Overview of Nuclear Medicine PET/CT Gastrointestinal Assessment, Protocols, and Interpretation
Simply put, digestive system cancers are a varied group of cancers that can behave and spread differently depending on their location, cell type, and growth pattern. Early accurate detection and proper characterization of the disease stage is extremely important to decide on a suitable treatment strategy. Innovations in the imaging techniques have proven helpful in non-invasively examining these cancers. However, the best way to confirm a cancer diagnosis is still through a tissue biopsy.
Positron emission tomography/computed tomography (PET/CT), a sophisticated imaging tool that combines the strengths of PET and CT, is commonly used in cancer diagnosis and management. To understand how this functions, consider that cancer cells in the gastrointestinal tract (GIT), or your digestive system, have an unusual ability to convert sugar into lactic acid for energy. This is known as the “Warburg effect.” These cancer cells are in a fast growth mode and so they require even more glucose than usual.
These cells have an increased number of glucose transporter 1 (GLUT-1) and an enzyme called hexokinase, which are responsible for the uptake and processing of glucose. This is where PET imaging comes in. It works by using a special form of glucose called fluorodeoxyglucose (18F-FDG). This compound is absorbed by the cancer cells, but doesn’t get used for making energy as regular glucose does, giving us the opportunity to visualize the high uptake areas on a PET scan, which indicates the presence of cancer cells.
In other cases, gallium PET/CT imaging is used particularly for specific types of tumors called neuroendocrine tumors (NETs), which overexpress certain types of receptors (SSTRs), and can capture images of these tumors and their spread in the body. This emphasizes how different imaging methods are judiciously combined to get the most accurate picture of a patient’s cancer.
Anatomy and Physiology of Nuclear Medicine PET/CT Gastrointestinal Assessment, Protocols, and Interpretation
The esophagus, the tube that carries food from your mouth to your stomach, and its junction (the place where it meets the stomach) are extremely important when it comes to treating esophageal cancers. Doctors need to know where exactly the cancer is located in order to provide the best possible treatment plan. This is particularly true if the treatment includes radiation therapy or surgery, which need to be executed with precision to avoid hurting useful cells.
The esophagus is divided into 4 parts for the purpose of treatment; knowing which part the cancer is in helps ensure that the entire tumor receives the treatment. Furthermore, the esophagus’ junction with the stomach is classified into 3 groups depending on the location of cancer’s centerpoint. Besides, there are several lymph nodes (small, kidney-shaped organs of the immune system distributed widely throughout the body) associated with the esophagus that are checked for cancer spread.
Your large intestine (also known as colon), is the section of your digestive tract that starts from a region named cecum and ends at another area known as anal verge – the area where the anus meets the skin. In case of colon cancer, a noteworthy detail is the cancer’s relationship with a lining called peritoneal reflection. If it has affected this lining, that means the cancer is more advanced and the chances are increased that it has, or will, spread.
The inside of your belly and its organs are lined with a thin membrane called the peritoneum, which has different arrangements along the colon. The peritoneum also forms a space in the pelvic area, this space differs slightly in males and females, and it only partially covers the rectum, the final portion of your intestine. The anal canal is the end of the digestive tract and consists of two key muscle layers which control bowel movements.
Colon cancer can spread along the “pathways” of blood vessels to lymph nodes, where they can grow and multiply. Tumors on the right side usually spread to the lymph nodes near the superior mesenteric artery whereas those on the left side spread to the ones near the inferior mesenteric artery. Tumors in the rectum can spread to nodes around the superior rectal artery and from there either onwards to the nodes by the aorta, the main artery, or down into the pelvic nodes. Anal cancer tends to spread via the lymph system rather than the blood vessels. Cancer above a certain point in the anal canal behaves like rectal cancer whereas cancer below that point spreads to lymph nodes that are along the leg.
Next, the topic of neuroendocrine tumors – these are formed from cells related to both nerves and hormone production, and can occur in almost any part of your digestive tract. Depending on the rate at which these cells multiply, these tumors can behave very differently. Some grow slowly and cause little issues, while others grow rapidly and are dangerous. A protein called Ki67 is used as a sign to categorize these tumors into three groups based on their characteristics and behavior.
The common way to look inside the body to view these tumors is through a type of scan called Positron Emission Tomography/Computed Tomography Imaging, or PET/CT Imaging for short. This involves a radioactive material totally safe for human body. When scanning, this material will collide with particles in your body, emit rays and then be detected by the machine. This processed is called ‘positron emission’ from which the name of the machine is derived.
