Overview of Nuclear Medicine PET/CT Head and Neck Cancer Assessment, Protocols, and Interpretation
PET-CT, or Positron Emission Tomography-Computed Tomography, is a type of scan used mainly for finding, assessing, and keeping an eye on certain tissues in the body that have a high metabolic rate, usually linked to cancer. PET part of the scan looks at how the body is functioning, while CT part gives a detailed picture of the body’s structure. Using these techniques together can – but not always – offer better results in terms of survival rates or finding the least invasive treatment option.
This overview is about how PET-CT is used for head and neck cancers. The term ‘head and neck cancer’ encompasses several types of cancers based on their location in the body. Most of these cancers are associated with tobacco and alcohol use and usually start in the lining of the mouth and throat area, where they are referred to as squamous cell carcinoma.
Another common cause of throat cancer is high-risk strains of the human papillomavirus (HPV). This type of cancer affects younger people more often and usually has better outcomes compared to head and neck cancers not caused by HPV. Another group of this type of cancer develops in the lymph nodes, the tiny, bean-shaped organs that produce and store cells that fight infection. Lymphomas, cancers of the glands (like the thyroid and salivary glands), and skin cancers are less common types of head and neck cancer and have different progression patterns and treatment options. This overview doesn’t cover these, but you can find more information in the National Comprehensive Cancer Network guidelines.
Anatomy and Physiology of Nuclear Medicine PET/CT Head and Neck Cancer Assessment, Protocols, and Interpretation
The treatment of squamous cell carcinoma in the head and neck (HNC SCC) is dependent on where the primary tumor is located. There are several areas where these tumors can develop, including the nasopharynx (top part of the throat behind the nose), larynx (voice box), hypopharynx (lower part of the throat), oral cavity (mouth), oropharynx (middle part of the throat), nasal cavity (inside of the nose), and paranasal sinuses (small hollow spaces in the skull around the nose).
Every one of these specific areas has its own function in the body, and therefore requires different surgical approaches and treatment strategies. How HNC spreads also depends on where the cancer primarily started.
Medical bodies like the American Joint Committee on Cancer (AJCC) and American Academy of Otolaryngology-Head and Neck Surgery (AAO-HNS) classify neck lymph nodes into seven categories, or ‘levels.’ This classification, which has remained the same since the 1990s, is key to understanding test results and maintaining consistent communication between medical professionals. It’s crucial to properly identify the main tumor site and describe whether the lymph nodes are involved in the cancer or not based on this AJCC/AAO-HNS classification.
A technique called positron emission tomography (PET) scans, approved by the FDA, is often used for HNC diagnosis. In this procedure, the patient is injected with a radioactive form of fluorine, which is linked to a type of sugar molecule. This radioactive sugar is safe to use and will concentrate in fast-growing cells, like cancer cells, that need more energy (provided by sugar). This allows the doctors to clearly see cancerous cells on the scan images.
Certain normal tissues also absorb this radioactive sugar faster, like the heart muscle, brain, the lining of the gastrointestinal tract (digestive system), and the small tubes in the kidneys. Other tissues, like brown fat, skeletal muscle, and the ducts in breasts, can also absorb it if they are performing their normal functions. But, if other types of tissue absorb this radioactive sugar, it could indicate a disease process, such as tumor growth, infection, or inflammation.
The amount of radioactive sugar absorbed by the cells can be measured and assigned a value, known as the standard uptake value (SUV). But, it’s important to note that the SUV is a relative measure and not absolute. It can be affected by many factors – from the calibration of the scanner to the patient’s blood glucose level. Therefore, defining what SUV levels should be considered ‘normal’ or ‘abnormal,’ or when to differentiate between benign (non-cancerous) and malignant (cancerous) tumor activity, can be subjective and can sometimes lead to incorrect scan interpretations.
