and dividing. Cells that are growing and dividing rapidly, like many cancer cells, take up more FDG and so show up brighter on the PET scan image.

FDG PET scans are good at finding sites of many types of metastatic disease throughout the body. For people with inflammatory breast cancer, studies suggest that FDG PET scans may detect metastases not found by CT and bone scans in 25% to 50% of patients.

In general, experts say that an FDG PET scan is most appropriate for more advanced (stage IIB and stage III) breast cancer because these cancers have a higher risk of metastasizing. It’s also appropriate for metastatic breast cancer (stage IV).

FDG PET scans can be less accurate at detecting whether invasive lobular breast cancer has metastasized. This is because this type of breast cancer doesn’t metabolize glucose quickly.

Research suggests that FDG PET scans also aren’t very good at detecting whether small amounts of stage I or stage II breast cancer has spread to the lymph nodes. 

So an FDG PET scan isn’t recommended for people with lower-stage breast cancer because it may lead to false-positive results when used to image cancers with a low risk of metastasis. A false-positive result is when a test shows an area that looks suspicious but turns out to not be cancer.

FES stands for fluoroestradiol, the tracer in an FES PET scan. FES is marketed under the brand name Cerianna.

FES is a type of estrogen and attaches to hormone receptors. Doctors use FES PET scans to find out if hormone receptor-positive breast cancer has metastasized or recurred. FES PET scans will not detect hormone receptor-negative breast cancer.

Your doctor may also order an FES PET scan if the cancer grows while you’re on hormonal therapy medicine. The results of the scan can help your doctor decide whether to switch to a different hormonal therapy medicine or move to chemotherapy.

There are some major limitations on when an FES PET scan can be done. Several hormonal therapy medicines used to treat hormone receptor-positive breast cancer can interfere with the ability of FES to attach to hormone receptors. This includes all SERMs, like tamoxifen and Evista (chemical name: raloxifene) and all SERDs, like Faslodex (chemical name: fulvestrant) and Orserdu (chemical name: elacestrant). Aromatase inhibitors don’t interfere with FES.

If you’re on a SERM or a SERD, you need to stop taking it for a certain amount of time before an FES PET scan to allow the medicine to completely wash out of your system. The U.S. Food and Drug Administration says you should stop tamoxifen for six weeks before an FES PET scan and stop Faslodex for six months before an FES PET scan. Orserdu, which is a newer SERD, has a shorter wash-out period, about two or three weeks.

Stopping treatment for that long to have an FES PET scan can be challenging for both people with breast cancer and their doctors. Depending on your situation, your doctor may recommend you have an FES scan before you start hormonal therapy.

An FES PET scan also can’t detect breast cancer that has spread to the liver because the liver takes up a lot of FES whether there is cancer there or not. If your doctor suspects liver metastases, another test, such as a CT scan or an MRI, will be done.

For either type of PET scan, a small amount of the tracer is given into a vein, usually on the inside of your elbow or in the back of your hand. This is done about an hour before the scan because it takes that long for your body to absorb the tracer.

For an FDG PET scan, you have to lie still while the tracer is absorbed. For an FES PET scan, you don’t necessarily have to lie still.

After the tracer is absorbed, you will lie on a narrow, padded table that slides into the PET scanner. The scan takes about 15 to 30 minutes. You have to lie very still during the scan because moving can affect the results.

The entire PET scan process takes about two hours.

For both types of scans, you’ll need to wear loose, comfortable clothing and leave jewelry and valuables at home. Don’t wear clothes that have metal in or on them, such as hooks, buckles, or wires.

If you’re breastfeeding, stop breastfeeding for four hours before your appointment. You’ll also need to wait for 12 hours after the scan to breastfeed, so you’ll want to plan ahead for these feedings.

Tell your doctor if you’re afraid of enclosed spaces (claustrophobia). Your doctor may give you medicine to help you feel calm.

A PET scan itself causes no pain, but you may feel a sting when the tracer is injected into your vein. There is no recovery time unless you were given a medicine to help you relax.

You’ll be asked to avoid strenuous activity or activities that make you repeat motions for 24 to 48 hours before the scan. This includes running, jogging, cycling, or swimming. Doing these activities can lower the quality of the scan images.

For 24 to 48 hours before the scan, you can’t eat any carbohydrates or sugar (bread, pasta, potatoes, rice, candy, etc.).

You’ll also be asked to stay warm the day before and the day of your scan. If you get cold, it affects a type of fat in your body and makes it harder to get clear images from the scan.

You can’t eat anything in the six hours before the scan — not even cough drops or gum — and can drink only water.

If you have diabetes and wear a continuous glucose monitor or insulin pump, you’ll have to remove them. Talk to the doctor who manages your diabetes care about how to manage your glucose while you’re not wearing your device. Make sure you have an extra device with you to put on after the scan.

If the cancer is being treated with a SERM or a SERD, you will have to stop taking the medicine for a certain number of weeks before the scan. Your doctor should have discussed this with you before the scan was ordered.

You’ll be asked to drink between 32 and 64 ounces of water two hours before your appointment.

For one hour after the scan, don’t have skin-to-skin contact or sleep in the same bed with a pregnant person or anyone younger than 18. This is because the tracer contains a small amount of radioactive material.

Drink water and other liquids during the rest of the day to help remove the tracer from your body. You can start eating your regular diet right away, unless your doctor gives you other instructions.

It’s safe to be around people who are older than 18. You can spend time near others, kiss, hug, hold, and touch them.

If you’re breastfeeding, stop breastfeeding for 12 hours after the scan. During that time, you can still pump milk and then either throw it away or store it for 24 hours before giving it to the baby. During this 12-hour period, you can feed the baby with milk you’ve previously pumped or use formula.

PET scans have almost no side effects. Fewer than 1% of people who have a PET scan have pain at the injection site or changes in their senses of smell and taste for one or two days.

A radiologist reads the PET scan and sends the results to your doctor, usually within a few days. If the facility where you had the PET scan or your doctor’s office uses portal technology, the PET scan results will be available there.

Your doctor will help you understand what the PET scan results mean.