In some scans, doctors may use a gallium-68 radiotracer – a radioactive substance that can attach itself to a specific type of hormone receptor on the cancer cells. They use this in combination with another chemical to make it easier for the substance to bind with the receptors. This method shines in revealing slow growing tumors, while aggressive, rapidly growing cells may be better seen using glucose (a type of sugar), as they consume glucose at a high rate for energy.
Why do People Need Nuclear Medicine PET/CT Gastrointestinal Assessment, Protocols, and Interpretation
The UK Royal College of Radiologists has provided guidelines for using a type of imaging test called FDG-PET/CT for different digestive system tumors. They published this in 2016.
For esophageal and gastroesophageal cancers (types of throat and stomach cancers), this test can be used to figure out how big the cancer is and if it has spread, including in patients who have already received a treatment that tries to shrink the tumor before surgery. This test can be very helpful in checking if the cancer has come back when other imaging tests can’t give a clear answer. It can also be used to see how treatments are working or to plan radiation therapy, which is a type of treatment that uses high energy to kill cancer cells.
For colorectal cancers (a type of bowel cancer), this test is recommended when there are signs of the disease spread to other organs including the lungs and liver from the beginning. It can be used to figure out the stage of the cancer when the disease comes back, to check how treatments are working, to look for recurrence when blood tests show there may still be cancer in the body, even if other tests can’t find it. It can also be used to check uncertain masses in the presacral area, which is the space between the sacrum and rectum, in the lowest part of your spine after treatment. In anal cancer (cancer in the opening through which stool leaves the body), this test is used to decide the cancer stage in those patients who need strong treatments. Planning radiation therapy or checking how the treatment is working are other reasons it might be used.
In a type of cancer called NETs, which can start anywhere throughout your body, this test is used to decide the stage of the disease or to check the disease stage when it comes back, but only in those tumors that do not show up on two other types of imaging tests that are commonly used for NETs. Another imaging test called 68Ga-DOTANOC PET/CT is used for less aggressive NET tumors (grades 1 and 2), to find unknown primary tumors (the tumor where the cancer started), or to check if cancer has come back.
When a Person Should Avoid Nuclear Medicine PET/CT Gastrointestinal Assessment, Protocols, and Interpretation
Getting a PET/CT scan needs careful planning. Things such as poor control over blood sugar levels, not fasting properly before the scan, or taking insulin right before the scan can all affect how the tracer, a substance used to highlight specific areas within the body on the scan, spreads in your body. Too much sugar in the blood can interfere with FDG, the tracer, binding to certain cell transporters, called ‘GLUT-1 transporters’. Also, if insulin levels are high, much of the FDG can be redirected to muscles and fat tissue instead.
The ideal time for a PET/CT scan would be at least 6 weeks after radiation therapy or surgery, and a minimum of 2 weeks after putting in a stent. This is to avoid inflammation at the site of intervention or treatment, which could make the disease appear more severe than it really is.
If a patient has a fear of enclosed spaces, also known as claustrophobia, they might need sedation for the scan. PET/CT is generally not suitable for expecting moms, so a different method should be used for checking the disease in them. Moms who are breastfeeding should avoid being close to their infants after the scan. Typically, it is advised that babies of such mothers should be bottle-fed by a third person for up to 12 hours after the scan.
Equipment used for Nuclear Medicine PET/CT Gastrointestinal Assessment, Protocols, and Interpretation
The substance used for the scan is made in a special machine called a cyclotron and brought into the hospital in containers that are lined with lead early in the morning. Special care is needed during this process to make sure nobody gets exposed to any radiation. The substance, called a radiotracer, is put into the patient’s body using a special type of syringe that is kept behind a screen made of lead.
The scanning machine (PET/CT) is essentially two different scanners housed in one unit. The patient lies on a bed that moves between the two parts of the machine during the scanning process.
The scanner works by using a scintillator – a material that converts radiation into light, which can then be turned into electrical signals that the machine can understand. The scanner is made up of thousands of these scintillators arranged in blocks, formed into a ring shape. In the past, bismuth germanate detectors were used, but now we use lutetium oxyorthosilicate detectors.
These detectors absorb energy from specific events within the body and create visible light photons, proportional to the energy they absorb. Different types of events are detected, some of which include two photons from the same event or from separate events.
The light photons are then directed towards the photodetectors. The most common photodetectors in a PET scanner are photomultiplier tubes (PMTs), which are vacuum tubes with photoreceptive surfaces. When the light photons interact with these surfaces, they create electrons which get amplified into an electrical signal that relates to the energy deposited by the PET scanner photons.