Why do People Need Nuclear Medicine PET/CT Head and Neck Cancer Assessment, Protocols, and Interpretation
If you believe you have head and neck cancer (HNC), one of the most accurate ways to diagnose it is to use an imaging test known as 18-FDG PET-CT. This test is particularly helpful when a doctor can’t tell if a tumor is present by just looking or taking a biopsy. This test is great at detecting hidden or hard-to-find throat cancers with an accuracy rate of over 90%. The National Comprehensive Cancer Network (NCCN), a group of cancer experts, recommends this type of imaging when diagnosing various head and neck cancers that are hard to detect during a typical physical examination. These cancers can be found in areas such as the oral cavity (mouth), oropharynx (middle part of the throat), glottic larynx (vocal cords), and others.
The NCCN also recommends a PET-CT scan for suspected cancers that are hard to see or determine during physical exams or other imaging tests like CT or MRI. This advice applies specifically if it’s believed that the cancer may be in a particular group of lymph nodes in the neck.
PET-CT scans also play a crucial part in cancer staging, which is determining how advanced the cancer is. Doctors can use this type of scan to detect if the cancer has spread to your neck’s lymph nodes. It’s also the preferred method to find if the cancer has spread far from its initial location (distant metastases), but there are exceptions. For example, in the case of nasopharyngeal cancer (a type of head and neck cancer located in the upper throat), a chest CT scan is equally effective. However, MRI is a better choice than PET-CT for detecting if the cancer has spread to the brain since PET-CT is not as accurate in this situation.
After treatment, PET-CT scans can help identify whether the cancer has returned or the area has just inflamed benignly. Research from non-clinical trials suggests getting a scan as early as 6 weeks following surgery, but the NCCN advises a scan 12 weeks after surgery. Doctors use this scan to check if there is any sign of the cancer returning. If none is found and you have no symptoms, no further imaging is needed unless new symptoms pop up.
Also, the NCCN recommends the use of PET-CT or CT for patients initially treated with a form of chemotherapy before surgery. These scans help monitor the cancer’s progression and guide the decision whether to go for a surgery or not.
If the cancer is advanced and doctors treat it with radiation and chemotherapy instead of surgery, the NCCN recommends a PET-CT scan 3 to 6 months after treatment to check for any remaining cancer. Doing a scan too soon or too late after treatment could give inaccurate results.
For check-ups or surveillance after treatment, the NCCN does not see a significant difference between using a PET-CT, CT, or MRI. They recommended doing PET-CT scans at 12 or 24-month intervals after the treatment. Still, if there’s no sign of cancer three months after treatment, it’s likely no further scans at longer intervals will provide additional benefit.
Equipment used for Nuclear Medicine PET/CT Head and Neck Cancer Assessment, Protocols, and Interpretation
When you have an imaging procedure like a PET-CT, here’s what’s happening: An unstable form of Fluorine (18-F) in your body breaks down and releases particles called positrons. These positrons travel a short distance in your body before they come into contact with electrons and emit high-energy light, known as gamma rays. These gamma rays strike special crystals in the scanner that surrounds your body during the procedure.
These crystals, that can vary depending on the maker of the machine, turn the energy into electrical signals. This data is then converted by a computer to create images of the inside of your body. The scanner is set up to only pick up signals from gamma rays that come from the same location, so the computer knows exactly where in your body the radiation is coming from. This method generally can pick up details as small as 8 mm. However, if there are any changes in the body smaller than this, the scanner might not be able to detect them.
On the other hand, a CT scan uses an external radiation source to capture pictures of your body. As the radiation passes through your body, detectors pick up the amount that has been absorbed. This creates pictures showing the size, shape, and composition of the organs and any abnormal areas within your body.
The PET-CT procedure requires extensive additional equipment including a lab setup to handle the pharmaceuticals used. The Fluorine-18 is typically shipped to the location of the scan based on a pre-calculated dose per patient. This is because it is produced using a particle accelerator, which few medical facilities own.
The way that PET-CT images are interpreted may also vary because different computer programs can process and display the images in different ways. Some may superimpose PET images onto the CT images or present PET images in a 3D-like fashion.