The first image produced by the PET scanner is not perfect. It can be influenced by the absorption of the PET scanner photons in the body, variations in the detector’s efficiency, and the simultaneous recording of different types of events. So, the image is corrected using a CT scan performed at the same time. The CT scan also provides additional information about the body’s structure that is not captured in the original PET images.
The final images are quantified (measured numerically) based on the concentration of the radioactive substance (in kilobecquerels per milliliter, or kBq/ml). This is then converted into a standardized uptake value (SUV) by adjusting for the patient’s weight and the amount of radioactivity injected. This measure allows us to pinpoint how much glucose metabolism is happening in specific areas within the body.
Preparing for Nuclear Medicine PET/CT Gastrointestinal Assessment, Protocols, and Interpretation
Preparing for an FDG-PET scan, which looks at how your body tissues are working, can be a bit complicated. This is because the scan uses a special substance related to glucose (that’s a type of sugar your body uses for energy), which is taken up by parts of your body that use glucose normally. Some things, like more activity in cells or higher sugar levels in your blood, can interfere with the results. So, to get the most accurate results, it’s important to control blood sugar levels and limit activities that aren’t related to the scan.
Controlling your blood sugar
Patients must not eat for 4 to 6 hours before the scan so there’s less interference between glucose and the special substance at the receptors in cells where both substances can enter. Having food can cause more insulin (a hormone that helps your cells use glucose) to be released, which can sometimes send glucose and the special substance to muscles. For people with diabetes, it’s usually best to have the scan in the morning after not eating overnight. Also, insulin should not be given before the scan. Before the scan, your blood sugar levels will be checked to make sure they’re lower than 200 milligrams per deciliter.
Limiting unnecessary muscle activity
Patients should avoid exercise the day before and on the day of the scan. Also, you must stay relaxed during the scan and avoid swallowing or talking. This is to prevent the substance used in the scan from going into the muscles involved in speech.
Managing brown fat activity
Brown fat is a type of fat that can cause misleading results on the scan because it uses glucose when you’re in a cold environment. To prevent this, a warm setting is provided for the patient.
Staying well hydrated
Patients should drink a lot of water before the scan. This helps your kidneys get rid of the glucose-like substance used in the scan. Also, patients are told to use the toilet right before the scan to reduce activity in the lower belly area caused by a full bladder. After the scan, you must drink plenty of water to help remove the radioactive material from the body. Drinks with caffeine aren’t recommended.
Getting ready for the scan
The procedure is explained to patients in detail to reduce anxiety. Patients will then sit in a room shielded with lead, where a small, thin tube will be inserted into a vein to minimize radiation exposure to the staff. A type of tubbing set known as a butterfly infusion set is used. Information about your vaccination and where you were injected is recorded to avoid confusion with scan results. This is because inflammation at the injection site can cause changes in the lymph nodes, which can be misleading on the scan.
How is Nuclear Medicine PET/CT Gastrointestinal Assessment, Protocols, and Interpretation performed
Doctors recommend different doses of 18F-FDG, an injector for medical scans, based on your age and weight. Adults usually get between 370 to 740 MBq, while kids usually get between 4 to 5 MBq per kilogram of their weight. After the injection, you’ll rest in a warm room for about an hour. This helps the 18F-FDG spread properly within your body. If any 18F-FDG leaks out of your vein during the injection, they’ll make a note of it. You’ll also be asked to empty your bladder right before the scan.
Typically, the scan doesn’t require any additional contrast agents added to your body. Instead, doctors advise you to drink water. This helps differentiate your gut during the scan.
When it’s time for the scan, you’ll lie down on a special bed that moves through the imaging machine. The first part of the scan generates a topogram, which is like a map of the scan area. For gastrointestinal cancers, this map usually covers the area from the base of your skull down to your mid-thigh. After creating the topogram, you’ll go through a quick CT scan. Then, the bed moves to the PET scan part of the machine to finish the combined scan process. The first images made by the PET scan are uncorrected images. Then these uncorrected images are improved or ‘corrected’ using the CT scan images.
The PET/CT images are then analyzed mainly based on what we call ‘maximum SUV’ or ‘SUV max’ measurements. This is a way to measure your body’s overall metabolism. To have a reference point, doctors will also record the SUV max of areas like your mediastinum (the central part of your chest where your heart and other organs are located) and your liver.
Please note though that no single SUV max value can accurately distinguish between benign (non-cancerous) and malignant (cancerous) processes. Doctors tend to use the following general guidelines:
- Low uptake: SUV max less than 2.5
- Intermediate uptake: SUV max between 2.5 and 5
- High uptake: SUV max greater than 5
- Intense uptake: SUV max greater than 10
This means, depending on how much of the 18F-FDG your body absorbs, they might grade your ‘uptake’ as low, intermediate, high, or intense. But this is just one part of the overall analysis.