In 2016, the American College of Radiology (ACR) outlined some criteria for these machines, such as the resolution of the images, sensitivity of photon detection, thickness of the slices in the scan, and scan time. However, they left some factors like the type of scanner, dose amounts, and whether or not to use contrast, up to the discretion of the individual facility.
For example, regarding dose amounts, some facilities may choose to inject the same amount of pharmaceuticals into each patient, or they may adjust the dose based on factors like body mass or the specific settings of the machine.
Another area of flexibility is when it comes to the use of a contrasting agent. While it can sometimes add more diagnostic information, it can also lower the readings of certain values, though this usually doesn’t interfere with interpreting the scan.
Finally, the time between injecting the pharmaceuticals and taking the scan might vary depending on the situation. Most facilities start taking images about 60 minutes after the injection, but this could be a little less or more based on specific cases or if the facility is participating in clinical trials.
Preparing for Nuclear Medicine PET/CT Head and Neck Cancer Assessment, Protocols, and Interpretation
The 2016 guidelines from the American College of Radiology (ACR) suggest that the doctor ordering a PET-CT scan should provide the imaging center with enough information to meet insurance requirements for the scan. This includes details about the patient’s symptoms, any past or current medical conditions, and previous treatments like surgeries or chemotherapy. Having this information helps the people conducting the scan to do it right and interpret the results accurately.
A PET-CT scan requires the patient to follow certain instructions to make sure the scan works well. The ACR suggests:
* Patients should avoid exercising and exposure to things that could cause inflammation (like smoking, alcohol) for 24 hours before the scan.
* A low-carb diet should be followed for the day leading up to the scan.
* Patients need to fast (avoid eating or drinking anything with sugar) for at least 4 hours before the scan.
* Patients should drink around 1 liter of water two hours before the appointment.
To make sure the scan can accurately measure the body’s use of sugar for energy, patients need to have their blood sugar tested just before the scan. If the blood sugar is too high or too low, the scan may need to be rescheduled.
Patients have things called brown fat and muscles in their body that can sometimes look like a disease on the scan. There are tricks to avoid this like keeping the patient warm, giving certain medications, and making sure the patient is relaxed and still during the scan.
In short, prepping and performing a PET-CT scan is a team effort between the doctor, the imaging center, and the patient. Following these guidelines helps to make sure the scan is accurate and useful for diagnosis or treatment.
How is Nuclear Medicine PET/CT Head and Neck Cancer Assessment, Protocols, and Interpretation performed
For a clear and accurate look at the head and neck, your doctor will scan you with your arms down. This helps to avoid distortions, or artifacts, in the scan that could blur the image of the head and neck. They might also use a device to keep your neck still during the scan, helping to prevent any movement that could affect the image’s clarity. Your doctor can also adjust the timing of the scan to match your breathing. This ensures the images are not affected by your chest’s movement while you breathe, although it might mean the scan takes a bit longer.
Depending on what the doctor is looking for, the intensity of the scan might vary. If they simply need to figure out how much the tumor has grown, a low-dose CT scan will be used. But if they’re planning your treatment, they’ll use a regular-dose CT scan for a clearer image and better analysis.
In determining whether an area is cancerous or not, doctors use something called the standard uptake value (SUV), the average of which can tell them if the process is likely benign (not cancerous) or malignant (cancerous). Many studies use 2.5 as the dividing line – if the average SUV is less than this, it’s likely benign; if it’s more, it could be malignant. This takes into account the expected ratio for non-cancerous tissue.
However, some doctors prefer comparing the SUV to what’s found in your blood. They argue it’s a more accurate way to tell if an area is likely to be cancerous. They might also consider the shape of the abnormality to aid their diagnosis, with a linear pattern often indicating a benign issue, such as inflammation after radiation treatment.
It’s important to remember that interpreting a scan is part science and part art. It requires the skill, experience, and judgment of the radiologist to make the most accurate assessment.