Possible Complications of Nuclear Medicine PET/CT Gastrointestinal Assessment, Protocols, and Interpretation
A PET/CT scan is a safe procedure that exposes you to a low amount of radiation. The amount of radiation you’re exposed to during a routine diagnostic PET/CT scan is less than what you’d receive from a CT scan that uses contrast dye (a special dye that helps doctors see certain areas more clearly).
However, there can be some challenges with interpreting the results of a PET/CT scan because of certain things that can lead to “false positive” or “false negative” results. A false positive means the scan shows something abnormal, but it isn’t a disease. A false negative means the scan looks normal even though disease is present.
There could be a few reasons for false positives on a PET/CT scan. For example, if you move or breathe during the scan, this can lead to the images not matching up properly, which can make it look like there’s a problem when there isn’t. This is especially tricky with pictures of the abdomen, where movement in your bowels can lead to confusion when interpreting the images.
Brown fat, a type of fat in your body, can light up brightly on PET scans if you’re cold. When this happens in the area above your collarbones, it can look like there’s disease in your lymph nodes when there isn’t. To avoid this confusion, doctors will look more closely at the CT images to see if the bright areas are solid masses (which may signal disease) or the low-density fatty tissue of brown fat.
Accidentally injecting the radiotracer (the substance that “lights up” on a scan) outside the vessel can light up lymph nodes in your armpit, which can be mistaken for disease. Also, if you’ve had a vaccination, this can also cause a reaction in your lymph nodes. Both of these situations should be noted so that doctors know to take them into consideration when reading your scan.
Inflammation from recent radiotherapy, surgery, or biopsy may also light up on a PET scan. Similarly, chemotherapy can cause temporary changes in your bone marrow that make it light up on the scan. In both these cases, the timing of the scan is key to making the right interpretation.
With regard to false negatives, certain types of cancer that contain mucous might not “light up” brightly on a PET scan due to fewer cells, lack of glucose transporter proteins, and poor glucose metabolism. For such cases, a Gallium PET scan might be advised instead.
Sometimes, metallic or vascular stents, or a full bladder, can get in the way of seeing small diseases on the scan. Also, taking metformin, a drug used to treat diabetes, can cause the bowel to light up uniformly, which makes it challenging to evaluate any abnormalities in the colon.
What Else Should I Know About Nuclear Medicine PET/CT Gastrointestinal Assessment, Protocols, and Interpretation?
Esophageal cancer is a type of cancer that can develop anywhere along the length of the esophagus, the tube that runs from your throat to your stomach. The first step in diagnosing esophageal cancer is to examine a tissue sample from the esophagus under a microscope. There are two main types of esophageal cancer: squamous cell carcinoma, which typically affects the upper part of esophagus and often linked with alcohol consumption and smoking. The other type is adenocarcinoma which mostly affects the lower part of the esophagus. This type is most often caused by acid reflux and smoking.
After diagnosis, the severity of the cancer is determined using what is called the TNM classification, which is a standard system to describe the extent of cancer spread. This information helps doctors decide on the necessary treatments. For example, surgery may be the first step for early-stage cancers. For more advanced cancers, a combination of chemotherapy and radiation therapy may be used before surgery.
To decide which type of treatment is necessary, doctors will usually carry out a series of tests. These tests include a scan with a machine called a PET/CT scanner. This pinpoints where the cancer is in your body, and how fast it is growing.
Subsequently, doctors might carry out other tests, like an endoscopic ultrasound (a kind of ultrasound that is taken from inside your body), a CT scan and other types of tests to get more detailed information about where the cancer is located and how much it has spread.
It is also essential to assess other organs to understand if the cancer has spread. If cancer has spread to organs such as the lungs or liver, the PET/CT scan, MRI, or CT scans are used for the assessment. These tests will help to plan the surgery, if necessary.
Relating to colon cancer, it is a type of cancer that starts in the colon or rectum which are parts of large intestine. Most colon cancers start as small, noncancerous (benign) clumps of cells called adenomatous polyps. Early diagnosis of these polyps can help to prevent colon cancer.
However, diagnosing colon cancer may be a bit difficult because of several issues, like the movements of bowel and metabolic activity around it. The PET/CT scan may be used in some cases or a magnetic resonance imaging (MRI) scan or CT scan can be used.
In addition to these, some patients can develop rare anal cancers. Simple though it sounds, it is a form of cancer that affects the tissues of the anus, the opening at the end of the digestive tract. Just like with esophageal and colon cancer, a PET/CT scan may help with imaging and diagnosing the cancer.