Possible Complications of Nuclear Medicine PET/CT Head and Neck Cancer Assessment, Protocols, and Interpretation
False-positive interpretations in medical imaging can occur for various reasons. These include image distortion, natural uptake of 18-FDG (a type of radioactive glucose used in PET scans), non-cancerous inflammatory conditions and tumors.
Image distortion can come from high-density objects in the body, like metal implants or contrast dyes used in imaging. This can make the PET scan show higher readings than there actually are. Also, if the PET and CT scan images do not align properly, this can result in incorrect readings. For such cases, doctors may interpret the PET images without using the computer software designed to correct for these errors.
Normal bodily function can also lead to false positives. The 18-FDG tracer can naturally be taken up by various tissues in the head and neck. These may include the thyroid and salivary glands, certain fat and lymphoid tissues, muscles around the eyes, involved in speech and swallowing, and any other muscles that were used prior to or during the exam. Increased muscle uptake can result from high insulin levels. To distinguish between this normal uptake and abnormal tissue, doctors use CT scans to measure the tissue’s density.
Several non-cancerous conditions can also mimic cancer on scans. Post-surgery inflammation or infection, for instance, can show up on PET-CT scans. Hence, if patients have an active infection or a collection of blood outside blood vessels, the scan could be falsely positive.
Non-cancerous tumors or conditions that cause excessive growth or abnormal development of cells can also give high readings in the head and neck tissues. For instance, bone marrow hyperplasia (overgrowth) after chemotherapy can increase the reading.
Some situations might result in false negatives, which means the PET-CT scan fails to detect a present issue. This could happen if the lesions are too small, or the patient has high blood sugar or insulin levels. If someone recently had medical treatments like chemotherapy, radiotherapy, or steroid therapy, it might also lower the chances of detecting the issue.
Lastly, the PET-CT scan might correctly identify presence of cancer, but it may not be able to distinguish between different types of cancer. For example, it can be difficult to tell the difference between lymphoma and head and neck cancer based on the imaging alone. However, the presence of related mucosal lesions and/or necrotic nodes might suggest diagnosis of head and neck cancer.
What Else Should I Know About Nuclear Medicine PET/CT Head and Neck Cancer Assessment, Protocols, and Interpretation?
Head and Neck Cancer (HNC) affects many people in the US every year, mainly due to tobacco use, alcohol abuse, and the Human Papillomavirus (HPV). People with HNC might have symptoms such as pain in the mouth or neck, discomfort or difficulty swallowing, earache, and painful swallowing.
HNC is more commonly seen in people over the age of 40. The chance of surviving for 5 years after diagnosis depends on how far the cancer has spread:
- If the cancer is only in the place it started: 77% to 92%
- If the cancer has spread to nearby areas: 38% to 60%
- If the cancer has spread to distant parts of the body: 20% to 39%
Interestingly, young patients with HPV-related HNC have a better 5-year survival rate of 85-90%.
Diagnosis of HNC is typically based on a physical exam. However, if this is not possible,
then computerized tomography (CT) scans with a special dye can detect most tumors because they cause changes in normal tissue size and density. Magnetic Resonance Imaging (MRI) can provide additional information by detecting inflammation that does not result in tissue size or density changes.
In certain situations, Positron Emission Tomography-Computed Tomography (PET-CT), a type of scan that shows how tissues and organs are functioning, can be helpful:
- When the original location of the cancer cannot be found despite swelling in the lymph nodes
- When one or more lymph nodes appear abnormal, but the suspicion of cancer is low.
- When there’s suspicion of cancer spreading, but other imaging tests have not detected it.
Even though PET-CT may not specifically distinguish cancer from other inflammatory conditions, it can guide doctors where to take a biopsy for the best and safest diagnosis.
PET-CT’s main benefit is in avoiding unnecessarily extensive surgeries that include removal of lymph nodes and surrounding structures from the neck, which may be more risky. By following specific guidelines, the use of PET-CT can be limited to appropriate clinical situations, reducing the number of PET-CTs that add no additional diagnostic information compared to other less expensive methods.
